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The relationship among the premonitory factors of landslide dam failure caused by seepage: an experimental study

The relationship among the premonitory factors of landslide dam failure caused by seepage: an... Background: A landslide dam always has the potential for catastrophic failure with high risk for life, cost and, property damage at the downstream site. The formation of a landslide dam is a natural process; thus, minimizing the risk due to its failure is important. Landslide dam failure can be categorized into three types: seepage failure, overtopping and slope failure. As described by other researchers, the established premonitory factors of landslide dam failure are hydraulic gradients, seepage and turbidity as well as vertical displacement and inflow into the reservoir. Methodology: This study only considered seepage failure and used flume experiments to understand it. Three groups of samples which represented fine, medium and coarse particle sizes, respectively, were prepared by Silica sand S4, S5, S6 and S8 of different proportion. These samples were used to conduct the flume experiments of failure and not failure case. Result: For failure cases, it was found that GI samples have a higher hydraulic gradient and that the seepage water takes time to exit the dam body—however, the seepage water has more TSS. GII samples also had a higher hydraulic gradient, while the flow of seepage water was faster than that of the fine sample with a low TSS. For GIII samples, the hydraulic gradient was very low in comparison with the GI and GII samples. The GIII samples had TSS values that were quite a bit higher than those of the GII samples and lower than those of the GI samples. Experiments on GI samples failed at each attempt; however, the GI samples with kaolinite did not fail and had a higher TSS value. For a GII sample of a non-failed case, the hydraulic gradient was lower than for GI samples and the seepage water flow was faster but the vertical displacement was constant and TSS was on a decreasing order. For a GIII sample, the hydraulic gradient became constant after reaching its initial peak value and TSS was on a decreasing order with an initially increasing vertical displacement that would become constant. Conclusion: Seepage failure of a landslide dam can be predicted by understanding the nature of its premonitory factors. These factors behave differently in different particle size samples. The TSS trend line may be the initial factor for checking the stability of a dam crest. A landslide dam with an increasing TSS order will fail and a decreasing order may not fail. Based on all experiments, it can be concluded that the hydraulic gradient has three stages: 1) it starts to increase and reaches a peak value; 2) it starts to decrease from the peak value and reaches a minimum; and 3) it starts to increase again where the seepage water begins to come out and the vertical displacement starts to increase. Dam failures always occur when seepage water comes out with an increasing TSS and an increasing vertical displacement. Repeated experiments on samples having more fine particles show that if a landslide dam is formed by fine particles, then there would be a high chance of its failure. In case of a constant hydraulic gradient, the landslide dam would be stable whenever there is an increasing vertical displacement and presence of TSS. Similarly, in case of a constant vertical displacement and a decreasing TSS, a landslide dam would be stable. Keywords: Landslide dam, Seepage, Hydraulic gradient, Total suspended solids (TSS) * Correspondence: civildhungana@gmail.com Department of Earth Science, Shimane University, 1060 Nishikawatsu-Cho, Matsue, Shimane 690-8504, Japan © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 2 of 20 Introduction management teams of the life and property safety at the Landslides or rock avalanches can form landslide dams downstream site. if their moving mass is sufficient to change the hydro- It has been shown that the failure sequence of a dam logical dynamics of a river channel and form a reservoir can be divided into four periods: 1) the emerging of (Costa and Schuster 1988; Canuti et al. 1988; Ermini and seepage water and front wetting, 2) the hyper- Casagli 2003; Kourp et al. 2010; Tacconi et al. 2018). concentrated flow discharge, 3) the emergence and The life span of these natural dams depends upon differ- development of a dam crest and 4) the failure of a dam ent natural factors. The failure of these dams creates crest with a sharp increase in its subsidence (Wang additional and catastrophic disasters. According to the et al. 2018). The additional question is: What will be history of landslide dam failure, about 34% of landslide the conditions for the failure or stability of a landslide dams have failed within a day of their formation. Simi- dam? larly, 87% of all landslide dams fail within a year of their The inflow rate into the reservoir as well as the magni- formation (Fig. 1). These statistics also indicate that tude, dam size and dam material are relevant for the fail- about 40% of landslide dams have a medium life span. ure of a landslide dam (Schuster and Costa 1986). An These dams should be investigated after within a short approach utilizing the Dimensionless Blockage Index period of their formation for a risk reduction plan to be (DBI) has previously been proposed for the stability ana- made for saving the life and property located down- lysis of landslide dams, as shown below (Eq. 1): stream of it. A better understanding of premonitory factors, which can easily be measured or observed in actual landslide dams that are at high risk of failure, is DBI ¼ Log A  ð1Þ important for disaster reduction (Wang et al. 2018). A V landslide dam that has not failed for more than one year could allow enough time for investigation, resulting in a where A is the area of a basin or reservoir, H is the b d high accuracy of prediction in comparison to those land- dam height and V is the volume of the dam material. slides that have a life span between two days and several DBI is directly related to the geometry of a dam struc- months. In this scenario, those landslide dams with a ture and reservoir size. Statistical analysis has indicated short life span are very important for the study of the that a dam is stable when DBI is < 2.75, quasi-stable premonitory factors, especially to discover in which when it is 2.75 < DBI < 3.08 and unstable when DBI is > conditions they would fail. These studies would directly 3.08 (Ermini and Casagli 2003). However, some records support the engineers and decision-makers of disaster did not satisfy this equation. Some of them, having large Fig. 1 Age of landslide dam at the time of failure (240 cases) (Peng and Zhan 2012) Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 3 of 20 DBI values showing their instability, have existed for a analysis for landslide dams may be one premonitory fac- very long time and vice-versa (Storm 2013). tor in the field. According to Cedergren (1977), seepage The hydraulic gradient is defined as a head loss, with failures have two types: (1) failure caused by erosion of respect to the distance travelled by a flow of water soil particles and (2) failure caused by saturation and through a media, as seen in Eq. 2: seepage forces. Jones (1981) has suggested that piping processes involve the dispersion of clay. The Dispersion Index method has been developed by Richie (1963)to i ¼ −Δh=L ð2Þ determine the dispersity of soil. Richie (1963) has de- fined 33% of the soil fractions, with less than 0.004 mm where i = the hydraulic gradient, Δh = the head loss and dispersing after being shaken in water for 10 min, as L = the distance travelled by water. Similarly, the flow of indicative of potential failure by tunnelling for earth seepage volume can be calculated as seen in Eq. 3: dams in Australia. Thus, fine particles are responsible for piping failure. Q ¼ kiA ð3Þ Rather than being initiated by a Darcian flow at an exit where Q is the seepage discharge, k is the hydraulic point, internal erosion is initiated by the erosive force of conductivity, i is the hydraulic gradient and A is the area water along a pre-existing planar opening (Richards and through which the discharge flows. In a laboratory, util- Reddy 2007). When pore water pressure increases on the izing a pore water sensor, the total head in the defined downstream side of the dam, the competent cohesion of positions can be measured. Using the formula of pore the soil would decrease. Reduction in cohesion reduces water pressure (u = γ h), the total head can be calculated the resistance force and increases the seepage force that considering the dam and flume tank geometry. Seepage can erode the soil particles, as described by Eq. 5: water is a very important factor for a landslide dam, which is visible on its downstream side at the actual landslide dam field. The parameters related to seepage F ¼ γ i ð5Þ water can enlighten the failure process of a landslide dam. Darcy (1856, cited in Fredlund et al. 2012) and where Fs = the seepage force per unit volume, i = the Okeke and Wang (2016a) have noted that the seepage hydraulic gradient and γ = the unit weight of water. flow velocity into a dam is directly dependent upon the Detailed research on seepage erosion for slope failures hydraulic gradient, as shown in Eq. 4: has been conducted by Rinaldi and Casagli (1999), Lobkovsky et al. (2004), Wilson et al. (2007), Fox et al. (2007) and many more. dh v ¼ k ð4Þ In situ, the turbidity of downstream water provides w w dz therateoferosion from thedam material,which plays where V = the flow rate of water (m/s), k is the perme- a direct role in the subsidence and stability of a dam in w w ability coefficient with respect to the water phase (m/s) the presence of a seepage water flow. According to and dh /dz = the hydraulic gradient in the z-direction. Wang et al. (2018), the monitory factors remain un- Due to the pressure difference between the upward slope changed at the initial stage as well as in the second and the downward slope of a landslide dam, the seepage stage; the turbidity and vertical displacement starts to flow occurs in those dams that produce a seepage force. slightly increase. Total suspended soils (TSS) also sup- At the time of seepage flow, when the seepage force port to understand the erosion into the dam material. becomes greater than the erosion resistance force, soil Turbidity and TSS are identical premonitory factors particles begin to move with the seepage water. that can be measured in both the field and laboratory Internal erosion is a major cause of embankment dam settings. Fine particles, which are in between the failure (Fell et al. 2003). Internal erosion that is caused coarser grains, are almost free from effective overbur- by flow along pre-existing openings, such as cracks in den and capable to migrate by a very low-velocity seep- cohesive material or voids along with a contact between age flow (Takaji and Yusuke 2008). Such eroded the soil-structures (Richards and Reddy 2007), has a particles can be measured as TSS. higher possibility of occurrence in landslide dams be- By causing light to be scattered, the concentration of cause of their formation process. Erosion as the cause of suspended particles may have a meaningful correlation landslide dam failure has previously been addressed by to turbidity. Although a variety of parameters, such as researchers (Wang et al. 2018; Okeke and Wang 2016b; density, size and shape of particles as well as water Richards and Reddy 2007). Unfortunately, this potential colour, may affect the relationship between the values of failure mode cannot be completely analysed using nu- TSS and turbidity (Nasrabadi et al. 2016). The