Abstract
The seepage evolution characteristics and stability change rule of granite residual soil inside the high cutting slope under the action of rainfall infiltration were studied. Based on the saturated-unsaturated seepage theory, the numerical simulation of the rainwater infiltration path inside the granite residual soil slope was carried out under the condition of rainwater infiltration. The evolution characteristics of the seepage field of the granite residual soil cutting high slope under the condition of rainfall infiltration were revealed, and the influence of the rainwater infiltration on the stability of the granite residual soil slope was mainly studied. The research results show that with the accumulation of rainfall infiltration, the pore water pressure of the granite residual soil slope increases continuously, and the pore water pressure of the soil below the platform increases the fastest. With the increase in elevation, the water content of the slope presents a wave crest distribution. In the early stage of rainfall, the seepage velocity vector distribution on the slope surface is relatively uniform. In the middle stage of rainfall, the seepage velocity vector is the most dense at the platform, and the rainwater migration rate is accelerated. In the later stage of rainfall, the infiltrated rainwater at the slope angle of the first-level slope spreads to the surrounding area. With the continuous rainfall, the slope stability safety factor gradually decreased unevenly, and after the rainfall stopped, it gradually increased with the rainwater infiltration.
1. Introduction
When constructing roads in hilly areas, due to the influence of terrain, in order to ensure the smoothness of the route, some high terrains are often excavated, forming a large number of cutting slopes. In southwestern China, especially in Guangxi, granite is widely distributed. Due to the subtropical monsoon climate, the annual average rainfall is abundant, and the granite is severely weathered, forming the local unique soil granite residual soil [1, 2], which is the residual granite after physical and chemical weathering. The debris in situ has special composition and structural characteristics, and its engineering geological characteristics are different from those of general loess. It is a regional special soil with high void ratio, high strength, low density, and low compressibility. During the excavation process of the granite residual soil slope, the stability of the slope is reduced due to the disturbance of the original stress field. Whenever the rainy season comes, a large amount of rainwater seeps into the slope, resulting in an increase in the bulk density of the soil on the cutting slope, thus accelerating the occurrence of slope instability [3].
At present, many scholars have carried out a lot of research on the stability of cutting slopes. Wang et al. [4] discussed the different catastrophic modes that may exist in the design, construction, and application of cutting slopes and proposed a full life cycle analysis model of cutting slopes based on Hall’s three-dimensional structure. It was of great significance to the stability assessment of cutting slopes. Qi et al. [5] analyzed the slope stability under earthquake and rainfall, and Yao and Wu [6] analyzed the stability of double-layer soil slope from rainfall-induced groundwater level rise. By analyzing a large number of high cutting slope landslide instability cases, Qiao et al. [7, 8] discussed the main causes and main instability models of high cutting slopes and proposed four corresponding reinforcement schemes for the reinforcement of road cutting slopes. The design provides a theoretical reference. Li [9] focused on the analysis of the stability change process of the cutting slope under the condition of spring melting and deduced the stability calculation method of this kind of slope. Aladejare et al. [10, 11] conducted a stability check calculation for a typical slope model and analyzed the sensitivity of the slope shear strength to the slope stability. Kristo et al. [12] investigated the effect of hysteresis in SWCC on the stability of unsaturated residual soil slopes in granite. Satyanaga et al. [13] studied the variation law of the safety factor of dual-porosity soil slopes under the bimodal water retention curve and bimodal unsaturated permeability.
To sum up, many scholars have some limitations in the research on the stability of granite residual soil high cutting slopes, and few scholars have deeply analyzed the influence of rainwater infiltration on the stability of granite residual soil high cutting slopes from the perspective of seepage. However, the phenomenon of unstable landslides on high cutting slopes with granite residual soil caused by rainwater infiltration has occurred frequently.
From the perspective of seepage, this paper uses numerical software to establish a seepage model of granite residual soil slope, and through the setting of seepage boundary conditions, the seepage path of rainwater in the granite residual soil cutting high slope under the condition of rainfall infiltration is described. The seepage characteristics of the granite residual soil high cutting slope under the action of rainfall infiltration were analyzed, and the stability evolution law of the granite residual soil cutting high slope under the action of seepage was revealed.
2. Theoretical Basis for Stability Analysis of High Slope with Granite Residual Soil under Seepage
2.1. Rainwater Seepage Theory
Under the action of seepage, the seepage evolution law of rainwater in high soil cutting slope can be described by two-dimensional cross section. When rainwater infiltrates into the interior of the slope, the volumetric moisture content of the soil inside the slope gradually increases, and the original saturated-unsaturated balance of the slope is broken, resulting in dynamic redistribution of the saturated-unsaturated zone [14, 15], and the distribution flow velocity satisfies Darcy’s law:where is the Darcy flow rate when rainwater seeps; is the saturated permeability coefficient; and is the hydraulic gradient in the y direction.
Under the condition of rainfall infiltration, the permeability coefficient of the granite residual soil in the saturated area of the high cutting slope is a fixed value. In the unsaturated area, the permeability coefficient increases gradually with the increase in the matrix suction [16, 17]. The seepage control differential equation is as follows:where is the pressure head; is the relative permeability coefficient; is the seepage tensor; is the unit water storage coefficient; is the water volume; and is the source-sink term.
