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Plugging efficiency of pouring aggregates through multiple boreholes into an inundated tunnel to prevent groundwater inrush disasters

Plugging efficiency of pouring aggregates through multiple boreholes into an inundated tunnel to... This paper presents an experimental investigation of the plugging efficiency of aggregate pouring through multiple boreholes under flowing water conditions into an inundated mine tunnel. Aggregate pouring into an inundated mine tunnel has been widely used and constitutes the premise for the salvage of flooded underground mines through fur - ther grouting. However, corresponding in-depth research is relatively limited due to the concealment of underground engineering. A visual experimental setup for aggregate pouring into a tunnel replica was built based on the theory of similarity between sediment movement and slurry pipeline transportation. Four factors, each with four levels, including the aggregate particle size (0.25–0.5, 0.5–1, 1–2 and 2–5 mm), distance between boreholes (0.5, 0.75, 1 and −2 −2 −2 1.5 m), initial water flow rate (0, 1.5 × 10 m/s, 2.4 × 10 m/s and 3 × 10 m/s) and tunnel inclination (0°, 3°, 5° and 8°) were selected in orthogonal experiments to investigate the plugging efficiency. Range and variance analysis of the four-level orthogonal array experimental results indicated that the factors influencing the plugging efficiency, varying between 83.96 and 98.15%, could be ranked in descending order as the initial water flow rate, aggregate particle size, distance between boreholes and tunnel inclination. The former two factors yielded a more significant influence than that of the latter two factors. The measured water pressure difference ranging from 16 to 32% between the front and back ends of the formed aggregate mass in the pouring process indicated that there remained a high resistance to water flow, even if the aggregate mass was not capped but reached a certain length. Plugging criteria for aggregate pouring into horizontal and inclined tunnels were then proposed. Moreover, the optimal distance between boreholes to form an effective bulkhead was determined, which could be defined as the distance between boreholes when the aggregate mass exhibits the fastest build-up and the plugging capacity is reached. Keywords: Inundated tunnel, Groundwater inrush disaster, Plugging efficiency, Aggregate pouring, Critical velocity, Sedimentation, Plugging criterion processes in an underground coal mine tunnel for inun- Introduction dation mitigation. The purpose of the aggregate pouring Grouting and sealing projects for emergency relief in stage is to form an accumulated aggregate section in the inundated mine tunnels are usually implemented in two tunnel to transform pipeline flow into permeate flow. The stages: aggregate pouring and grouting. Figure  1 shows purpose of the grouting stage is to reinforce the aggre- a schematic of the aggregate pouring and grouting gate section with cement grout to eliminate water flow (Jiang et al. 2020). Therefore, to achieve efficient sealing, *Correspondence: suiwanghua@cumt.edu.cn the first stage plays an important role in successful sal - vage efforts. Table  1 lists successful cases of this method School of Resources and Geosciences, Institute of Mine Water Hazard Prevention and Control Technology, China University of Mining in emergency relief and salvage of mines by pouring and Technology, Xuzhou 221116, China aggregates and subsequent grouting. In the treatment Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 2 of 18 Fig. 1 Schematic of the aggregate pouring and grouting processes for water inrush disaster treatment in a coal mine of extra-large water inrush disasters, a resistance sec- propagate along the aggregate accumulation mass and tion with a length of several hundred meters is generally improve the blocking effect (Zhang et  al. 2020). Inte - needed. grated technology, differences and difficulties of bulkhead To choose appropriate parameters for groundwater construction involving aggregate pouring and grouting in inrush disaster control using this approach, pouring tri- a tunnel under flowing water conditions were examined als should be conducted before aggregate pouring if the via a comparison to the general pregrouting method. The −3 3 unit water injection exceeds 10–20 × 10 m /min∙m. In evaluation of the hydrogeological conditions of the inun- pouring trials, different particle sizes, water–solid mass dated mine and identification of water sources, geological ratios and pouring rates should be systematically con- structure, cause of water inrush disasters and technical sidered and explored. The literature indicates that the flood control methods were considered (Guo 2005; Jiang process of aggregate accumulation and plugging in tun- 2009; Li 2010). nels can be divided into three stages: the bottom laying Innovative methods and materials have been used in stage, filling stage and plugging stage (Wang 2012; Mou recent years in the construction of plugging sections in et al. 2020). This division of plugging stages was based on inundated tunnels, such as grouting involving directional analysis; this division has not been verified in practice or drilling boreholes (Shao et  al. 2011; Ji 2014; Wang et  al. via laboratory testing. It has been found that the aggre- 2011). A successful project of water-conducting path- gate pouring speed and particle size should be continu- way plugging in a karst collapsed column completely ously adjusted to further reduce the seepage speed and eliminated groundwater flow along the main horizontal create favorable conditions for grouting after successful direction with 4 horizontal branch boreholes for aggre- capping. At this time, the water head difference between gate pouring into the subsurface and subsequent cement the two sides should be increased so that the slurry can grout injection (Zheng 2018). Other examples include Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 3 of 18 Table 1 Case histories of groundwater inrush disaster treatment in mine tunnels via aggregate pouring in China Coal mine Time of Groundwater Pressure Flowrate Time of Aggregate Cement Sealing effect accident resources and (MPa) (m /h) aggregate consumption grouts (%) 3 3 occurrence pathway pouring (m ) (× 10  kg) Longmen coal 11 December Cambrian 3.0 2,200.0 20 June–22 3100.5 757.0 97.1 mine, Henan 1994 karst aquifer, August 1994 through a geo- logic structure Renlou coal 4 March 1996 Ordovician 5.0 11,854.0– 25 April–25 129.9 15,032.0 85.0–90.0 mine, Anhui karst collapsed 34,570.0 May 1996 column Wucun coal 15 November Ordovician N/A 2,145.0, 18 January to 1,535.0 3,182.6 97.0–100.0 mine, Henan 1999 karst aquifer 2,378.0 (maxi- 10 March 2000 intersected by mum) faults and col- lapsed column Dongpang 12 April 2003 Ordovician 5.0 7,000.0 10 May–11 42,837.0 26,396.0 98.7 coal mine, karst collapsed June 2003 Hebei column Sanshuping 7 August 2011 Ordovician 3.0 8,000.0, 15 October–9 25,716.0 60,383.0 98.7 coal mine, karst aquifer 13,200.0 (maxi- November Shaanxi mum) 2011 Taoyuan coal 2 February Karst collapsed N/A 30,000.0 24 Febru- N/A 220,843.0 100.0 mine, Anhui 2013 column ary–16 May Panji coal mine 25 May 2017 Ordovician N/A 3,024.0 20 June–27 21,141.0 15,349.0 100.0 No. 2, Anhui karst collapsed July 2017 column Yiliang Mining 2 March 2019 Permian N/A 1,650.0 15 June–30 3900.0 910.0 98.0 Company, Maokou and October 2019 Yunnan Qixia Forma- tion aquifers successful plugging with an efficiency of 99% in the Luo - as the Eulerian–Lagrangian method, two-fluid model tuoshan coal mine, Inner Mongolia, and Yuchang coal (TFM) and CFD–DEM (discrete element method) cou- mine, Shaanxi, and the successful use of the method of pled method (Ekambara et  al. 2009;  Kaushal et  al. 2012; grouting bag emplacement to achieve rapid and control- Capecelatro and Desjardins 2013; Messa et  al. 2014; lable blocking of tunnels under high flow rates (Wang Arolla and Desjardins 2015; Messa and Malavasi 2015; et al. 2013; Zhu 2015). Uzi and Levy 2018; Messa and Matoušek 2020; Mou Experimental investigation and numerical simula- 2021). tion methods are commonly used methods to better To provide a theoretical reference and better under- understand the mechanism of aggregate accumulation stand the movement and deposition of aggregates in tun- and grouting sealing in inundated underground tun- nels, existing studies of slurry flow in pipelines are helpful nels. Experiments of aggregate pouring into a horizon- because of the similarity between aggregate accumula- tal pipeline through a single borehole indicated that tion in tunnels and slurry hydraulic transport in pipe- the influencing factors of the plugging efficiency in lines (Wilson 1979). Durand (1952) classified the state of descending order are the aggregate particle size, initial solid–liquid mixtures into three categories, i.e., homo- water flow velocity, and water–solid mass ratio (Wang geneous, intermediate, and heterogeneous mixtures, et  al. 2013; Li 2010; Zhang et  al. 2020). A large-scale based on suspension flow of sand and gravel in water. model was designed with opaque materials consider- Wasp et  al. (1977) categorized the flow regime of solids ing a water pressure of 5 MPa to simulate the movement in pipelines into homogeneous and heterogeneous types. of grout-conserving bags under high-pressure condi- Fei (1994) divided solid particles into bed loads and sus- tions (Dong et  al. 2020). Numerical simulations with pended loads. Soepyan et  al. (2013) and Zouaoui et  al. the computational fluid dynamics (CFD) technique have (2016) investigated the transport of sand-water slurries been widely conducted in the investigation, design and along a horizontal pipeline, and Miedema (2016) deter- management of slurry pipe flow in recent years, such mined the transition of slurry flow from heterogeneous Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 4 of 18 into homogeneous flow. There exists an important con - Consequently, the main purpose of this study was to cept to define the state of deposition and movement of experimentally investigate the plugging effects of aggre - particles in fluid. The critical velocity has been defined gate pouring through multiple boreholes under flow - as the minimum velocity of water at which aggregate ing water conditions in an inundated mine tunnel using particles transition from a static state to a moving state a visual test platform. Different factors influencing the regarding aggregate pouring in tunnels (Zhang et  al. plugging effect of aggregates and the sedimentation and 2020). Similar terms have been proposed for slurry trans- accumulation mechanism of aggregates were analyzed portation in pipelines. The concept of the limit deposit based on the experimental results. The results indicated velocity was used by Durand (1952) to denote the criti- that the initial water flow velocity and aggregate particle cal deposit velocity, and a representative Durand equa- size exert a more significant influence on the plugging tion was provided. The critical flow rate was proposed efficiency than that of the distance between boreholes and defined as the velocity at which solid particles are and tunnel inclination. The water pressure difference deposited from a suspended state to produce a fixed bed between the front and back ends of the formed aggregate (Graf et  al. 1970). Fei (1994) defined the critical velocity mass indicates resistance against water flow. In addition, of water as the velocity at which aggregate particles can this paper proposed a plugging criterion for aggregate be transported without settling. In accordance with the pouring and the optimal distance between boreholes to assumptions of Thomas (1979), a model for the predic - establish an effective bulkhead. The results can facilitate tion of the deposition velocity of a slurry comprising a better understanding of the effects of aggregate accu - fine particles was developed by Wasp and Slatter (2004). mulation mass formation, water pressure loss and aggre- A semiempirical equation to predict the critical deposit gate pouring into multiple boreholes on the plugging velocity was proposed (Pinto et al. 2014), and the effects efficiency. of the pipeline shape, particle size and inclination angle on the critical flow velocity were investigated (Kim et al. Materials and methods 2008; Bratland 2010; Corredor et  al. 2016). Moreover, Similarity the reduction in pressure along the pipeline can be used Pipeline flow of solids represents a special case of pipe - to evaluate friction loss in two-phase flow (Newitt et  al. line dynamics. This model is a nonconstant flow pipeline 1955; Turian et al. 1971; Turian and Yuan 1977; Ni et al. transportation model completely filled with fluid and sol - 1989; Vlasak et  al. 2012; Miedema 2015). The hydraulic ids. It is difficult to achieve complete pipeline geometric gradient is influenced by many factors, such as the par - similarity to the test requirements. The Froude criterion ticle size, specific gravity and fluid viscosity (Duckworth (gravity similarity criterion), Reynolds criterion and Euler and Argyros 1972; Allahvirdizadeh et al. 2016). However, criterion are commonly used similarity criteria in fluid water pressure changes and distribution along tunnels in modeling. Previous investigation and analysis studies of the multiple-borehole pouring process and the influence pipeline hydraulic simulation models have indicated that on the formation of water-blocking segments have not tunnels containing moving water can be mainly consid- yet been thoroughly investigated. ered with the Euler and gravity similarity criteria, fol- In summary, the abovementioned theoretical, experi- lowed by the Reynolds criterion. Therefore, the gravity mental and numerical studies were fundamental to similarity criterion was used for hydraulic simulation of a explain the accumulation and plugging mechanism for flooded tunnel. The fluid in this test was clean water with groundwater control and salvaging. However, in-depth a temperature of approximately 7 °C. The Reynolds num - research is relatively limited and lacks universality due ber Re > 2000 was calculated according to the minimum −2 to the concealment of underground engineering. For water flow velocity of approximately 1.5 × 10   m/s. example, the actual process of aggregate sedimentation Therefore, the flow state was turbulent in this test. The during pouring should be further investigated despite tunnel wall generally comprises concrete material with a the proposed division into three stages. The interaction roughness ratio ranging from 0.014 to 0.017, and plexi- between aggregate pouring into different boreholes and glass with a roughness ratio of 0.008–0.0095 was used in the borehole layout should be further examined. Because this test. According to the gravity similarity criterion, the multiple boreholes are usually used to achieve better roughness ratio could be obtained as  =  . This indi - and swifter plugging in practical projects, there exist few cates that the geometric ratio should be higher than 1:30 cases describing the use of a single borehole. Factors such to meet the gravity similarity criterion  (Chanson 2008). as the distance between boreholes and tunnel inclination Therefore, a geometric ratio between the model and pro - should be considered in orthogonal array design (OAD) totype of 1:20 was selected after comprehensive consid- experiments to study aggregate sedimentation, migra- eration of the laboratory facilities and similarity (Yalin tion and accumulation and criteria for plugging effects. 