Research shows that there is an obvious negative correlation between resistivity and water content (zhang et al.2020). After the measurement, select the excavation model within the range of 1.05-1.19m of the abscissa, and take samples every 14cm along the depth direction to measure the water content. The fitting curve of resistivity and water content is drawn according to the measured data, as shown in (Fig. 6). The functional relationship between the two can be expressed by formula (3).
According to formula (3), the water content and saturation of the slope increase with the rainfall seepage process, and the resistivity gradually decreases. The main existing form of water in soil changes with the increase of water content, the charge mobility increases with the increase of water content, and the soil resistivity decreases. At the initial stage of rainfall, the resistivity drops rapidly, the form of water in soil changes from adsorbed water to capillary water, and the current changes from electric double-layer conduction along the surface of soil particles to conduction along the surface of soil particles and the series path conduction of water and soil (mojid et al.2008). With the increase of the internal conductive path, the ion mobility increases and the resistivity decreases rapidly. With the increase of water content, the continuity of pore water is enhanced, the current is mainly conducted along the continuous pore water, and the reduction rate of soil resistivity slows down. When the water content is greater than 20%, the soil mainly exists in the form of gravity water, the continuity of pore water is good (Fukue et al.1999), and the soil tends to be saturated. The increase of water content has little effect on the continuity of pore water, the current is mainly conducted by pore water, and the soil resistivity tends to be stable. The test results show that when it is close to saturation the water content is 34.7% and the corresponding resistivity is 39.4 Ω\(\bullet\)M.
2.4 Law of water movement and crack development in slope
According to the change of resistivity, the typical rainfall time is selected to draw the inversion diagram to illustrate the seepage and fracture development law in the survey line area, as shown in (Fig. 7).
In the initial stage of rainfall, there is no runoff on the surface, only rapid infiltration on the top of the slope. The resistivity distribution in the survey line area is clear without abnormal resistance. After the 5th rainfall, the cumulative rainfall is 10.5mm, the resistivity of each layer in the survey line area tends to be evenly distributed, the average depth of the wetting peak is about 5cm, and the lowest shallow resistivity is about 19 Ω·M. With the rainfall process, water continuously seeps into the slope, ponding occurs at the top of the slope, and the shallow area of the slope reaches saturation. No obvious crack or damage is found on the slope top (Fig. 7a).
After the 33th rainfall, the cumulative rainfall was 69.3mm, and there was a significant difference in water infiltration on the slope; The resistivity of the shallow slope in the survey line area changes unevenly. The depth of the wetting peak on the left side of the slope is low, and the average depth of the wetting peak is 6cm. The water at the top of the slope gradually diffuses to the shoulder, and the infiltration rate at the left side is faster than that in other areas. The results show that during the cycle of rainfall infiltration and measurement, the left side of the slope first produces small expansion and contraction cracks, forming a water infiltration channel (Fig. 7b).
After the 80th rainfall, the cumulative rainfall is 168mm, and the average depth of wetting peak is 21cm. Runoff erosion occurs at many places on the slope surface, and obvious tension cracks appear at the rear of the slope top. Rainfall seeps into the slope through the cracks at the top of the slope. As shown in (Fig. 7c), many obvious low resistance areas are formed inside the slope, indicating that the rainfall seepage in different areas is uneven, and many water migration paths are formed inside the slope.
After the 91th rainfall, the cumulative rainfall was 191.1mm, and the moisture content of the soil mass in the upper part of the slope gradually became saturated. Under the influence of micro topography and other conditions on the slope, the flow on the slope is connected and converged with each other, the bearing capacity of water on soil particles is further enhanced, and multiple rills are formed on the slope. The slope slides locally, and a large amount of ponding appears at the left slope toe. According to (Fig. 7d) shows that the depth of wetting front moves down, and the infiltration rate of No. 14–16 measuring points in the central region is faster. There are many relatively high resistance areas and obvious low resistance areas in the slope. The high resistivity zone is the fissure formed by the expansion, contraction and tension of the slope; The low resistance area is the water flow path and ponding area.
After the 93th rainfall, the cumulative rainfall is 195.3mm. Under the influence of expansion and contraction force and tensile force, there are many transverse cracks in the runoff area on the left slope, and many longitudinal cracks on the right slope and the top, forming a new water seepage channel; At the left toe of the slope, sliding occurs due to rill erosion and the increase of soil moisture content. As shown in (Fig. 7e), the banded low resistance area inside the slope is connected with the water bearing area of the slope, and the soft structural plane is gradually developed in the middle of the slope.
After the 104th rainfall, the cumulative rainfall reached 218.4mm. As the weight of the slope increases, the transverse fracture on the top of the slope are further widened and gradually develop towards the shoulder, forming a network of secondary fracture on the top of the slope. With the increase of the number and width of cracks, a large amount of water enters the slope along the cracks. There are many crisscross fracture in the slope, traction sliding occurs in the middle of the slope, and the slope toe collapses. As shown in (Fig. 7f), within the range of No. 16–17 measuring points on the surface layer of the slope, obvious high resistance anomaly is detected, which corresponds to the position where cracks are found on the top of the slope; There are many high resistivity abnormal areas in the slope, indicating that with the rainfall and evaporation cycle process, many cracks are formed in the slope, and the slope is seriously damaged.