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Investigation for the initiation of a loess landslide based on the unsaturated permeability and strength theory

Investigation for the initiation of a loess landslide based on the unsaturated permeability and... Background: The Yanlian landslide, occurring on 21-22 October 2010, destroyed many facilities of a big oil refinery in Shaan’xi Province, China. It led to a suspending of the refinery work for a week and caused near 700 million RMB economic losses. Results: Site exploration shows that the sliding mass is unsaturated-saturated loess. The groundwater is rich in the landslide and shortage in the surrounding slopes. Further investigation finds that the water drop released from the vapor heating furnaces on the top of the slope is the only source of the groundwater. Laboratory tests were performed to get the unsaturated strength parameters and hydraulic conductivity of the loess layers which were applied to a pre-failure slope model to simulate the water infiltration process and the stress field based on which the factor of safety is figured out to analyze the slope stability. Analysis shows that during the first ten years, the factor of safety has no prominent decrease. In the following 5 years, the slope stability decreases significantly till failure. Conclusions: A little water infiltration has minor effects on slope stability for some time. As a result it is easy to be ignored. However, when the period of water infiltration is long enough to raise the groundwater level, it will have detrimental influence on the stability. In conclusion, any minor water produced by engineering or other activities for a long period may have harmful effect on slope stability. Therefore it is essential to take account of this kind of water and adopt measures to curb the surface water infiltration and to drain the groundwater. Keywords: Unsaturated soil; Loess; Landslide; Strength parameters; Soil-water characteristic curve; Hydraulic conductivity function Background moves down intermittently for two days and the people Loess landslides occurring in China lead to a huge eco- working on it has enough time to escape. So there is nomic losses every year (Liao et al. 2008; Bai et al. 2012; no fatality in the accident. The cause of the landslide Li et al. 2012). It has been proved that rainfall, irrigation, is unclear and deeply concerned by the managers and water canal leakage, submerging of reservoir, melted researchers. The aims of the paper are to find the trigger- snow and earthquake are the main factors triggering ing factors by site exploration and to study the failure loess landslides (Lei, 1994; Dai and Lee 2001; Tu et al. mechanism by laboratory tests and numerical analysis. 2009; Li et al. 2013; Zhang & Peng 2014). However, there are no water resources and earthquake mentioned above Methods, results and discussions before the Yanlian landslide occurred, which is located at Characters of the landslides Luochuan county, Shaanxi province, China as shown in The landslide occurred on the erosive bank of the Luohe Fig. 1, and the slopes nearby are stable even though they river which is the second tributary of the Yellow river. are higher and steeper than the failed. The landslide The first and the third terraces of the river develop on that bank and the second terrace is eroded off. The first terrace, about 5 m above the river bed, is flat and wide * Correspondence: dcdgx07@chd.edu.cn on which there is an oil refinery named Yanlian (Fig. 2). Department of Geological Engineering, Chang’an University, Xi’an, Shaan’xi, China Against the rear of the first terrace is a 20 m high rock © 2015 Li et al. 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. Li et al. Geoenvironmental Disasters (2015) 2:24 Page 2 of 11 Fig. 3 The spring at the toe of the slope Fig. 1 Location of the Yanlian landslide (Fig. 5). The refinery had to stop working until 7 days cliff composed of hard horizontal sandstone of the upper later when the facilities were recovered. There were no Triassic formation. The rock cliff is the base of the third fatalities but it caused 700 million RMB losses. terrace which is overlain by about 2 m thick alluvial peb- The surface gradient of the slope before failure is about ble and nearly 50 m thick Quaternary loess successively. 30° and the vertical height is 47 m. The failure mass has a The pebble is filled with silt-sand and cemented by leached width of 240 m, a length of 150 m, a thickness of carbonate calcium, so it has a very low permeability. A 13.8 m and a total volume of 400,000 m .The slopeis spring flows out from the top of the pebble with an average composed of the unsaturated loess of Late (Q loess), flow quantity of about 0.5 L/s (Fig. 3). The sliding mass, Medium (Q loess) and Early (Q loess) Pleistocene 2 1 shearing out on the pebble bed, is totally loess. Around formations as showninFig.6.The groundwaterlevel the failed slope, there are 34 large oil tanks lying on is only 7 m deep at the center of the landslide, but no the top and 24 oil pipelines on the toe. A road wanders groundwater is found in the surrounding slopes. The on the slope from the toe to the top. A boiler house region is generally shortage of groundwater because of and a coal storehouse stand on the first terrace against the small precipitation (annually 596 mm/a) and the the cliff (Fig. 2). deep cut landform. The rainfall during the rainy season Several days before October 21, 2010, a small part of may quickly run off on the surface. Investigation on loess at the toe of slope started to collapse from where a the top platform where oil tanks stand finds that there spring flowed out and buried part of the coal storage is lots of water vapor released from the pressure house. When the workers were trying to clear the col- adjusters on the heating pipes which are used to keep lapsed material, the whole slope moved downwards for the oil in the transfer pipes from frozen in winter two days. (Fig. 7). The hot vapor condenses in winter and partly The landslide buried part of the coal storehouse (Fig. 2) penetrates into the ground. The condensed water of the and cut off all the 24 oil pipelines (Fig. 4) and the road. vapor is possibly the water source in the landslide The oil tanks were only 5 m away from the head scarp Fig. 2 The Yanlian landslide Fig. 4 The oil transfer pipes cut off by the landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 3 of 11 Fig. 5 The oil tanks hanging over the top of the head scarp Fig. 7 The release water vapor from the pressure adjust valves on the heating pipes which has been ignored at the beginning. Therefore it is assumed that the groundwater in the landslide was (>0.05 mm) less than 10 %. From Q to Q and Q 3 2 1 generated by unsaturated infiltration of the condensed loess, the mean particle size and the void ratio decrease water drops and the landslide was trigged by the rising as well as dry density increases. It demonstrates that of the groundwater level. the lower the loess layer lies, the longer the time of pe- dogenesis is and the heavier the load of consolidation Determination of the loess physical properties experiences, the more compacted the loess is. The sliding mass and surface are composed of loess. Basic physical properties of loess include permeability, Determination of SWCC and HCF water content, density, plasticity, grain components. Soil-water characteristic curves (SWCC) were mea- The coefficient of saturated permeability was measured sured in laboratory with undisturbed block specimens through