Investigation for the initiation of a loess landslide based on the unsaturated permeability and strength theory
- Ping Li^{1}Email author,
- Xingting Zhang^{1} and
- Hao Shi^{1}
Received: 7 October 2014
Accepted: 27 April 2015
Published: 17 September 2015
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 functionBackground
Methods, results and discussions
Characters of the landslides
Several days before October 21, 2010, a small part of loess at the toe of slope started to collapse from where a spring flowed out and buried part of the coal storage house. When the workers were trying to clear the collapsed material, the whole slope moved downwards for two days.
Determination of the loess physical properties
The basic physical properties of the loess samples
Loess | Density | Moisture content | Specific gravity | Dry density | Void ratio | Volumetric moisture content | Liquid limit | Plastic limit | Plastic index | Coefficient of permeability |
---|---|---|---|---|---|---|---|---|---|---|
ρ/(g/cm^{3}) | w/(%) | Gs | ρ _{d}/(g/cm^{3}) | e_{0} | θ/(%) | w _{L}/(%) | w _{P}/(%) | I_{P}/(%) | k/(m/day) | |
Q_{3} | 1.67 | 16.8 | 2.71 | 1.43 | 0.895 | 24.0 | 30.9 | 18.9 | 12.0 | 0.110 |
Q_{2} | 1.86 | 20.5 | 2.71 | 1.54 | 0.760 | 31.6 | 37.0 | 22.8 | 14.2 | 0.044 |
Q_{1} | 1.89 | 19.9 | 2.71 | 1.58 | 0.715 | 31.4 | 33.7 | 20.8 | 12.9 | 0.027 |
Determination of SWCC and HCF
Soil-water characteristic curves (SWCC) were measured in laboratory with undisturbed block specimens of 300 mm × 300 mm × 300 mm in dimensions. The specimens were collected from Q_{1}, Q_{2} and Q_{3} loess respectively. The matric suctions were measured in wetting processby the TEN−15 tensiometer and the corresponding moisture contents by the oven-drying method. It was accomplished through the following steps: First, a block of specimen was air dried and a hole of 80 mm in depth and 300 mm in diameter was punched in the center of one face of the specimen. Second, insert a tensiometer into the hole with the ceramic head saturated in advance and fill the gap with the same soil powder. Third, the specimen was enclosed with melted wax to make the moister distribution uniform. Some time later, read the data when the matric suction recorded by the gauge is stable. Then remove part of the enclosed wax and collect a bit of soil to measure the water content. So a set of data was obtained. In the next cycle, dropped some water in the specimen and enclosed it again. Repeat the above process to get a new set of data till the suction is 0.
Where C _{r} is a constant related to the matric suction corresponding to the residual volumetric moisture content, a typical value for it is 1500 kPa.
Measurement of the unsaturated strength parameters c _{0} ', φ' and φ ^{ b }
Where τ _{ f } is the unsaturated shearing strength; c′_{0} is the effective cohesion; φ′ is the effective friction angle; σ-u _{a} is the pure normal strength; u _{a}-u _{w} is the matric suction and φ ^{b} is the friction angle related to matric suction.
In which, c′_{0} and φ′ can be measured by conventional direct shear test or triaxial test with saturated specimens. φ ^{b} can be measured by suction-controlled direct shear test or triaxial test.
Here the total cohesion c′ can be determined against u _{a}-u _{w} by the conventional direct shear test or triaxial test.
For the three layers of Q_{3}, Q_{2} and Q_{1} loess as shown in Fig. 6, Q_{3} lies on the top of the slope and fails due to the tension cracks, so Q_{3} loess should have lost its strength before the slope failure. Q_{2} loess fails at a shear zone in the middle of the slope, so consolidated-undrained triaxial tests for Q_{2} loess were conducted to simulate the stress state. Q_{1} loess shears outward on the top of the pebble bed horizontally which has a fixed shear direction, so consolidated quick direct tests were conducted on Q_{1} loess to simulate the confined shear plane.
The moisture content of the loess underground is generally above 5 % while the saturated moisture content is about 30 %. Specimens with initial moisture contents of 5, 10, 15, 20, 25 and 30 % were prepared for Q_{2} loess and Q_{1} loess repectively to do the laboratory tests. To make the specimen with intended moisture content, shape and dry each specimen first, then put it on a balance and drop water around it till the total weight is equivalent to the moisture content. After that, the specimens are enclosed in a rubber membrance for a week at least to make the moisture distribution uniform.