In the process of rainwater seepage, set any tiny three-dimensional space unit in the high-slope space of granite residual soil as dxdydz. According to the law of mass conservation and seepage control differential equation, select any time period dt for integration, and the continuous equation of water flow in the high-cutting slope of granite residual soil can be obtained [18, 19]:where is the porosity and is the density of water.
Simplify the above equation to get
2.2. Slope Stability Analysis
The famous French scholar Cullen conducted a corresponding experimental study on the shear strength of the soil. According to the test data, it is considered that the saturated shear strength of the soil satisfies the following formula [20], specifically,
Under the condition of rainfall infiltration, the rainwater migrates in the soil road slope, resulting in the existence of saturated and unsaturated areas in the soil above the groundwater level of the slope. The shear strength of soil in the unsaturated zone is calculated using the unsaturated shear strength formula proposed by Fredlund and Rahardjo [21].where is the air intake value; when the soil pore opening is connected to the atmosphere, is 0; is the effect of soil suction on friction strength, which is generally 14°; and is the pore water pressure.
Under the action of rainfall infiltration, on the one hand, the infiltration of rainwater increases the effective weight of the soil and increases the sliding force of the slope; on the other hand, it reduces the shear strength of the granite residual soil and reduces the slope anti-skid [22, 23]. In this paper, the strength reduction method is used to continuously reduce the shear strength parameters of the granite residual soil slope according to and . The stability safety factor of the slope is obtained by iterative calculation.
3. Rainfall Infiltration Scheme
3.1. Design Rainfall Program
Atmospheric rainfall is the main cause of landslide instability in high-cutting slopes with granite residual soil [24, 25]. In order to study the stability change law of granite residual soil road cutting high slope under the action of seepage, the rainfall intensity was selected as 7.0 × 10−4 mm/s, the rainfall lasted for 48 hours, and the accumulated rainfall reached 120 mm, as shown in Table 1.
3.2. High Slope Calculation Parameters
According to the field investigation data, the granite residual soil road cutting high slope studied in this paper is a granite residual soil layer as a whole, and its soil-water characteristic curve was fitted by the typical Van Genuchten model [26], and the fitting result is shown in Figure 1.
Through indoor and outdoor related experiments, the saturated volumetric water content of granite residual soil under the natural compaction degree is 0.18, the residual volumetric water content is 0.11, the permeability coefficient is 1.83 × 10−7 cm/s, the weight is 22 kN/m3, and the cohesion and the angle of internal friction are 25 kPa and 22°, respectively. In addition, referring to the relevant literature [27], the selected physical and mechanical parameters are shown in Table 2.
3.3. Numerical Computation Model
Based on the high cutting slope of granite residual soil in a certain section of a county, the slope was excavated in two stages as a whole. The first stage slope was 12 meters high and the slope ratio was 1 : 1.75, and the second stage slope was eight meters high and the slope ratio was 1 : 1.5. Relying on the project overview, set up three monitoring points on the surface of the first and second grade slopes and two meters below the center of the platform. Three monitoring sections a, b, and c are set at the first and second grade slope angles and slope tops, and the corresponding numerical calculation model is established in numerical software (relying on the slope module of GeoStudio numerical software for calculation) as shown in Figure 2. In order to make the numerical calculation more realistic to simulate the general situation of the project, the established numerical calculation model was divided into grids, groundwater level setting, and boundary condition constraints. The slope was divided into 2536 units as a whole; the elevation of groundwater level on the left side of the model was set to five meters, and that on the right side was set to two meters; the surface and top of granite residual soil slope were permeable boundary, and rainfall conditions were applied. The bottom of the model was impermeable boundary, with displacement constraints in X and Y directions, and displacement constraints in X direction are applied on both sides of the model.
4. Numerical Simulation Calculation Results
4.1. Variation Law of Slope Pore Water Pressure
The variation law of pore water pressure monitoring points of granite residual soil cutting high slope under rainfall infiltration is shown in Figure 3. It can be seen from Figure 3 that the pore water pressure of monitoring point 1 increased from −71 kPa to about 5 kPa, the pore water pressure of monitoring point 2 increased from −55 kPa to about 10 kPa, and the pore water pressure of monitoring point 3 increased from −37 kPa to about 7 kPa. During the entire study duration, the pore water pressure of monitoring point 2 increases abruptly at the first and the fastest rate. This is because under the condition of rainfall infiltration, rainwater gradually infiltrates on the slope surface. When the rainwater seepage moves to the monitoring point, the pore water pressure of the soil at the monitoring point begins to increase. With the continuous accumulation of rainfall infiltration, the pore water pressure continues to increase. Monitoring point 2 is located below the platform, and the infiltration amount of rainwater at the platform is larger than that at the midpoint of the slopes at all levels. Therefore, the pore water pressure of monitoring point 2 has the largest change and the fastest increase.