the falling head method. The water content of 300 mm × 300 mm × 300 mm in dimensions. The and density were measured by oven drying and metal specimens were collected from Q ,Q and Q loess 1 2 3 ring respectively. The plastic limit and liquid limit were respectively. The matric suctions were measured in measured by hand rolling and Cassgrande apparatus wetting processby the TEN−15 tensiometer and the respectively. The results are shown in Table 1. Laser corresponding moisture contents by the oven-drying Particle Analysis were utilized to measure the grain size method. It was accomplished through the following distributions of Q ,Q ,Q loess as shown in Fig. 8. It steps: First, a block of specimen was air dried and a 1 2 3 is shown that major component is silt (0.005−0.05 mm) hole of 80 mm in depth and 300 mm in diameter taking up 60 %. The second is clay (<0.005 mm) taking was punched in the center of one face of the speci- up 20−40 %, and the minor component is fine sand men. Second, insert a tensiometer into the hole with Fig. 6 The main longitude section of the Yanlian landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 4 of 11 Table 1 The basic physical properties of the loess samples Loess Density Moisture Specific Dry Void Volumetric moisture Liquid Plastic Plastic Coefficient of content gravity density ratio content limit limit index permeability 3 3 ρ/(g/cm ) w/(%) Gs ρ /(g/cm )e θ/(%) w /(%) w /(%) I /(%) k/(m/day) d 0 L P P Q 1.67 16.8 2.71 1.43 0.895 24.0 30.9 18.9 12.0 0.110 Q 1.86 20.5 2.71 1.54 0.760 31.6 37.0 22.8 14.2 0.044 Q 1.89 19.9 2.71 1.58 0.715 31.4 33.7 20.8 12.9 0.027 the ceramic head saturated in advance and fill the gap Where C is a constant related to the matric suction cor- with the same soil powder. Third, the specimen was responding to the residual volumetric moisture content, enclosed with melted wax to make the moister distri- a typical value for it is 1500 kPa. bution uniform. Some time later, read the data when The best fitting values of the parameters a, m, n and the matric suction recorded by the gauge is stable. the regressed SWCC are shown in Fig. 9, in which the Then remove part of the enclosed wax and collect a bit dots are the measured values with tensiometer. of soil to measure the water content. So a set of data The hydraulic conductivity functions (HCF) is esti- was obtained. In the next cycle, dropped some water in mated empirically by the model proposed by Childs the specimen and enclosed it again. Repeat the above and Collis-Gorge (1950) and improved by Marshall process to get a new set of data till the suction is 0. (1958) and Kunze et al. (1968). The procedure is per- Because the range of tensiometer is less than 100 kPa, formed by dividing SWCC into n equal moisture content the measured data is applied to regress with the Fredlund increments. The relationship between the HCF and the &Xing’s (1994) eq. (1) to extend the curves to a higher matric suctions is calculated by the model. With the mea- suction range: sured saturated coefficients of permeability in Table 1 and the SWCC curves in Fig. 9, the HCF are predicted and shown in Fig. 10. θ ¼ CðÞ ψ ð1Þ w m fg ln½ e þðÞ ψ=a Measurement of the unsaturated strength parameters Where θ is the volumetric moisture content; θ is the w s c ', φ' and φ saturated volumetric moisture content; ψ is the matric Fredlund et al. (1978) unsaturated shearing strength is suction; a is theapproximateair-entryvalueofthesoil; expressed as eq. (3). n is a parameter that controls the slope at the inflec- tion point in the SWCC; m is a parameter related to ′ ′ b the residual moisture content; e is the natural number, τ ¼ c þðÞ σ−u tanφ þðÞ u −u tanφ ð3Þ f a a w 2.71828; and C(ψ) is the correcting function defined by eq. (2). Where τ is the unsaturated shearing strength; c′ is the f 0 effective cohesion; φ′ is the effective friction angle; σ-u ln 1 þ is thepurenormalstrength; u -u is the matric suction CðÞ ψ ¼ 1− ð2Þ a w ln 1 þ 10 =C and φ is the friction angle related to matric suction. Fig. 9 The regressive SWCC curves of the loess, the dots are the Fig. 8 Distribution curves of the particle size for the Q ,Q ,Q loess measured values with TEN-10 tensiometer 3 2 1 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 5 of 11 enclosed in a rubber membrance for a week at least to make the moisture distribution uniform. Triaxial tests for Q loess were performed on the spec- imens of 80 mm in height and 39 mm in diameter. The confining pressures were set at 100 kPa, 200 kPa, 300 kPa, 400 kPa and 500 kPa rspectively. The rate of axial displacement was set at 0.04 mm/min for all the tests. Direct shear tests for Q loess were conducted using the disk specimens of 20 mm in height and 50 mm in diameter. Normal stress was applied with 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa and the shear displacement rate was set at 0.02 mm/min. The triaxial test results are showned in the forms of the maximam shear stress q= (σ -σ )/2 against 1 2 axial strain ε and q against average principle stess Fig. 10 The HCF curves of the Q ,Q ,Q loess 3 2 1 p= (σ + σ )/2 in Fig. 11. It can be seen that the 1 2 specimens with moisture contents lower than 10 % have no pore watetr presure produced in the whole In which, c′ and φ′ can be measured by conventional shearing process, but those with higer moisture con- direct shear test or triaxial test with saturated specimens. tents produce pore water presure. The higher the φ can be measured by suction-controlled direct shear moisture content, the higher the pore water presure test or triaxial test. is. Specimens with 5 % moisture contents have re- Let markable peak values than that of 10 % moisture contents under low confining pressure of 100 kPa. ′ ′ b c ¼ c þðÞ u −u tanφ a w 0 Meanwhile, most of the other specimens have realis- tic elasto-plastic and hardening stress–strain forms. Where, c′ can be considered as the total cohesion which Some of the high moisture content specimens have includes the friction produced by matric suction. So, gentle softening stress–strain curves, while the strain there is soften can be attributed to the increase of pore ′ ′ water pressure. It manifests that the loess could c − c tanφ ¼ ð4Þ maintain the stuctural strength in the condition of u − u a w low moisture content and low confning pressure. For Here the total cohesion c′ can be determined against high moisture content and high confining pressure, u -u by the conventional direct shear test or triaxial the stucture would be broken during consolidation a w test. prior to shearing. For the three layers of Q ,Q and Q loess as shown With the stress path q-p curves, the effective strength 3 2 1 in Fig. 6, Q lies on the top of the slope and fails due to K lines can be drawn as the common tangent lines of 3 f the tension cracks, so Q loess should have lost its effective stress paths, and the total effective cohesion c' strength before the slope failure. Q loess fails at a shear and friction angle φ' can be calculated by intercept b and zone in the middle of the slope, so consolidated- gradient angles α of K lines with eq. (5). The obtained undrained triaxial tests for Q loess were conducted to unsaturated strength parameters, the moisture contents, simulate the stress state. Q loess shears outward on the the calculated volumetric moisture contents and the ma- top of the pebble bed horizontally which has a fixed shear tric suction obtained from the SWCC curves are listed direction, so consolidated quick direct tests were con- in Table 2. ducted on Q loess to simulate the confined shear