In this study, a series of experiments of the full-scale physical model was employed to investigate the hydromechanical behaviours of the postearthquake landslide evolution, in forms of rill erosion and shallow headward failure on the rill bank slopes under unsaturated conditions. Soil-water characteristic curves (SWCC) were established using the Brooks-Corey (BC) and van Genuchten (VG) models. The stability of the shallow failure was then analyzed via a one-dimensional and unsaturated stability analysis model of the infinite slope. This measurement revealed that the preferential flow and the matrix flow coexisted when infiltration occurred and the VG model performs better in fitting the SWCC than the BC model. Consistent feedback between stability calculations and experimental observations enables the analysis of mechanisms of rill erosion and slope failure of postseismic landslide under the impact of preferential flow. Furthermore, the seepage-induced internal erosion phenomenon was observed in the experiment. This work thus provides a new perspective on the triggering mechanisms of debris flow during the postseismic period.
The Wenchuan earthquake in 2008 undermined the geologic stability and thoroughly changed the microgeomorphic conditions in the Longmenshan region of Sichuan province, southwestern China [
Understanding the evolution mechanism of shallow headward failure and rill erosion of earthquake-induced landslide deposits in the headwater valleys caused by heavy rainfall is key to developing an early warning system of postseismic debris flow disasters [
With the development of soil mechanics theories, greater attention has been paid to unsaturated soil materials and their failure mechanisms are inconsistent with the principles and concepts of classical, saturated soil mechanics. Unlike the simpler saturated sands, silts, and clays, these materials puzzled scientists for decades [
Based on the patterns of postearthquake mass transportation attributable to the shallow failure mode of the landslide deposits in the region affected by the Wenchuan earthquake, a series of full-scale physical model tests were conducted using the natural, coarse-textured soils from earthquake-induced landslide deposits. With the aid of an unsaturated infinite slope stability analysis model, changes in the one-dimensional unsaturated factor of safety (FS) of the experimental slope with artificial rainfall were computed at first. The experimental and calculation results were then compared and analyzed. By providing experimental data for predicting the infiltration characteristics and stability of landslide deposits composed of coarse-textured gravelly soils with rill erosion and shallow headward failure under unsaturated conditions, this study is aimed at providing new insights into the failure modes and mechanisms of rainfall-induced landslides and debris flows during the postseismic period.
The study focuses on the shallow failure mechanisms of the Yindongzi landslide in the Baisha River Basin. This landslide occurred in the Yindongzi Trench, Lianhe Village, Hongkou Township, Dujiangyan county, Chengdu City, Sichuan province, China, which is the meizoseismal area of the Wenchuan earthquake, and it is filled with typical earthquake-induced landslide deposits [
(a) Location and scale of Yindongzi landslide. (b) Free surfaces of rill banks on the landslide deposit. (c) A schematic diagram of shallow headward failure in rill erosion in the landslide deposit. (d) An outbreak of debris flows in Yindongzi gully.
In order to reproduce the rainfall-induced failure process of natural rill banks as realistically as possible, a full-scale (1 : 1) model was built according to the observed rill banks (Figure
Pictures of the physical model: (a) front view and (b) side view.
Schematic diagram of embedded sensors.
Experimental strategies.
Series | Rainfall intensity (mm/h) | Slope angle (°) |
---|---|---|
Test 1 | 170 | 60 |
Test 2 | 140 | 60 |
Test 3 | 110 | 60 |
The model tests were conducted on an indoor landslide simulation platform (Figure
Rainfall-induced landslide simulation platform.
The shallow failure occurred mainly at the depth of 0.5 m, where the gravelly soil exhibited high permeability. Therefore, the seepage observed in the natural rill banks was vertical infiltration with no groundwater rising and failure occurred in an unsaturated condition instead of failure with lateral saturated seepage. Permeable boundary conditions are typical for shallow failure with unsaturated infiltration. To simulate the permeable boundary, 13
The measurement system included a data acquisition system (Figure
Pictures of the measuring system for the model test: (a) data logger; (b) pore water pressure sensor; (c) the wireless tiltmeter; (d) soil moisture sensor.