4.2. Slope Volumetric Water Content Distribution Law
The distribution law of volumetric moisture content of three monitoring sections a, b, and c of granite residual soil cutting high slope under rainfall infiltration is shown in Figure 4. Within the elevation range of 25∼35 m, the volumetric moisture content of the monitoring section a gradually increases with the elevation. Within the elevation of 8∼25 m, the soil volumetric moisture content of the monitoring section a is low and stable at about 0.12. Within the elevation of 0∼8 m, the volumetric water content gradually decreases with the increase in elevation. The whole shows a wave crest distribution state.
The distribution law of volumetric water content of monitoring sections b and c is the same as that of section a. Because monitoring section c is located at the foot of the first-level slope, affected by infiltration, water, and groundwater levels, the moisture content of soil within 15 meters of elevation is high and remains above 0.8.
The reasons for the above phenomena are that in the low elevation, due to the existence of the groundwater level, the volumetric water content of the soil below the groundwater level is in a saturated state. Within a certain elevation above the groundwater level, the volumetric moisture content decreases gradually with the increase in the elevation. On the other hand, in a certain range below the slope surface, under the influence of seepage, rainwater gradually migrates to the interior of the slope, resulting in a gradual increase in soil volumetric moisture content within a certain range of the slope. Therefore, under the action of seepage, with the increase in elevation, the volume water content of slope presents a wave crest distribution.
4.3. Variation Law of Slope Seepage Velocity Vector
The variation law of seepage velocity vector of granite residual soil cutting high slope under the condition of rainfall infiltration is shown in Figure 5. When the rainfall lasted for 3 hours, the seepage velocity vector on the slope surface is relatively uniform, denser at the platform, and the length of the seepage velocity vector is short. When the rainfall lasted for 12 hours, the seepage velocity vector on the slope surface gradually extended to the interior of the slope, with a longer length, and the arrow direction as a whole went down along the slope surface. When the rainfall lasted for 24 hours, the seepage velocity vector gradually diffused to the periphery at the slope angle of the first-level slope. When the rainfall reaches 48 h, the seepage velocity vector at the slope angle of the first-level slope has extended to the groundwater level line, causing the groundwater level line to increase.
(a)
(b)
(c)
(d)
The reason is that in the initial stage of rainfall, the rainwater flows uniformly from the surface of the slope, resulting in the uniform distribution characteristics of the seepage velocity vector on the surface of the slope. In the middle stage of rainfall, the infiltration at the platform of the slope is the largest, and the surface of the seepage velocity vector is dense. With the continuous accumulation of rainfall infiltration, the migration rate of rainwater accelerates, resulting in the increase in the length of the arrow of the seepage velocity vector. In the later stage of rainfall, the rainwater at the corner of the first grade slope diffuses around and gradually spreads downward to replenish the groundwater level, resulting in the increase in the groundwater level.
4.4. Change Law of Slope Safety Factor
The stability change law of granite residual soil cutting high slope under rainfall infiltration is shown in Figure 6. With the increase in rainfall duration, the safety factor of the slope shows a nonuniform decrease. After the rainfall stops, the safety factor of the slope gradually increases slowly. Within 48 hours of rainfall, the safety factor of the slope decreased from 1.46 to 1.09, a decrease of 25%. At this time, the slope was already in an unstable state. This phenomenon occurs because under the condition of rainfall infiltration, rainwater infiltrates from the surface of the slope. With the accumulation of rainfall infiltration, the pore water pressure of the slope gradually increases, and the volumetric water content gradually increases. The seepage velocity vector gradually expands to the interior of the slope, resulting in a gradual decrease in the shear strength of the slope soil, a gradual increase in the sliding force, and a gradual decrease in the antisliding force. Therefore, the slope safety factor gradually shows a decrease in unevenness with the increase in rainfall duration.
5. Conclusions
(1)With the continuous accumulation of rainfall infiltration, the pore water pressure of the slope continues to increase, and the pore water pressure of the soil below the platform position increases the fastest; under the action of seepage, as the elevation increases, the volumetric water content of the slope presents a wave crest distribution.(2)In the early stage of rainfall, rainwater infiltrates uniformly on the slope surface, resulting in a relatively uniform distribution of seepage velocity vectors on the slope surface. In the middle of the rainfall, the seepage velocity vector is the most dense at the platform. With the continuous accumulation of rainfall infiltration, the speed of rainfall migration is accelerated. In the later period of rainfall, the infiltrated rainwater at the slope angle of the first-level slope spreads to the periphery and gradually spreads downward.(3)Under the condition of rainfall infiltration, the safety factor of slope stability first gradually decreased unevenly under the action of rainfall seepage. After the rain stopped, the safety factor gradually increased with the infiltration of rain.(4)Rainfall has a great influence on the stability of the granite residual soil slope. After the slope is formed, an effective drainage system should be set up, and stability monitoring should be carried out on the middle key parts of the slope.Data Availability
The experimental data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
We acknowledge the support received from the Railway Subgrade Safety Emergency Technology Innovation and Demonstration Team of Zhengzhou Railway Vocational and Technical College (Grant no. 21KJCXTD02) and the key scientific research project plan of Henan Province Colleges and Universities Program on intelligent identification method of railway subgrade muddying and muddying based on ground penetrating radar (Grant no. 22A580007).