1971). Based on previous research results and similarity Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 5 of 18 −2 criteria, a pipeline was simulated with a cross-sectional orthogonal array was set at four levels: 0, 1.5 × 10  m/s, 2 −2 −2 area of 12  m , a length of 80 m and a water inrush flow of 2.4 × 10  m/s and 3 × 10  m/s. 6440  m /h. Table 2 lists the scale of different parameters. Tunnel inclination D. In rescue operations in flooded mines, grouting and blocking are mostly selected in Factors and materials horizontal or uphill tunnels relative to the movement The test parameters were determined referring to the direction of water flow. In this experiment, the tunnel cases listed in Table 1. inclination was selected at the 4 levels of 0°, 3°, 5° and 8°. Aggregate particle size A. The aggregate particle size in The aggregate injection rate in this test is the rate the experiments was selected at the four levels of 0.25– generated by the gravitational action of water flow and 0.5  mm, 0.5–1  mm, 1–2  mm and 2–5  mm. These levels aggregates. The rate is proportional to the length of were determined according to the similarity rule and the pouring borehole. If sidewall friction is ignored, on-site investigation of general coal mine water block- the final rate at which aggregates enter the tunnel is ing projects, where the selected aggregates included yel- v = 2gh = 3.13 m/s, where h is the distance from the top low sand (0.7–3.0  mm), rice-sized and centimeter-scale surface of the water–sand mixture in the funnel to the stone (3–5 mm), melon seed-sized stone (5–10 mm) and top of the tunnel. slag particles (10–20 mm). Sand and gravel particles were screened and washed to remove finer soil particles and Experimental setup impurities to ensure a high visualization clarity of the The plugging efficiency of aggregate injection into a sub - aggregate movement process. merged tunnel under water flow is affected by many fac - Distance between boreholes B. According to the layout tors, including natural and engineering factors, such as of grouting and blocking boreholes in submerged tun- the tunnel inclination, water flow rate, distance between nels, multiple boreholes are required for pouring and boreholes, aggregate particle size, water–solid mass ratio grouting, and the distance between boreholes generally and injection rate. The experimental setup could pro - varies between 15 and 30 m. The distance was selected at vide a constant water head and adjustable and recyclable four levels, namely, 0.5 m, 0.75 m, 1 m and 1.5 m, accord- water flow conditions at different flow rates. The system ing to the geometric scale in this study. Figure  2c shows could control the water–solid mass ratio and aggre- details of the borehole layout in this test. gate pouring rate. A data collection system could collect Initial water flow rate C. In flooded tunnels, plugging flowrate and water pressure data in real time. Images of sections are generally selected in concentrated water- aggregate accumulation and migration in the pipeline ways, exhibiting high water flow rates. For example, in were captured for process analysis. Figure  2 shows a the Dongpang coal mine in Hebei, the flow rate in the schematic and photo of the experimental setup. −2 water inrush tunnel reached as high as 13.88 × 10  m/s, and the flow rate in Panji coal mine No. 2 in Anhui Orthogonal array for experiment design −2 reached 7 × 10   m/s. In this test, the flow rate scal - Table  3 provides an orthogonal array with four factors, ing ratio is 4.47, and the initial water flow rate in the each with four levels. Hence, L 4 = 16 trials are needed in this experiment. The array was designed according to the OAD method proposed by C. R. Rao and used in industrial experimentation by Taguchi (Hedayat et  al. 1999). Table 2 Model scale set based on the Froude criterion Scale for F r Results and analysis Plugging efficiency Inertial force/gravity v /Lg Residual water flow channel Length   = 20 L L Under static water conditions, the resulting aggregate Time = 4.47 accumulation mass is tapered due to the angle of repose. Flow velocity v The cross section of the simulated tunnel model is circu - = 4.47 lar, and the contact conditions between the aggregates Flow quantity = 1788.85 and pipe wall cannot be completely matched, resulting Pressure   = 20 p L in a gap between the accumulation mass and pipe inner Force F   = 8000 L wall, namely, a residual water flow channel (Zhang et  al. Power 2 2020). = 35777 1 The residual water flow channel can be divided into a Roughness n 6 = 1.65 minimum residual channel and a final residual channel. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 6 of 18 Fig. 2 Experimental setup of the aggregate pouring process into an inundated mine tunnel through multiple boreholes under flowing water conditions. 1-water supply with a constant head; 2-water flow rate adjusting device; 3-water meter; 4-funnel for aggregate pouring; 5-tunnel replica; 6-inclination adjuster; 7-water cycling device; 8-camera; 9-PC; 10-data logger; 11-water pressure sensor; 12-electronical balance The minimum residual water flow channel is the channel former case occurs due to the existence of the angle of with the smallest cross-sectional area for water flow at a repose resulting in a gap with the tunnel wall, while the certain moment in the pipeline under aggregate accumu- latter case can be attributed to the existence of a balance lation. The minimum residual channel can be formed in between the resistance of the aggregate particles and the two cases, i.e., static and flowing water conditions. The carrying capacity of water flow. Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 7 of 18 Table 3 Orthogonal array L 4 for aggregate pouring into a tunnel with water flow Trial number Symbol of the trial Aggregate particle Distance between the Initial water flow Tunnel Plugging −2 size A (mm) boreholes B (m) rate C (× 10  m/s) inclination efficiency PE D (°) (%) 1 A1B1C1D1 0.25–0.5 0.5 0 0 98.15 2 A1B2C2D2 0.25–0.5 0.75 1.5 3 94.65 3 A1B3C3D3 0.25–0.5 1.0 2.4 5 88.79 4 A1B4C4D4 0.25–0.5 1.5 3.0 8 83.96 5 A2B1C2D3 0.5–1.0 0.5 1.5 5 94.80 6 A2B2C1D4 0.5–1.0 0.75 0 8 97.83 7 A2B3C4D1 0.5–1.0 1.0 3.0 0 91.86 8 A2B4C3D2 0.5–1.0 1.5 2.4 3 92.65 9 A3B1C3D4 1.0–2.0 0.5 2.4 8 93.20 10 A3B2C4D3 1.0–2.0 0.75 3.0 5 91.23 11 A3B3C1D2 1.0–2.0 1.0 0 3 97.74 12 A3B4C2D1 1.0–2.0 1.5 1.5 0 94.94 13 A4B1C4D2 2.0–5.0 0.5 3.0 3 96.09 14 A4B2C3D1 2.0–5.0 0.75 2.4 0 97.12 15 A4B3C2D4 2.0–5.0 1.0 1.5 8 97.19 16 A4B4C1D3 2.0–5.0 1.5 0 5 96.27 Fig. 3 Minimum residual and final channel under a hydrostatic conditions and b flowing water conditions Figure  3a shows a residual channel under static water The final residual water flow channel is the channel conditions. formed between the surface of the sand bed formed after Under flowing water conditions, aggregates could accu - stable accumulation of aggregates in the tunnel and the mulate behind the pouring boreholes because of water tunnel wall. With increasing accumulation, a stable sand flow. Due to friction between the aggregates and pipe bed is finally formed comprising sand with a certain par - wall, at a certain moment, there could occur a minimum ticle size after flow rate stabilization. At this time, the water flow channel in the pipe. The surface of the aggre - surface of the deposited mass below the water flow chan - gated mass below the channel exhibits a certain curvature nel is nearly flat and straight, and the aggregates move with lower values at the middle and higher values on the as a bedload under the action of water flow. Figure  3b sides, and eventually, the surface can become horizontal. shows a final residual water channel under flowing water conditions. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 8 of 18 When the water flow rate near the aggregate accumu - mass is continuously pushed forward until the outlet is lation mass in the tunnel remains stable and aggregate reached and overflow occurs. pouring continues, the bed sand height remains basically unchanged for a certain time. Therefore, the effect of Criterion for the plugging efficiency aggregate plugging remains unchanged. However, water The final plugging efficiency (PE ) is the ratio of the cross- flow can carry aggregates along the roadway toward the sectional area of the final aggregate accumulation mass to front end. With decreasing kinetic energy of the par- the cross-sectional area of the tunnel. ticles, aggregates can accumulate on the sand bed until almost reaching the top and filling the entire cross sec - PE(%) = (1) tion (Fig.  3b). The important factors influencing the size and type of the sand bed mass are the water flow rate, where P is the cross-sectional area of the final aggregate aggregate particle size and water flow depth. The smallest accumulation mass and P is the cross-sectional area of residual channel is formed before the final residual chan - 2 the tunnel, 31,415.93  mm . nel. At this time, the flow rate is the highest, the channel Figure  4 shows the cross-sectional area of the final is usually unstable, and the channel is also the closest to aggregate accumulation mass: 2 2 πD D 2H 2H −1 −1 (2) P = − cos 1 − − sin cos 1 − 4 8 D D the capping moment. At this time, the pouring amount where D is the diameter of the tunnel and H is the dis and aggregate particle size should be increased over time. tance between the aggregate accumulation mass surface In the end, a smaller residual channel results in a better and the top of the tunnel; H was measured by photo- plugging effect. When the flow rate in the final residual grammetry of the accumulation masses during pouring. channel is very high in the test and exceeds the critical velocity of the introduced aggregates, the accumulation Fig. 4 Cross-sectional area of aggregate deposits in a tunnel Table 4 Range analysis of the main factors of the plugging efficiency in a tunnel with flowing water Levels Aggregate particle size A (mm) Distance between boreholes B Initial water flow rate C Tunnel −2 (m) (10  m/s) inclination D (°) PE 91.39 95.56 97.50 95.52 PE 94.29 95.21 95.39 95.28 PE 94.28 93.90 92.94 92.77 PE 96.67 91.95 90.78 93.05 Range 5.28 3.60 6.71 2.74 Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 9 of 18 Fig. 5 Response graph for the main factors according to Table 4 Main effects boreholes was increased from B1 to B4, the plugging effi - Range and variance analysis methods are commonly used ciency tended to decrease. A smaller distance between methods for orthogonal experiments. These experiments boreholes resulted in faster agglomeration of the accu- can reveal the degree of influence of various factors on the mulation masses to form a water blocking section. In results, primary and secondary orders and optimal com- addition, the plugging effect was better. As the initial bination. Table 3 lists the final plugging efficiency (PE ). water flow rate was increased from C1 to C4, the plug - ging efficiency decreased. Because the water flow rate Range analysis increased, the ability to carry aggregates increased. The Table  4 shows the results of range analysis for the differ - aggregate accumulation mass height hardly increased ent factors, and Fig. 5 shows a response graph. at a high water flow rate. The larger the residual water As the aggregate particle size was increased from A1 channel is, the poorer the water blocking effect. As the to A4, the plugging efficiency tended to increase. This is tunnel inclination was increased from D1 to D4, the plug- related to the critical velocity of the aggregate particles. ging efficiency exhibited a decreasing trend. The tunnel The accumulation mass of coarse-grained aggregates is inclination angle was selected as lower than 10°, which is rapidly established, and the final height of the accumula - lower than the friction angle of sand. Therefore, this fac - tion mass is larger than that consisting of fine aggregates. tor imposed a relatively smaller effect on aggregate settle - Therefore, with increasing particle size, the plugging effi - ment and the water blocking effect than that of the other ciency increases. However, under static water conditions, factors. the aggregate accumulation mass exhibits an underwater The result of R > R > R > R indicates that the plug- C A B D angle of repose. When the particle size is greater than ging efficiency is influenced by these four factors in the 0.25 mm, the angle of repose increases with the aggregate following descending order: initial water flow rate, aggre - particle size, the residual channel area increases, and the gate particle size, distance between boreholes and tunnel plugging efficiency decreases. As the distance between inclination. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 10 of 18 Table 5 Variance analysis of the main factors of PE Source of deviation Deviation sum of Degrees of Mean square F ratio F (3,3) critical F (3,3) 0.025 0.05 squares freedom error value critical value Particle size of aggregates A 55.99 3 18.66 12.93 15.44 9.28 Distance between boreholes B 31.95 3 10.65 7.38 15.44 9.28 Initial water flow rate C 102.18 3 34.06 23.59 15.44 9.28 Tunnel inclination D 25.04 3 8.35 5.78 15.44 9.28 Error 4.33 3 1.44 Analysis of variance and the distance between boreholes (0.5–0.75  m) are, Table  5 lists the analysis of variance (ANOVA) results the higher the plugging efficiency, i.e., a fine aggregate for the factors of the aggregate water blocking effect. The particle size should be used in the case of a small drill- reliability of ANOVA is 1 − α , where α is set to 0.025 and ing spacing. The second one is located in the right and 0.5. upper parts. This region indicates that a good plugging F > F . The initial water flow rate is signifi - efficiency can be obtained for intermediate particle sizes C 0.025(3,3) cant at the level of α = 0.025 , with a credibility of 0.975. (0.75–1.5  mm) in the case of a large borehole spacing. Combined with the range analysis results, it can be found For larger particle sizes (2–5  mm), the overall plugging that the initial water flow rate is the most important fac - efficiency is high, and the influence of the borehole spac - tor affecting the rate of aggregate plugging. Therefore, ing decreases. The main reason for the formation of these the initial water flow rate is the first factor that should be two regions is the interaction between gravity and drag considered in the design of water plugging projects. forces. Regarding fine aggregates, the accumulation pro - F > F . The aggregate particle size is signifi - cess is slow due to low gravity forces, and multiple bore- 0.05(3,3) cant at the level of α = 0.05 , with a credibility of 0.95. holes pouring with a small spacing can easily and quickly Combined with the above range analysis results, it can be form a continuous accumulation mass. Given a small observed that the aggregate particle size is second only to borehole spacing, an accumulation mass can easily form, the initial water flow rate, so the aggregate particle size and the residual water channel is smaller. However, the notably influences the process of aggregate pouring. The accumulation mass can be easily washed away at a high effect of the distance between the boreholes and tunnel water flow rate. In contrast, the gravity forces of coarse inclination on the plugging efficiency is slight. particles are higher, and the deposition process in water The above analysis indicates that the factors influenc - occurs fast, resulting in a long continuous accumulation ing the plugging efficiency are mainly the initial water mass in the aggregate pouring process involving multiple flow rate and aggregate particle size, followed by the boreholes with a larger spacing. distance between boreholes and tunnel inclination. The In actual engineering, fine aggregates should be poured initial water flow rate is not an artificially controllable under a small borehole spacing. Coarser aggregates factor. The results also demonstrate that the optimal should be poured via more widely spaced boreholes or combination is A4B1C1D1, but this combination was not at certain intervals. At the same time, when selecting included in this experiment; the closest combination is the distance between boreholes, consideration should be A1B1C1D1. In actual engineering, we should choose the given to reducing the influence of water turbulence on remaining three factors of the aggregate particle size, dis- the pouring process to quickly form a bulk mass. tance between boreholes and tunnel inclination (or loca- tion) to achieve the best water plugging effect. Aggregate particle size and initial water flow rate Aggregates of different particle sizes achieve unique criti - Interaction between factors cal velocities, and it is important to choose aggregates Aggregate particle size and distance between boreholes of an appropriate particle size to realize fast aggregate The two factors of the aggregate particle size and distance accumulation and deposition and water blocking in a between boreholes can be artificially selected in actual given tunnel. Figure 6b shows that when the initial water −2 −2 engineering, and the experimental results indicate that flow rate ranges from 2 × 10 to 3 × 10  m/s, the plug- there exists interaction between these two factors. Fig- ging efficiency is proportional to the aggregate particle ure 6a shows that there are two regions of a high plugging size, and coarser aggregates can yield a better plugging efficiency. The first one in the left lower part suggests effect. When the initial water flow speed is lower than −2 that the smaller the aggregate particle size (0.25–0.5 mm) 2 × 10  m/s, aggregates with a particle size smaller than Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 11 of 18 Fig. 6 PE under various combinations of the different factors Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 12 of 18 0.5  mm can provide the best plugging effect. When the aggregate particle size varies between 1 and 2  mm, the plugging efficiency is lower than that achieved with the other aggregate particle sizes. The plugging efficiency for the different aggregate particle sizes is more greatly affected by the initial water flow rate, which does not sug - gest that the aggregate particle size determines the final effect. In actual engineering, it is necessary to choose an appropriate particle size for different initial water flow rates. Distance between boreholes and initial water flow rate According to the Reynolds criterion, turbulence is more likely to occur at a high flow rate when the cross section and characteristic length of the tunnel are fixed. It was found that the turbulence due to water flow restricted the pouring speed, and the turbulence exhibited a cer- tain relationship with the initial water flow rate. Consid - ering that the flow velocity of moving water is relatively high in actual projects, it is meaningful to choose a suit- able distance between boreholes. Figure  6c shows that the influence of the initial water flow rate on the plug - ging efficiency is very obvious, and the initial water flow rate exerts a greater influence than that of the distance between boreholes. When the initial water flow rate is low and the distance between boreholes is smaller than 0.75 m, a higher plugging efficiency can be obtained (red area). The plugging efficiency is low when the distance between boreholes ranges from 0.75 to 1.5 m. When the distance between boreholes is small, the turbulence more significantly affects the pouring process, and boreholes can become severely blocked. When the initial water flow −2 rate is higher than 2.4 × 10   m/s, the influence of the distance between boreholes on PE is relatively limited. The contour of high PE values is biased toward the direc- tion of a higher initial water flow rate. Water pressure during plugging The layout of water pressure sensors is shown in Fig.  2c. The water pressure measurements during the pour - ing process reveal that there exists a pressure difference between the front and back ends of the formed aggregate accumulation mass. Even if the aggregate accumulation mass has not been capped, water flow can still be greatly impeded if a certain length is reached. Figure  7a shows changes in the water pressure in the tunnel in Trial No. 2. In the figure, t is the time to start pouring, t is the time to end pouring from funnel No. 2, t is the time to end pouring from funnel No. 1, and t is the time when the accumulation mass form final Fig. 7 Water pressures during aggregate pouring in a Trials No. 2; b has stabilized. Before the start of pouring, due to fric- No. 3; c No. 12; and d No. 14 tion between the wall and water flow, the pressure in Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 13 of 18 the tunnel can be reduced along the flow direction. The pressure difference varying between 16.0% and 31.8% pressures measured by the six pressure sensors gradually occurs before and after accumulation mass stabilization decrease, but the difference is small. At time t , the water (from time t to t ) in the tunnel. In the case of the same 0 1 2 pressure in the tunnel instantaneously rises because the particle size, the faster the water flow is, the smaller the water–sand mixture in the funnel reaches inside the tun- pressure difference. Even if the aggregate accumulation nel and becomes connected with the tunnel. As pouring mass is not capped, under the premise of continuous proceeds, the accumulation masses below the two bore- pouring, the aggregate accumulation mass still provides a holes become connected, and the pressure at pressure suitable resistance effect on water flow. sensor No. 1 rises faster than that at the other sensors. This indicates that the aggregate accumulation mass has Discussion contacted the top of the tunnel and that the resistance to Sedimentation of aggregates water flow has increased. Then, at time t , the pressure Shape of the aggregate accumulation mass suddenly drops because pouring stops in borehole No. 2 The sand and water used in this experiment were mixed and the water pressure originating from the funnel drops. according to a water–solid mass ratio of 1:1–2:1, and the Afterward, as pouring is continued in borehole No. 1, the two boreholes were filled simultaneously. Figure  8 shows pressure at sensors No. 1 and No. 2 begins to rise to a the final shape of the aggregate accumulation mass. The stable value, and the pressure at sensors No. 3, 4 and 5 extension of the accumulation mass along the direction falls, resulting in a large pressure difference between the of water flow can be mainly divided into three segments: front and back ends of the accumulation mass. At time an accumulation mass on the front surface, an accumu- t , the pouring process in borehole No. 1 stops, and the lation mass on the back surface and an accumulation pressure finally drops. At this time, the smallest residual mass between the holes. The details for this division are channel is gradually stabilized by the water flow. At time described in the literature (Zhang et al. 2020). The shape t , the final residual channel is formed, and the water of the accumulation mass between the boreholes when final pressure in the tunnel eventually matches the initial pres- capped is controlled by the shape of the tunnel. The larger sure before pouring. the aggregate particle size is, the steeper the edge of the The water pressure changes in the other trials, as accumulation mass. Aggregate particles are subjected shown in Fig.  7b, c and d, are relatively similar to those to a water flow impulse on the water-facing surface, and in this trial. The energy source of aggregate movement in the accumulation mass occurs in convex contact with the the tunnel is the potential energy of water flow (or pres - tunnel wall on the upstream side and in concave contact sure difference), kinetic energy of the internal turbulence on the downstream side. An obvious difference in the and potential energy of the aggregate particles. A change accumulation mass shape in the experiments is mainly in water pressure reflects the water head loss, and a large manifested upon a reduction in aggregate particle size, Fig. 8 Aggregate accumulation mass in a Trial No. 6 (A = 0.5–1 mm, B = 0.75 m, C = 0, and D = 8°); b Trial No. 2 (A = 0.25–0.5 mm, B = 0.75 m, −2 −2 C = 1.5 × 10 m/s, and D = 3°); c Trial No. 10 (A = 1–2 mm, B = 0.75 m, C = 3 × 10 m/s, and D = 5°) and d Trial No. 14 (A = 2–5 mm, B = 0.75 m, −2 C = 2.4 × 10 m/s, and D = 0°) Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 14 of 18 increase in the water flow rate, increase in tunnel inclina - in particle accumulation. Before the aggregate mass tion and increase in pouring volume. In particular, there reaches its maximum height for the first time, particles is a notable difference in the shape of the aggregate accu - mainly accumulate under vertical settlement. When mulation mass between static and flowing water condi - the water flow rate increases close to the critical condi - tions in the tunnel. tion of particle movement, the particles begin to move Figure 8a shows the shape of the sand aggregate accu- mainly along the direction of water flow, and the accu - mulation mass in an 8° inclined tunnel under static mulation masses slowly become connected. When the water conditions. The aggregate settlement is simi - initial water flow rate is high, the critical condition of lar to that under free sedimentation in a river under particles is easily reached, and the time for the aggre- steady-state flow. Under gravity, buoyancy, drag and gates to settle is shortened. The maximum height of the other forces, aggregates accelerate at a certain ini- aggregate accumulation mass is smaller than that under tial speed, settle at the bottom of the tunnel, and are a low flow rate. Under the same conditions, the remain - quickly capped. When the aggregates contact the top ing water passages are larger, and the water blocking wall of the tunnel, they do not easily settle, particu- effect is poorer. The critical flow rate for coarse par - larly coarse particles, so the borehole can easily become ticles is higher than that for fine particles. The height plugged. In horizontal pipes, the accumulation masses of the coarse sand accumulation