plane. −1 The moisture content of the loess underground is gen- c ¼ bcos φ ð5Þ −1 erally above 5 % while the saturated moisture content is φ ¼ sin ðÞ tanα about 30 %. Specimens with initial moisture contents of 5, 10, 15, 20, 25 and 30 % were prepared for Q loess and Q loess repectively to do the laboratory tests. To Where b is the intercept and α is the gradient of K line 1 f make the specimen with intended moisture content, shape respectively. and dry each specimen first, then put it on a balance and The data in Table 2 shows that the effective friction drop water around it till the total weight is equivalent angle φ' is independent of the moisture content. The to the moisture content. After that, the specimens are range of the angles is between 24.5° and 25.5°. The mean Li et al. Geoenvironmental Disasters (2015) 2:24 Page 6 of 11 Fig. 11 Stress–strain curves and stress paths for the specimens of definite moisture contents (a1)-(a6) are the q-ε curvesand (b1)-(b6) are the stress path q-p curves for the specimens with w = 5, 10, 15, 20, 25, 30 % respectively, wherein q =(σ -σ )/2, p =(σ + σ )/2 1 2 1 2 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 7 of 11 Table 2 The measured effective strength parameters and the contents and the matric suction determined by SWCC relevant indexes for Q loess 2 curves are also listed in Table 3. Specimen Moisture Volumetric Matric Total Effective Similar to the triaxial results for Q loess, the effective no. content moisture suction effective friction friction angle is independent of the moisture content. content cohesion angle The range of the angles is between 29.4° and 30.5°. The w/(%) θ /(%) U -u /(kPa) c′/(kPa) φ′/(°) w a w mean value is φ′ = 30.0°. The total effective cohesion T 5 7.7 200.5 74.6 25.5 decreases with the increase of the moisture content and T 10 15.4 67.0 27.0 24.8 changes prominently in the low range. It reaches to the T 15 23.1 28.7 16.7 25.0 minimum value of 5.0 kPa or so as the moisture content decreases to plastic limit (PL = 20.8 %). So c′ = 5.0 kPa. T 20 30.8 10.6 6.8 24.9 Through the curve of c′-c′ verse u -u as shown in 0 a w T 25 38.5 0.2 3.8 24.5 Fig. 15, φ can be determined to be 25.3°. T 30 46.2 0.0 3.5 25.2 Strength parameters φ = 19.5° c′ = 3.50 kPa φ′ = 25.0° Analysis for the landslide initiation mechanism To analyze the initiation mechanism of the landslide, a value is φ′ = 25.0°. The total effective cohesion c'has a 2D FEM model of the slope before failure is built up to reverse relation with the moisture content. It decreases simulate the water seepage processes and to monitor the prominently in the low moisture content range. As the moisture field. The stress field and the strength variation moisture content exceeds the plastic limit (PL = 22.8 %), with the moisture change are also analyzed. Fig. 16 it reaches to its minimum value 3.50 kPa or so and shows the FEM model. Geo-studio SEEP and SIGMA keeps constant. So c′ is accepted as 3.50 kPa. software are utilized to simulate the seepage process and The curve of c′-c′ verse u -u , shown in Fig. 12, ex- the stress fields, respectively. 0 a w presses that the relationship of c′-c′ and u -u is linear The HCF of Q ,Q ,Q loess, the SWCC and the re- 0 a w 1 2 3 and the gradient angle of the linear curve is 19.5°. lated strength parameters of Q ,Q loess have been fully 1 2 The stress–strain curves of the direct shear for Q documented above. The strength of Q loess can be 1 3 loess are shown in Fig. 13. It can be seen that the neglected, so a low cohesion is assigned to it empirically marked peak values appear in the cases of low normal and the friction angle is taken as zero. The deformation stresses and low moisture contents, such as in the mois- parameters of elastic module and Poisson’s ratio have ture contents of 5 % and 10 % under normal stress from minor effect on the stress field, so values of these param- 100 kPa to 400 kPa. For the remaining specimens, the eters for all the strata are assigned empirically. The stress-strains show realistic elasto-plastic behavior or strength of the bedrock is much higher than the loess hardening, similar to the triaxial results of Q loess. As- strata, so a high cohesion and friction angle are assigned. suming 6 mm shear displacement as the failure point, The values for all the parameters needed in the simula- the shear strength against mormal stress for each mois- tion are summarized in Table 4. ture content are ploted in Fig. 14, from which the total The initial condition starts as water begins to drop in effective cohesions c′ and effective friction angles φ′can the zone of the oil tanks standing on top of the slope. By be calculated by eq. (5) and are listed in Table 3. The estimating the voulme of the dropping water into the moisture contents, the calculated volumetric moisture ground in winter, a 20 mm high and 5 m wide water col- umn is supposed to seep into the ground perday and continue for 100 days (winter days) a year. The rainfall is negleted. In addition, the initial moisture contents of the loess layers are defined by the moister content log- ging in the borehole in the nearby slope. Geo-studio SEEP is used to simulate the seepage pro- cesses. The moisture field (expressed as pore water pres- sure field) is added into the Geo-studio SIGMA to simulate the stress and the strength variation under water infiltration. The sliding surface is fixed as the real occurred one, and the shear stress as well as shear strength at each point on the sliding surface can be figured out with the simulated stress states. The factor of safety Fs is defined by eq. (6) based on limit equilib- Fig. 12 The relation between c′-c′ against u -u of the Q loess 0 a w 2 rium theory. Li et al. Geoenvironmental Disasters (2015) 2:24 Page 8 of 11 Fig. 13 The shear stress–strain curves of the test results for different moisture contents (a)w = 5 %; (b)w =10 %; (c)w =15 %; (d)w =20 %; (e)w =25 %; (f)w =30 % Table 3 The measured effective strength parameters and the relevant indexes for Q loess Specimen Moisture Volumetric Matric Total Effective no. content moisture suction effective friction content cohesion angle w/(%) θ /(%) u -u /(kPa) c′/(kPa) φ′/(°) w a w D 5 7.9 225.2 105.7 30.0 D 10 15.8 116.9 67.9 30.1 D 15 23.7 60.3 42.0 30.5 D 20 31.6 24.3 19.4 29.4 D 25 39.5 0.0 7.9 29.6 D 30 47.4 0.0 5.0 29.5 Fig. 14 The shear strength vs normal stress for different moisture contents Strength parameters φ = 25.3° c′ = 5.0 kPa φ′ = 30.0° 0 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 9 of 11 Table 4 The values of the parameters for the slope simulation Strata Q loess Q loess Q loess Sandstone 3 2 1 2 3 3 5 Elastic module E/(kPa) 10 10 10 10 Poisson’s ratio ν 0.35 0.35 0.33 0.20 Effective friction angle φ′/(°) 0.0 25.0 30.0 45.0 Friction angle related to 0.0 195. 