correla- merical formulae or models. Seepage monitoring and tions between TSS and turbidity have been discussed in Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 4 of 20 detail in a wide range of case studies. A common linear itsTSS. Theintention wastorelatethe TSStothe relationship may be defined as shown in Eq. 6: hydraulic gradient and the vertical displacement dur- ing the failure process using a combination of differ- ent grade of artificial sand particles. The main aim of TSS ¼ mTurbidityfg NTU ð6Þ this study was to identify the real conditions for fail- ure that can be measured or understood in the field. Rügner et al. (2013) have found linear relationships Only seepage failure was considered. between TSS and turbidity with m values of 1–2.8 mg l −1NTU − 1 (average 1.9 mg l −1NTU − 1) for natur- Materials and methods ally suspended sediments in rivers in southern Experimental setup Germany. Otherstudies report slightly lowerorhigher A flume tank, 0.45 m by 0.45 m (height * width) and 2.0 m m values (e.g., 1.1 mg l −1NTU − 1 for particles from long, was designed to collect the seepage water from the karstic springs or up to 3 mg l −1NTU −1for sus- downstream side of a dam. The seepage water was pended sediments in the Lake Tahoe basin, respect- collected using holes that were 0.75 m away from the dam ively) (Schwarz et al. 2011; Stubblefield et al. 2007). In centre, as shown in Fig. 2. The dam height was 0.2 m and the laboratory, the flume tank can be designed to col- the upstream and downstream slopes were 45 degrees and lect seepage water for conducting TSS test. Sample 35 degrees, respectively. The width of the dam crest was collection time can be simulated to the time of com- 0.1 m. At the floor of the flume tank, double-sided tape puter using different methods and can relate to other was used and dry silica sand 6 was poured over it to main- monitoring factors. tain the roughness between the dam material and flume Remote sensing is an important monitoring tool in the tank floor. The flume tank was built using Plexiglas due to sphere of natural disaster research these days. Using which visibility was possible. Based on practice, to obtain geographic information system (GIS) and interferometric a seepage failure, the bed slope of the flume was designed synthetic aperture (InSAR) technology, the displacement as 1:40 slope. Three pore water pressure sensors, with a of dams can be monitored regularly. Commercial and rated capacity of 50 kPa, were used—hereafter called non-commercial satellite images are available from Pwp1, Pwp2 and Pwp3—for the downstream and up- different agencies. Images from both before and after an stream sides of the dam body and at the reservoir, respect- event can be analysed to monitor the landslide dam. ively, as shown in Fig. 2. These sensors were connected to Studies, based on GIS and remote sensing, provide the dam from the base of the flume tank facing upwards. useful results for management and engineers. The sub- Pwp1 and Pwp2 were covered by the filter material to sidence of landslide dam crest can be monitored in situ control the flow of sand. The CMOS multi-function using simple technology for example laser levelling ma- analogue laser sensors were used to measure the vertical chine can be used. Since subsidence can be monitored, displacement from the top of the flume tank using a the relation of vertical displacement to other monitoring wooden frame—hereafter called as Vdr and Vdl, for the factors would be very useful to predict the failure of right and left sides, respectively. Laser sensors monitored landslide dam. the dam crest at two fixed points continuously. A half-cut However, studies have been conducted on different polyvinyl chloride (PVC) pipe was used to collect the type of landslide dam failure likely overtopping, pip- seepage water from the downstream. A pipe was fixed ing and seepage. Most of these studies have below the holes with a gentle slope. highlighted failure patterns and some studies have fo- cused on seepage failure and internal erosion like Fell et al. 2003; Okeke and Wang 2016a. Conducting an Materials actual comparative study for understanding the stable Artificial silica sand was selected as the sample and a and failure conditions of landslide dam is still neces- combination of silica sand S4, S5, S6 and S8 were used sary. The effect of erosion on TSS and its relation to in different proportions, as shown in Table 1. Silica sand other premonitory factors of landslide dam has not S5 and S6 were considered to be the main dam material been well thought out yet. constituents and silica sand S4 and S8 played the role of Hence, this research aimed to establish the relation- coarse and fine particles, respectively. Based on this, ship between the premonitory factors of landslide samples SAM1, SAM2 and SAM5 had more fine parti- dams during the failure process. Here, the hydraulic cles i.e. Silica sand S8 and samples SAM3 and SAM6 gradient was measured using pore water pressure sen- had more coarse particles i.e. silica sand S4—hereafter sors and the vertical displacement was measured referred to as GI and GIII samples, respectively. Simi- using a laser sensor at the dam crest and from the larly, sample SAM 4 had the same content of silica sand seepage water collected from the dam site to measure S4 and S8—hereafter referred to as GII samples. Based Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 5 of 20 Fig. 2 Experimental setup of flume tank. a 3D view b cross-section and c longitudinal section on these samples, experiments were conducted for failed Methods and not failed dam conditions (Table 2). A mixing machine was used for mixing the dam ma- Before conducting the final experiment, a series of terials. Initially, the materials were weighed and experiments were performed for selecting the sand mix poured into a mixing machine and mixed for five mi- ratio and initial water content for creating the desired nutes. Before creating the dam in the flume tank, a dam shape. Silica sand is artificial sand but, in the field, sample was collected to find its initial water content the presence of different soil minerals plays a vital role and index properties. Before creating the dam, sensors in grain size percentage and turbidity. Kaolinite, which is were also placed in their respective positions. The one of the soil minerals present in most natural soils, dam was prepared through a layer to layer compac- was used here to understand the effects of minerals on tion divided into four parts—each layer consisted of seepage water, hydraulic gradient and vertical displace- about 9 kg of sample, and about 1to 2 kg sample was ment. The grain size distribution of all samples is shown used to make the final shape of the dam. Real-time in Fig. 3. data was collected using universal recorders (KYOWA Table 1 Silica sands and kaolinite mixed ratio of samples Sample number SS 4 (kg) SS 5 (kg) SS 6 (kg) SS 8 (kg) Kaolinite (kg) Water (kg) Total (kg) SAM 1 0.5 4.5 5.0 0.5 0.5 0.5 11.5 SAM 2 0.5 4.5 5.0 1.0 – 0.5 11.5 SAM 3 1.0 4 5.5 0.5 – 0.5 11.5 SAM 4 0.5 4.5 5.5 0.5 – 0.5 11.5 SAM 5 – 5.0 5.0 1.0 – 0.5 11.5 SAM 6 1.0 5.0 5.0 –– 0.5 11.5 Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 6 of 20 Table 2 Sample groups based on percentage of fine and time of failure of dam crest decreases with increase in in- coarser particle and, failure condition flow rate into the reservoir (Okeke and Wang 2016a). Description Group I (GI) Group II (GII) Group III (GIII) Similarly decrease in inflow rate will increase the stability and failure time. However, experiments of GI samples Failure SAM 5, SAM 2 SAM 4 SAM 3, SAM 6 EXP5FR1, EXP5FR2, EXP5FR3 are failed in all attempts, Not failure SAM 1 SAM 4 SAM 3, SAM 6 in spite of low inflow rates than in the GII sample of not failed condition. Thus, it can be concluded that fine sam- ples can fail easily. Additional figures in the annex cover PCD 300B and PCD 400). Sampling frequency was additional experiments with GIII and GI samples of failure two numbers of data per second. Seepage water was and not failure cases. collected to measure the TSS and the time of seepage The reservoir was connected by a pipeline to the main water collection was recorded using a stopwatch. water supply in the laboratory room. When the reservoir Seepage water was collected using a half-cut PVC started to fill up, Pwp3 sensor started to respond. Seep- pipe under the flume tank, facing upwards. Each sam- age began instantaneously and, as the water level in the ple was collected for about 10 s (± 2 s). After collect- reservoir increased, the water pressure also began to in- ing a sample for TSS, volume was measured and crease in the dam body and Pwp2 started to respond. oven-dried using 105 °C temperatures. The weight of Pwp1 sensors also responded after some time. Due to the dried sample was measured and TSS was pressure head differences between Pwp1 and Pwp2, the calculated. hydraulic gradient began to increase and reached the peak value. Seepage of water continued to flow down- Results and discussion wards and the pore water pressure at Pwp1 started to in- In this work, experiments are conducted to test the failure crease and the hydraulic gradient started to decrease. and stable conditions of a dam crest. Table 3 shows the Using the formula for pore water pressure (u = γ h), the experiment numbers and their statuses (either failed or water height has been calculated and, using Eq. 2, the stable). Experiments are conducted with GIII, GII, and GI hydraulic gradient is calculated by considering the slope samples to compare the failed and not failed conditions angle of the flume tank. The rapidly increasing water with respect to the hydraulic gradient, vertical displace- content in the dam material supports the seepage water ment, and TSS. The inflow rate into the reservoir is the flow out of the dam. If the upward seepage forces on a key to obtaining the failed and not failed conditions. The body of soil exceed the gravitational forces at the point inflow rate in this study can be understood from the reser- of exit, the vertical critical gradient will exceeded and voir level, i.e. pore water pressure at Pwp3. Stability and soil particles may be removed from this area (Terzaghi Fig. 3 Grain size distribution curves of samples used in experiments Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 7 of 20 Table 3 Initial state of samples and result of experiments Experiment no. Sample no Initial Moisture Dry density (kg/m ) Result of content (%) experiment EXP 2F1 SAM 2 4.3 1529 Failure EXP 3F SAM 3 4.5 1227 EXP 4F SAM 4 4.3 1272 EXP 5FR1 SAM 5 4.4 1479 EXP 5FR2 4.7 1479 EXP 5FR3 4.3 1526 EXP 6F SAM 6 4.3 1287 EXP 3NF SAM 3 3.9 1247 Not failure EXP 4NF SAM 4 4.4 1262 EXP 6NF SAM 6 4.3 1299 EXP 1NF SAM 1 4.4 1294 et al. 1996). When the reservoir starts to fill up, the vertical displacement was about 2.5 mm just prior to the seepage force will increase and will exceed the gravita- failure of the dam crest. The hydraulic gradient began to tional forces and seepage water starts to come out with decrease from its peak value and the seepage water soil particles. The collected seepage water sample was started to come out on the dam’s downstream side. An oven-dried to measure the TSS. The dam and reservoir initial value of TSS was quite a bit higher in most exper- size, the slope of the flume tank, the position of sensors iments. TSS initially decreased and then began to in- and the seepage water collection position was fixed for crease slowly. Wang et al. (2018) also present that the all experiments. turbidity of downstream seepage water has increased be- fore the failure. The vertical displacement rate was very Characteristics of the premonitory factor for failure cases low before the seepage water came out and, after