2.1 Landslide position and geological conditions
The Sanmendong landslide is located on the right bank of the Qinggan River, 8.5 km from the river’s mouth, in Meiping village of Shazhen Town, Zigui County (coordinates: 110°34'52.4"E, 30°58'17.9"N; China Geodetic Coordinate System, CGCS 2000). The landslide is located within a subtropical monsoon climate zone, where the average annual temperature and rainfall are 17–19℃ and 1,493 mm, respectively. The annual rainfall distribution is uneven and is mainly concentrated from June to October, with average monthly rainfall of between 150 mm and 457.6 mm. Historical rainfall data obtained from the Institute of Heavy Rain of China Meteorological Administration, Wuhan (L. Wang et al., 2022) report the occurrence of > 100 mm of rainfall three to four times annually (mainly in June and July) and a maximum daily rainfall amount of 358 mm. According to field surveys, heavy rainfall is one of the main drivers of Sanmendong landslide deformation.
The geographical location of the landslide and a lithology distribution map of the stratum are shown in Fig. 1. Faults are not developed in the area near the Sanmendong landslide, but the landslide is located on the southern side of the Baifuping anticline, which is a monocline. The bedrock under the sliding mass has relatively well-developed fissures, and the cutting of fissures is conducive for collapse occurrence, and it thus provides the material basis for the formation of this landslide. According to the field survey, the sliding mass is mainly composed of quaternary slide sediments, with a thickness of 10–20 m, and it comprises gravelly soil and silty clay with gravel, where the gravel mainly originates from quartz sandstone and mudstone. The soil composition is clay, sand, and cultivated soil, with a soil-rock ratio of 5:5–7:3. The sliding mass accumulation is loose, and the sliding surface is the contact surface between the bedrock and soil. The bedrock under the sliding mass is purplish-red or purplish-grey mudstone and siltstone of the Middle-lower Jurassic (J1 − 2n). Bedrock outcrops can be seen on the eastern scarp, western ridge, and back edge of the landslide. The angle of the bedrock layer is 110°∠32°, and the main sliding direction of the landslide is 66°.
2.2 Morphological characteristics of the landslide
The Sanmendong landslide is a typical soil landslide comprised of slide sediments, and the sliding mass is tongue-shaped in plane view (see Fig. 2). Planar and profile maps of the landslide were prepared based on the engineering geological survey and geological mapping and are shown in Figs. 3 and 4, respectively. As seen from Fig. 4, the front of the landslide slopes gently, the central and back edges are relatively steep, the mass is low in the west and high in the east, and the landslide has a concave shape. The elevation of the front of the landslide (140 m) is below the reservoir water level. The back edge of the landslide is arc-shaped, bounded by bedrock, and has an elevation of 350 m. The right and left sides of the landslide are bounded by the scarp and the bedrock ridge, respectively; the average landslide slope is 15°, the average width is 300 m, and the sliding mass has a length, area, and volume of 830 m, 24.9×104 m2, and 448×104 m3, respectively.
2.3 Deformation characteristics of the landslide
The Sanmendong landslide is an old landslide. Before 2003, deformation of the landslide was inconspicuous, but it was mainly caused by local collapse of the steep slope, which was induced by rainfall. Since the Three Gorges Reservoir became operational in June 2003, the degree of landslide deformation has been exacerbated, and various parts of the landslide now exhibit different degrees of annual change in subsidence, collapse, cracks, and other deformation signs. To prevent a geological disaster, protect the lives and homes of villagers living next the landslide, and monitor the landslide with the aim of providing an early warning system, the authorities have implemented and simultaneously employed GPS monitoring. In addition, they have also conducted surface macroscopic surveys of the Sanmendong landslide since 2006.
2.3.1 Macroscopic deformation characteristics of the landslide
The macroscopic survey of the surface of Sanmendong landslide conducted in the field has revealed different degrees of annual deformation, especially during the rainy season from May to September. As the reservoir water level decreases but episodes of heavy rainfall increase, deformation of all parts of the landslide is exacerbated compared with that during other periods.
