A large number of deposit landslides are induced by rainfall, and those with different weak layers may be subject to catastrophic failure. This research investigates the rainfall infiltration effect on the stability of deposit landslides with a weak layer at different slope angles. Four rainfall physical model tests were conducted with fixed double penetration artificial rainfall technique and dynamic sensor technologies by using the rainfall test methods as modified in the paper. Deformation and mechanics parameters, as well as water content parameters in the key position in the deposit landslide, were monitored by means of various displacement monitoring sensors, dynamic soil pressure sensors, pore water pressure (PWP) monitoring sensors, and water content sensors. The results show that, under the same rainfall conditions, the rule of displacement and mechanical changes of deposit slope with different angles are similar, that the displacement, soil pressure, and PWP are characterized by two stages of rising and falling, and that the displacement of deposit slope with weak layer remains creep after rainfall. In addition, the displacement at the rear edge of the slope with a small angle is larger than that at the front of the steep slope, but the displacement in the front of the slope is opposite. Furthermore, the slope with a smaller angle is prone to form a tensile crack in the back of the slope, and its deformation and failure have the characteristics of a progressive and thrust-type landslide. While the failure in front of a steep slope (slope angle more than 60°) occurred first, the slope failure was characterized by sudden and retrogressive modes. The mathematical analysis of the model is also conducted which shows that deformation and failure can be divided into three stages, i.e., creep inoculation, accumulation uplift, and speed-up sliding. The test results can provide a reference for the investigation, design, and assessment of similar deposit slopes.
Rainfall infiltration has a great influence on the stability of deposit slopes, especially the deposit slope with weak layers, which is prone to landslide [
A physical model was used to conduct rainfall simulation tests with different slopes for accumulation landslide and a warning model of rainfall-type landslide based on rainfall duration and rainfall was proposed [
These experimental or numerical studies at the lab or field have mostly focused on the relationship between landslide failure and rainfall of deposit landslide. The effect on deposit landslide with a weak layer under different slope angles induced by rainfall has seldom been considered. On the one hand, due to the complexity of the geological structure of accumulation slope with a weak layer, the displacement or mechanical parameters may be random by indoor simulation or numerical method. On the other hand, rainfall infiltration has both horizontal and vertical infiltration on the deposit slope with different slope angles, while the traditional landslide hydrogeological cycle takes the change of the same seepage field into account. Nevertheless, there are few studies on the two-way seepage problems such as soil-water contact fracture zone, dissolution zone, and vertical crack under complex engineering-geological conditions.
In this paper, the Nanheng Bridge deposit landslide in Longnan county, Jiangxi province, China, which was induced by rainfall on 20 March 2016 is investigated. This landslide is mainly characterized by a three-layer structure: the sliding bed composed of carboniferous limestone and sandstone, the slide zones composed of silty clay, and the sliding body mainly composed of gravel deposit. Another typical feature is that there are three transverse landslide cracks in the back of the slope. In order to measure the mechanical and deformation parameters of the landslide with special geological structure, three strata of the artificial rainfall simulation test model and four angle slope models are developed, and several monitors are set in the different positions of the slide zone and main body. The characteristics and deformation failure mechanism of the deposit landslide with the weak layer on different angles under artificial rainfall are analyzed. Numerical analysis of rainfall infiltration is also conducted in this paper using the improved Green-Ampt model, and the effect of nonuniform changes of the wetting front along vertical cracks and inclined weak layers has been considered.
The prototype landslide is located 1.0 km downstream of Nanheng town. The main body of the landslide is composed of clastic rock, fill soil, silty clay, and gravel soil with an average particle size of 0.5 m (accounting for 50–60%), and the underlying bedrock is limestone and sandstone with a dip angle of 28–35°. The main structural traces in this deposit are SW-NE about 560 m wide and 616 m long, with a relative height difference of 224 m, and the total volume was estimated to be 1.2 × 106 m3. Rainfall in this area mainly occurs in March and June accounting for 53.5% of the annual precipitation. Some of the rainfall is turned into the surface water downflow to the Wo river at the foot of the landslide, and others infiltrate into the ground through surface cracks and overburden interstices, forming a weak and muddy sliding surface that is not conducive to slope stability. The geological section of the deposits landslide is shown in Figure
Geological section of deposit landslide in Nanheng.