the seep- Results of GI sample age water flow, the rate of vertical displacement in- Experiment No. EXP 2F was conducted for the GI sam- creased. The hydraulic gradient slowly started to ple. The reservoir began to fill up, with an increase in increase as the downward slope failure increased and, pore water pressure in Pwp3, resulting in the wetting of at the same time, the reservoir level also increased—fi- dam material front. The initial moisture content of the nally, the dam crest failed. sample was 4.3% only. The saturation level has been in- creasing continuously, and the colours of the dam ma- Results of GII sample terial also change from light to dark. The water level has Experiment No. EXP 4F was conducted for the GII sam- increased at the Pwp2 sensor after about 600 s of Pwp3. ple. The wetting front was rapidly increased just after The difference between the two-pore pressure inside the the beginning of the reservoir fill up. The pore water dam—i.e., Pwp2 and Pwp1—was high. The hydraulic pressure was increased at Pwp3 as the reservoir started conductivity of the soil would be affected by the particle filling up and Pwp2 also began to increase after Pwp3 size; the finer particles have low permeability. The ex- started. Pwp1 began at nearly the same time as Pwp2. perimental results are presented in Fig. 4. The hydraulic The pore water pressure at Pwp2 is increased very gradient was increased rapidly as pore water pressure in- quickly and, as a result, the hydraulic gradient also in- creased in Pwp2 and reached the peak value, highest creased very quickly, from about 0.2 to 0.6. Due to a within this study. The hydraulic gradient began to de- sudden failure of a small soil mass block from the upper crease from the peak value as Pwp1 started to increase. part of the slope of the dam’s downstream side, the seep- The vertical displacement began to increase slowly at a age water flow is stopped and the water pressure at nearly constant rate and then it rapidly increased prior Pwp1 is increased, which also affected the hydraulic to the dam crest failure. Wang et al. (2018), Okeke and gradient. Figure 5 shows the details of the experiment re- Wang 2016a are also presented same pattern of hy- sults. Initially, the hydraulic gradient reached the peak draulic gradient for the real sample of landslide dam fail- value but it does not decrease again to the minimum ure and for silica sand respectively. The downstream value, unlike in the other experiments in this study, and slope was continuously changing its topography due to again started to increase instead. The hydraulic gradient the increase in water content and seepage failure. The was unsteady. Reasons for the fluctuation of the hydraulic Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 8 of 20 Fig. 4 Experiment results of experiment No. EXP 2F of GI sample. a Pore water pressure and vertical displacement, b Hydraulic gradient and TSS gradient are: 1) the release of water from the downstream value. Considering the time gap, due to the position of into different pocket areas of the downstream slope and 2) the seepage collection point, the vertical displacement the failure of the downward slope and the decreased began to increase when TSS is measured. With the position of the flow line. The vertical displacement was changes in the hydraulic gradient and the increasing nearly constant at the initial stage and, as the hydraulic TSS, the dam became unstable and, finally, failed. gradient is increased, the vertical displacement also increased. Due to the appearance and disappearance of Results of GIII sample minor cracks in the dam crest, the vertical displace- Experiment No. EXP 6F was conducted for the GIII ment is increased and later decreased. The vertical sample. Here, the seepage water has a great effect on the displacement is about 0.85 mm prior to the failure of erosion and stability of the dam body. The coarser soil the dam crest. Figure 5c shows that seepage began had a higher hydraulic conductivity and a higher chance when the hydraulic gradient reached the initial peak of erosion of the fine particles. The pore water pressures Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 9 of 20 Fig. 5 Experiment results of experiment No. EXP 4F of GII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS at Pwp2 and Pwp1 began to increase very quickly in and down movement of the vertical displacement is the comparison to the GI and GII samples. The pore water result of the sudden presence and absence of minor pressure at Pwp2 increased rather quickly and, after ap- cracks at the dam crest. The hydraulic gradient reached proximately 250 s, the pore pressure at Pwp2 and Pwp1 the peak value and started to decrease as the pore pres- become equal. The water level increased in the down- sure increased in Pwp1. After reaching the low value of stream side of the dam, as a result of which the slope the hydraulic gradient, it slowly increased as the water failed. As the slope failed, the position of the flow line level increased in the reservoir and slope edge failed of changed and the pore pressure at Pwp1 increased with the downstream slope. The hydraulic gradient changed its decreasing rate. The results are presented in Fig. 6. with the change in the topography of the downward The vertical displacement started to increase as the hy- slope. As the pore water pressure reached approximately draulic gradient increased. This could be the effect of 1.4 kPa at Pwp3, the vertical displacement increased rap- changes in the water content of the dam body. The up idly and the dam crest failed. The seepage water began Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 10 of 20 Fig. 6 Experiment results of experiment No. EXP 6F of GIII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS to come out at about 500 s of the peak hydraulic gradi- experiment to understand the effect of minerals on pre- ent. The GIII sample has big voids, due to which the fine monitory factors. The pore water pressure at Pwp2 soil particles, which were in-between the coarse parti- began to respond 500 s after it began to respond at cles, came out with the seepage water—resulting in Pwp3. The pore water pressure at Pwp2 increased more higher turbidity. The TSS value is approximately 1.2 g/lt, quickly and became nearly equal to that of Pwp3. Simi- which was higher in comparison to that of the GII and larly, Pwp1 also increased about 250 s after Pwp2 started. GI samples without kaolinite. Figure 7 shows the experiment results. The hydraulic process of this experiment is nearly the same as in the Characteristics of premonitory factors for the non-failure other experiments in which kaolinite is not used. The cases vertical displacement increased as the hydraulic gradient Results of GI sample started to decrease from the peak value. This experiment Experiment No. EXP 1NF was conducted on the GI is continued for about 7000 s and it is stopped and de- sample with kaolinite. Kaolinite is only used in this fined as a non-failure case when the pore water pressure Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 11 of 20 Fig. 7 Experiment results of experiment No. EXP 1NF of GI (with Kaolinite) sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS at Pwp1, Pwp2 and Pwp3 became nearly constant. The would erode more particles with a low velocity. vertical displacement and hydraulic gradient became Here, in this experiment, due to the presence of constant as Pwp2 and Pwp1 become nearly constant. kaolinite, theTSS valueisveryhighincontrastto The maximum hydraulic gradient is about 0.47 and the that of other experiments. The seepage water came vertical displacement is less than 1 mm during the out from the dam after the hydraulic gradient de- experiment. creased to its minimum value from its peak value. The seepage velocity inside the landslide dam Similarly, displacement has been noticed as the seep- would be very low and distant travel by the seepage age water began to come out. Fine samples without water would not occur in a straight line. Thus, the kaolinite are also has the same nature of curves of eroded particles would travel in different directions the hydraulic gradient, TSS, and the vertical dis- and, finally, come out with the seepage water. If placement but the value of TSS is significantly low there are more fine particles, the seepage water in these experiments. Here, the constant hydraulic Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 12 of 20 gradient and decreasing TSS are the causes of the formation. The downstream slope topography was dam crest not-failure. continuously changing due to the increase in the water level and seepage failure. After approximately Result of GII sample 4000 s, the pore water pressure at Pwp3 became Experiment No. EXP 4NF was conducted for the nearly constant. Figure 8 shows the results of the ex- non-failure case of the GII sample. As in the failure periment. This condition can be considered as the case (EXP 4F), initially, the pore water pressure in- inflow rate into the reservoir and the seepage water creased at Pwp2 and Pwp1 together. The hydraulic rate from the dam body is the same. At the same gradient reached the peak value and started to time, the pore water pressure at Pwp2 is also con- decrease and the vertical displacement is constant at stant, which additionally proved that the dam is about 0.5 mm during the experiment; however, at stable. Although the maximum value of the hydraulic last, it reached 2.5 mm due to a small crack gradient is about 1.2 at 3000 s, the dam crest is Fig. 8 Experiment results of experiment No. EXP 4NF of GII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 13 of 20 stable, which shows the importance of the vertical Characteristics of TSS for the failure and non-failure cases displacement and the seepage water TSS for failure. Figures 10 and 11 show the TSS characteristics dur- The seepage water began to come out after the hy- ing both the failure and non-failure cases, respect- draulic gradient decreased to the minimum value ively. These TSS graphs are conscripted after the from its peak value. The hydraulic gradient reached removal of the initial and final data for the failure the maximum value and started to decrease, while cases and the initial data for the non-failure cases. the TSS value also decreased. During the experi- Except for sample no. 3, for the failure case, all ex- ment, it is visualized that the turbidity of the water periments show that TSS increased before the failure decreased. Finally, the TSS became zero. The vertical of dam crest. The TSS trend lines for the different displacement is nearly constant throughout the ex- experiments are presented in Figs. 10 and 11 with periment, at less than 0.5 mm. The changes in the equations. From the Fig. 10,it can be understood water content in the dam material and at the dam that the nature of TSS in failure cases increased crest surface could have an effect on the vertical dis- before the failure but the rate of TSS increment is placement, which can be noticed in this experiment. diverse in different samples. The TSS is high for Here, the constant vertical displacement and the de- sample GII, medium for sample GIII and lowers for creasing TSS are the main causes of the dam crest sample GI. Fine particles, which are in between the non-failure. coarser grains, are almost free from effective over- burden and capable to migrate by a very low-velocity of seepage flow (Takaji and Yusuke 2008). As sample Result of GIII sample GII has both the silica sand S4 and S8 in equal per- Experiment no. EXP 3NF was conducted for the non- centage, the TSS is measured higher. An interesting failure case of the GIII sample. The hydraulic gradient characteristic is noticed for non-failure—that the result obtained in this experiment is