4.1 Slope deformation
Figure 10 shows the time history curve of each displacement monitoring point under different rainfall conditions to analyze the slope deformation during rainfall.
(1) Under different rainfall intensities, the displacement change trend of each monitoring point was basically consistent. With the progress of rainfall, the rainwater infiltrated along the pores or microcracks, the physical strength of the landslide decreased, and the displacement began to increase. The slope underwent asymptotic deformation and failure. At the end of rainfall, the displacement reached the maximum and gradually stabilized. With higher rainfall intensities, the surface displacement was greater, and the time to reach the maximum was shorter. Under the 50-year rainfall (N1), the slope was unstable for about 1.0 h, and the displacement reached the maximum for about 1.75 h. Under heavy rain (N2), the slope was unstable for about 1.5 h, and the displacement reached the maximum in 2.1 h. However, under rainstorm (N3), the slope displacement continued to increase at the end of rainfall.
(2) Under different rainfall intensities, the surface deformation had a certain lagging effect due to the different rainfall infiltration rates. With greater rainfall intensities, the deformation lagging time was shorter. The lagging effect of working conditions 1 to 3 was 0.4 h, 0.6 h, and 1.2 h, respectively.
(3) The spatial distribution pattern of deformation was analyzed. Under the same rainfall conditions, small deformation always appeared first at the foot of the slope before the rear and middle edges. When the slope was completely destroyed, the deformation rate of the middle edge decreased sharply, while the turning points of the deformation rates of the front and rear edges lagged behind. The middle edge (T2) had the largest deformation, followed by the trailing edge (T1), and the toe (T3) had the smallest deformation.
4.2 Soil pressure
Figure 11 shows the time history curves at each soil pressure monitoring point under different rainfall conditions to analyze the response process of soil pressure to rainfall infiltration.
(1) The trends of soil pressure variation curves at each measuring point under different rainfall intensities were basically consistent and had good correspondence with the development of slope deformation and the change of pore water pressure. During rainfall, the soil mass was gradually saturated, the weight of the soil above the soil pressure sensors increased, and the soil pressure increased accordingly. In the meantime, the slope surface expanded and shrunk, forming tension cracks acting as the dominant flow channels of rainwater infiltration, and further strengthening the seepage effect. After the rainfall, the water in the slope was discharged along the seepage channels, the soil weight decreased, and the soil pressure decreased slowly.
(2) According to slope deformation pattern analysis, gradual failures occurred, and the stress redistribution led to sudden stress changes. With the appearance of local and overall failures, vertical tensile failures occurred in the soil mass, and the earth pressure dropped suddenly. The sudden earth pressure drops appeared at the foot of the slope first, and the vertical earth pressure at the middle and rear parts began to decrease immediately afterward.
(3) Under greater rainfall intensities, the soil pressure changed faster due to water infiltration, and the trends were more apparent.
4.3 Pore water pressure
Rainfall infiltration caused local saturation in the slope, changing the pore water pressure. Figure 12 shows the time history curves at each pore water pressure monitoring point under different rainfall conditions to analyze the response process of pore water pressure to rainfall infiltration.
(1) The changes in pore water pressure trends at each measuring point under different rainfall intensities are basically consistent, with slow increases first, rapid increases next, and rapid decreases at last. At the beginning of the test, the pore pressure at some measuring points was negative because of the curved liquid air interface between soil particles in the unsaturated soil and the matrix suction. The soil moisture content gradually increased with the progress of rainfall. In the meantime, the seepage effect in the soil increased, and stable seepage channels gradually formed while the pore pressure slowly increased. Before the landslide, the soil mass underwent significant compression deformations, and excess pore water pressure was generated due to the untimely drainage, causing the surge of pore pressure. When the whole slope body was damaged, the pore water dissipated rapidly while the pore pressure decreased rapidly. At the end of rainfall, the seepage gradually stopped, the static pore water pressure gradually dissipated, and the pore pressure decreased.
(2) In terms of spatial pattern, the pore water pressure changes in different parts of the landslide at the same depth were different. The response time of the front edge (P8, P9) and middle part (P4, P5, P6, P7) of the landslide were short as the rainwater accumulated in the front and middle of the landslide, forming the groundwater level, and the pore pressure accumulated first.
(3) At the same monitoring profile, the variation of pore water pressure at different depths was also different. The variation history of water pressure in the shallow soil mass of the slope was closely related to the water seepage in the vertical cracks. The pore pressure variation range of the slope surface (P2, P4, P6, P8) was greater than that of the internal (P1, P3, P5, P7, P9). After the rainfall, the surface pore pressure decreased rapidly while the internal pore pressure continued to rise, with a hysteresis effect.
(4) In addition, higher rainfall intensities led to higher infiltration rates, greater seepage effects, and shorter soil saturation time, shortening the pore pressure response time and increasing the change rate.