mass is larger than below the two boreholes are basically symmetrical on that of the fine sand accumulation mass. Coarse sand the left and right sides of the holes. With a smaller extends less along the direction of water flow than does distance between boreholes and finer aggregates, the fine sand. accumulation masses can be quickly connected to Gravity component forces alter the shape of the form a section with a suitable water plugging effect, aggregate accumulation mass in an inclined tunnel. for instance, in Trial No. 1. However, under a large The larger the angle is, the more obvious the difference distance between boreholes, there occur two separate in the morphology of the accumulation mass between accumulation masses in the inclined tunnel that are both sides of the pouring borehole. similar to an asymmetrical cone shape (Fig.  8a). The larger the angle and the smaller the particle size are, the Process of plugging more obvious the difference between the two sides of Figure  9a shows the accumulation process of aggregates the accumulation mass. Under static water conditions, with a particle size ranging from 2 to 5  mm in static the particle size generates an underwater friction angle water in Trial No. 6. In a tunnel inclined at 8° under static (angle of repose). To continuously move the aggregates water conditions, a mixture of water and fine sand enters along the horizontal direction, the impact force caused the tunnel at a certain initial water flow rate. Under the by pouring must be greater than the shear strength of action of gravity and drag forces in the water flow, the the aggregate accumulation mass with a certain shape. aggregates are scattered at the bottom of the tunnel. The accumulation masses become connected to form a Over time, the settlement process continues to intensify water blocking section with a certain length. Therefore, beneath the borehole, and aggregate settlement is biased reasonable selection of the distance between boreholes toward low-lying locations in the tunnel, with increased and aggregate particle size is very important. The con - extension. It is found that coarse sand accumulates faster tact conditions of the aggregate accumulation masses than do fine particles. The pouring speed in borehole No. in static water within the tunnel are not very stable. 2 is relatively high, so the aggregate accumulation mass When a certain water flow rate is applied, the particles below this borehole is formed faster. at the top of the aggregate accumulation mass move. In Figure  9b shows that a coarse sand-water mixture this case, the pouring speed or aggregate particle size enters a horizontal tunnel under flowing water condi - should be adjusted to form a blocking section that is tions at a certain initial rate. The instantaneous initial appropriate for further grouting reinforcement. speed of pouring is much higher than the water flow Figure  8b, c, and d show the accumulation pat- rate inside the tunnel; thus, the aggregates settle quickly. terns for three aggregate particle sizes under flowing However, with increasing height of the accumulation water conditions, at tunnel inclinations of 3°, 5° and mass, the size of the water flow channel decreases, and 0°, respectively. The factors influencing the accumula - the water flow rate continues to increase until it is higher tion and distribution of aggregates in flowing water are than the startup critical velocity of the particles. There - more complex than those in static water. The forces fore, particles are carried forward by the water flow and include gravity, lifting, drag, interparticle, and seep- mainly settle on the downstream side of the accumula- age forces. The state of water flow and movement of tion mass under bedload conditions, and fewer sand sand particles in flowing water should be considered Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 15 of 18 Fig. 9 Plugging process of aggregates (the arrows indicate water flow) particles settle on the side upstream. However, when the a water blocking section. For example, in Trial No. 14, aggregate mass approaches its maximum height for the aggregates filled the tunnel continuously until the space first time, the water flow becomes highly turbulent. The between the two boreholes was completely eliminated water flow accelerates at the front and decelerates at the (Fig. 9b). back. Due to the sudden change in flow velocity, the pres - Under the condition of the same distance between sures in the upper and lower parts of the back side of the boreholes, the required amount and time for aggre- accumulation mass below borehole No. 1 are inconsist- gate pouring are the issues to be considered in the ent. A circulation current occurs, which is similar to flow rapid construction of the aggregate accumulation mass. around a cylinder in hydrodynamics. Where the residual Connecting the accumulation masses between the channel is small, the water flow is turbulent, and trail - boreholes during the shortest time with the minimum ing vortices occur. For example, vortices can be clearly injection volume is important for plugging section con- observed in Trials No. 2 and No. 3. The finer the particles struction. As indicated above, coarse particles more are, the more severe the disturbance of the sand-carrying easily and more rapidly produce accumulation masses water flow, and the more particles in the suspended mass than do fine particles. However, the water pressure dif - are deposited on the back side of the accumulation mass. ference between the front and back ends of the accu- Under downstream extension, the space between the two mulation mass is more notable for finer particles than masses is continuously filled, which eventually produces for coarse particles under the same distance between boreholes. The greater the water head loss, the easier it Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 16 of 18 The third stage is defined as the capping and accumu - lation stage. As shown in Fig.  10c, the aggregate accu- mulation mass in the tunnel is similar to that on a static riverbed. The introduced aggregates move mainly in the form of a bedload and some suspended matter in the residual channel at the top of the accumulation mass. The aggregates are deposited on the back side of the entire accumulation mass. After reaching a certain length, the sand-carrying capacity of the water flow cannot move the mass. The particles are mainly dominated by contact and jumping movements. The aggregates continue to accu - mulate. A stable water blocking segment can eventually be formed. Plugging criterion The water-blocking section of an accumulation mass formed via aggregate pouring in a concentrated water- way initially resists water flow impulses and reduces the water flow intensity. At this time, there is still much Fig. 10 Aggregate movement at the different stages of the seepage in the water-blocking section, facing the dan- establishment of the water-blocking section. a Settlement and ger of being washed away by water at any time. It is accumulation stage; b Connection and accumulation stage; c Capping and accumulation stage necessary to implement grouting. Therefore, the aggre - gate plugging criterion is of great importance to deter- mine the timing of grouting. The above force balance analysis and plugging efficiency can be used as plug - is for the aggregate mass to become capped. Therefore, ging criteria in experiments and projects. In practical a reasonable combination of fine and coarse particles is engineering, the plugging criterion can be summarized the key to the rapid construction of the water-blocking as (Guo 2005): (1) the top interface of the aggregates section. in the borehole should be higher than the elevation of the tunnel. After sweeping the borehole bottom, the Movement and deposition of aggregates poured aggregates hardly settle, and the height of the The movement pattern of pipeline sand particles in this accumulation mass is higher than the top of the water test can be roughly divided into three stages based on the channel. (2) The water-blocking section can resist water three stages reported in the literature (Wang 2012). flow. There exists a large constant water level difference The first stage is denoted as the settlement and accu - between the front and back ends of the water-blocking mulation stage. Figure  10a shows that the aggregates section, and the friction between the water-blocking quickly and vertically accumulate before reaching the top section and tunnel wall can resist water flow impulses, for the first time. The cross-sectional area of the water ensuring that this section is not washed away in a channel decreases, the particles mainly move as a cohe- short time. (3) The unit water injection is lower than sive mass, and very few particles move as suspended −3 3 16 × 10  m /min∙m. Usually, all the above criteria are particles. met to determine that a stable water-blocking section The second stage is the connection accumulation stage. is built. At the same time, a critical condition for the As shown in Fig.  10b, the accumulation mass reaches its movement of particles in a horizontal or an inclined maximum height for the first time, namely, the water flow tunnel can be used to evaluate the stability of aggre- carrying capacity is higher than the critical condition for gates (Zhang et al. 2020). particle movement. At this time, the water flow is turbu - lent and accompanied by vortices. The settling particles move in the form of bed and suspended masses under Limitations and further study the action of turbulent water flow and vortices. The bed The injection speed in the aggregate pouring process in mass includes contact and jumping masses, which settle this experiment is different from the actual speed. The downstream. The accumulation mass extends forward pouring speed affects the porosity and other proper - and finally is connected into a whole. ties of the aggregate accumulation mass. The angle of Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 17 of 18 the tunnel selected in this study is very low. When the is greatly restricted, even if the aggregate mass is not tunnel inclination is greater than the angle of repose capped but reaches a certain length. of the aggregate accumulation mass, there are certain The critical condition for particle sedimentation and changes. It is therefore meaningful to study the influ - accumulation is that the equilibrium state of the result- ence of the injection speed, sequence of aggregate ant forces should reach zero before the deceleration pouring, and mixing of different particle sizes on the motion of the particles. A plugging criterion for aggre- plugging effect. Generally, the actual tunnel cross sec - gate deposition in horizontal and inclined tunnels was tion is not circular. The sedimentation and migration proposed. Moreover, it was proposed that the mini- processes of particles in different cross-sectional shapes mum plugging length of the aggregate deposit should will vary. In the next step, a cross-sectional shape closer be greater than the theoretical minimum length of to that in actual projects will be selected during aggre- effective grouting. When the aggregate pouring bore - gate pouring into a large-passage tunnel. In addition, holes are not influenced by each other, a small spacing the pressure in this test is low. Certain properties of between boreholes should be selected. high-pressure water flow can influence aggregate accu - Acknowledgements mulation, including the state, density, and permeability. The authors thank the Natural Science Foundation of China for the provided In actual projects, the water pressure reaches higher support under Grant No. 41877238. than 3  MPa. Further research should be conducted to Author contributions investigate the effect of high-pressure water flow on the WS and GZ proposed the main idea of this study and designed the experi- properties of aggregate accumulation. Another limi- ments. SH performed the experiments and analyzed the results. JC and SY performed the field investigation and edited and wrote part of the paper. All tation is that the model wall is relatively  smooth, and authors have read and approved the final manuscript. the roughness is not considered. A follow-up study should focus on the effect of different roughnesses on Funding This study was supported by the Natural Science Foundation of China under the blocking effect of the aggregate accumulation mass Grant No. 41877238. under the premise of ensuring visualization. Availability of data and materials All data generated or analyzed in this study are included in the published Conclusions article. An experimental investigation on the plugging effects of aggregate pouring through multiple boreholes under Declarations flowing water conditions in an inundated tunnel was conducted, and the results were examined in this paper. Competing interests The authors declare that they have no competing interests. A visual test platform for aggregate pouring into a tunnel was built. This platform could realize the visuali - Author details zation of the aggregate pouring process into the tunnel School of Resources and Geosciences, Institute of Mine Water Hazard Prevention and Control Technology, China University of Mining and Technol- under multiple factors and levels and realize real-time ogy, Xuzhou 221116, China. China Design Group Co., Ltd., Nanjing 210014, collection of image, pressure and flow data. 3 China. National Coal Mine Water Hazard Prevention Engineering Technology A four-level orthogonal array experimental test was Research Center, Suzhou 234000, Anhui, China. Institute of Mining Engineer- ing Research and Design, BGRIMMM Technology Group, Beijing 100160, China. performed. 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Plugging efficiency of pouring aggregates through multiple boreholes into an inundated tunnel to prevent groundwater inrush disasters

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2197-8670