typical in this study, slopeangle of trendlineofTSS is nearly same for where the hydraulic gradient reached the peak value and the GI, GII and GIII samples. It can be concluded become constant. The pore water pressure at Pwp2 and that, if the TSS trend line slope is larger and de- Pwp1 increased at the same time as in the failure case creasing, then it could be predicted that a landslide experiment. The rate of increase for Pwp2 and Pwp1 dif- dam would not fail. The velocity of seepage water fered from that in the failure case for the same sample. depends upon the hydraulic gradient. The seepage The vertical displacement increased from when the hy- velocity plays a role in the erosion of soil particles. draulic gradient began to increase—i.e. when the water In this report, when comparing the results of the ex- level started to increase in the dam body. The vertical periments performed, it is found that the higher the displacement increased very slowly, up to about 3.0 and value of the hydraulic gradient, higher the TSS value also. 4.0 for Vdl and Vdr, respectively. Finally, the vertical dis- The TSS value is higher for the GII and GIII samples than placement became constant and the pore water pressure for the GI sample; however, the fine sample with kaolinite in the reservoir started to decrease, which may be due to has the highest TSS value. the higher rate of seepage water than of inflow into the reservoir. Figure 9 shows the experiment results. The Conclusion maximum hydraulic gradient of this experiment is ap- The seepage failure of a landslide dam can be pre- proximately 0.67, which is higher than in the failure case dicted by understanding the nature of its premoni- for a coarse sample. The seepage water came out after tory factors. These factors behave differently in 1850 s—i.e. just after the hydraulic gradient reached the different particle size samples. The TSS trend line peak value. The vertical displacement increased simul- may represent an initial factor to check the stability taneously with seepage water. After reaching 1.5 mm, of a dam crest. A dam crest would fail with increas- the vertical displacement increased rapidly until 3.7 mm ing TSS and it may be stable with decreasing TSS. and became constant. This experiment shows that the The sample having coarser particle would have a presence of TSS and the increment of the vertical dis- higher TSS even with a low hydraulic gradient. For placement are not the only satisfactory conditions for samples having more fine particles, the vertical dis- failure but that the role of the hydraulic gradient also placement would be very low and it would start to needs to be considered. The hydraulic gradient should increase just prior to the failure of a dam crest. reach the peak value, then decrease to the minimum Most experiments with samples having more fine value and once again start to increase as in the failure particles fail suddenly. For samples having higher case presented in this report. In this experiment, the hy- coarser particle, the failure of a dam is possible with draulic gradient is the main cause behind the dam crest a low hydraulic gradient. The seepage failure of the not-failure. downstream side slope would be smooth for samples Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 14 of 20 Fig. 9 Experiment results of experiment No. EXP 3NF of GIII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS having higher percentage of fine particles, whereas a displacement starts to increase. Dam failures always mass block failure would occur for samples having occur when the seepage water comes out with an in- higher percentage of medium and coarse particle. A creasing TSS tendency and an increasing vertical dis- damcrest wouldbestableifits hydraulicgradient placement while, at the same time, the hydraulic becomes constant, which is especially possible for gradient is at its third stage. Experiments with GI, samples having higher percentage of coarse particle. GII and, GIII samples of the non-failed condition Based on all experiments, it can be concluded that show that there would be either no hydraulic gradi- the hydraulic gradient has three stages: 1) it begins ent increase, no increment in the vertical displace- to increase and reaches peak value, 2) it begins to ment or a decreasing TSS or any two of them. In decrease from the peak value and reaches the mini- the field, if we could monitor the seepage water and mum value and 3) it begins to increase again when the vertical displacement, it would be easy to predict the seepage water starts to come out and the vertical potential dam failure. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 15 of 20 Fig. 10 Trend of TSS for different samples (failure condition) Fig. 11 Trend of TSS for different samples (not failure condition) Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 16 of 20 Appendix Fig. 12 Experiment results of experiment No. EXP 6NF of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 17 of 20 Fig. 13 Experiment results of experiment No. EXP 3F of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 18 of 20 Fig. 14 Experiment results experiment No. EXP 5F2, of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 19 of 20 Fig. 15 Photographs of experimental setup. a Side view of flume tank during experiment. b Downstream slope of dam with laser sensor at the top of flume tank. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 20 of 20 Acknowledgments Richie JA (1963) Earthwork tunneling and the application of soil testing The first author would like to express warm gratitude to the Rotary procedures. J Soil Water Conserv 19:111–129 Yoneyama Memorial Foundation and the Izumo South Rotary Club for Rinaldi M, Casagli N (1999) Stability of streambanks formed in partially saturated providing a scholarship for the first author. The authors would also like to soils and effects of negative pore-water pressures: the Sieve River (Italy). thank the anonymous reviewers for reviewing the draft version of the Geomorphology 26(4):253–277 manuscript. Rügner H, Schwientek M, Beckingham B, Kuch B, Grathwohl P (2013) Turbidity as a proxy for total suspended solids (TSS) and particle facilitated transport in catchments. Environ Earth Sci 69(2):373–380 Availability of data and material Schuster RL, Costa JE (1986) A perspective on landslide dam. In: Shuster RL (ed) The data sets used and analysed during the current study are available from Landslide dams: Processes, risk, and mitigation. Proceedings of a session in the corresponding author on reasonable request. All data used in this study conjunction with the ASCE convention. ASCE (Geotechnical Special Publ n 3), were produced in the department of Earth science laboratory of Shimane New York, pp 1–20 University. Schwarz K, Gocht T, Grathwohl P (2011) Transport of polycyclic aromatic hydrocarbons in highly vulnerable karst systems. Environ Pollut 159:133–139 Author’s contributions Storm A (2013) Geological prerequisites for landslide dams’ disaster assessment DP conducted the laboratory work with close coordination of FW. FW and mitigation in central Asia. In: Wang F, Miyajima M, Li T, Shan W, Fathani provided guidance and support for data analysis and presentation. DP T (eds) Progress of geo-disaster mitigation technology in Asia. Environmental drafted the manuscript and all authors read and approved the manuscript. Science and Engineering (Environmental Engineering). Springer, Berlin, pp 17–53 Funding Stubblefield AP, Reuter JE, Dahlgren RA, Goldman CR (2007) Use of turbidometry This study was financially supported by the fund “Initiation and motion to characterize suspended sediment and phosphorus fluxes in the Lake mechanisms of long runout landslides due to rainfall and earthquake in the Tahoe basin, California, USA. Hydrol Process 21:281–291. https://doi.org/10. falling pyroclastic deposit slope area” (JSPS-B-19H01980, Principal 1002/hyp.6234 Investigator: Fawu Wang). Tacconi C, Vilímek V, Emmer A, Catani F (2018) Morphological analysis and features of the landslide dams in the cordillera Blanca. Landslides 15(3): 507–521 Competing interests Takaji K, Yusuke F (2008) Effect of particle gradation on seepage failure in The authors declare that they have no competing interests. granular soils. In: Sekiguchi H (ed) Proceedings of the 4th international conference on scour and erosion (ICSE-4), November 5–7, 2008. The Received: 8 August 2019 Accepted: 7 November 2019 Japanese Geotechnical Society, Tokyo, pp 497–504 Terzaghi K, Peck RB, Mesri G (1996) Soil Mechanics in Engineering Practice (third ed.), John Wiley and Sons, INC. References Wang WF, Dai Z, Okeke CAU, Mitani Y, Yang H (2018) Experimental study to Canuti P, Casagli N, Ermini L (1988) Inventory of landslide dam in the northern identify premonitory factors of landslide dam failures. Eng Geol 232:123–134 Apennine as a model for induced flood hazard forecasting. In: Andah K (ed) Wilson GV, Periketi RK, Fox GA, Dabney SM, Shields FD, Cullum RF (2007) Soil Managing hydro- geological disaster in a vulnerable environment. Grifo Pub, properties controlling seepage erosion contributions to streambank failure. Perugia, pp 189–202 Earth Surf Process Landf 32(3):447–459 Cedergren HR (1977) Seepage, drainage, and flow nets. Wiley, New York Costa JE, Schuster RL (1988) The formation and failure of natural dams. Geol Soc Am Bull 100(7):1054–1068 Publisher’sNote Ermini L, Casagli N (2003) Prediction of the behavior of landslide dam using a Springer Nature remains neutral with regard to jurisdictional claims in geomorphological dimensionless index. Earth Surf Process Landf 28:31–47 published maps and institutional affiliations. Fell R, Wan CF, Cyganiewicz J, Foster M (2003) Time for development of internal erosion and piping in embankment dams. J Geotech Geoenviron 129(4):307–314 Fox GA, Wilson GV, Simon A, Langendoen EJ, Akay O, and Fuchs JW (2007) Measuring streambank erosion due to groundwater seepage: correlation to bank pore water pressure, precipitation and stream stage. Earth Surface Processes and Landforms 32(10):1558–73 Fredlund DG, Rahardjo H, Fredlund MD (2012) Unsaturated soil mechanics in engineering practice. Wiley, New York Jones JAA (1981) The nature of soil piping: a review of research. In: Volume 3 of British Geomorphological Research Group, research monograph series. Geo Books: Norwich Kourp O, Densmore AL, Schlunegger F (2010) The role of landslide in mountain ranve evolution. Geomorphology 120(1):77–90 Lobkovsky AE, Jensen B, Kudrolli A, Rothman DH (2004) Threshold phenomena in erosion driven by subsurface flow. J Geophys Res Earth Surf 109:F04010. https://doi.org/10.1029/2004JF000172 Nasrabadi T, Ruegner H, Sirdari ZZ, Schwientek M, Grathwohl P (2016) Using total suspended solids (TSS) and turbidity as proxies for evaluation of metal transport in river water. Appl Geochem 68:1–9 Okeke ACU, Wang F (2016a) Critical hydraulic gradients for seepage induced failure of landslide dams. Geoenviron Dis. https://doi.org/10.1186/s40677- 016-0043-z Okeke ACU, Wang F (2016b) Hydromechanical constraints on the piping failure of landslide dams: an experimental investigation. Geoenviron Dis. https://doi. org/10.1186/s40677-016-0038-9 Peng M, Zhan LM (2012) Breaching parameters of landslide dam. Landslides 9: 13–31. https://doi.org/10.1007/s10346-011-0271-y Richards KS, Reddy KR (2007) Critical appraisal of piping phenomena in earth dams. B Eng Geol Environ 66(4):381–402 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

The relationship among the premonitory factors of landslide dam failure caused by seepage: an experimental study

Geoenvironmental Disasters , Volume 6 (1) – Nov 29, 2019

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Springer Journals
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Copyright © 2019 by The Author(s).