25.3 - suction φ /(°) Saturated effective cohesion 3.0 3.5 5.0 1000 c′ /(kPa) Density γ/(g/cm ) 1.65 1.86 1.89 2.45 Fig. 15 Relation between c′-c′ and u -u of the Q loess 0 a w 1 Saturated permeability 0.110 0.044 0.027 - K/(m/d) Initial moisture content w /(%) 16.8 20.5 19.9 - x 0 τ dx Initial volumetric moisture 24.0 31.6 31.4 - content θ /(%) Fs ¼ Z ð6Þ Initial matric suction 18.3 9.2 25.1 - τdx (u -u \ /(kPa) x a w 0 Where τ is the shear strength of the sliding surface as th defined by eq. (3); τ is the shear stress on the sliding stability has no significant change. In the 9 year, the surface; x and x are the horizontal coordinate of the groundwater rises up to submerge the lower portion of the A B end points of the sliding surface. sliding surface and the slope stability starts to decrease. The simulations are conducted from the beginning During the drop break, the groundwater continues flowing of water dropping and the factor of safety is calculated from the inner to the slope toe. As a result the ground- as well till it fails. At last, it keeps stable for 15 years water level beneath the slope toe continues rising. So before failure. The period is agreement with the age of in all the yearly periods, the slope stability becomes the oil tanks. worse even after the water stops infiltrating. Figure 17 shows the simulated pore water pressure Figure 18 shows the evolution of the factor of safety fields at specified time. For each year, the results at the during water infiltration. During the first eight years, the ends of 100 days water dropping and after 265 days factor of safety has no prominent decreasing and begins th th break are provided. It shows that in the first year, the to lower in the 9 -10 years. The time coincides with the moisture perches at the upper layer of the slope. And period that the groundwater level begins to touch the po- when stopping infiltrating, the moisture scatters and tential sliding surface and rises continuously. In addition, th negative pore pressure decreases. In the 4 year, mois- the moisture content of the slope above the groundwater ture moves down to the impermeable rock bed and mi- level also increases which reduces the strength of the loess. grates along the boundary of Q and Q loess as well as All of these factors lead to the slope failure. 3 2 the bedrock. When the infiltration stops, the wetting front moves laterally and the groundwater level decreases with Conclusions the water extends outwards. However, at this time, just a Loess is a typical unsaturated soil which is sensitive to littlemoistureinvades the sliding surface, so the slope moisture. In this paper, investigation demonstrates that Fig. 16 FEM meshes of the slope model before failure (Fs = 1.44) Li et al. Geoenvironmental Disasters (2015) 2:24 Page 10 of 11 Fig. 17 The simulated results of the pore water pressure in the slope at different times (a) At the end of 100 days, Fs=1.43; (b) At the end of the first year, th th th Fs=1.43; (c)Atthe endof the 4 year and 100 days, Fs = 1.44; (d)Atthe endof the 5 year, Fs = 1.43; (e)Atthe endofthe 9 year and 100 days, th th th Fs=1.36; (f)Atthe endof the 9 year, Fs = 1.29; (g) At the end of the 14 year and 100 days, Fs = 1.03; (h) At the end of the 14 year, Fs = 1.01 Fig. 18 The factor of safety varied with time Li et al. Geoenvironmental Disasters (2015) 2:24 Page 11 of 11 long term condensed water vapor released from the pres- Li TL, Wang P, Xi Y (2013). Mechanisms for initiation and motion of Chinese loess landslides, Ed. by FawuWang, Masakatsu Miyajima, Tonglu Li,Wei Shan, Teuku sure adjusters on the heating pipes which are used to keep Faisal Fathani, Progress of Geo-Disaster Mitigation Technology in Asia, the oil in the transfer pipes from frozen in winter is the Springer, Verlag Berlin Heidelberg:105-122. triggering factor of the Yanlian landslide. Laboratory tests Liao HJ, Su LJ, Li ZD, Pan YB, Fukuoka H (2008) Testing study on the strength and deformation characteristics of soil in loess landslides. Ed. by Chen Zuyu, including conventional triaxial tests, consolidation quick Zhang Jianmin, Li Zhongkui, Wu Faquan, Ho Ken, Landslides and Engineered shear tests and SWCC measurement are performed to Slopes (from the Past to the Future), CRC Press, Leiden, The Netherlands, Vol estimate the unsaturated strength parameters and water 1:443-447. Marshall TJ (1958) A relation between permeability and size distribution of pores. conductivity. J Soil Sci 9:1–8 The safety of factor can be figured out based on the Tu XB, Kwong AKL, Dai FC, Tham LG, Min H (2009) Field monitoring of rainfall moisture field and the stress field simulated by the FEM infiltration in a loess slope and analysis of failure mechanism of rainfall-induced landslides. J Eng Geol 105(1):134–150 model with the parameters. The result shows that a little Zhang JQ, Peng JB (2014) A coupled slope cutting—a prolonged rainfall-induced water infiltration has minor effects on slope stability for loess landslide: a 17 October 2011 case study. Bull Eng Geol Environ some time. As a result it is easy to be ignored. However, 73(4):997–1011 when the period of water infiltration is long enough to raise the groundwater level, it will have detrimental in- fluence on the stability and the slope will fail at last. The analysis above illustrates that any minor water produced by engineering or other activities may have harmful effect on slope stability for a long period of action. Therefore it is essential to take account of this kind of water and adopt measures to curb the surface water infiltration and to drain the groundwater. Competing interests The authors declare that they have no competing interests. Authors' contributions All authors participated the field investigations. HS & XZ conducted sampling and all the laboratory tests. XZ conducted graphing and analysis for the test results. HS conducted the numerical simulations. PL drafted the manuscript. All authors read and approved the final manuscript. Acknowledgements This research was funded by one of National Basic Research Program of China (2014CB744701) and National Natural Science Foundation of China (Program no. 41372329). The authors wish to acknowledge Dr. Austin ChukwuelokaOkeke, Department of Geoscience, Shimane University, for reading the manuscript and revising the English language. Our thanks also give to Dr. Xianli Xing, Department of Geological Engineering, Chang’an University, for her valuable guidance of the laboratory tests. Received: 7 October 2014 Accepted: 27 April 2015 References Bai MZ, Du YQ, Kuang X (2012) Warning Method and System in Risk Management for Loess Engineering Slopes. J Perform Constr Facil 26(2):190–196 Chillds EC, Collis-Geoge GN (1950) The permeability of porous materials. Proc R Soc London, Ser A 201:392–405 Dai FC, Lee CF (2001) Frequency-volume relation and prediction of rainfall- Submit your manuscript to a induced landslides. Eng Geol 59(3/4):253–266 journal and benefi t from: Fredlund DG, Morgenstern NR, Widger RA (1978) The shear strength of unsaturated soils. Can Geotech J 15:313–321 7 Convenient online submission Fredlund DG, Xing A (1994) Equations for the soil-water characteristic curve. Can 7 Rigorous peer review Geotech J 31(3):521–532 Kunze RJ, Uehara G, Graham K (1968) Factors important in the calculation of 7 Immediate publication on acceptance hydraulic conductivity. Soil Sci Soc Am Proc 32:760–765 7 Open access: articles freely available online Lei XY (1994) The hazards of loess landslides in the southern tableland of 7 High visibility within the fi eld Jingyang County, Shaanxi and their relationship with the channel water 7 Retaining the copyright to your article into fields (In Chinese). J Eng Geol 3(1):56–64 Li P, Zhang B, Li TL (2012) Study on Regionalization for Characteristic and Destruction Rule of Slope in Loess Plateau (In Chinese). J Earth Sci Submit your next manuscript at 7 springeropen.com Environ 34(3):89–98 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Investigation for the initiation of a loess landslide based on the unsaturated permeability and strength theory

Li, Ping; Zhang, Xingting; Shi, Hao
Geoenvironmental Disasters , Volume 2 (1) – Sep 17, 2015
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Springer Journals
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Copyright © 2015 by Li et al.