Geophysical prospecting, drilling, trenching, and well exploration have been taken into survey and analysis of this landslide, and it was found that the landslide occurred mainly above the bed weak layer with a dip angle about 30°. The thickness of the main body is 2 m to 45 m, and the leading edge is mainly composed of filling the soil with a maximum thickness of 14 m. The sliding zone is developed in the weak layer at the junction of highly weathered limestone and accumulation body. The weak layer is about 0.8–3.0 m thick with a buried depth of 10–32 m with a water content of 35–41%. Rainfall is the causal factor of this landslide, and mudding of the slide zone soil is the main failure reason of the Nanheng landslide, which is a kind of planar landslide in the deep layer (see Figure
The model setting was conducted to understand the geological features in the area and the mechanism of the landslide deposits with a weak layer. The field data and experimental data including different dip angles are used, and the main slide section which has formed three tension cracks [
The theoretical basis of the simulation experiment is the similarity principle, that is, the model is required to be similar to the entity (prototype), and the model can reflect the situation of the entity. The similarity theory is the main basis of the simulation experiment. According to the mechanical mechanisms, the similarity theory mainly includes mechanical similarity, material similarity, initial condition similarity, and boundary condition similarity [
The model is similar to the research object, which needs to satisfy certain relations in terms of geometric conditions, force conditions, and friction coefficient. In summary, the similarity principle can be expressed as follows: if two systems are similar (model and prototype), their geometric characteristics and each physical quantity must be proportional to each other. It can be defined as follows.
Similarity coefficient of geometric conditions:
Similarity coefficient of force conditions:
It is placed in a steel frame reinforced glass boxes with 2,300 mm in length, 1,200 mm in width, and 1,000 mm in height. This glass box is fully enclosed except the top and a 60 mm diameter drainage pipe with a switch was installed at the front of the landslide. Meanwhile, in order to study the deformation and failure of the landslide with different dip angles, two model boxes were made with each separated by a 70 mm thick plank, and four models were tested simultaneously. The schematic cross section and the test model of the artificial simulation of the test are shown in Figure
Design of artificial rainfall simulator for the test model. (a) Schematic cross section. (b) Photograph of the test model.
In order to observe the data, the model box is made transparent around, and a 10 mm by 10 mm grid is affixed in the direction of the profile. Test slope base of 371 mm in height, 880 mm in length, and 513 mm in width was built by 49 black bricks which are 230 mm long, 110 mm wide, and 53 mm thick. To simulate the infiltration of rainfall into the weak interlayer of the main body along the transverse cracks of the landslide, 10 filter pipes (Johnson pipes) with a diameter of 12 mm were inserted in the rear edge of the landslide, and a layer of plastic rain cloth with a thickness of 0.5 mm was laid on the blue brick to form the waterproof layer. A weak interlayer mainly composed of fine sand about 25 mm thick with a dip angle 26° was built on the rain cloth, and the particle size of the sand is 20–40 mesh. 280 mm thick granite weathering soil was placed on top of a weak layer. The granite weathered soil was screened with a mesh of less than 50 mm × 50 mm to remove impurities such as plant roots.
The artificial rainfall system is composed of a control console, a large function pump, a spraying system, a data monitoring, and an automatic collection system. The rainfall system is composed of four subrainfall zones (three downspout zones and one side spout zone), and each subrainfall subsystem could be operated independently. The total area of effective rainfall is 784 m2, and the total area of rainfall has 3 downcast areas and 1 side spray area. The effective rainfall height is 18 m, and the rain chute system is set in the three downcast areas, which can well solve the ineffective rain before and after the rain in downcast areas. The continuous variation range of rain strength is 10–200 mm/h in the downflow area. The side spray area is 30–300 mm/h, and various rainfall conditions of 10–500 mm/h can be simulated by the under spray and side spray tissues. Rainfall uniformity is greater than 0.80, and the storage capacity of the collector is more than 32,000 pieces. The rainfall sampling interval is 10–9,999 seconds.
According to the principles of the geometric similarity ratio, the similarity ratio of this experimental model is 1 : 100. Given that the mechanical mechanism of prototype landslide is a thrust-type landslide, we select 1 : 1 for the gravity similarity ratio. Other physical indexes between the prototype and the model can be derived according to the similarity theory and they are listed in Table
Physical parameter similarity ratio.
Parameter | Definition | Similarity ratio |
---|---|---|
Length | 100 | |
Volume weight | 1 | |
Rainfall intensity | 10 | |
Rainfall duration | 10 | |
Cohesion | 100 | |
Angle of friction | 1 | |
Displacement | 100 | |
Shear stress | 100 | |
Coefficient of permeability | 10 | |
Modulus of compression | 100 |
Field investigation has shown that the water content of the shallow surface rock and soil of this landslide deposit was extremely low. Rainfall infiltration was along the tension cracks and soil pore into the weak layer. Deposit slope will experience skipping and damage along the shear plane if the sliding force of the potential shear plane (weak layer) exceeds the antiskipping force landslide [
Main parameter similarity ratio of rainfall model.