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Environment; Environment, general; Earth Sciences, general; Geography, general; Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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2197-8670
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10.1186/s40677-019-0135-7
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Abstract

Background: A landslide dam always has the potential for catastrophic failure with high risk for life, cost and, property damage at the downstream site. The formation of a landslide dam is a natural process; thus, minimizing the risk due to its failure is important. Landslide dam failure can be categorized into three types: seepage failure, overtopping and slope failure. As described by other researchers, the established premonitory factors of landslide dam failure are hydraulic gradients, seepage and turbidity as well as vertical displacement and inflow into the reservoir. Methodology: This study only considered seepage failure and used flume experiments to understand it. Three groups of samples which represented fine, medium and coarse particle sizes, respectively, were prepared by Silica sand S4, S5, S6 and S8 of different proportion. These samples were used to conduct the flume experiments of failure and not failure case. Result: For failure cases, it was found that GI samples have a higher hydraulic gradient and that the seepage water takes time to exit the dam body—however, the seepage water has more TSS. GII samples also had a higher hydraulic gradient, while the flow of seepage water was faster than that of the fine sample with a low TSS. For GIII samples, the hydraulic gradient was very low in comparison with the GI and GII samples. The GIII samples had TSS values that were quite a bit higher than those of the GII samples and lower than those of the GI samples. Experiments on GI samples failed at each attempt; however, the GI samples with kaolinite did not fail and had a higher TSS value. For a GII sample of a non-failed case, the hydraulic gradient was lower than for GI samples and the seepage water flow was faster but the vertical displacement was constant and TSS was on a decreasing order. For a GIII sample, the hydraulic gradient became constant after reaching its initial peak value and TSS was on a decreasing order with an initially increasing vertical displacement that would become constant. Conclusion: Seepage failure of a landslide dam can be predicted by understanding the nature of its premonitory factors. These factors behave differently in different particle size samples. The TSS trend line may be the initial factor for checking the stability of a dam crest. A landslide dam with an increasing TSS order will fail and a decreasing order may not fail. Based on all experiments, it can be concluded that the hydraulic gradient has three stages: 1) it starts to increase and reaches a peak value; 2) it starts to decrease from the peak value and reaches a minimum; and 3) it starts to increase again where the seepage water begins to come out and the vertical displacement starts to increase. Dam failures always occur when seepage water comes out with an increasing TSS and an increasing vertical displacement. Repeated experiments on samples having more fine particles show that if a landslide dam is formed by fine particles, then there would be a high chance of its failure. In case of a constant hydraulic gradient, the landslide dam would be stable whenever there is an increasing vertical displacement and presence of TSS. Similarly, in case of a constant vertical displacement and a decreasing TSS, a landslide dam would be stable. Keywords: Landslide dam, Seepage, Hydraulic gradient, Total suspended solids (TSS) * Correspondence: civildhungana@gmail.com Department of Earth Science, Shimane University, 1060 Nishikawatsu-Cho, Matsue, Shimane 690-8504, Japan © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 2 of 20 Introduction management teams of the life and property safety at the Landslides or rock avalanches can form landslide dams downstream site. if their moving mass is sufficient to change the hydro- It has been shown that the failure sequence of a dam logical dynamics of a river channel and form a reservoir can be divided into four periods: 1) the emerging of (Costa and Schuster 1988; Canuti et al. 1988; Ermini and seepage water and front wetting, 2) the hyper- Casagli 2003; Kourp et al. 2010; Tacconi et al. 2018). concentrated flow discharge, 3) the emergence and The life span of these natural dams depends upon differ- development of a dam crest and 4) the failure of a dam ent natural factors. The failure of these dams creates crest with a sharp increase in its subsidence (Wang additional and catastrophic disasters. According to the et al. 2018). The additional question is: What will be history of landslide dam failure, about 34% of landslide the conditions for the failure or stability of a landslide dams have failed within a day of their formation. Simi- dam? larly, 87% of all landslide dams fail within a year of their The inflow rate into the reservoir as well as the magni- formation (Fig. 1). These statistics also indicate that tude, dam size and dam material are relevant for the fail- about 40% of landslide dams have a medium life span. ure of a landslide dam (Schuster and Costa 1986). An These dams should be investigated after within a short approach utilizing the Dimensionless Blockage Index period of their formation for a risk reduction plan to be (DBI) has previously been proposed for the stability ana- made for saving the life and property located down- lysis of landslide dams, as shown below (Eq. 1): stream of it. A better understanding of premonitory factors, which can easily be measured or observed in actual landslide dams that are at high risk of failure, is DBI ¼ Log A  ð1Þ important for disaster reduction (Wang et al. 2018). A V landslide dam that has not failed for more than one year could allow enough time for investigation, resulting in a where A is the area of a basin or reservoir, H is the b d high accuracy of prediction in comparison to those land- dam height and V is the volume of the dam material. slides that have a life span between two days and several DBI is directly related to the geometry of a dam struc- months. In this scenario, those landslide dams with a ture and reservoir size. Statistical analysis has indicated short life span are very important for the study of the that a dam is stable when DBI is < 2.75, quasi-stable premonitory factors, especially to discover in which when it is 2.75 < DBI < 3.08 and unstable when DBI is > conditions they would fail. These studies would directly 3.08 (Ermini and Casagli 2003). However, some records support the engineers and decision-makers of disaster did not satisfy this equation. Some of them, having large Fig. 1 Age of landslide dam at the time of failure (240 cases) (Peng and Zhan 2012) Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 3 of 20 DBI values showing their instability, have existed for a analysis for landslide dams may be one premonitory fac- very long time and vice-versa (Storm 2013). tor in the field. According to Cedergren (1977), seepage The hydraulic gradient is defined as a head loss, with failures have two types: (1) failure caused by erosion of respect to the distance travelled by a flow of water soil particles and (2) failure caused by saturation and through a media, as seen in Eq. 2: seepage forces. Jones (1981) has suggested that piping processes involve the dispersion of clay. The Dispersion Index method has been developed by Richie (1963)to i ¼ −Δh=L ð2Þ determine the dispersity of soil. Richie (1963) has de- fined 33% of the soil fractions, with less than 0.004 mm where i = the hydraulic gradient, Δh = the head loss and dispersing after being shaken in water for 10 min, as L = the distance travelled by water. Similarly, the flow of indicative of potential failure by tunnelling for earth seepage volume can be calculated as seen in Eq. 3: dams in Australia. Thus, fine particles are responsible for piping failure. Q ¼ kiA ð3Þ Rather than being initiated by a Darcian flow at an exit where Q is the seepage discharge, k is the hydraulic point, internal erosion is initiated by the erosive force of conductivity, i is the hydraulic gradient and A is the area water along a pre-existing planar opening (Richards and through which the discharge flows. In a laboratory, util- Reddy 2007). When pore water pressure increases on the izing a pore water sensor, the total head in the defined downstream side of the dam, the competent cohesion of positions can be measured. Using the formula of pore the soil would decrease. Reduction in cohesion reduces water pressure (u = γ h), the total head can be calculated the resistance force and increases the seepage force that considering the dam and flume tank geometry. Seepage can erode the soil particles, as described by Eq. 5: water is a very important factor for a landslide dam, which is visible on its downstream side at the actual landslide dam field. The parameters related to seepage F ¼ γ i ð5Þ water can enlighten the failure process of a landslide dam. Darcy (1856, cited in Fredlund et al. 2012) and where Fs = the seepage force per unit volume, i = the Okeke and Wang (2016a) have noted that the seepage hydraulic gradient and γ = the unit weight of water. flow velocity into a dam is directly dependent upon the Detailed research on seepage erosion for slope failures hydraulic gradient, as shown in Eq. 4: has been conducted by Rinaldi and Casagli (1999), Lobkovsky et al. (2004), Wilson et al. (2007), Fox et al. (2007) and many more. dh v ¼ k ð4Þ In situ, the turbidity of downstream water provides w w dz therateoferosion from thedam material,which plays where V = the flow rate of water (m/s), k is the perme- a direct role in the subsidence and stability of a dam in w w ability coefficient with respect to the water phase (m/s) the presence of a seepage water flow. According to and dh /dz = the hydraulic gradient in the z-direction. Wang et al. (2018), the monitory factors remain un- Due to the pressure difference between the upward slope changed at the initial stage as well as in the second and the downward slope of a landslide dam, the seepage stage; the turbidity and vertical displacement starts to flow occurs in those dams that produce a seepage force. slightly increase. Total suspended soils (TSS) also sup- At the time of seepage flow, when the seepage force port to understand the erosion into the dam material. becomes greater than the erosion resistance force, soil Turbidity and TSS are identical premonitory factors particles begin to move with the seepage water. that can be measured in both the field and laboratory Internal erosion is a major cause of embankment dam settings. Fine particles, which are in between the failure (Fell et al. 2003). Internal erosion that is caused coarser grains, are almost free from effective overbur- by flow along pre-existing openings, such as cracks in den and