From 2006 to 2016, the landslide deformation area was mainly distributed near a rural road in the central part of the landslide. In June 2007, a tension crack (GC1) with a length and width of approximately 20 m and 15 cm, respectively, formed on the slope of the left side of the landslide (toward 270°) 20 m below the central road (elevation: > 200 m; see GC1 in Fig. 5a). In May 2009, a tension crack (GC2) with a length and width of approximately 30 m and 10 cm, respectively, and toward 110° appeared at the middle road (elevation: 220 m) on the left side of the landslide, as shown in Fig. 5b. In June 2009, both GC1 and GC2 expanded, and their widths increased to 20 m. In June 2014, a new tension crack (GC3) emerged at the mid-elevation of > 240 m on the right side of the landslide toward 45°, with a length and width of approximately 20 m and 5–10 cm, respectively (Fig. 5c). The landslide is roughly divided into upper and lower parts by cracks GC2 and GC3. A concentrated area of deformation signs in located in the middle of the landslide, and this indicates that the amount of deformation in the lower part of the landslide is larger than that in the upper part.
In October 2017, the landslide was affected by the “Autumn Rainfall of West China,” and the middle and back of the landslide exhibited greater signs of deformation than its other parts. The road foundation at the central boundary on the right side of the landslide sank, tension cracks (GC4) were driven by pavement tension cracks (with widths of 10 cm), and the soil sank by 30–40 cm (Fig. 6a and Fig. 6b). The road near a house on the center-left side of the landslide also exhibited a tension crack and sink deformation because the soil sank by approximately 20–60 cm; the width of crack (GC5) reached 24 cm, and its length extended over 20–30 m (Fig. 6c). The road at the boundary of the rear edge of the landslide was cut into multiple sections by the tension cracks. Moreover, cracks appeared in the cistern wall (FP1), and its water storage function was lost and it could no longer be used, as shown in Fig. 6d.
From 2018 to 2020, deformation signs on (1) residential buildings on the center-left side of the landslide and the nearby roads, (2) roads on the center-right side of the landslide boundary, and (3) roads at the rear border of the landslide were exacerbated, but no distinct deformation signs were evident on the other parts of the landslide. However, all parts of the landslide exhibited exacerbation of deformation signs in 2021, and the degrees of deformation increased after periods of rainfall and in relation to rises and falls in the reservoir water level.
From January to May 2021, the water level of the TGRA gradually decreased from its normal level (+ 175 m) to that of flood control water level (+ 145 m), and the amount of rainfall increased monthly. Existing cracks expanded, and new signs of deformation were evident in numerous parts of the landslide. New tension cracks were triggered in the village road on the center-left side of the landslide (+ 200 m, GC6) and the road on the center-middle part of the landslide (+ 280 m, GC7) (see Fig. 7a and Fig. 7b, respectively). The foundations of many residences on the left boundary of the central and rear parts of the landslide sank, resulting in cracks in the retaining wall (FP2), as shown in Fig. 7c.
During the 2021 rainy season (June to August), the TGRA was operated at a low water level (+ 145 m). During this period, the landslide was subjected to periods of long-duration rainfall, and the deformation degree of the landslide was aggravated. The active deformation area of the landslide was the 20 m area between the two road cracks, GC2 and GC6. The gully on the right side of the road between the two road cracks collapsed after a long period of heavy rainfall from June 12 to June 27 (Fig. 8a), and the area of collapse measured 8 m (width) by 6 m (length) with a volume of ~ 100 m3. Affected by the long period of heavy rainfall from August 15 to August 25, the right side of the road in the front of the landslide was cut off, and the pavement was fragmented by cracks (GC8) with widths ranging from 2 cm to 4 cm (Fig. 8b).
From September to December 2021, the water level of the TGRA gradually rose to a normal level, the amount of rainfall decreased monthly, and the landslide slope body showed no new deformation signs. During this period, the local authorities filled the roadbed on the left and right sides of the landslide and subsequently levelled the road surface. The survey taken at the end of the year recorded no new road surface deformation signs.