Subject
Environment; Environment, general; Earth Sciences, general; Geography (general); Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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2197-8670
DOI
10.1186/s40677-015-0032-7
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Abstract

Background: The Yanlian landslide, occurring on 21-22 October 2010, destroyed many facilities of a big oil refinery in Shaan’xi Province, China. It led to a suspending of the refinery work for a week and caused near 700 million RMB economic losses. Results: Site exploration shows that the sliding mass is unsaturated-saturated loess. The groundwater is rich in the landslide and shortage in the surrounding slopes. Further investigation finds that the water drop released from the vapor heating furnaces on the top of the slope is the only source of the groundwater. Laboratory tests were performed to get the unsaturated strength parameters and hydraulic conductivity of the loess layers which were applied to a pre-failure slope model to simulate the water infiltration process and the stress field based on which the factor of safety is figured out to analyze the slope stability. Analysis shows that during the first ten years, the factor of safety has no prominent decrease. In the following 5 years, the slope stability decreases significantly till failure. Conclusions: A little water infiltration has minor effects on slope stability for some time. As a result it is easy to be ignored. However, when the period of water infiltration is long enough to raise the groundwater level, it will have detrimental influence on the stability. In conclusion, any minor water produced by engineering or other activities for a long period may have harmful effect on slope stability. Therefore it is essential to take account of this kind of water and adopt measures to curb the surface water infiltration and to drain the groundwater. Keywords: Unsaturated soil; Loess; Landslide; Strength parameters; Soil-water characteristic curve; Hydraulic conductivity function Background moves down intermittently for two days and the people Loess landslides occurring in China lead to a huge eco- working on it has enough time to escape. So there is nomic losses every year (Liao et al. 2008; Bai et al. 2012; no fatality in the accident. The cause of the landslide Li et al. 2012). It has been proved that rainfall, irrigation, is unclear and deeply concerned by the managers and water canal leakage, submerging of reservoir, melted researchers. The aims of the paper are to find the trigger- snow and earthquake are the main factors triggering ing factors by site exploration and to study the failure loess landslides (Lei, 1994; Dai and Lee 2001; Tu et al. mechanism by laboratory tests and numerical analysis. 2009; Li et al. 2013; Zhang & Peng 2014). However, there are no water resources and earthquake mentioned above Methods, results and discussions before the Yanlian landslide occurred, which is located at Characters of the landslides Luochuan county, Shaanxi province, China as shown in The landslide occurred on the erosive bank of the Luohe Fig. 1, and the slopes nearby are stable even though they river which is the second tributary of the Yellow river. are higher and steeper than the failed. The landslide The first and the third terraces of the river develop on that bank and the second terrace is eroded off. The first terrace, about 5 m above the river bed, is flat and wide * Correspondence: dcdgx07@chd.edu.cn on which there is an oil refinery named Yanlian (Fig. 2). Department of Geological Engineering, Chang’an University, Xi’an, Shaan’xi, China Against the rear of the first terrace is a 20 m high rock © 2015 Li et al. 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. Li et al. Geoenvironmental Disasters (2015) 2:24 Page 2 of 11 Fig. 3 The spring at the toe of the slope Fig. 1 Location of the Yanlian landslide (Fig. 5). The refinery had to stop working until 7 days cliff composed of hard horizontal sandstone of the upper later when the facilities were recovered. There were no Triassic formation. The rock cliff is the base of the third fatalities but it caused 700 million RMB losses. terrace which is overlain by about 2 m thick alluvial peb- The surface gradient of the slope before failure is about ble and nearly 50 m thick Quaternary loess successively. 30° and the vertical height is 47 m. The failure mass has a The pebble is filled with silt-sand and cemented by leached width of 240 m, a length of 150 m, a thickness of carbonate calcium, so it has a very low permeability. A 13.8 m and a total volume of 400,000 m .The slopeis spring flows out from the top of the pebble with an average composed of the unsaturated loess of Late (Q loess), flow quantity of about 0.5 L/s (Fig. 3). The sliding mass, Medium (Q loess) and Early (Q loess) Pleistocene 2 1 shearing out on the pebble bed, is totally loess. Around formations as showninFig.6.The groundwaterlevel the failed slope, there are 34 large oil tanks lying on is only 7 m deep at the center of the landslide, but no the top and 24 oil pipelines on the toe. A road wanders groundwater is found in the surrounding slopes. The on the slope from the toe to the top. A boiler house region is generally shortage of groundwater because of and a coal storehouse stand on the first terrace against the small precipitation (annually 596 mm/a) and the the cliff (Fig. 2). deep cut landform. The rainfall during the rainy season Several days before October 21, 2010, a small part of may quickly run off on the surface. Investigation on loess at the toe of slope started to collapse from where a the top platform where oil tanks stand finds that there spring flowed out and buried part of the coal storage is lots of water vapor released from the pressure house. When the workers were trying to clear the col- adjusters on the heating pipes which are used to keep lapsed material, the whole slope moved downwards for the oil in the transfer pipes from frozen in winter two days. (Fig. 7). The hot vapor condenses in winter and partly The landslide buried part of the coal storehouse (Fig. 2) penetrates into the ground. The condensed water of the and cut off all the 24 oil pipelines (Fig. 4) and the road. vapor is possibly the water source in the landslide The oil tanks were only 5 m away from the head scarp Fig. 2 The Yanlian landslide Fig. 4 The oil transfer pipes cut off by the landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 3 of 11 Fig. 5 The oil tanks hanging over the top of the head scarp Fig. 7 The release water vapor from the pressure adjust valves on the heating pipes which has been ignored at the beginning. Therefore it is assumed that the groundwater in the landslide was (>0.05 mm) less than 10 %. From Q to Q and Q 3 2 1 generated by unsaturated infiltration of the condensed loess, the mean particle size and the void ratio decrease water drops and the landslide was trigged by the rising as well as dry density increases. It demonstrates that of the groundwater level. the lower the loess layer lies, the longer the time of pe- dogenesis is and the heavier the load of consolidation Determination of the loess physical properties experiences, the more compacted the loess is. The sliding mass and surface are composed of loess. Basic physical properties of loess include permeability, Determination of SWCC and HCF water content, density, plasticity, grain components. Soil-water characteristic curves (SWCC) were mea- The coefficient of