capable to migrate by a very low-velocity seep- cohesive material or voids along with a contact between age flow (Takaji and Yusuke 2008). Such eroded the soil-structures (Richards and Reddy 2007), has a particles can be measured as TSS. higher possibility of occurrence in landslide dams be- By causing light to be scattered, the concentration of cause of their formation process. Erosion as the cause of suspended particles may have a meaningful correlation landslide dam failure has previously been addressed by to turbidity. Although a variety of parameters, such as researchers (Wang et al. 2018; Okeke and Wang 2016b; density, size and shape of particles as well as water Richards and Reddy 2007). Unfortunately, this potential colour, may affect the relationship between the values of failure mode cannot be completely analysed using nu- TSS and turbidity (Nasrabadi et al. 2016). The correla- merical formulae or models. Seepage monitoring and tions between TSS and turbidity have been discussed in Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 4 of 20 detail in a wide range of case studies. A common linear itsTSS. Theintention wastorelatethe TSStothe relationship may be defined as shown in Eq. 6: hydraulic gradient and the vertical displacement dur- ing the failure process using a combination of differ- ent grade of artificial sand particles. The main aim of TSS ¼ mTurbidityfg NTU ð6Þ this study was to identify the real conditions for fail- ure that can be measured or understood in the field. Rügner et al. (2013) have found linear relationships Only seepage failure was considered. between TSS and turbidity with m values of 1–2.8 mg l −1NTU − 1 (average 1.9 mg l −1NTU − 1) for natur- Materials and methods ally suspended sediments in rivers in southern Experimental setup Germany. Otherstudies report slightly lowerorhigher A flume tank, 0.45 m by 0.45 m (height * width) and 2.0 m m values (e.g., 1.1 mg l −1NTU − 1 for particles from long, was designed to collect the seepage water from the karstic springs or up to 3 mg l −1NTU −1for sus- downstream side of a dam. The seepage water was pended sediments in the Lake Tahoe basin, respect- collected using holes that were 0.75 m away from the dam ively) (Schwarz et al. 2011; Stubblefield et al. 2007). In centre, as shown in Fig. 2. The dam height was 0.2 m and the laboratory, the flume tank can be designed to col- the upstream and downstream slopes were 45 degrees and lect seepage water for conducting TSS test. Sample 35 degrees, respectively. The width of the dam crest was collection time can be simulated to the time of com- 0.1 m. At the floor of the flume tank, double-sided tape puter using different methods and can relate to other was used and dry silica sand 6 was poured over it to main- monitoring factors. tain the roughness between the dam material and flume Remote sensing is an important monitoring tool in the tank floor. The flume tank was built using Plexiglas due to sphere of natural disaster research these days. Using which visibility was possible. Based on practice, to obtain geographic information system (GIS) and interferometric a seepage failure, the bed slope of the flume was designed synthetic aperture (InSAR) technology, the displacement as 1:40 slope. Three pore water pressure sensors, with a of dams can be monitored regularly. Commercial and rated capacity of 50 kPa, were used—hereafter called non-commercial satellite images are available from Pwp1, Pwp2 and Pwp3—for the downstream and up- different agencies. Images from both before and after an stream sides of the dam body and at the reservoir, respect- event can be analysed to monitor the landslide dam. ively, as shown in Fig. 2. These sensors were connected to Studies, based on GIS and remote sensing, provide the dam from the base of the flume tank facing upwards. useful results for management and engineers. The sub- Pwp1 and Pwp2 were covered by the filter material to sidence of landslide dam crest can be monitored in situ control the flow of sand. The CMOS multi-function using simple technology for example laser levelling ma- analogue laser sensors were used to measure the vertical chine can be used. Since subsidence can be monitored, displacement from the top of the flume tank using a the relation of vertical displacement to other monitoring wooden frame—hereafter called as Vdr and Vdl, for the factors would be very useful to predict the failure of right and left sides, respectively. Laser sensors monitored landslide dam. the dam crest at two fixed points continuously. A half-cut However, studies have been conducted on different polyvinyl chloride (PVC) pipe was used to collect the type of landslide dam failure likely overtopping, pip- seepage water from the downstream. A pipe was fixed ing and seepage. Most of these studies have below the holes with a gentle slope. highlighted failure patterns and some studies have fo- cused on seepage failure and internal erosion like Fell et al. 2003; Okeke and Wang 2016a. Conducting an Materials actual comparative study for understanding the stable Artificial silica sand was selected as the sample and a and failure conditions of landslide dam is still neces- combination of silica sand S4, S5, S6 and S8 were used sary. The effect of erosion on TSS and its relation to in different proportions, as shown in Table 1. Silica sand other premonitory factors of landslide dam has not S5 and S6 were considered to be the main dam material been well thought out yet. constituents and silica sand S4 and S8 played the role of Hence, this research aimed to establish the relation- coarse and fine particles, respectively. Based on this, ship between the premonitory factors of landslide samples SAM1, SAM2 and SAM5 had more fine parti- dams during the failure process. Here, the hydraulic cles i.e. Silica sand S8 and samples SAM3 and SAM6 gradient was measured using pore water pressure sen- had more coarse particles i.e. silica sand S4—hereafter sors and the vertical displacement was measured referred to as GI and GIII samples, respectively. Simi- using a laser sensor at the dam crest and from the larly, sample SAM 4 had the same content of silica sand seepage water collected from the dam site to measure S4 and S8—hereafter referred to as GII samples. Based Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 5 of 20 Fig. 2 Experimental setup of flume tank. a 3D view b cross-section and c longitudinal section on these samples, experiments were conducted for failed Methods and not failed dam conditions (Table 2). A mixing machine was used for mixing the dam ma- Before conducting the final experiment, a series of terials. Initially, the materials were weighed and experiments were performed for selecting the sand mix poured into a mixing machine and mixed for five mi- ratio and initial water content for creating the desired nutes. Before creating the dam in the flume tank, a dam shape. Silica sand is artificial sand but, in the field, sample was collected to find its initial water content the presence of different soil minerals plays a vital role and index properties. Before creating the dam, sensors in grain size percentage and turbidity. Kaolinite, which is were also placed in their respective positions. The one of the soil minerals present in most natural soils, dam was prepared through a layer to layer compac- was used here to understand the effects of minerals on tion divided into four parts—each layer consisted of seepage water, hydraulic gradient and vertical displace- about 9 kg of sample, and about 1to 2 kg sample was ment. The grain size distribution of all samples is shown used to make the final shape of the dam. Real-time in Fig. 3. data was collected using universal recorders (KYOWA Table 1 Silica sands and kaolinite mixed ratio of samples Sample number SS 4 (kg) SS 5 (kg) SS 6 (kg) SS 8 (kg) Kaolinite (kg) Water (kg) Total (kg) SAM 1 0.5 4.5 5.0 0.5 0.5 0.5 11.5 SAM 2 0.5 4.5 5.0 1.0 – 0.5 11.5 SAM 3 1.0 4 5.5 0.5 – 0.5 11.5 SAM 4 0.5 4.5 5.5 0.5 – 0.5 11.5 SAM 5 – 5.0 5.0 1.0 – 0.5 11.5 SAM 6 1.0 5.0 5.0 –– 0.5 11.5 Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 6 of 20 Table 2 Sample groups based on percentage of fine and time of failure of dam crest decreases with increase in in- coarser particle and, failure condition flow rate into the reservoir (Okeke and Wang 2016a). Description Group I (GI) Group II (GII) Group III (GIII) Similarly decrease in inflow rate will increase the stability and failure time. However, experiments of GI samples Failure SAM 5, SAM 2 SAM 4 SAM 3, SAM 6 EXP5FR1, EXP5FR2, EXP5FR3 are failed in all attempts, Not failure SAM 1 SAM 4 SAM 3, SAM 6 in spite of low inflow rates than in the GII sample of not failed condition. Thus, it can be concluded that fine sam- ples can fail easily. Additional figures in the annex cover PCD 300B and PCD 400). Sampling frequency was additional experiments with GIII and GI samples of failure two numbers of data per second. Seepage water was and not failure cases. collected to measure the TSS and the time of seepage The reservoir was connected by a pipeline to the main water collection was recorded using a stopwatch. water supply in the laboratory room. When the reservoir Seepage water was collected using a half-cut PVC started to fill up, Pwp3 sensor started to respond. Seep- pipe under the flume tank, facing upwards. Each sam- age began instantaneously and, as the water level in the ple was collected for about 10 s (± 2 s). After collect- reservoir increased, the water pressure also began to in- ing a sample for TSS, volume was measured and crease in the dam body and Pwp2 started to respond. oven-dried using 105 °C temperatures. The weight of Pwp1 sensors also responded after some time. Due to the dried sample was measured and TSS was pressure head differences between Pwp1 and Pwp2, the calculated. hydraulic gradient began to increase and reached the peak value. Seepage of water continued to flow down- Results and discussion wards and the pore water pressure at Pwp1 started to in- In this work, experiments are conducted to test the failure crease and the hydraulic gradient started to decrease. and stable conditions of a dam crest. Table 3 shows the Using the formula for pore water pressure (u = γ h), the experiment numbers and their statuses (either failed or water height has been calculated and, using Eq. 2, the stable). Experiments are conducted with GIII, GII, and GI hydraulic gradient is calculated by considering the slope samples to compare the failed and not failed conditions angle of the flume tank. The rapidly increasing water with respect to the hydraulic gradient, vertical displace- content in the dam material supports the seepage water ment, and TSS. The inflow rate into the reservoir is the flow out of the dam. If the upward seepage forces on a key to obtaining the failed and not failed conditions. The body of soil exceed the gravitational forces at the point inflow rate in this study can be understood from the reser- of exit, the vertical critical gradient will exceeded and voir level, i.e. pore water