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10.1186/s40677-022-00217-2
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Abstract

This paper presents an experimental investigation of the plugging efficiency of aggregate pouring through multiple boreholes under flowing water conditions into an inundated mine tunnel. Aggregate pouring into an inundated mine tunnel has been widely used and constitutes the premise for the salvage of flooded underground mines through fur - ther grouting. However, corresponding in-depth research is relatively limited due to the concealment of underground engineering. A visual experimental setup for aggregate pouring into a tunnel replica was built based on the theory of similarity between sediment movement and slurry pipeline transportation. Four factors, each with four levels, including the aggregate particle size (0.25–0.5, 0.5–1, 1–2 and 2–5 mm), distance between boreholes (0.5, 0.75, 1 and −2 −2 −2 1.5 m), initial water flow rate (0, 1.5 × 10 m/s, 2.4 × 10 m/s and 3 × 10 m/s) and tunnel inclination (0°, 3°, 5° and 8°) were selected in orthogonal experiments to investigate the plugging efficiency. Range and variance analysis of the four-level orthogonal array experimental results indicated that the factors influencing the plugging efficiency, varying between 83.96 and 98.15%, could be ranked in descending order as the initial water flow rate, aggregate particle size, distance between boreholes and tunnel inclination. The former two factors yielded a more significant influence than that of the latter two factors. The measured water pressure difference ranging from 16 to 32% between the front and back ends of the formed aggregate mass in the pouring process indicated that there remained a high resistance to water flow, even if the aggregate mass was not capped but reached a certain length. Plugging criteria for aggregate pouring into horizontal and inclined tunnels were then proposed. Moreover, the optimal distance between boreholes to form an effective bulkhead was determined, which could be defined as the distance between boreholes when the aggregate mass exhibits the fastest build-up and the plugging capacity is reached. Keywords: Inundated tunnel, Groundwater inrush disaster, Plugging efficiency, Aggregate pouring, Critical velocity, Sedimentation, Plugging criterion processes in an underground coal mine tunnel for inun- Introduction dation mitigation. The purpose of the aggregate pouring Grouting and sealing projects for emergency relief in stage is to form an accumulated aggregate section in the inundated mine tunnels are usually implemented in two tunnel to transform pipeline flow into permeate flow. The stages: aggregate pouring and grouting. Figure  1 shows purpose of the grouting stage is to reinforce the aggre- a schematic of the aggregate pouring and grouting gate section with cement grout to eliminate water flow (Jiang et al. 2020). Therefore, to achieve efficient sealing, *Correspondence: suiwanghua@cumt.edu.cn the first stage plays an important role in successful sal - vage efforts. Table  1 lists successful cases of this method School of Resources and Geosciences, Institute of Mine Water Hazard Prevention and Control Technology, China University of Mining in emergency relief and salvage of mines by pouring and Technology, Xuzhou 221116, China aggregates and subsequent grouting. In the treatment Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 2 of 18 Fig. 1 Schematic of the aggregate pouring and grouting processes for water inrush disaster treatment in a coal mine of extra-large water inrush disasters, a resistance sec- propagate along the aggregate accumulation mass and tion with a length of several hundred meters is generally improve the blocking effect (Zhang et  al. 2020). Inte - needed. grated technology, differences and difficulties of bulkhead To choose appropriate parameters for groundwater construction involving aggregate pouring and grouting in inrush disaster control using this approach, pouring tri- a tunnel under flowing water conditions were examined als should be conducted before aggregate pouring if the via a comparison to the general pregrouting method. The −3 3 unit water injection exceeds 10–20 × 10 m /min∙m. In evaluation of the hydrogeological conditions of the inun- pouring trials, different particle sizes, water–solid mass dated mine and identification of water sources, geological ratios and pouring rates should be systematically con- structure, cause of water inrush disasters and technical sidered and explored. The literature indicates that the flood control methods were considered (Guo 2005; Jiang process of aggregate accumulation and plugging in tun- 2009; Li 2010). nels can be divided into three stages: the bottom laying Innovative methods and materials have been used in stage, filling stage and plugging stage (Wang 2012; Mou recent years in the construction of plugging sections in et al. 2020). This division of plugging stages was based on inundated tunnels, such as grouting involving directional analysis; this division has not been verified in practice or drilling boreholes (Shao et  al. 2011; Ji 2014; Wang et  al. via laboratory testing. It has been found that the aggre- 2011). A successful project of water-conducting path- gate pouring speed and particle size should be continu- way plugging in a karst collapsed column completely ously adjusted to further reduce the seepage speed and eliminated groundwater flow along the main horizontal create favorable conditions for grouting after successful direction with 4 horizontal branch boreholes for aggre- capping. At this time, the water head difference between gate pouring into the subsurface and subsequent cement the two sides should be increased so that the slurry can grout injection (Zheng 2018). Other examples include Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 3 of 18 Table 1 Case histories of groundwater inrush disaster treatment in mine tunnels via aggregate pouring in China Coal mine Time of Groundwater Pressure Flowrate Time of Aggregate Cement Sealing effect accident resources and (MPa) (m /h) aggregate consumption grouts (%) 3 3 occurrence pathway pouring (m ) (× 10  kg) Longmen coal 11 December Cambrian 3.0 2,200.0 20 June–22 3100.5 757.0 97.1 mine, Henan 1994 karst aquifer, August 1994 through a geo- logic structure Renlou coal 4 March 1996 Ordovician 5.0 11,854.0– 25 April–25 129.9 15,032.0 85.0–90.0 mine, Anhui karst collapsed 34,570.0 May 1996 column Wucun coal 15 November Ordovician N/A 2,145.0, 18 January to 1,535.0 3,182.6 97.0–100.0 mine, Henan 1999 karst aquifer 2,378.0 (maxi- 10 March 2000 intersected by mum) faults and col- lapsed column Dongpang 12 April 2003 Ordovician 5.0 7,000.0 10 May–11 42,837.0 26,396.0 98.7 coal mine, karst collapsed June 2003 Hebei column Sanshuping 7 August 2011 Ordovician 3.0 8,000.0, 15 October–9 25,716.0 60,383.0 98.7 coal mine, karst aquifer 13,200.0 (maxi- November Shaanxi mum) 2011 Taoyuan coal 2 February Karst collapsed N/A 30,000.0 24 Febru- N/A 220,843.0 100.0 mine, Anhui 2013 column ary–16 May Panji coal mine 25 May 2017 Ordovician N/A 3,024.0 20 June–27 21,141.0 15,349.0 100.0 No. 2, Anhui karst collapsed July 2017 column Yiliang Mining 2 March 2019 Permian N/A 1,650.0 15 June–30 3900.0 910.0 98.0 Company, Maokou and October 2019 Yunnan Qixia Forma- tion aquifers successful plugging with an efficiency of 99% in the Luo - as the Eulerian–Lagrangian method, two-fluid model tuoshan coal mine, Inner Mongolia, and Yuchang coal (TFM) and CFD–DEM (discrete element method) cou- mine, Shaanxi, and the successful use of the method of pled method (Ekambara et  al. 2009;  Kaushal et  al. 2012; grouting bag emplacement to achieve rapid and control- Capecelatro and Desjardins 2013; Messa et  al. 2014; lable blocking of tunnels under high flow rates (Wang Arolla and Desjardins 2015; Messa and Malavasi 2015; et al. 2013; Zhu 2015). Uzi and Levy 2018; Messa and Matoušek 2020; Mou Experimental investigation and numerical simula- 2021). tion methods are commonly used methods to better To provide a theoretical reference and better under- understand the mechanism of aggregate accumulation stand the movement and deposition of aggregates in tun- and grouting sealing in inundated underground tun- nels, existing studies of slurry flow in pipelines are helpful nels. Experiments of aggregate pouring into a horizon- because of the similarity between aggregate accumula- tal pipeline through a single borehole indicated that tion in tunnels and slurry hydraulic transport in pipe- the influencing factors of the plugging efficiency in lines (Wilson 1979). Durand (1952) classified the state of descending order are the aggregate particle size, initial solid–liquid mixtures into three categories, i.e., homo- water flow velocity, and water–solid mass ratio (Wang geneous, intermediate, and heterogeneous mixtures, et  al. 2013; Li 2010; Zhang et  al. 2020). A large-scale based on suspension flow of sand and gravel in water. model was designed with opaque materials consider- Wasp et  al. (1977) categorized the flow regime of solids ing a water pressure of 5 MPa to simulate the movement in pipelines into homogeneous and heterogeneous types. of grout-conserving bags under high-pressure condi- Fei (1994) divided solid particles into bed loads and sus- tions (Dong et  al. 2020). Numerical simulations with pended loads. Soepyan et  al. (2013) and Zouaoui et  al. the computational fluid dynamics (CFD) technique have (2016) investigated the transport of sand-water slurries been widely conducted in the investigation, design and along a horizontal pipeline, and Miedema (2016) deter- management of slurry pipe flow in recent years, such mined the transition of slurry flow from heterogeneous Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 4 of 18 into homogeneous flow. There exists an important con - Consequently, the main purpose of this study was to cept to define the state of deposition and movement of experimentally investigate the plugging effects of aggre - particles in fluid. The critical velocity has been defined gate pouring through multiple boreholes under flow - as the minimum velocity of water at which aggregate ing water conditions in an inundated mine tunnel using particles transition from a static state to a moving state a visual test platform. Different factors influencing the regarding aggregate pouring in tunnels (Zhang et  al. plugging effect of aggregates and the sedimentation and 2020). Similar terms have been proposed for slurry trans- accumulation mechanism of aggregates were analyzed portation in pipelines. The concept of the limit deposit based on the experimental results. The results indicated velocity was used by Durand (1952) to denote the criti- that the initial water flow velocity and aggregate particle cal deposit velocity, and a representative Durand equa- size exert a more significant influence on the plugging tion was provided. The critical flow rate was proposed efficiency than that of the distance between boreholes and defined as the velocity at which solid particles are and tunnel inclination. The water pressure difference deposited from a suspended state to produce a fixed bed between the front and back ends of the formed aggregate (Graf et  al. 1970). Fei (1994) defined the critical velocity mass indicates resistance against water flow. In addition, of water as the velocity at which aggregate particles can this paper proposed a plugging criterion for aggregate be transported without settling. In accordance with the pouring and the optimal distance between boreholes to assumptions of Thomas (1979), a model for the predic - establish an effective bulkhead. The results can facilitate tion of the deposition velocity of a slurry comprising a better understanding of the effects of aggregate accu - fine particles was developed by Wasp and Slatter (2004). mulation mass formation, water pressure loss and aggre- A semiempirical equation to predict the critical deposit gate pouring into multiple boreholes on the plugging velocity was proposed (Pinto et al. 2014), and the effects efficiency. of the pipeline shape, particle size and inclination angle on the critical flow velocity were investigated (Kim et al. Materials and methods 2008; Bratland 2010; Corredor et  al. 2016). Moreover, Similarity the reduction in pressure along the pipeline can be used Pipeline flow of solids represents a special case of pipe - to evaluate friction loss in two-phase flow (Newitt et  al. line dynamics. This model is a nonconstant flow pipeline 1955; Turian et al. 1971; Turian and Yuan 1977; Ni et al. transportation model completely filled with fluid and sol - 1989; Vlasak et  al. 2012; Miedema 2015). The hydraulic ids. It is difficult to achieve complete pipeline geometric gradient is influenced by many factors, such as the par - similarity to the test requirements. The Froude criterion ticle size, specific gravity and fluid viscosity (Duckworth (gravity similarity criterion), Reynolds criterion and Euler and Argyros 1972; Allahvirdizadeh et al. 2016). However, criterion are commonly used similarity criteria in fluid water pressure changes and distribution along tunnels in modeling. Previous investigation and analysis studies of the multiple-borehole pouring process and the influence pipeline hydraulic simulation models have indicated that on the formation of water-blocking segments have not tunnels containing moving water can be mainly consid- yet been thoroughly investigated. ered with the Euler and gravity similarity criteria, fol- In summary, the abovementioned theoretical, experi- lowed by the Reynolds criterion. Therefore, the gravity mental and numerical studies were fundamental to similarity criterion was used for hydraulic simulation of a explain the accumulation and plugging mechanism for flooded tunnel. The fluid in this test was clean water with groundwater control and salvaging. However, in-depth a temperature of approximately 7 °C. The Reynolds num - research is relatively limited and lacks universality due ber Re > 2000 was calculated according to the minimum −2 to the concealment of underground engineering. For water flow velocity of approximately 1.5 × 10   m/s. example, the actual process of aggregate sedimentation Therefore, the flow state was turbulent in this test. The during pouring should be further investigated despite tunnel wall generally comprises concrete material with a the proposed division into three stages. The interaction roughness ratio ranging from 0.014 to 0.017, and plexi- between aggregate pouring into different boreholes and glass with a roughness ratio of 0.008–0.0095 was used in the borehole layout should be further examined. Because this test. According to the gravity similarity criterion, the multiple boreholes are usually used to achieve better roughness ratio could be obtained as  =  . This indi - and swifter plugging in practical projects, there exist few cates that the geometric ratio should be higher than 1:30 cases describing the use of a single borehole. Factors such to meet the gravity similarity criterion  (Chanson 2008). as the distance between boreholes and tunnel inclination Therefore, a geometric ratio between the model and pro - should be considered in orthogonal array design (OAD) totype of 1:20 was selected after comprehensive consid- experiments to study aggregate sedimentation, migra- eration of the laboratory facilities and similarity (Yalin tion and accumulation and criteria for plugging effects. 