After the Wenchuan earthquake in 2008, years of heavy rainfall caused landslide deposits to become unconsolidated, develop high water permeability, and lose fine particles. To reproduce the preearthquake gradation characteristics of the soil in this region by inferring the grading curves of the analogous landslide deposits from the earthquake-stricken area [
Basic parameters for original and model soils.
Soil types | Soil unit weight (kN·m-3) | Maximum dry density (g·cm-3) | Saturated hydraulic conductivity (cm·s-1) | Poisson ratio (-) | Effective cohesion (kPa) | Effective friction angle (°) | Initial moisture (-) | Porosity (-) |
---|---|---|---|---|---|---|---|---|
Original | 18.9 | 1.90 | 0.26 | 19-25 | 35-40 | 0.113-0.133 | 0.23 | |
Model | 17.0-20.0 | 2.20 | 0.28 | 28.6 | 30-35 | 0.095-0.125 | 0.27 |
Particle grading curves of original and model soils.
The coarse-textured gravelly soil gathered from the landslide site has a loose porous structure, which enables preferential flow to run through certain pathways [
(a) Diagram of the soil porosity measurement device and (b) relationship between soil porosity and density.
Preferential flow in the gravelly soil column with different densities: (a)
In this study, the two most commonly used models, the Brooks-Corey (BC) and van Genuchten (VG) models, were utilized to describe the soil-water characteristic curve (SWCC) of the experimental soil. The SWCC can be fitted by the measured matric suction and the corresponding volumetric water content. A relatively simple equation of the soil-water characteristic curve was proposed by Brooks [
Genuchten [
The stability of rainfall-induced shallow landslides is usually evaluated with the one-dimensional limit equilibrium model named as “infinite slope stability model” [
The suction stress of the VG model is
The unsaturated infinite slope stability analysis model is
Equation (
Experimental observations reveal the shallow failure mode of the slope with a gradient of 60°. The typical failure process of the slope is test series 3 shown in Figure
Slope failure process and phenomenon at time stages. Failure regions and cracks are outlined by yellow solid lines and dashed lines, respectively. (a)
The variations of matric suction and volumetric water content in test series 1-3 under rainfall intensities of 170 mm/h, 140 mm/h, and 110 mm/h are shown in Figure
Matric suction and volumetric water content changes under different rainfall conditions: (a) test 1,
Under heavy rainfall conditions, the preferential flow can rapidly flow through fractures and macropores causing rapid increases of water content and pore water pressure, thus affecting the infiltration process and the stability of the bank slope. To understand the influence of preferential flow on the stability of landslide deposits under different rainfall intensities, it is necessary to first establish the soil-water characteristic curve representing the impact of preferential flow. The soil-water characteristic curves described with the BC model and the VG model can be fitted with the measured matric suction and volumetric water content at different depths during the wetting process. Curve fitting is done in Origin software with its analysis and fitting module, in which user-defined functions can be created (e.g., BC model and VG model) and the fitting curve can be accomplished automatically according to the fitting data and the input expression. The fitting algorithm of Origin is based on a nonlinear least squares method, and fitting results and graphs can be exported when the fitting process is over. As shown in Figure
SWCC at different depths in test series 3: (a)
Fitting parameters for wetting process at different depths in test series 3.
Fitting model | Depth (cm) | |||
Van Genuchten | 25 | 0.6067 | 68.6838 | 0.9947 |
50 | 0.4609 | 175.0497 | 0.9639 | |
75 | 0.5369 | 9.2780 | 0.9261 | |
100 | 0.4749 | 77.3908 | 0.9765 | |
Depth (cm) | ||||
Brooks-Corey | 25 | 0.0081 | 0.0706 | 0.1766 |
50 | 0.1326 | 0.3233 | 0.5256 | |
75 | 0.3656 | 0.5348 | 0.5291 | |
100 | 0.1314 | 0.5786 | 0.7244 |
Stability of bank slopes is related to the gravity and the matric suction of the unsaturated soil. Under unsaturated conditions, the changes in water content caused by rainfall infiltration result in changes in the moisture and matric suction of the soil and thus greatly influence slope stability. Because rainfall-induced landslides occur under the influence of water infiltration, the wetting process is usually more important in describing the physical processes of slope failure. Meanwhile, the camera recorded the behaviour and timing of deformation during experiments and the surface tiltmeter was able to accurately capture the tilt angle changes of the soil at measuring points. By comparing the slope failure processes under different rainfall intensities with the rainfall duration, it was found that the surface inclination variations match with the failure stages observed.