4.4 Water content
Figure 13 shows the time history curves at each water content monitoring point under different rainfall conditions to analyze the response process of water content to rainfall infiltration.
(1) The trends of soil pressure curves at each measuring point under different rainfall intensities were basically consistent, and the initial water content was 5.8–18.3%, with slow growth first, rapid growth next, and dynamic stability at last. Rainfall first infiltrated through the slope surface pores. As the wetting front reached the monitoring point, the volume moisture content increased. With the progress of rainfall, the tension cracks caused by slope deformation provided the dominant seepage channels for rainfall infiltration, rapidly increasing the internal water content of the landslide.
(2) The infiltration speed at different locations of the slope varied, and the monitoring points at the rear edge and surface layer of the slope responded more quickly than those at the front edge and deep part of the slope. Among them, the water content of the rear edge points (H1, H2, H3) reached saturation sooner. The reason was the vertical infiltration of rainfall, and the rear edge of the landslide was steep and thin. The rear edge soon reached saturation under rainfall conditions and then infiltrated forward.
(3) At the same monitoring section, the deep water content of the slope was significantly higher than that near the surface because the rainwater began to gather and finally saturate after reaching the slip zone (H1, H3, H5, H7, H9) through fissure infiltration. In the meantime, the slope surface (H2, H4, H6, H8) remained stable after the balance between downward infiltration and rainfall recharge was reached.
(4) With greater rainfall intensities, the water content growth rate was higher, and the time to reach the maximum water content was shorter.
(5) In addition, under the rainfall conditions of N1 and N2, the slope toe (H8, H9) decreased because the drainage path of the front edge of the landslide gradually increased, and the water storage capacity decreased under the effect of rain erosion.
4.5 Matric suction
Figure 14 shows the time history curves of each water content monitoring point under different rainfall conditions. The response pattern of matrix suction to rainfall infiltration was similar to that of water content.
(1) The matric suction changed from stable to decreasing and then stabilizing with the rainfall. The response time of matrix suction and volume water content at the measuring point was relatively consistent, showing a negative correlation, i.e., the matrix suction decreased with the increase of volume water content. As the volume water content at the measuring point reached saturation, the matrix suction dissipated to 0 kPa.
(2) The matric suction at the top of the slope showed the fastest changes, followed by the middle of the slope, and the bottom responded the slowest. The matrix suction change rate increased with the increase in rainfall intensity.
4.6 Failure mode
Figures 15 and 16 show the landslide deformation processes under conditions N1 and N2, respectively. According to the model test results and the responses of each parameter to rainfall infiltration, the failure process of completely weathered granite induced by rainfall can be divided into four stages shown in Fig. 17.
(1) The infiltration erosion stage is shown in Fig. 17a. At the beginning of rainfall, the slope surface was saturated, and the soil surface was slightly eroded. The reasons for the erosion damage were twofold. One was that the kinetic energy generated by the falling raindrops caused splash and damage of the soil particles on the slope. The other was that the loose soil on the slope surface was washed away with the continuous infiltration of the slope, forming multiple erosion pits.
(2) The surface deformation stage is shown in Fig. 17b. As the rainfall continued, the loose materials accumulated on the slope were gradually washed away, and the high porosity of the strongly weathered soil accelerated the water erosion and lateral erosion in the ditch. At the same time, the earth pressure and pore water pressure increased, the stress redistributed, and edge cracks and cracks formed at the positions of tensile stress concentration and continued to expand. The resulting dominant inflow seepage further strengthens the seepage effect.
(3) The damage-deepening stage is presented in Figs. 17c and 17d. With rainfall infiltration, the soil mass gradually saturated, and the interior of the slope started to destabilize, creeping toward the free surface. Under the combined action of rainwater scouring and seepage, the gully head at the toe of the front slope collapsed, and the side cracks and tension cracks of the slope gradually deepened and interconnected under tensile stress. As a result, the soil around the cracks softened further, the matrix suction dissipated, and the soil at the rear edge was affected by multiple tension cracks. The shear strength and effective stress were insufficient to offset the sliding force and cause soil flow damage. The middle and rear edges form a "drum-shaped bulge" under the load of the upper soil. At the same time, due to the different sliding speeds of the middle edge soil, "sheet sliding" appeared.
(4) The overall instability stage is shown in Figs. 17e and 17f. With continuous rainfall, the surface soil of the slope was almost saturated, especially after the saturation of the soil in the sliding zone, which provided a potential sliding surface for the landslide. In addition, the formation of the groundwater level exerted an uplifting force on the landslide soil, the anti-sliding force of the slope body decreased, and the slope body destabilized as a whole. The front and middle edges showed "sheet slip", which slowly accumulated at the foot of the slope in the form of "debris flow". The effective free face of the landslide appeared, and the rear edge disintegrated completely, resulting in overall pushing damage.