1971). Based on previous research results and similarity Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 5 of 18 −2 criteria, a pipeline was simulated with a cross-sectional orthogonal array was set at four levels: 0, 1.5 × 10  m/s, 2 −2 −2 area of 12  m , a length of 80 m and a water inrush flow of 2.4 × 10  m/s and 3 × 10  m/s. 6440  m /h. Table 2 lists the scale of different parameters. Tunnel inclination D. In rescue operations in flooded mines, grouting and blocking are mostly selected in Factors and materials horizontal or uphill tunnels relative to the movement The test parameters were determined referring to the direction of water flow. In this experiment, the tunnel cases listed in Table 1. inclination was selected at the 4 levels of 0°, 3°, 5° and 8°. Aggregate particle size A. The aggregate particle size in The aggregate injection rate in this test is the rate the experiments was selected at the four levels of 0.25– generated by the gravitational action of water flow and 0.5  mm, 0.5–1  mm, 1–2  mm and 2–5  mm. These levels aggregates. The rate is proportional to the length of were determined according to the similarity rule and the pouring borehole. If sidewall friction is ignored, on-site investigation of general coal mine water block- the final rate at which aggregates enter the tunnel is ing projects, where the selected aggregates included yel- v = 2gh = 3.13 m/s, where h is the distance from the top low sand (0.7–3.0  mm), rice-sized and centimeter-scale surface of the water–sand mixture in the funnel to the stone (3–5 mm), melon seed-sized stone (5–10 mm) and top of the tunnel. slag particles (10–20 mm). Sand and gravel particles were screened and washed to remove finer soil particles and Experimental setup impurities to ensure a high visualization clarity of the The plugging efficiency of aggregate injection into a sub - aggregate movement process. merged tunnel under water flow is affected by many fac - Distance between boreholes B. According to the layout tors, including natural and engineering factors, such as of grouting and blocking boreholes in submerged tun- the tunnel inclination, water flow rate, distance between nels, multiple boreholes are required for pouring and boreholes, aggregate particle size, water–solid mass ratio grouting, and the distance between boreholes generally and injection rate. The experimental setup could pro - varies between 15 and 30 m. The distance was selected at vide a constant water head and adjustable and recyclable four levels, namely, 0.5 m, 0.75 m, 1 m and 1.5 m, accord- water flow conditions at different flow rates. The system ing to the geometric scale in this study. Figure  2c shows could control the water–solid mass ratio and aggre- details of the borehole layout in this test. gate pouring rate. A data collection system could collect Initial water flow rate C. In flooded tunnels, plugging flowrate and water pressure data in real time. Images of sections are generally selected in concentrated water- aggregate accumulation and migration in the pipeline ways, exhibiting high water flow rates. For example, in were captured for process analysis. Figure  2 shows a the Dongpang coal mine in Hebei, the flow rate in the schematic and photo of the experimental setup. −2 water inrush tunnel reached as high as 13.88 × 10  m/s, and the flow rate in Panji coal mine No. 2 in Anhui Orthogonal array for experiment design −2 reached 7 × 10   m/s. In this test, the flow rate scal - Table  3 provides an orthogonal array with four factors, ing ratio is 4.47, and the initial water flow rate in the each with four levels. Hence, L 4 = 16 trials are needed in this experiment. The array was designed according to the OAD method proposed by C. R. Rao and used in industrial experimentation by Taguchi (Hedayat et  al. 1999). Table 2 Model scale set based on the Froude criterion Scale for F r Results and analysis Plugging efficiency Inertial force/gravity v /Lg Residual water flow channel Length   = 20 L L Under static water conditions, the resulting aggregate Time = 4.47 accumulation mass is tapered due to the angle of repose. Flow velocity v The cross section of the simulated tunnel model is circu - = 4.47 lar, and the contact conditions between the aggregates Flow quantity = 1788.85 and pipe wall cannot be completely matched, resulting Pressure   = 20 p L in a gap between the accumulation mass and pipe inner Force F   = 8000 L wall, namely, a residual water flow channel (Zhang et  al. Power 2 2020). = 35777 1 The residual water flow channel can be divided into a Roughness n 6 = 1.65 minimum residual channel and a final residual channel. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 6 of 18 Fig. 2 Experimental setup of the aggregate pouring process into an inundated mine tunnel through multiple boreholes under flowing water conditions. 1-water supply with a constant head; 2-water flow rate adjusting device; 3-water meter; 4-funnel for aggregate pouring; 5-tunnel replica; 6-inclination adjuster; 7-water cycling device; 8-camera; 9-PC; 10-data logger; 11-water pressure sensor; 12-electronical balance The minimum residual water flow channel is the channel former case occurs due to the existence of the angle of with the smallest cross-sectional area for water flow at a repose resulting in a gap with the tunnel wall, while the certain moment in the pipeline under aggregate accumu- latter case can be attributed to the existence of a balance lation. The minimum residual channel can be formed in between the resistance of the aggregate particles and the two cases, i.e., static and flowing water conditions. The carrying capacity of water flow. Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 7 of 18 Table 3 Orthogonal array L 4 for aggregate pouring into a tunnel with water flow Trial number Symbol of the trial Aggregate particle Distance between the Initial water flow Tunnel Plugging −2 size A (mm) boreholes B (m) rate C (× 10  m/s) inclination efficiency PE D (°) (%) 1 A1B1C1D1 0.25–0.5 0.5 0 0 98.15 2 A1B2C2D2 0.25–0.5 0.75 1.5 3 94.65 3 A1B3C3D3 0.25–0.5 1.0 2.4 5 88.79 4 A1B4C4D4 0.25–0.5 1.5 3.0 8 83.96 5 A2B1C2D3 0.5–1.0 0.5 1.5 5 94.80 6 A2B2C1D4 0.5–1.0 0.75 0 8 97.83 7 A2B3C4D1 0.5–1.0 1.0 3.0 0 91.86 8 A2B4C3D2 0.5–1.0 1.5 2.4 3 92.65 9 A3B1C3D4 1.0–2.0 0.5 2.4 8 93.20 10 A3B2C4D3 1.0–2.0 0.75 3.0 5 91.23 11 A3B3C1D2 1.0–2.0 1.0 0 3 97.74 12 A3B4C2D1 1.0–2.0 1.5 1.5 0 94.94 13 A4B1C4D2 2.0–5.0 0.5 3.0 3 96.09 14 A4B2C3D1 2.0–5.0 0.75 2.4 0 97.12 15 A4B3C2D4 2.0–5.0 1.0 1.5 8 97.19 16 A4B4C1D3 2.0–5.0 1.5 0 5 96.27 Fig. 3 Minimum residual and final channel under a hydrostatic conditions and b flowing water conditions Figure  3a shows a residual channel under static water The final residual water flow channel is the channel conditions. formed between the surface of the sand bed formed after Under flowing water conditions, aggregates could accu - stable accumulation of aggregates in the tunnel and the mulate behind the pouring boreholes because of water tunnel wall. With increasing accumulation, a stable sand flow. Due to friction between the aggregates and pipe bed is finally formed comprising sand with a certain par - wall, at a certain moment, there could occur a minimum ticle size after flow rate stabilization. At this time, the water flow channel in the pipe. The surface of the aggre - surface of the deposited mass below the water flow chan - gated mass below the channel exhibits a certain curvature nel is nearly flat and straight, and the aggregates move with lower values at the middle and higher values on the as a bedload under the action of water flow. Figure  3b sides, and eventually, the surface can become horizontal. shows a final residual water channel under flowing water conditions. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 8 of 18 When the water flow rate near the aggregate accumu - mass is continuously pushed forward until the outlet is lation mass in the tunnel remains stable and aggregate reached and overflow occurs. pouring continues, the bed sand height remains basically unchanged for a certain time. Therefore, the effect of Criterion for the plugging efficiency aggregate plugging remains unchanged. However, water The final plugging efficiency (PE ) is the ratio of the cross- flow can carry aggregates along the roadway toward the sectional area of the final aggregate accumulation mass to front end. With decreasing kinetic energy of the par- the cross-sectional area of the tunnel. ticles, aggregates can accumulate on the sand bed until almost reaching the top and filling the entire cross sec - PE(%) = (1) tion (Fig.  3b). The important factors influencing the size and type of the sand bed mass are the water flow rate, where P is the cross-sectional area of the final aggregate aggregate particle size and water flow depth. The smallest accumulation mass and P is the cross-sectional area of residual channel is formed before the final residual chan - 2 the tunnel, 31,415.93  mm . nel. At this time, the flow rate is the highest, the channel Figure  4 shows the cross-sectional area of the final is usually unstable, and the channel is also the closest to aggregate accumulation mass: 2 2 πD D 2H 2H −1 −1 (2) P = − cos 1 − − sin cos 1 − 4 8 D D the capping moment. At this time, the pouring amount where D is the diameter of the tunnel and H is the dis and aggregate particle size should be increased over time. tance between the aggregate accumulation mass surface In the end, a smaller residual channel results in a better and the top of the tunnel; H was measured by photo- plugging effect. When the flow rate in the final residual grammetry of the accumulation masses during pouring. channel is very high in the test and exceeds the critical velocity of the introduced aggregates, the accumulation Fig. 4 Cross-sectional area of aggregate deposits in a tunnel Table 4 Range analysis of the main factors of the plugging efficiency in a tunnel with flowing water Levels Aggregate particle size A (mm) Distance between boreholes B Initial water flow rate C Tunnel −2 (m) (10  m/s) inclination D (°) PE 91.39 95.56 97.50 95.52 PE 94.29 95.21 95.39 95.28 PE 94.28 93.90 92.94 92.77 PE 96.67 91.95 90.78 93.05 Range 5.28 3.60 6.71 2.74 Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 9 of 18 Fig. 5 Response graph for the main factors according to Table 4 Main effects boreholes was increased from B1 to B4, the plugging effi - Range and variance analysis methods are commonly used ciency tended to decrease. A smaller distance between methods for orthogonal experiments. These experiments boreholes resulted in faster agglomeration of the accu- can reveal the degree of influence of various factors on the mulation masses to form a water blocking section. In results, primary and secondary orders and optimal com- addition, the plugging effect was better. As the initial bination. Table 3 lists the final plugging efficiency (PE ). water flow rate was increased from C1 to C4, the plug - ging efficiency decreased. Because the water flow rate Range analysis increased, the ability to carry aggregates increased. The Table  4 shows the results of range analysis for the differ - aggregate accumulation mass height hardly increased ent factors, and Fig. 5 shows a response graph. at a high water flow rate. The larger the residual water As the aggregate particle size was increased from A1 channel is, the poorer the water blocking effect. As the to A4, the plugging efficiency tended to increase. This is tunnel inclination was increased from D1 to D4, the plug- related to the critical velocity of the aggregate particles. ging efficiency exhibited a decreasing trend. The tunnel The accumulation mass of coarse-grained aggregates is inclination angle was selected as lower than 10°, which is rapidly established, and the final height of the accumula - lower than the friction angle of sand. Therefore, this fac - tion mass is larger than that consisting of fine aggregates. tor imposed a relatively smaller effect on aggregate settle - Therefore, with increasing particle size, the plugging effi - ment and the water blocking effect than that of the other ciency increases. However, under static water conditions, factors. the aggregate accumulation mass exhibits an underwater The result of R > R > R > R indicates that the plug- C A B D angle of repose. When the particle size is greater than ging efficiency is influenced by these four factors in the 0.25 mm, the angle of repose increases with the aggregate following descending order: initial water flow rate, aggre - particle size, the residual channel area increases, and the gate particle size, distance between boreholes and tunnel plugging efficiency decreases. As the distance between inclination. Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 10 of 18 Table 5 Variance analysis of the main factors of PE Source of deviation Deviation sum of Degrees of Mean square F ratio F (3,3) critical F (3,3) 0.025 0.05 squares freedom error value critical value Particle size of aggregates A 55.99 3 18.66 12.93 15.44 9.28 Distance between boreholes B 31.95 3 10.65 7.38 15.44 9.28 Initial water flow rate C 102.18 3 34.06 23.59 15.44 9.28 Tunnel inclination D 25.04 3 8.35 5.78 15.44 9.28 Error 4.33 3 1.44 Analysis of variance and the distance between boreholes (0.5–0.75  m) are, Table  5 lists the analysis of variance (ANOVA) results the higher the plugging efficiency, i.e., a fine aggregate for the factors of the aggregate water blocking effect. The particle size should be used in the case of a small drill- reliability of ANOVA is 1 − α , where α is set to 0.025 and ing spacing. The second one is located in the right and 0.5. upper parts. This region indicates that a good plugging F > F . The initial water flow rate is signifi - efficiency can be obtained for intermediate particle sizes C 0.025(3,3) cant at the level of α = 0.025 , with a credibility of 0.975. (0.75–1.5  mm) in the case of a large borehole spacing. Combined with the range analysis results, it can be found For larger particle sizes (2–5  mm), the overall plugging that the initial water flow rate is the most important fac - efficiency is high, and the influence of the borehole spac - tor affecting the rate of aggregate plugging. Therefore, ing decreases. The main reason for the formation of these the initial water flow rate is the first factor that should be two regions is the interaction between gravity and drag considered in the design of water plugging projects. forces. Regarding fine aggregates, the accumulation pro - F > F . The aggregate particle size is signifi - cess is slow due to low gravity forces, and multiple bore- 0.05(3,3) cant at the level of α = 0.05 , with a credibility of 0.95. holes pouring with a small spacing can easily and quickly Combined with the above range analysis results, it can be form a continuous accumulation mass. Given a small observed that the aggregate particle size is second only to borehole spacing, an accumulation mass can easily form, the initial water flow rate, so the aggregate particle size and the residual water channel is smaller. However, the notably influences the process of aggregate pouring. The accumulation mass can be easily washed away at a high effect of the distance between the boreholes and tunnel water flow rate. In contrast, the gravity forces of coarse inclination on the plugging efficiency is slight. particles are higher, and the deposition process in water The above analysis indicates that the factors influenc - occurs fast, resulting in a long continuous accumulation ing the plugging efficiency are mainly the initial water mass in the aggregate pouring process involving multiple flow rate and aggregate particle size, followed by the boreholes with a larger spacing. distance between boreholes and tunnel inclination. The In actual engineering, fine aggregates should be poured initial water flow rate is not an artificially controllable under a small borehole spacing. Coarser aggregates factor. The results also demonstrate that the optimal should be poured via more widely spaced boreholes or combination is A4B1C1D1, but this combination was not at certain intervals. At the same time, when selecting included in this experiment; the closest combination is the distance between boreholes, consideration should be A1B1C1D1. In actual engineering, we should choose the given to reducing the influence of water turbulence on remaining three factors of the aggregate particle size, dis- the pouring process to quickly form a bulk mass. tance between boreholes and tunnel inclination (or loca- tion) to achieve the best water plugging effect. Aggregate particle size and initial water flow rate Aggregates of different particle sizes achieve unique criti - Interaction between factors cal velocities, and it is important to choose aggregates Aggregate particle size and distance between boreholes of an appropriate particle size to realize fast aggregate The two factors of the aggregate particle size and distance accumulation and deposition and water blocking in a between boreholes can be artificially selected in actual given tunnel. Figure 6b shows that when the initial water −2 −2 engineering, and the experimental results indicate that flow rate ranges from 2 × 10 to 3 × 10  m/s, the plug- there exists interaction between these two factors. Fig- ging efficiency is proportional to the aggregate particle ure 6a shows that there are two regions of a high plugging size, and coarser aggregates can yield a better plugging efficiency. The first one in the left lower part suggests effect. When the initial water flow speed is lower than −2 that the smaller the aggregate particle size (0.25–0.5 mm) 2 × 10  m/s, aggregates with a particle size smaller than Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 11 of 18 Fig. 6 PE under various combinations of the different factors Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 12 of 18 0.5  mm can provide the best plugging effect. When the aggregate particle size varies between 1 and 2  mm, the plugging efficiency is lower than that achieved with the other aggregate particle sizes. The plugging efficiency for the different aggregate particle sizes is more greatly affected by the initial water flow rate, which does not sug - gest that the aggregate particle size determines the final effect. In actual engineering, it is necessary to choose an appropriate particle size for different initial water flow rates. Distance between boreholes and initial water flow rate According to the Reynolds criterion, turbulence is more likely to occur at a high flow rate when the cross section and characteristic length of the tunnel are fixed. It was found that the turbulence due to water flow restricted the pouring speed, and the turbulence exhibited a cer- tain relationship with the initial water flow rate. Consid - ering that the flow velocity of moving water is relatively high in actual projects, it is meaningful to choose a suit- able distance between boreholes. Figure  6c shows that the influence of the initial water flow rate on the plug - ging efficiency is very obvious, and the initial water flow rate exerts a greater influence than that of the distance between boreholes. When the initial water flow rate is low and the distance between boreholes is smaller than 0.75 m, a higher plugging efficiency can be obtained (red area). The plugging efficiency is low when the distance between boreholes ranges from 0.75 to 1.5 m. When the distance between boreholes is small, the turbulence more significantly affects the pouring process, and boreholes can become severely blocked. When the initial water flow −2 rate is higher than 2.4 × 10   m/s, the influence of the distance between boreholes on PE is relatively limited. The contour of high PE values is biased toward the direc- tion of a higher initial water flow rate. Water pressure during plugging The layout of water pressure sensors is shown in Fig.  2c. The water pressure measurements during the pour - ing process reveal that there exists a pressure difference between the front and back ends of the formed aggregate accumulation mass. Even if the aggregate accumulation mass has not been capped, water flow can still be greatly impeded if a certain length is reached. Figure  7a shows changes in the water pressure in the tunnel in Trial No. 2. In the figure, t is the time to start pouring, t is the time to end pouring from funnel No. 2, t is the time to end pouring from funnel No. 1, and t is the time when the accumulation mass form final Fig. 7 Water pressures during aggregate pouring in a Trials No. 2; b has stabilized. Before the start of pouring, due to fric- No. 3; c No. 12; and d No. 14 tion between the wall and water flow, the pressure in Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 13 of 18 the tunnel can be reduced along the flow direction. The pressure difference varying between 16.0% and 31.8% pressures measured by the six pressure sensors gradually occurs before and after accumulation mass stabilization decrease, but the difference is small. At time t , the water (from time t to t ) in the tunnel. In the case of the same 0 1 2 pressure in the tunnel instantaneously rises because the particle size, the faster the water flow is, the smaller the water–sand mixture in the funnel reaches inside the tun- pressure difference. Even if the aggregate accumulation nel and becomes connected with the tunnel. As pouring mass is not capped, under the premise of continuous proceeds, the accumulation masses below the two bore- pouring, the aggregate accumulation mass still provides a holes become connected, and the pressure at pressure suitable resistance effect on water flow. sensor No. 1 rises faster than that at the other sensors. This indicates that the aggregate accumulation mass has Discussion contacted the top of the tunnel and that the resistance to Sedimentation of aggregates water flow has increased. Then, at time t , the pressure Shape of the aggregate accumulation mass suddenly drops because pouring stops in borehole No. 2 The sand and water used in this experiment were mixed and the water pressure originating from the funnel drops. according to a water–solid mass ratio of 1:1–2:1, and the Afterward, as pouring is continued in borehole No. 1, the two boreholes were filled simultaneously. Figure  8 shows pressure at sensors No. 1 and No. 2 begins to rise to a the final shape of the aggregate accumulation mass. The stable value, and the pressure at sensors No. 3, 4 and 5 extension of the accumulation mass along the direction falls, resulting in a large pressure difference between the of water flow can be mainly divided into three segments: front and back ends of the accumulation mass. At time an accumulation mass on the front surface, an accumu- t , the pouring process in borehole No. 1 stops, and the lation mass on the back surface and an accumulation pressure finally drops. At this time, the smallest residual mass between the holes. The details for this division are channel is gradually stabilized by the water flow. At time described in the literature (Zhang et al. 2020). The shape t , the final residual channel is formed, and the water of the accumulation mass between the boreholes when final pressure in the tunnel eventually matches the initial pres- capped is controlled by the shape of the tunnel. The larger sure before pouring. the aggregate particle size is, the steeper the edge of the The water pressure changes in the other trials, as accumulation mass. Aggregate particles are subjected shown in Fig.  7b, c and d, are relatively similar to those to a water flow impulse on the water-facing surface, and in this trial. The energy source of aggregate movement in the accumulation mass occurs in convex contact with the the tunnel is the potential energy of water flow (or pres - tunnel wall on the upstream side and in concave contact sure difference), kinetic energy of the internal turbulence on the downstream side. An obvious difference in the and potential energy of the aggregate particles. A change accumulation mass shape in the experiments is mainly in water pressure reflects the water head loss, and a large manifested upon a reduction in aggregate particle size, Fig. 8 Aggregate accumulation mass in a Trial No. 6 (A = 0.5–1 mm, B = 0.75 m, C = 0, and D = 8°); b Trial No. 2 (A = 0.25–0.5 mm, B = 0.75 m, −2 −2 C = 1.5 × 10 m/s, and D = 3°); c Trial No. 10 (A = 1–2 mm, B = 0.75 m, C = 3 × 10 m/s, and D = 5°) and d Trial No. 14 (A = 2–5 mm, B = 0.75 m, −2 C = 2.4 × 10 m/s, and D = 0°) Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 14 of 18 increase in the water flow rate, increase in tunnel inclina - in particle accumulation. Before the aggregate mass tion and increase in pouring volume. In particular, there reaches its maximum height for the first time, particles is a notable difference in the shape of the aggregate accu - mainly accumulate under vertical settlement. When mulation mass between static and flowing water condi - the water flow rate increases close to the critical condi - tions in the tunnel. tion of particle movement, the particles begin to move Figure 8a shows the shape of the sand aggregate accu- mainly along the direction of water flow, and the accu - mulation mass in an 8° inclined tunnel under static mulation masses slowly become connected. When the water conditions. The aggregate settlement is simi - initial water flow rate is high, the critical condition of lar to that under free sedimentation in a river under particles is easily reached, and the time for the aggre- steady-state flow. Under gravity, buoyancy, drag and gates to settle is shortened. The maximum height of the other forces, aggregates accelerate at a certain ini- aggregate accumulation mass is smaller than that under tial speed, settle at the bottom of the tunnel, and are a low flow rate. Under the same conditions, the remain - quickly capped. When the aggregates contact the top ing water passages are larger, and the water blocking wall of the tunnel, they do not easily settle, particu- effect is poorer. The critical flow rate for coarse par - larly coarse particles, so the borehole can easily become ticles is higher than that for fine particles. The height plugged. In horizontal pipes, the accumulation masses of the coarse sand accumulation mass is larger than below the two boreholes are basically symmetrical on that of the fine sand accumulation mass. Coarse