The relationship between the surface inclination and the factor of safety (FS) calculated by the VG model during the infiltration process in all tests is shown in Figure
Relations between inclinations and the factor of safety during the wetting path in different tests and the video shots of general failure: (a) test series 1,
Combining Figures
Soil parameters for slope stability analysis.
Depth |
Effective cohesion |
Effective friction angle |
Natural unit weight |
Slope angle |
---|---|---|---|---|
25 | 1.5 | 30 | 18 | 60 |
50 | 2.5 | 30 | 18 | 60 |
75 | 3.0 | 33 | 18 | 60 |
100 | 5.0 | 33 | 18 | 60 |
The seepage erosion causes changes in the soil pore structure, seepage field, and stress field and is one of the failure mechanisms in rainfall-induced landslides [
Sampling sites for particle migration analysis.
Characteristic particle size of soil samples at different depths in test series 2.
Grading curves for soil samples at different depths.
Figure
Real-time rainfall intensities and cumulative rainfall in (a) test series 1, (b) test series 2, and (c) test series 3.
The variations of rainfall intensities in the experiment were not significant for all the tests except for the first 10 minutes of test 3; consequently, the cumulative rainfall showed approximately linear increases with time (Figure
The phenomenon of grain coarsening due to vertical migration of the fine particles was observed in the tests, which also has been mentioned in other pieces of research [
This paper focuses on the phenomena in which earthquake-induced landslide deposits in the debris flow source region undergo rill erosion and failure under heavy rainfall conditions and are transformed into material sources for future debris flows after major earthquakes. The Yindongzi landslide in the Baisha River Basin, a typical meizoseismal area in the mountainous area of Sichuan province, southwestern China, was used as an example. A model test of shallow failures of rill banks was conducted indoors on a full-scale slope with a steep gradient under heavy rainfall conditions, using natural soils from the earthquake-induced landslide deposits. Rainfall infiltration characteristics were integrated with the hydromechanical coupling mechanism to investigate the evolution, outbreak, and propagation of rill bank failure under heavy rainfall. The conclusions are as follows:
Through experimental observations, the typical failure model of the unsaturated gravelly soil slope with a gradient of 60° can be summarized as local failure expansion causes large-scale overall collapse. With rainwater infiltrating into the slope, part of the slope changes from an unsaturated state to a saturated state at first. Secondly, the soil structure begins to change. The soil moisture and pore water pressure vary continuously and fine particles migrate both in the slope and at the surface. Then, soil strength reduces and failure initiates. The localized failure begins to occur and expands to large-scale, general slope failure later on, forming scarps at the back edge of the failure. Finally, the slope tends to be stable and the failure area no longer enlarges. The seepage characteristics of rainfall-induced slope failure in tests are the result of the interaction between preferential flow and matrix flow. The matrix flow is affected by matric suction, which shows a significant delay compared with the occurrence of preferential flow in soil layers of 75 cm and 100 cm in depth. The VG model provides a better fit to the soil-water characteristic curve, which reflects the dominance of preferential flow in coarse-textured gravelly soils ( Calculation results in FS show that the matric suction can contribute to the stability of shallow rill banks under unsaturated conditions. Sudden changes of the inclination captured by tiltmeters are well correlated with a reduction in the FS. Therefore, the inclination can be utilized as an index for early warning. The experiments verified the vertical migration of fine particles caused by rainfall infiltration. Statistics reveal that a significant increase in either rainfall duration or cumulative rainfall amount enables fine particle to migrate fully.
This study presents a new perspective on the formation mechanisms and the source transformation conditions of debris flows induced by rainfall after earthquakes. Further, it provides a framework for the development of early warnings for rainfall-induced landslides and debris flows during the postseismic period.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflict of interest.
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 41771021 and 41471012) and the 135 Strategic Program of the Institute of Mountain Hazards and Environment (Grant nos. SDS-135-1701 and SDS-135-1704).