saturated permeability was measured sured in laboratory with undisturbed block specimens through the falling head method. The water content of 300 mm × 300 mm × 300 mm in dimensions. The and density were measured by oven drying and metal specimens were collected from Q ,Q and Q loess 1 2 3 ring respectively. The plastic limit and liquid limit were respectively. The matric suctions were measured in measured by hand rolling and Cassgrande apparatus wetting processby the TEN−15 tensiometer and the respectively. The results are shown in Table 1. Laser corresponding moisture contents by the oven-drying Particle Analysis were utilized to measure the grain size method. It was accomplished through the following distributions of Q ,Q ,Q loess as shown in Fig. 8. It steps: First, a block of specimen was air dried and a 1 2 3 is shown that major component is silt (0.005−0.05 mm) hole of 80 mm in depth and 300 mm in diameter taking up 60 %. The second is clay (<0.005 mm) taking was punched in the center of one face of the speci- up 20−40 %, and the minor component is fine sand men. Second, insert a tensiometer into the hole with Fig. 6 The main longitude section of the Yanlian landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 4 of 11 Table 1 The basic physical properties of the loess samples Loess Density Moisture Specific Dry Void Volumetric moisture Liquid Plastic Plastic Coefficient of content gravity density ratio content limit limit index permeability 3 3 ρ/(g/cm ) w/(%) Gs ρ /(g/cm )e θ/(%) w /(%) w /(%) I /(%) k/(m/day) d 0 L P P Q 1.67 16.8 2.71 1.43 0.895 24.0 30.9 18.9 12.0 0.110 Q 1.86 20.5 2.71 1.54 0.760 31.6 37.0 22.8 14.2 0.044 Q 1.89 19.9 2.71 1.58 0.715 31.4 33.7 20.8 12.9 0.027 the ceramic head saturated in advance and fill the gap Where C is a constant related to the matric suction cor- with the same soil powder. Third, the specimen was responding to the residual volumetric moisture content, enclosed with melted wax to make the moister distri- a typical value for it is 1500 kPa. bution uniform. Some time later, read the data when The best fitting values of the parameters a, m, n and the matric suction recorded by the gauge is stable. the regressed SWCC are shown in Fig. 9, in which the Then remove part of the enclosed wax and collect a bit dots are the measured values with tensiometer. of soil to measure the water content. So a set of data The hydraulic conductivity functions (HCF) is esti- was obtained. In the next cycle, dropped some water in mated empirically by the model proposed by Childs the specimen and enclosed it again. Repeat the above and Collis-Gorge (1950) and improved by Marshall process to get a new set of data till the suction is 0. (1958) and Kunze et al. (1968). The procedure is per- Because the range of tensiometer is less than 100 kPa, formed by dividing SWCC into n equal moisture content the measured data is applied to regress with the Fredlund increments. The relationship between the HCF and the &Xing’s (1994) eq. (1) to extend the curves to a higher matric suctions is calculated by the model. With the mea- suction range: sured saturated coefficients of permeability in Table 1 and the SWCC curves in Fig. 9, the HCF are predicted and shown in Fig. 10. θ ¼ CðÞ ψ ð1Þ w m fg ln½ e þðÞ ψ=a Measurement of the unsaturated strength parameters Where θ is the volumetric moisture content; θ is the w s c ', φ' and φ saturated volumetric moisture content; ψ is the matric Fredlund et al. (1978) unsaturated shearing strength is suction; a is theapproximateair-entryvalueofthesoil; expressed as eq. (3). n is a parameter that controls the slope at the inflec- tion point in the SWCC; m is a parameter related to ′ ′ b the residual moisture content; e is the natural number, τ ¼ c þðÞ σ−u tanφ þðÞ u −u tanφ ð3Þ f a a w 2.71828; and C(ψ) is the correcting function defined by eq. (2). Where τ is the unsaturated shearing strength; c′ is the f 0 effective cohesion; φ′ is the effective friction angle; σ-u ln 1 þ is thepurenormalstrength; u -u is the matric suction CðÞ ψ ¼ 1− ð2Þ a w ln 1 þ 10 =C and φ is the friction angle related to matric suction. Fig. 9 The regressive SWCC curves of the loess, the dots are the Fig. 8 Distribution curves of the particle size for the Q ,Q ,Q loess measured values with TEN-10 tensiometer 3 2 1 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 5 of 11 enclosed in a rubber membrance for a week at least to make the moisture distribution uniform. Triaxial tests for Q loess were performed on the spec- imens of 80 mm in height and 39 mm in diameter. The confining pressures were set at 100 kPa, 200 kPa, 300 kPa, 400 kPa and 500 kPa rspectively. The rate of axial displacement was set at 0.04 mm/min for all the tests. Direct shear tests for Q loess were conducted using the disk specimens of 20 mm in height and 50 mm in diameter. Normal stress was applied with 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa and the shear displacement rate was set at 0.02 mm/min. The triaxial test results are showned in the forms of the maximam shear stress q= (σ -σ )/2 against 1 2 axial strain ε and q against average principle stess Fig. 10 The HCF curves of the Q ,Q ,Q loess 3 2 1 p= (σ + σ )/2 in Fig. 11. It can be seen that the 1 2 specimens with moisture contents lower than 10 % have no pore watetr presure produced in the whole In which, c′ and φ′ can be measured by conventional shearing process, but those with higer moisture con- direct shear test or triaxial test with saturated specimens. tents produce pore water presure. The higher the φ can be measured by suction-controlled direct shear moisture content, the higher the pore water presure test or triaxial test. is. Specimens with 5 % moisture contents have re- Let markable peak values than that of 10 % moisture contents under low confining pressure of 100 kPa. ′ ′ b c ¼ c þðÞ u −u tanφ a w 0 Meanwhile, most of the other specimens have realis- tic elasto-plastic and hardening stress–strain forms. Where, c′ can be considered as the total cohesion which Some of the high moisture content specimens have includes the friction produced by matric suction. So, gentle softening stress–strain curves, while the strain there is soften can be attributed to the increase of pore ′ ′ water pressure. It manifests that the loess could c − c tanφ ¼ ð4Þ maintain the stuctural strength in the condition of u − u a w low moisture content and low confning pressure. For Here the total cohesion c′ can be determined against high moisture content and high confining pressure, u -u by the conventional direct shear test or triaxial the stucture would be broken during consolidation a w test. prior to shearing. For the three layers of Q ,Q and Q loess as shown With the stress path q-p curves, the effective strength 3 2 1 in Fig. 6, Q lies on the top of the slope and fails due to K lines can be drawn as the common tangent lines of 3 f the tension cracks, so Q loess should have lost its effective stress paths, and the total effective cohesion c' strength before the slope failure. Q loess fails at a shear and friction angle φ' can be calculated by intercept b and zone in the middle of the slope, so consolidated- gradient angles α of K lines with eq. (5). The obtained undrained triaxial tests for Q loess were conducted to unsaturated strength parameters, the moisture contents, simulate the stress state. Q loess shears outward on the the calculated volumetric moisture contents and the ma- top of the pebble bed horizontally which has a fixed shear tric suction obtained