Triaxial tests for Q_{2} loess were performed on the specimens 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_{1} 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 measured effective strength parameters and the relevant indexes for Q_{2} loess
Specimen no. | Moisture content | Volumetric moisture content | Matric suction | Total effective cohesion | Effective friction angle |
---|---|---|---|---|---|
w/(%) | θ _{ w }/(%) | U _{ a } -u _{ w }/(kPa) | c′/(kPa) | φ′/(°) | |
T_{1} | 5 | 7.7 | 200.5 | 74.6 | 25.5 |
T_{2} | 10 | 15.4 | 67.0 | 27.0 | 24.8 |
T_{3} | 15 | 23.1 | 28.7 | 16.7 | 25.0 |
T_{4} | 20 | 30.8 | 10.6 | 6.8 | 24.9 |
T_{5} | 25 | 38.5 | 0.2 | 3.8 | 24.5 |
T_{6} | 30 | 46.2 | 0.0 | 3.5 | 25.2 |
Strength parameters | φ ^{b} = 19.5° | c′ _{0} = 3.50 kPa | φ′ = 25.0° |
Where b is the intercept and α is the gradient of K _{f} line respectively.
The data in Table 2 shows that the effective friction angle φ' is independent of the moisture content. The range of the angles is between 24.5° and 25.5°. The mean value is φ′ = 25.0°. The total effective cohesion c' has a reverse relation with the moisture content. It decreases prominently in the low moisture content range. As the moisture content exceeds the plastic limit (PL = 22.8 %), it reaches to its minimum value 3.50 kPa or so and keeps constant. So c′ _{0} is accepted as 3.50 kPa.
The measured effective strength parameters and the relevant indexes for Q_{1} loess
Specimen no. | Moisture content | Volumetric moisture content | Matric suction | Total effective cohesion | Effective friction angle |
---|---|---|---|---|---|
w/(%) | θ _{ w }/(%) | u _{ a } -u _{ w }/(kPa) | c′/(kPa) | φ′/(°) | |
D_{1} | 5 | 7.9 | 225.2 | 105.7 | 30.0 |
D_{2} | 10 | 15.8 | 116.9 | 67.9 | 30.1 |
D_{3} | 15 | 23.7 | 60.3 | 42.0 | 30.5 |
D_{4} | 20 | 31.6 | 24.3 | 19.4 | 29.4 |
D_{5} | 25 | 39.5 | 0.0 | 7.9 | 29.6 |
D_{6} | 30 | 47.4 | 0.0 | 5.0 | 29.5 |
Strength parameters | φ ^{b} = 25.3° | c′ _{0} = 5.0 kPa | φ′ = 30.0° |
Analysis for the landslide initiation mechanism
The values of the parameters for the slope simulation
Strata | Q_{3 }loess | Q_{2} loess | Q_{1} loess | Sandstone |
---|---|---|---|---|
Elastic module E/(kPa) | 10^{2} | 10^{3} | 10^{3} | 10^{5} |
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 suction φ ^{b}/(°) | 0.0 | 195. | 25.3 | - |
Saturated effective cohesion c′ _{0}/(kPa) | 3.0 | 3.5 | 5.0 | 1000 |
Density γ/(g/cm^{3}) | 1.65 | 1.86 | 1.89 | 2.45 |
Saturated permeability K/(m/d) | 0.110 | 0.044 | 0.027 | - |
Initial moisture content w _{0}/(%) | 16.8 | 20.5 | 19.9 | - |
Initial volumetric moisture content θ _{ 0 }/(%) | 24.0 | 31.6 | 31.4 | - |
Initial matric suction (u _{ a } -u _{ w }\_{0}/(kPa) | 18.3 | 9.2 | 25.1 | - |
The initial condition starts as water begins to drop in the zone of the oil tanks standing on top of the slope. By estimating the voulme of the dropping water into the ground in winter, a 20 mm high and 5 m wide water column 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 logging in the borehole in the nearby slope.
Where τ _{ f } is the shear strength of the sliding surface as defined by eq. (3); τ is the shear stress on the sliding surface; x _{A} and x _{B} are the horizontal coordinate of the end points of the sliding surface.
The simulations are conducted from the beginning of water dropping and the factor of safety is calculated as well till it fails. At last, it keeps stable for 15 years before failure. The period is agreement with the age of the oil tanks.
Conclusions
Loess is a typical unsaturated soil which is sensitive to moisture. In this paper, investigation demonstrates that long term condensed water vapor released from the pressure adjusters on the heating pipes which are used to keep the oil in the transfer pipes from frozen in winter is the triggering factor of the Yanlian landslide. Laboratory tests including conventional triaxial tests, consolidation quick shear tests and SWCC measurement are performed to estimate the unsaturated strength parameters and water conductivity.
The safety of factor can be figured out based on the moisture field and the stress field simulated by the FEM model with the parameters. The result shows that 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 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.
Declarations
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.
Open AccessThis 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.
Authors’ Affiliations
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