Material | Type | Unit weight | Soil moisture | Cohesion | Friction angle Φ (°) | Infiltration coefficient | Modulus of compressibility |
---|---|---|---|---|---|---|---|
Residual soil | Prototype | 1.78 | 26.4 | 24.6 | 21.4 | 2.1 × 10−5 | 5.40 |
Model | 1.80 | 26.5 | 0.25 | 21.3 | 2.0 × 10−6 | 0.05 | |
Weak layer | Prototype | 1.96 | 41.2 | 33.1 | 19.3 | 2.9 × 10−6 | 5.85 |
Model | 1.95 | 40.5 | 0.34 | 19.5 | 3.0 × 10−7 | 0.06 |
In this work, we modified the rainfall test method proposed by Li et al. [
Artificial rainfall synthesis system.
A new automatic monitoring system has been designed for the model test, mainly including pore water pressure (PWP), soil pressure, displacement monitoring, and automatic camera monitoring system. Three CYY2 PWP dynamic sensors, three CYY9 dynamic soil pressure sensors, three CYY-TR-WY integrated bidirectional soil displacement sensors, and one RS485 digital soil-water sensor were deployed at different locations of each model. Four high-definition cameras were placed around the four models (see Figure
Monitoring equipment connected to the model test.
Test purposes | Type | Spec. | Number | Output |
---|---|---|---|---|
PWP | CYY2 | Precision: ±0.5%; range: 0 ∼ 2 kPa; | 2 per model | 0 ∼ 5VDC |
Soil pressure | CYY9 | Precision: ±0.25%; range: 0 ∼ 50 kPa; | 2 per model | 0 ∼ 5VDC |
Soil moisture | RS485 | Precision: ±3%; range: 0 ∼ 100%; | 1 per model | 0 ∼ 5VDC |
Displacement | CYY-TR-WY | Precision: ±0.5%; range 0 ∼ 500 mm; | 2 per model | 0 ∼ 5VDC |
Data acquisition and voltage stabilizer | CYY-58 | 100 channels | 1 | 0 ∼ 24VDC |
Webcam | DS-2CD2T25XY-SW (Hikvision) | Image size: 1920 × 1080 | 1 per model | 220VDC |
All sensors are digitized through the integration of high-frequency amplifiers; then, the voltage signal is converted into stress or strain signal by a specially designed conversion software. Hikvision DS-2CD2T25XY-SW with 3 million pixels are specifically designed to monitoring the test day and night. The advantages of the monitoring system include a large range, small size effect, durable water repellency, and all the deformation and mechanic parameters during the rainfall process are generated from the execution recording of the real-time software (Figure
Monitor of the test model. (a) Sensor. (b) CYY-58 data acquisition unit and voltage stabilizer.
Four models of rainfall experiment were carried out at the same time, and the slope angles of these models were set at 30°, 45°, 60°, and 75°. The main experimental procedures include the following: (1) installation and fixation of the base of the model box and then model making; (2) sieving granite weathering soil, fine sand, and so on according to the similarity ratio, preparing similar materials of landslide deposit and soft layer, and strictly controlling related parameters such as water content and density; (3) layering related materials according to the sequence of blue brick, rain cloth, weak layer, and landslide deposit, meanwhile installing various monitoring instruments in different depths according to the design; (4) laying the landslide deposit in four layers, with each layer of 7 cm in thickness, and the specifications marked on the side plate shall be used to control the stratification compaction; (5) after the commissioning of the various monitoring instruments, the rain test starts. When the rainfall is uniform, the formal test is conducted according to the predetermined rainfall conditions. If the slope is deformed and damaged, it must stop; and (6) monitoring the changes of water content, pore water pressure, soil pressure, and displacement in the process of rainfall, and continue monitoring until the full 24 hours after rainfall stops.
The largest displacement of the back edge was observed in the 30° slopes angle model followed by the 45° slope angle model, the 60° slope angle model, and the 75° slope angle mode. Conversely, the largest displacement occurred in the front of the slope model with 75° slope angle, followed by the 60° slope angle model, the 45° slope angle mode, and the 30° slope angle model (Figure
Photo of displacement after rainfall. Slope with (a) 30° angle, (b) 45° angle, (c) 60° angle, and (d) 75° angle.
In Figure
Characteristics of the displacement in the back part of the models (D1 and D2 sensor location).
Displacements in the weak layer of the test model (D2 sensor location) are compared with the measured data by the D2 sensor in Figure
The displacement of surface and front of these models with different slope angles is shown in Figure
Displacement of shallow surface in front of the models (D3 sensor location).
In order to illustrate the effect of different slopes on the mechanical mechanism of the deposit slope with weak layer, two CYY9 dynamic soil pressure sensors were set in 200 mm depth in the back part and middle part of the test model, and the other CYY9 dynamic soil pressure sensor was set in the 150 mm depth in the front of the test model. Figure
Characteristics of soil pressure on the test models. (a) Soil pressure on the back part of test models by sensor S1. (b) Soil pressure in the middle of the slope by sensor S2. (c) Soil pressure in front of the slope by sensor S3.