from the SWCC curves are listed direction, so consolidated quick direct tests were con- in Table 2. ducted on Q loess to simulate the confined shear plane. −1 The moisture content of the loess underground is gen- c ¼ bcos φ ð5Þ −1 erally above 5 % while the saturated moisture content is φ ¼ sin ðÞ tanα about 30 %. Specimens with initial moisture contents of 5, 10, 15, 20, 25 and 30 % were prepared for Q loess and Q loess repectively to do the laboratory tests. To Where b is the intercept and α is the gradient of K line 1 f make the specimen with intended moisture content, shape respectively. and dry each specimen first, then put it on a balance and The data in Table 2 shows that the effective friction drop water around it till the total weight is equivalent angle φ' is independent of the moisture content. The to the moisture content. After that, the specimens are range of the angles is between 24.5° and 25.5°. The mean Li et al. Geoenvironmental Disasters (2015) 2:24 Page 6 of 11 Fig. 11 Stress–strain curves and stress paths for the specimens of definite moisture contents (a1)-(a6) are the q-ε curvesand (b1)-(b6) are the stress path q-p curves for the specimens with w = 5, 10, 15, 20, 25, 30 % respectively, wherein q =(σ -σ )/2, p =(σ + σ )/2 1 2 1 2 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 7 of 11 Table 2 The measured effective strength parameters and the contents and the matric suction determined by SWCC relevant indexes for Q loess 2 curves are also listed in Table 3. Specimen Moisture Volumetric Matric Total Effective Similar to the triaxial results for Q loess, the effective no. content moisture suction effective friction friction angle is independent of the moisture content. content cohesion angle The range of the angles is between 29.4° and 30.5°. The w/(%) θ /(%) U -u /(kPa) c′/(kPa) φ′/(°) w a w mean value is φ′ = 30.0°. The total effective cohesion T 5 7.7 200.5 74.6 25.5 decreases with the increase of the moisture content and T 10 15.4 67.0 27.0 24.8 changes prominently in the low range. It reaches to the T 15 23.1 28.7 16.7 25.0 minimum value of 5.0 kPa or so as the moisture content decreases to plastic limit (PL = 20.8 %). So c′ = 5.0 kPa. T 20 30.8 10.6 6.8 24.9 Through the curve of c′-c′ verse u -u as shown in 0 a w T 25 38.5 0.2 3.8 24.5 Fig. 15, φ can be determined to be 25.3°. T 30 46.2 0.0 3.5 25.2 Strength parameters φ = 19.5° c′ = 3.50 kPa φ′ = 25.0° Analysis for the landslide initiation mechanism To analyze the initiation mechanism of the landslide, a value is φ′ = 25.0°. The total effective cohesion c'has a 2D FEM model of the slope before failure is built up to reverse relation with the moisture content. It decreases simulate the water seepage processes and to monitor the prominently in the low moisture content range. As the moisture field. The stress field and the strength variation moisture content exceeds the plastic limit (PL = 22.8 %), with the moisture change are also analyzed. Fig. 16 it reaches to its minimum value 3.50 kPa or so and shows the FEM model. Geo-studio SEEP and SIGMA keeps constant. So c′ is accepted as 3.50 kPa. software are utilized to simulate the seepage process and The curve of c′-c′ verse u -u , shown in Fig. 12, ex- the stress fields, respectively. 0 a w presses that the relationship of c′-c′ and u -u is linear The HCF of Q ,Q ,Q loess, the SWCC and the re- 0 a w 1 2 3 and the gradient angle of the linear curve is 19.5°. lated strength parameters of Q ,Q loess have been fully 1 2 The stress–strain curves of the direct shear for Q documented above. The strength of Q loess can be 1 3 loess are shown in Fig. 13. It can be seen that the neglected, so a low cohesion is assigned to it empirically marked peak values appear in the cases of low normal and the friction angle is taken as zero. The deformation stresses and low moisture contents, such as in the mois- parameters of elastic module and Poisson’s ratio have ture contents of 5 % and 10 % under normal stress from minor effect on the stress field, so values of these param- 100 kPa to 400 kPa. For the remaining specimens, the eters for all the strata are assigned empirically. The stress-strains show realistic elasto-plastic behavior or strength of the bedrock is much higher than the loess hardening, similar to the triaxial results of Q loess. As- strata, so a high cohesion and friction angle are assigned. suming 6 mm shear displacement as the failure point, The values for all the parameters needed in the simula- the shear strength against mormal stress for each mois- tion are summarized in Table 4. ture content are ploted in Fig. 14, from which the total The initial condition starts as water begins to drop in effective cohesions c′ and effective friction angles φ′can the zone of the oil tanks standing on top of the slope. By be calculated by eq. (5) and are listed in Table 3. The estimating the voulme of the dropping water into the moisture contents, the calculated volumetric moisture ground in winter, a 20 mm high and 5 m wide water col- umn is supposed to seep into the ground perday and continue for 100 days (winter days) a year. The rainfall is negleted. In addition, the initial moisture contents of the loess layers are defined by the moister content log- ging in the borehole in the nearby slope. Geo-studio SEEP is used to simulate the seepage pro- cesses. The moisture field (expressed as pore water pres- sure field) is added into the Geo-studio SIGMA to simulate the stress and the strength variation under water infiltration. The sliding surface is fixed as the real occurred one, and the shear stress as well as shear strength at each point on the sliding surface can be figured out with the simulated stress states. The factor of safety Fs is defined by eq. (6) based on limit equilib- Fig. 12 The relation between c′-c′ against u -u of the Q loess 0 a w 2 rium theory. Li et al. Geoenvironmental Disasters (2015) 2:24 Page 8 of 11 Fig. 13 The shear stress–strain curves of the test results for different moisture contents (a)w = 5 %; (b)w =10 %; (c)w =15 %; (d)w =20 %; (e)w =25 %; (f)w =30 % Table 3 The measured effective strength parameters and the relevant indexes for Q loess Specimen Moisture Volumetric Matric Total Effective no. content moisture suction effective friction content cohesion angle w/(%) θ /(%) u -u /(kPa) c′/(kPa) φ′/(°) w a w D 5 7.9 225.2 105.7 30.0 D 10 15.8 116.9 67.9 30.1 D 15 23.7 60.3 42.0 30.5 D 20 31.6 24.3 19.4 29.4 D 25 39.5 0.0 7.9 29.6 D 30 47.4 0.0 5.0 29.5 Fig. 14 The shear strength vs normal stress for different moisture contents Strength parameters φ = 25.3° c′ = 5.0 kPa φ′ = 30.0° 0 Li et al. Geoenvironmental Disasters (2015) 2:24 Page 9 of 11 Table 4 The values of the parameters for the slope simulation Strata Q loess Q loess Q loess Sandstone 3 2 1 2 3 3 5 Elastic module E/(kPa) 10 10 10 10 Poisson’s ratio ν 0.35 0.35 0.33 0.20 Effective friction angle φ′/(°) 0.0 25.0 30.0 45.0 Friction angle related to 0.0 195. 