The landslide deformation points recorded since the first surface macroscopic survey was taken in 2006 were marked on panoramic view and planar maps, and are shown in Fig. 2 and Fig. 3, respectively. The timing of the deformation signs recorded on each part of the landslide obtained from the macroscopic surface survey demonstrate that landslide deformation is related to rainfall and changes in the water level of the reservoir. The existence of an active deformation area in the middle of the landslide near the rural road at an elevation of + 220 m, and the active deformation area at the rear of the landslide near the monitoring point ZG363, indicate that the whole landslide has entered a creep deformation period. The active deformation area in the middle of the landslide was formed over a longer period than that at the back of the landslide, and it extends over a larger scale. This in turn indicates that the displacement rate at the front of the landslide is faster; in particular, the landslide exhibits deformation characteristics of a fast displacement rate at the front and a slow displacement rate at the rear, in addition to traction movement.
2.3.2 Landslide deformation characteristics based on GPS Monitoring
The GPS monitoring project has been observing the Sanmendong landslide since 2006, and two virtually parallel monitoring longitudinal profiles of the sliding mass have been obtained. There are GPS monitoring points at the front, middle, and rear of the two monitoring profiles that are consistent with the main sliding direction of the landslide, and there are a total of six monitoring points numbered ZG360–ZG365 (Fig. 2), where ZG360, ZG361, and ZG362 are located at the rear, middle, and front of the right side of the landslide, respectively, and ZG363, ZG364, and ZG365 are located at the rear, middle, and front of the left side of the landslide, respectively. Although macroscopic deformation of the landslide is evidently correlated with reservoir water level fluctuations and rainfall, deformation has not been visible after periods of rainfall or reservoir water level fluctuations. Therefore, it is critical to elucidate detailed deformation characteristics of the Sanmendong landslide using GPS monitoring data combined with meteorological and hydrological data. In this respect, the curves of cumulative displacement, monthly rainfall, and reservoir water level changes over time were plotted and are shown in Fig. 9.
As seen from Fig. 9, the six monitoring points were in a state of continuous displacement while showing synchronous and step deformation characteristics. The cumulative displacements of the six monitoring points from 2006 to 2021 were 1250.9 mm, 2414.8 mm, 2601.6 mm, 2604.4 mm, 2555.8 mm, and 3430.0 mm, respectively. The accumulated displacement indicates that the six monitoring points were in a state of displacement and that the entire Sanmendong landslide had entered a creep deformation period. The cumulative displacement monitored by the three monitoring points on the left side of the landslide (ZG363, ZG364, and ZG365) was generally higher than that of the three monitoring points on the right side of the landslide (ZG360, ZG361, and ZG362). More specifically, the slip rate on the left side of the sliding mass was stronger than that on the right side of the sliding mass. For the same monitoring profile, the cumulative displacement value at the front of the sliding mass was higher than that in the middle, and that of the rear was the lowest. In summary, the displacement rate at the front of the sliding mass was found to be greater than that at the rear.
There was good agreement between the steps in the cumulative displacement curve of the six monitoring points, the decline in the reservoir water level, and the high monthly average rainfall. Using 2015 as an example, we found that the steps in the cumulative displacement curve for each monitoring point occurred from March 1 to September 30, a period represented by a decline in the reservoir water level, low water level operation, and the highest monthly rainfall.
Overall, the deformation degree of the landslide exhibited a strong correlation with rainfall and fluctuations in the reservoir water level, and it exhibited the following deformation characteristics: a fast moving left side, a slow moving right side, a fast moving front, a slow moving rear, and traction movement. These findings agree with the deformation characteristics of the landslide’s movement, as indicated by the macroscopic surface survey. Therefore, after elucidating engineering geology conditions of the landslide and the main factors influencing landslide deformation, we used a numerical simulation method to analyze the mechanism of landslide deformation response to rainfall, reservoir water level fluctuation, and the coupling effect of the these two main factors.