In the middle part of the test model (Figure
The soil pressure changes in three stages of the slope as shown in Figure
Experimental observations have revealed that the PWP-time curve of the test model with different slope angles is characterized by an increase with time and then a decrease after rainfall (see Figure
Characteristics of PWP on the test models. (a) PWP in the back of the test model by sensor P1. (b) PWP in the weak layer by sensor P2. (c) PWP in front of the test model by sensor P3.
In Figure
As shown in Figure
In order to study the response characteristics of the slope failure with water content after the rain, four water content sensors were set at the 240 mm depth of the test model with different angles. As shown in Figure
Characteristics of water contents in the test model with different angles (W1 sensor location).
According to the physical model test of soil accumulation landslide, the Geostudio calculation model consistent with the landslide model is established (see Figure
Numerical model of Geostudio.
Taking the slope model of 30-degree slope angle accumulation model as an example, the calculation process is as follows: The model was defined by Geostudio software (Geo-slope of the Geostudio 2018 R2) and divided into 880 nodes and 821 cells according to the model size. The global cell size was 0.0275 m. According to the elastic solution method, the elastic model is used to calculate the initial in situ stress, and the convergence condition is that the ratio of the unbalanced force to the typical stress is less than 1 × 10−5 Under natural conditions, the side and the bottom of the model are set as a fixed boundary, and the At the slope surface for the rainfall infiltration boundary, rainfall condition is the same as the physical test; when the rainfall intensity is less than the saturated soil infiltration parameters for flow infiltration boundary, the soil is greater than the saturation coefficient for head boundary; the Van-Genuchen fitting equation was used in the Geostudio with a change in the parameters of layers to study the change of stress and strain
The slope with an angle of 30° is still taken as an example, and the stress and strain of the slope simulated by the limit equilibrium Morgenstern-Price method are shown in Figures
Figure
Initial values of total stress and total strain of landslide. (a) Initial value of total stress (
Figure
The total strain and stress after 6 hours of the accumulation landslide. (a) Total stress of landslide (
Figure
PWP and maximum principal stress of landslide 10 hours after rainfall. (a) PWP of the model (
By comparing the relationship between the time step and the changes of stress and strain, the landslide model is obviously correlated with the soft interlayer and slope angle in the process of rainfall, which verifies the characteristics of tensile crack at the back edge of the bedding gentle slope, extrusion at the front edge, and bedded deformation, as well as the characteristics of collapse at the front edge of the steep slope, descending at the back edge, and traction deformation and failure.
According to the model test and numerical analysis, it is concluded that two stages of displacement and three failure stages exist in deposit landslide with a weak layer with each having a special characteristic. Under the condition of the same rainfall, the displacement and soil pressure change in the trailing edge at 30° or 45° slope is greater than displacement and soil pressure in the front of the slope, and it has the characteristics of creep and retrogressive.
However, the displacement in the front of the test model with a 60° or 75° slope angle is greater than the displacement in the back of the slope, and it is prone to occur suddenly leading to traction landslide. In general, the rainfall infiltration causes weak intercalations, and PWP rises in a short time during the rain but still maintains a certain degree of growth after rainfall stops. It has some delay in time and a negative correlation with slope angles. In the weak layer, the PWP rises and dissipates for a longer time lag. Rainfall leads to the accumulation of PWP in the weak layer after the rainfall stops so that the sliding body maintains a long period of inertial creep. The gentler the slope is, the shorter the soil saturation time is and the faster the slope failure occurs. The deformation characteristics are generally characterized by the trailing edge crack and the front of the slope bulge, and the slope deformation and failure are generally divided into three stages: creep inoculation, accumulation uplift, and speed-up sliding.
Moreover, this test model can study complicated slope failure and deformation and can determine the factor of safety for the slope with a weak layer. From these tests, the weak layer should be paying more attention, the drainage system should be set at the back of slope to prevent surface water from infiltrating along the crack, and water guiding measures should be taken in the front of the slope to prevent the concentration of groundwater in the weak layer which can cause a sudden instability [
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was partially supported by the National Science Foundation of China (NSFC) (Nos. 41641023 and 51869012), the Natural Science Foundation of Jiangxi Province (No. 20171BAB213027), and Jiangxi Provincial Key Scientific Research Plan (No. 20177BBG70046). The authors thank Prof. Huang Runqiu and Prof. Pei XiangJun at the Chengdu University of Technology, whose reviews helped improve this manuscript. They would also like to thank the Soil and Water Conservation Ecological Science Park of Jiangxi Provincial for providing them with a good rainfall system and service support.