pressure at Pwp3. Stability and soil particles may be removed from this area (Terzaghi Fig. 3 Grain size distribution curves of samples used in experiments Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 7 of 20 Table 3 Initial state of samples and result of experiments Experiment no. Sample no Initial Moisture Dry density (kg/m ) Result of content (%) experiment EXP 2F1 SAM 2 4.3 1529 Failure EXP 3F SAM 3 4.5 1227 EXP 4F SAM 4 4.3 1272 EXP 5FR1 SAM 5 4.4 1479 EXP 5FR2 4.7 1479 EXP 5FR3 4.3 1526 EXP 6F SAM 6 4.3 1287 EXP 3NF SAM 3 3.9 1247 Not failure EXP 4NF SAM 4 4.4 1262 EXP 6NF SAM 6 4.3 1299 EXP 1NF SAM 1 4.4 1294 et al. 1996). When the reservoir starts to fill up, the vertical displacement was about 2.5 mm just prior to the seepage force will increase and will exceed the gravita- failure of the dam crest. The hydraulic gradient began to tional forces and seepage water starts to come out with decrease from its peak value and the seepage water soil particles. The collected seepage water sample was started to come out on the dam’s downstream side. An oven-dried to measure the TSS. The dam and reservoir initial value of TSS was quite a bit higher in most exper- size, the slope of the flume tank, the position of sensors iments. TSS initially decreased and then began to in- and the seepage water collection position was fixed for crease slowly. Wang et al. (2018) also present that the all experiments. turbidity of downstream seepage water has increased be- fore the failure. The vertical displacement rate was very Characteristics of the premonitory factor for failure cases low before the seepage water came out and, after the seep- Results of GI sample age water flow, the rate of vertical displacement in- Experiment No. EXP 2F was conducted for the GI sam- creased. The hydraulic gradient slowly started to ple. The reservoir began to fill up, with an increase in increase as the downward slope failure increased and, pore water pressure in Pwp3, resulting in the wetting of at the same time, the reservoir level also increased—fi- dam material front. The initial moisture content of the nally, the dam crest failed. sample was 4.3% only. The saturation level has been in- creasing continuously, and the colours of the dam ma- Results of GII sample terial also change from light to dark. The water level has Experiment No. EXP 4F was conducted for the GII sam- increased at the Pwp2 sensor after about 600 s of Pwp3. ple. The wetting front was rapidly increased just after The difference between the two-pore pressure inside the the beginning of the reservoir fill up. The pore water dam—i.e., Pwp2 and Pwp1—was high. The hydraulic pressure was increased at Pwp3 as the reservoir started conductivity of the soil would be affected by the particle filling up and Pwp2 also began to increase after Pwp3 size; the finer particles have low permeability. The ex- started. Pwp1 began at nearly the same time as Pwp2. perimental results are presented in Fig. 4. The hydraulic The pore water pressure at Pwp2 is increased very gradient was increased rapidly as pore water pressure in- quickly and, as a result, the hydraulic gradient also in- creased in Pwp2 and reached the peak value, highest creased very quickly, from about 0.2 to 0.6. Due to a within this study. The hydraulic gradient began to de- sudden failure of a small soil mass block from the upper crease from the peak value as Pwp1 started to increase. part of the slope of the dam’s downstream side, the seep- The vertical displacement began to increase slowly at a age water flow is stopped and the water pressure at nearly constant rate and then it rapidly increased prior Pwp1 is increased, which also affected the hydraulic to the dam crest failure. Wang et al. (2018), Okeke and gradient. Figure 5 shows the details of the experiment re- Wang 2016a are also presented same pattern of hy- sults. Initially, the hydraulic gradient reached the peak draulic gradient for the real sample of landslide dam fail- value but it does not decrease again to the minimum ure and for silica sand respectively. The downstream value, unlike in the other experiments in this study, and slope was continuously changing its topography due to again started to increase instead. The hydraulic gradient the increase in water content and seepage failure. The was unsteady. Reasons for the fluctuation of the hydraulic Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 8 of 20 Fig. 4 Experiment results of experiment No. EXP 2F of GI sample. a Pore water pressure and vertical displacement, b Hydraulic gradient and TSS gradient are: 1) the release of water from the downstream value. Considering the time gap, due to the position of into different pocket areas of the downstream slope and 2) the seepage collection point, the vertical displacement the failure of the downward slope and the decreased began to increase when TSS is measured. With the position of the flow line. The vertical displacement was changes in the hydraulic gradient and the increasing nearly constant at the initial stage and, as the hydraulic TSS, the dam became unstable and, finally, failed. gradient is increased, the vertical displacement also increased. Due to the appearance and disappearance of Results of GIII sample minor cracks in the dam crest, the vertical displace- Experiment No. EXP 6F was conducted for the GIII ment is increased and later decreased. The vertical sample. Here, the seepage water has a great effect on the displacement is about 0.85 mm prior to the failure of erosion and stability of the dam body. The coarser soil the dam crest. Figure 5c shows that seepage began had a higher hydraulic conductivity and a higher chance when the hydraulic gradient reached the initial peak of erosion of the fine particles. The pore water pressures Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 9 of 20 Fig. 5 Experiment results of experiment No. EXP 4F of GII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS at Pwp2 and Pwp1 began to increase very quickly in and down movement of the vertical displacement is the comparison to the GI and GII samples. The pore water result of the sudden presence and absence of minor pressure at Pwp2 increased rather quickly and, after ap- cracks at the dam crest. The hydraulic gradient reached proximately 250 s, the pore pressure at Pwp2 and Pwp1 the peak value and started to decrease as the pore pres- become equal. The water level increased in the down- sure increased in Pwp1. After reaching the low value of stream side of the dam, as a result of which the slope the hydraulic gradient, it slowly increased as the water failed. As the slope failed, the position of the flow line level increased in the reservoir and slope edge failed of changed and the pore pressure at Pwp1 increased with the downstream slope. The hydraulic gradient changed its decreasing rate. The results are presented in Fig. 6. with the change in the topography of the downward The vertical displacement started to increase as the hy- slope. As the pore water pressure reached approximately draulic gradient increased. This could be the effect of 1.4 kPa at Pwp3, the vertical displacement increased rap- changes in the water content of the dam body. The up idly and the dam crest failed. The seepage water began Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 10 of 20 Fig. 6 Experiment results of experiment No. EXP 6F of GIII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS to come out at about 500 s of the peak hydraulic gradi- experiment to understand the effect of minerals on pre- ent. The GIII sample has big voids, due to which the fine monitory factors. The pore water pressure at Pwp2 soil particles, which were in-between the coarse parti- began to respond 500 s after it began to respond at cles, came out with the seepage water—resulting in Pwp3. The pore water pressure at Pwp2 increased more higher turbidity. The TSS value is approximately 1.2 g/lt, quickly and became nearly equal to that of Pwp3. Simi- which was higher in comparison to that of the GII and larly, Pwp1 also increased about 250 s after Pwp2 started. GI samples without kaolinite. Figure 7 shows the experiment results. The hydraulic process of this experiment is nearly the same as in the Characteristics of premonitory factors for the non-failure other experiments in which kaolinite is not used. The cases vertical displacement increased as the hydraulic gradient Results of GI sample started to decrease from the peak value. This experiment Experiment No. EXP 1NF was conducted on the GI is continued for about 7000 s and it is stopped and de- sample with kaolinite. Kaolinite is only used in this fined as a non-failure case when the pore water pressure Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 11 of 20 Fig. 7 Experiment results of experiment No. EXP 1NF of GI (with Kaolinite) sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS at Pwp1, Pwp2 and Pwp3 became nearly constant. The would erode more particles with a low velocity. vertical displacement and hydraulic gradient became Here, in this experiment, due to the presence of constant as Pwp2 and Pwp1 become nearly constant. kaolinite, theTSS valueisveryhighincontrastto The maximum hydraulic gradient is about 0.47 and the that of other experiments. The seepage water came vertical displacement is less than 1 mm during the out from the dam after the hydraulic gradient de- experiment. creased to its minimum value from its peak value. The seepage velocity inside the landslide dam Similarly, displacement has been noticed as the seep- would be very low and distant travel by the seepage age water began to come out. Fine samples without water would not occur in a straight line. Thus, the kaolinite are also has the same nature of curves of eroded particles would travel in different directions the hydraulic gradient, TSS, and the vertical dis- and, finally, come out with the seepage water. If placement but the value of TSS is significantly low there are more fine particles, the seepage water in these experiments. Here, the constant hydraulic Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 12 of 20 gradient and decreasing TSS are the causes of the formation. The downstream slope topography was dam crest not-failure. continuously changing due to the increase in the water level and seepage failure. After approximately Result of GII sample 4000 s, the pore water pressure at Pwp3 became Experiment No. EXP 4NF was conducted for the nearly constant. Figure 8 shows the results of the ex- non-failure case of the GII sample. As in the failure periment. This condition can be considered as the case (EXP 4F), initially, the pore water pressure in- inflow rate into the reservoir and the seepage water creased at Pwp2 and Pwp1 together. The hydraulic rate from the dam body is the same. At the same gradient reached the peak value and started to time, the pore water pressure at Pwp2 is also con- decrease and the vertical displacement is constant at stant, which additionally proved that the dam is about 0.5 mm during the experiment; however, at