25.3 - suction φ /(°) Saturated effective cohesion 3.0 3.5 5.0 1000 c′ /(kPa) Density γ/(g/cm ) 1.65 1.86 1.89 2.45 Fig. 15 Relation between c′-c′ and u -u of the Q loess 0 a w 1 Saturated permeability 0.110 0.044 0.027 - K/(m/d) Initial moisture content w /(%) 16.8 20.5 19.9 - x 0 τ dx Initial volumetric moisture 24.0 31.6 31.4 - content θ /(%) Fs ¼ Z ð6Þ Initial matric suction 18.3 9.2 25.1 - τdx (u -u \ /(kPa) x a w 0 Where τ is the shear strength of the sliding surface as th defined by eq. (3); τ is the shear stress on the sliding stability has no significant change. In the 9 year, the surface; x and x are the horizontal coordinate of the groundwater rises up to submerge the lower portion of the A B end points of the sliding surface. sliding surface and the slope stability starts to decrease. The simulations are conducted from the beginning During the drop break, the groundwater continues flowing of water dropping and the factor of safety is calculated from the inner to the slope toe. As a result the ground- as well till it fails. At last, it keeps stable for 15 years water level beneath the slope toe continues rising. So before failure. The period is agreement with the age of in all the yearly periods, the slope stability becomes the oil tanks. worse even after the water stops infiltrating. Figure 17 shows the simulated pore water pressure Figure 18 shows the evolution of the factor of safety fields at specified time. For each year, the results at the during water infiltration. During the first eight years, the ends of 100 days water dropping and after 265 days factor of safety has no prominent decreasing and begins th th break are provided. It shows that in the first year, the to lower in the 9 -10 years. The time coincides with the moisture perches at the upper layer of the slope. And period that the groundwater level begins to touch the po- when stopping infiltrating, the moisture scatters and tential sliding surface and rises continuously. In addition, th negative pore pressure decreases. In the 4 year, mois- the moisture content of the slope above the groundwater ture moves down to the impermeable rock bed and mi- level also increases which reduces the strength of the loess. grates along the boundary of Q and Q loess as well as All of these factors lead to the slope failure. 3 2 the bedrock. When the infiltration stops, the wetting front moves laterally and the groundwater level decreases with Conclusions the water extends outwards. However, at this time, just a Loess is a typical unsaturated soil which is sensitive to littlemoistureinvades the sliding surface, so the slope moisture. In this paper, investigation demonstrates that Fig. 16 FEM meshes of the slope model before failure (Fs = 1.44) Li et al. Geoenvironmental Disasters (2015) 2:24 Page 10 of 11 Fig. 17 The simulated results of the pore water pressure in the slope at different times (a) At the end of 100 days, Fs=1.43; (b) At the end of the first year, th th th Fs=1.43; (c)Atthe endof the 4 year and 100 days, Fs = 1.44; (d)Atthe endof the 5 year, Fs = 1.43; (e)Atthe endofthe 9 year and 100 days, th th th Fs=1.36; (f)Atthe endof the 9 year, Fs = 1.29; (g) At the end of the 14 year and 100 days, Fs = 1.03; (h) At the end of the 14 year, Fs = 1.01 Fig. 18 The factor of safety varied with time Li et al. Geoenvironmental Disasters (2015) 2:24 Page 11 of 11 long term condensed water vapor released from the pres- Li TL, Wang P, Xi Y (2013). Mechanisms for initiation and motion of Chinese loess landslides, Ed. by FawuWang, Masakatsu Miyajima, Tonglu Li,Wei Shan, Teuku sure adjusters on the heating pipes which are used to keep Faisal Fathani, Progress of Geo-Disaster Mitigation Technology in Asia, the oil in the transfer pipes from frozen in winter is the Springer, Verlag Berlin Heidelberg:105-122. triggering factor of the Yanlian landslide. Laboratory tests Liao HJ, Su LJ, Li ZD, Pan YB, Fukuoka H (2008) Testing study on the strength and deformation characteristics of soil in loess landslides. Ed. by Chen Zuyu, including conventional triaxial tests, consolidation quick Zhang Jianmin, Li Zhongkui, Wu Faquan, Ho Ken, Landslides and Engineered shear tests and SWCC measurement are performed to Slopes (from the Past to the Future), CRC Press, Leiden, The Netherlands, Vol estimate the unsaturated strength parameters and water 1:443-447. Marshall TJ (1958) A relation between permeability and size distribution of pores. conductivity. J Soil Sci 9:1–8 The safety of factor can be figured out based on the Tu XB, Kwong AKL, Dai FC, Tham LG, Min H (2009) Field monitoring of rainfall moisture field and the stress field simulated by the FEM infiltration in a loess slope and analysis of failure mechanism of rainfall-induced landslides. J Eng Geol 105(1):134–150 model with the parameters. The result shows that a little Zhang JQ, Peng JB (2014) A coupled slope cutting—a prolonged rainfall-induced water infiltration has minor effects on slope stability for loess landslide: a 17 October 2011 case study. Bull Eng Geol Environ some time. As a result it is easy to be ignored. However, 73(4):997–1011 when the period of water infiltration is long enough to raise the groundwater level, it will have detrimental in- fluence on the stability and the slope will fail at last. The analysis above illustrates that any minor water produced by engineering or other activities may have harmful effect on slope stability for a long period of action. Therefore it is essential to take account of this kind of water and adopt measures to curb the surface water infiltration and to drain the groundwater. Competing interests The authors declare that they have no competing interests. Authors' contributions All authors participated the field investigations. HS & XZ conducted sampling and all the laboratory tests. XZ conducted graphing and analysis for the test results. HS conducted the numerical simulations. PL drafted the manuscript. All authors read and approved the final manuscript. Acknowledgements This research was funded by one of National Basic Research Program of China (2014CB744701) and National Natural Science Foundation of China (Program no. 41372329). The authors wish to acknowledge Dr. Austin ChukwuelokaOkeke, Department of Geoscience, Shimane University, for reading the manuscript and revising the English language. Our thanks also give to Dr. Xianli Xing, Department of Geological Engineering, Chang’an University, for her valuable guidance of the laboratory tests. Received: 7 October 2014 Accepted: 27 April 2015 References Bai MZ, Du YQ, Kuang X (2012) Warning Method and System in Risk Management for Loess Engineering Slopes. J Perform Constr Facil 26(2):190–196 Chillds EC, Collis-Geoge GN (1950) The permeability of porous materials. Proc R Soc London, Ser A 201:392–405 Dai FC, Lee CF (2001) Frequency-volume relation and prediction of rainfall- Submit your manuscript to a induced landslides. Eng Geol 59(3/4):253–266 journal and benefi t from: Fredlund DG, Morgenstern NR, Widger RA (1978) The shear strength of unsaturated soils. Can Geotech J 15:313–321 7 Convenient online submission Fredlund DG, Xing A (1994) Equations for the soil-water characteristic curve. Can 7 Rigorous peer review Geotech J 31(3):521–532 Kunze RJ, Uehara G, Graham K (1968) Factors important in the calculation of 7 Immediate publication on acceptance hydraulic conductivity. Soil Sci Soc Am Proc 32:760–765 7 Open access: articles freely available online Lei XY (1994) The hazards of loess landslides in the southern tableland of 7 High visibility within the fi eld Jingyang County, Shaanxi and their relationship with the channel water 7 Retaining the copyright to your article into fields (In Chinese). J Eng Geol 3(1):56–64 Li P, Zhang B, Li TL (2012) Study on Regionalization for Characteristic and Destruction Rule of Slope in Loess Plateau (In Chinese). J Earth Sci Submit your next manuscript at 7 springeropen.com Environ 34(3):89–98

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Published: Sep 17, 2015

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