sand the left and right sides of the holes. With a smaller extends less along the direction of water flow than does distance between boreholes and finer aggregates, the fine sand. accumulation masses can be quickly connected to Gravity component forces alter the shape of the form a section with a suitable water plugging effect, aggregate accumulation mass in an inclined tunnel. for instance, in Trial No. 1. However, under a large The larger the angle is, the more obvious the difference distance between boreholes, there occur two separate in the morphology of the accumulation mass between accumulation masses in the inclined tunnel that are both sides of the pouring borehole. similar to an asymmetrical cone shape (Fig.  8a). The larger the angle and the smaller the particle size are, the Process of plugging more obvious the difference between the two sides of Figure  9a shows the accumulation process of aggregates the accumulation mass. Under static water conditions, with a particle size ranging from 2 to 5  mm in static the particle size generates an underwater friction angle water in Trial No. 6. In a tunnel inclined at 8° under static (angle of repose). To continuously move the aggregates water conditions, a mixture of water and fine sand enters along the horizontal direction, the impact force caused the tunnel at a certain initial water flow rate. Under the by pouring must be greater than the shear strength of action of gravity and drag forces in the water flow, the the aggregate accumulation mass with a certain shape. aggregates are scattered at the bottom of the tunnel. The accumulation masses become connected to form a Over time, the settlement process continues to intensify water blocking section with a certain length. Therefore, beneath the borehole, and aggregate settlement is biased reasonable selection of the distance between boreholes toward low-lying locations in the tunnel, with increased and aggregate particle size is very important. The con - extension. It is found that coarse sand accumulates faster tact conditions of the aggregate accumulation masses than do fine particles. The pouring speed in borehole No. in static water within the tunnel are not very stable. 2 is relatively high, so the aggregate accumulation mass When a certain water flow rate is applied, the particles below this borehole is formed faster. at the top of the aggregate accumulation mass move. In Figure  9b shows that a coarse sand-water mixture this case, the pouring speed or aggregate particle size enters a horizontal tunnel under flowing water condi - should be adjusted to form a blocking section that is tions at a certain initial rate. The instantaneous initial appropriate for further grouting reinforcement. speed of pouring is much higher than the water flow Figure  8b, c, and d show the accumulation pat- rate inside the tunnel; thus, the aggregates settle quickly. terns for three aggregate particle sizes under flowing However, with increasing height of the accumulation water conditions, at tunnel inclinations of 3°, 5° and mass, the size of the water flow channel decreases, and 0°, respectively. The factors influencing the accumula - the water flow rate continues to increase until it is higher tion and distribution of aggregates in flowing water are than the startup critical velocity of the particles. There - more complex than those in static water. The forces fore, particles are carried forward by the water flow and include gravity, lifting, drag, interparticle, and seep- mainly settle on the downstream side of the accumula- age forces. The state of water flow and movement of tion mass under bedload conditions, and fewer sand sand particles in flowing water should be considered Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 15 of 18 Fig. 9 Plugging process of aggregates (the arrows indicate water flow) particles settle on the side upstream. However, when the a water blocking section. For example, in Trial No. 14, aggregate mass approaches its maximum height for the aggregates filled the tunnel continuously until the space first time, the water flow becomes highly turbulent. The between the two boreholes was completely eliminated water flow accelerates at the front and decelerates at the (Fig. 9b). back. Due to the sudden change in flow velocity, the pres - Under the condition of the same distance between sures in the upper and lower parts of the back side of the boreholes, the required amount and time for aggre- accumulation mass below borehole No. 1 are inconsist- gate pouring are the issues to be considered in the ent. A circulation current occurs, which is similar to flow rapid construction of the aggregate accumulation mass. around a cylinder in hydrodynamics. Where the residual Connecting the accumulation masses between the channel is small, the water flow is turbulent, and trail - boreholes during the shortest time with the minimum ing vortices occur. For example, vortices can be clearly injection volume is important for plugging section con- observed in Trials No. 2 and No. 3. The finer the particles struction. As indicated above, coarse particles more are, the more severe the disturbance of the sand-carrying easily and more rapidly produce accumulation masses water flow, and the more particles in the suspended mass than do fine particles. However, the water pressure dif - are deposited on the back side of the accumulation mass. ference between the front and back ends of the accu- Under downstream extension, the space between the two mulation mass is more notable for finer particles than masses is continuously filled, which eventually produces for coarse particles under the same distance between boreholes. The greater the water head loss, the easier it Hui et al. Geoenvironmental Disasters (2022) 9:15 Page 16 of 18 The third stage is defined as the capping and accumu - lation stage. As shown in Fig.  10c, the aggregate accu- mulation mass in the tunnel is similar to that on a static riverbed. The introduced aggregates move mainly in the form of a bedload and some suspended matter in the residual channel at the top of the accumulation mass. The aggregates are deposited on the back side of the entire accumulation mass. After reaching a certain length, the sand-carrying capacity of the water flow cannot move the mass. The particles are mainly dominated by contact and jumping movements. The aggregates continue to accu - mulate. A stable water blocking segment can eventually be formed. Plugging criterion The water-blocking section of an accumulation mass formed via aggregate pouring in a concentrated water- way initially resists water flow impulses and reduces the water flow intensity. At this time, there is still much Fig. 10 Aggregate movement at the different stages of the seepage in the water-blocking section, facing the dan- establishment of the water-blocking section. a Settlement and ger of being washed away by water at any time. It is accumulation stage; b Connection and accumulation stage; c Capping and accumulation stage necessary to implement grouting. Therefore, the aggre - gate plugging criterion is of great importance to deter- mine the timing of grouting. The above force balance analysis and plugging efficiency can be used as plug - is for the aggregate mass to become capped. Therefore, ging criteria in experiments and projects. In practical a reasonable combination of fine and coarse particles is engineering, the plugging criterion can be summarized the key to the rapid construction of the water-blocking as (Guo 2005): (1) the top interface of the aggregates section. in the borehole should be higher than the elevation of the tunnel. After sweeping the borehole bottom, the Movement and deposition of aggregates poured aggregates hardly settle, and the height of the The movement pattern of pipeline sand particles in this accumulation mass is higher than the top of the water test can be roughly divided into three stages based on the channel. (2) The water-blocking section can resist water three stages reported in the literature (Wang 2012). flow. There exists a large constant water level difference The first stage is denoted as the settlement and accu - between the front and back ends of the water-blocking mulation stage. Figure  10a shows that the aggregates section, and the friction between the water-blocking quickly and vertically accumulate before reaching the top section and tunnel wall can resist water flow impulses, for the first time. The cross-sectional area of the water ensuring that this section is not washed away in a channel decreases, the particles mainly move as a cohe- short time. (3) The unit water injection is lower than sive mass, and very few particles move as suspended −3 3 16 × 10  m /min∙m. Usually, all the above criteria are particles. met to determine that a stable water-blocking section The second stage is the connection accumulation stage. is built. At the same time, a critical condition for the As shown in Fig.  10b, the accumulation mass reaches its movement of particles in a horizontal or an inclined maximum height for the first time, namely, the water flow tunnel can be used to evaluate the stability of aggre- carrying capacity is higher than the critical condition for gates (Zhang et al. 2020). particle movement. At this time, the water flow is turbu - lent and accompanied by vortices. The settling particles move in the form of bed and suspended masses under Limitations and further study the action of turbulent water flow and vortices. The bed The injection speed in the aggregate pouring process in mass includes contact and jumping masses, which settle this experiment is different from the actual speed. The downstream. The accumulation mass extends forward pouring speed affects the porosity and other proper - and finally is connected into a whole. ties of the aggregate accumulation mass. The angle of Hui  et al. Geoenvironmental Disasters (2022) 9:15 Page 17 of 18 the tunnel selected in this study is very low. When the is greatly restricted, even if the aggregate mass is not tunnel inclination is greater than the angle of repose capped but reaches a certain length. of the aggregate accumulation mass, there are certain The critical condition for particle sedimentation and changes. It is therefore meaningful to study the influ - accumulation is that the equilibrium state of the result- ence of the injection speed, sequence of aggregate ant forces should reach zero before the deceleration pouring, and mixing of different particle sizes on the motion of the particles. A plugging criterion for aggre- plugging effect. Generally, the actual tunnel cross sec - gate deposition in horizontal and inclined tunnels was tion is not circular. The sedimentation and migration proposed. Moreover, it was proposed that the mini- processes of particles in different cross-sectional shapes mum plugging length of the aggregate deposit should will vary. In the next step, a cross-sectional shape closer be greater than the theoretical minimum length of to that in actual projects will be selected during aggre- effective grouting. When the aggregate pouring bore - gate pouring into a large-passage tunnel. In addition, holes are not influenced by each other, a small spacing the pressure in this test is low. Certain properties of between boreholes should be selected. high-pressure water flow can influence aggregate accu - Acknowledgements mulation, including the state, density, and permeability. The authors thank the Natural Science Foundation of China for the provided In actual projects, the water pressure reaches higher support under Grant No. 41877238. than 3  MPa. Further research should be conducted to Author contributions investigate the effect of high-pressure water flow on the WS and GZ proposed the main idea of this study and designed the experi- properties of aggregate accumulation. Another limi- ments. SH performed the experiments and analyzed the results. JC and SY performed the field investigation and edited and wrote part of the paper. All tation is that the model wall is relatively  smooth, and authors have read and approved the final manuscript. the roughness is not considered. A follow-up study should focus on the effect of different roughnesses on Funding This study was supported by the Natural Science Foundation of China under the blocking effect of the aggregate accumulation mass Grant No. 41877238. under the premise of ensuring visualization. Availability of data and materials All data generated or analyzed in this study are included in the published Conclusions article. An experimental investigation on the plugging effects of aggregate pouring through multiple boreholes under Declarations flowing water conditions in an inundated tunnel was conducted, and the results were examined in this paper. Competing interests The authors declare that they have no competing interests. A visual test platform for aggregate pouring into a tunnel was built. This platform could realize the visuali - Author details zation of the aggregate pouring process into the tunnel School of Resources and Geosciences, Institute of Mine Water Hazard Prevention and Control Technology, China University of Mining and Technol- under multiple factors and levels and realize real-time ogy, Xuzhou 221116, China. China Design Group Co., Ltd., Nanjing 210014, collection of image, pressure and flow data. 3 China. National Coal Mine Water Hazard Prevention Engineering Technology A four-level orthogonal array experimental test was Research Center, Suzhou 234000, Anhui, China. Institute of Mining Engineer- ing Research and Design, BGRIMMM Technology Group, Beijing 100160, China. performed. 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Journal

Geoenvironmental Disasters – Springer Journals

Published: Jul 7, 2022

Keywords: Inundated tunnel; Groundwater inrush disaster; Plugging efficiency; Aggregate pouring; Critical velocity; Sedimentation; Plugging criterion

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