stable. Although the maximum value of the hydraulic last, it reached 2.5 mm due to a small crack gradient is about 1.2 at 3000 s, the dam crest is Fig. 8 Experiment results of experiment No. EXP 4NF of GII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 13 of 20 stable, which shows the importance of the vertical Characteristics of TSS for the failure and non-failure cases displacement and the seepage water TSS for failure. Figures 10 and 11 show the TSS characteristics dur- The seepage water began to come out after the hy- ing both the failure and non-failure cases, respect- draulic gradient decreased to the minimum value ively. These TSS graphs are conscripted after the from its peak value. The hydraulic gradient reached removal of the initial and final data for the failure the maximum value and started to decrease, while cases and the initial data for the non-failure cases. the TSS value also decreased. During the experi- Except for sample no. 3, for the failure case, all ex- ment, it is visualized that the turbidity of the water periments show that TSS increased before the failure decreased. Finally, the TSS became zero. The vertical of dam crest. The TSS trend lines for the different displacement is nearly constant throughout the ex- experiments are presented in Figs. 10 and 11 with periment, at less than 0.5 mm. The changes in the equations. From the Fig. 10,it can be understood water content in the dam material and at the dam that the nature of TSS in failure cases increased crest surface could have an effect on the vertical dis- before the failure but the rate of TSS increment is placement, which can be noticed in this experiment. diverse in different samples. The TSS is high for Here, the constant vertical displacement and the de- sample GII, medium for sample GIII and lowers for creasing TSS are the main causes of the dam crest sample GI. Fine particles, which are in between the non-failure. coarser grains, are almost free from effective over- burden and capable to migrate by a very low-velocity of seepage flow (Takaji and Yusuke 2008). As sample Result of GIII sample GII has both the silica sand S4 and S8 in equal per- Experiment no. EXP 3NF was conducted for the non- centage, the TSS is measured higher. An interesting failure case of the GIII sample. The hydraulic gradient characteristic is noticed for non-failure—that the result obtained in this experiment is typical in this study, slopeangle of trendlineofTSS is nearly same for where the hydraulic gradient reached the peak value and the GI, GII and GIII samples. It can be concluded become constant. The pore water pressure at Pwp2 and that, if the TSS trend line slope is larger and de- Pwp1 increased at the same time as in the failure case creasing, then it could be predicted that a landslide experiment. The rate of increase for Pwp2 and Pwp1 dif- dam would not fail. The velocity of seepage water fered from that in the failure case for the same sample. depends upon the hydraulic gradient. The seepage The vertical displacement increased from when the hy- velocity plays a role in the erosion of soil particles. draulic gradient began to increase—i.e. when the water In this report, when comparing the results of the ex- level started to increase in the dam body. The vertical periments performed, it is found that the higher the displacement increased very slowly, up to about 3.0 and value of the hydraulic gradient, higher the TSS value also. 4.0 for Vdl and Vdr, respectively. Finally, the vertical dis- The TSS value is higher for the GII and GIII samples than placement became constant and the pore water pressure for the GI sample; however, the fine sample with kaolinite in the reservoir started to decrease, which may be due to has the highest TSS value. the higher rate of seepage water than of inflow into the reservoir. Figure 9 shows the experiment results. The Conclusion maximum hydraulic gradient of this experiment is ap- The seepage failure of a landslide dam can be pre- proximately 0.67, which is higher than in the failure case dicted by understanding the nature of its premoni- for a coarse sample. The seepage water came out after tory factors. These factors behave differently in 1850 s—i.e. just after the hydraulic gradient reached the different particle size samples. The TSS trend line peak value. The vertical displacement increased simul- may represent an initial factor to check the stability taneously with seepage water. After reaching 1.5 mm, of a dam crest. A dam crest would fail with increas- the vertical displacement increased rapidly until 3.7 mm ing TSS and it may be stable with decreasing TSS. and became constant. This experiment shows that the The sample having coarser particle would have a presence of TSS and the increment of the vertical dis- higher TSS even with a low hydraulic gradient. For placement are not the only satisfactory conditions for samples having more fine particles, the vertical dis- failure but that the role of the hydraulic gradient also placement would be very low and it would start to needs to be considered. The hydraulic gradient should increase just prior to the failure of a dam crest. reach the peak value, then decrease to the minimum Most experiments with samples having more fine value and once again start to increase as in the failure particles fail suddenly. For samples having higher case presented in this report. In this experiment, the hy- coarser particle, the failure of a dam is possible with draulic gradient is the main cause behind the dam crest a low hydraulic gradient. The seepage failure of the not-failure. downstream side slope would be smooth for samples Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 14 of 20 Fig. 9 Experiment results of experiment No. EXP 3NF of GIII sample. a Pore water pressure and vertical displacement b Hydraulic gradient and TSS having higher percentage of fine particles, whereas a displacement starts to increase. Dam failures always mass block failure would occur for samples having occur when the seepage water comes out with an in- higher percentage of medium and coarse particle. A creasing TSS tendency and an increasing vertical dis- damcrest wouldbestableifits hydraulicgradient placement while, at the same time, the hydraulic becomes constant, which is especially possible for gradient is at its third stage. Experiments with GI, samples having higher percentage of coarse particle. GII and, GIII samples of the non-failed condition Based on all experiments, it can be concluded that show that there would be either no hydraulic gradi- the hydraulic gradient has three stages: 1) it begins ent increase, no increment in the vertical displace- to increase and reaches peak value, 2) it begins to ment or a decreasing TSS or any two of them. In decrease from the peak value and reaches the mini- the field, if we could monitor the seepage water and mum value and 3) it begins to increase again when the vertical displacement, it would be easy to predict the seepage water starts to come out and the vertical potential dam failure. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 15 of 20 Fig. 10 Trend of TSS for different samples (failure condition) Fig. 11 Trend of TSS for different samples (not failure condition) Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 16 of 20 Appendix Fig. 12 Experiment results of experiment No. EXP 6NF of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 17 of 20 Fig. 13 Experiment results of experiment No. EXP 3F of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 18 of 20 Fig. 14 Experiment results experiment No. EXP 5F2, of GIII sample. a Pore water pressure and vertical displacement curves b Hydraulic gradient and TSS curves Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 19 of 20 Fig. 15 Photographs of experimental setup. a Side view of flume tank during experiment. b Downstream slope of dam with laser sensor at the top of flume tank. Dhungana and Wang Geoenvironmental Disasters (2019) 6:17 Page 20 of 20 Acknowledgments Richie JA (1963) Earthwork tunneling and the application of soil testing The first author would like to express warm gratitude to the Rotary procedures. J Soil Water Conserv 19:111–129 Yoneyama Memorial Foundation and the Izumo South Rotary Club for Rinaldi M, Casagli N (1999) Stability of streambanks formed in partially saturated providing a scholarship for the first author. The authors would also like to soils and effects of negative pore-water pressures: the Sieve River (Italy). thank the anonymous reviewers for reviewing the draft version of the Geomorphology 26(4):253–277 manuscript. Rügner H, Schwientek M, Beckingham B, Kuch B, Grathwohl P (2013) Turbidity as a proxy for total suspended solids (TSS) and particle facilitated transport in catchments. Environ Earth Sci 69(2):373–380 Availability of data and material Schuster RL, Costa JE (1986) A perspective on landslide dam. In: Shuster RL (ed) The data sets used and analysed during the current study are available from Landslide dams: Processes, risk, and mitigation. Proceedings of a session in the corresponding author on reasonable request. All data used in this study conjunction with the ASCE convention. ASCE (Geotechnical Special Publ n 3), were produced in the department of Earth science laboratory of Shimane New York, pp 1–20 University. Schwarz K, Gocht T, Grathwohl P (2011) Transport of polycyclic aromatic hydrocarbons in highly vulnerable karst systems. Environ Pollut 159:133–139 Author’s contributions Storm A (2013) Geological prerequisites for landslide dams’ disaster assessment DP conducted the laboratory work with close coordination of FW. FW and mitigation in central Asia. In: Wang F, Miyajima M, Li T, Shan W, Fathani provided guidance and support for data analysis and presentation. DP T (eds) Progress of geo-disaster mitigation technology in Asia. Environmental drafted the manuscript and all authors read and approved the manuscript. Science and Engineering (Environmental Engineering). Springer, Berlin, pp 17–53 Funding Stubblefield AP, Reuter JE, Dahlgren RA, Goldman CR (2007) Use of turbidometry This study was financially supported by the fund “Initiation and motion to characterize suspended sediment and phosphorus fluxes in the Lake mechanisms of long runout landslides due to rainfall and earthquake in the Tahoe basin, California, USA. Hydrol Process 21:281–291. https://doi.org/10. falling pyroclastic deposit slope area” (JSPS-B-19H01980, Principal 1002/hyp.6234 Investigator: Fawu Wang). Tacconi C, Vilímek V, Emmer A, Catani F (2018) Morphological analysis and features of the landslide dams in the cordillera Blanca. Landslides 15(3): 507–521 Competing interests Takaji K, Yusuke F (2008) Effect of particle gradation on seepage failure in The authors declare that they have no competing interests. granular soils. In: Sekiguchi H (ed) Proceedings of the 4th international conference on scour and erosion (ICSE-4), November 5–7, 2008. 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Published: Nov 29, 2019

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