About 18 years ago, a large-scale discontinuous layer in properties and colour was found in the new Fengjie town at the shore of the Three Gorges Reservoir area in China. There are many resettled residents and buildings on the sloping area, the safety of which is potentially affected by this layer, so it has become the focus of attention. Before this study started there were two viewpoints regarding the origin of this layer. One was that is was from a huge ancient slide and the other was that is was from a fault graben. In order to find out how it was formed and to be able to carry out a stability analysis of the slope the authors have carried out a research program, including geological field investigations and mapping, a deep drilling hole, a geotechnical centrifuge model test, and a simulation analysis. The results of the research led to the conclusion that the layer is the sliding plane of a huge deep-seated ancient rock slide, which we called the Sanmashan landslide. An important argument for the conclusion is the recognition of a regional compressive tectonic stress field in this area, which cannot lead to the formation of a fault graben because it needs a tensional tectonic stress field. Moreover, numerous unique geological features, sliding marks, and other relics of the ancient slide have been discovered in the field. The formation process of the ancient slide could be repeated in a large geotechnical centrifuge model test. The test shows that a deformation and failure process of “creep–crack–cut” has occurred. The type of the ancient slide can be classified as a “successive rotational rock slide”. Finally, the role of seepage in the stability of the Sanmashan landslide has been analysed. Our final conclusions are that, during rainfall and filling–drawdown cycles in the Three Gorges Reservoir, the Sanmashan landslide as a whole is dormant and stable and the secondary landslides in the toe area of the slope are presently stable but can be reactivated. This research provides an important basis for the remedial measures and land use planning in the new Fengjie town, and a well-documented case history for researchers worldwide.
Before the old town of Fengjie was submerged in the water of the Three Gorges Reservoir, the Sanmashan slope area was recommended as the resettlement site for building a new Fengjie in 1997 (Fig. 1). During the construction process of the new Fengjie town, a large-scale discontinuous weak zone was found in the Sanmashan slope area in 1998 (Fig. 2). To determine the safety of the slope by geotechnical analysis, it was necessary to determine the origin and characteristics of the weak zone.
Location map of the Sanmashan landslide (31
The identification and formation mechanism of deep-seated ancient rock slides is an important research subject (Crosta, 1996; Crosta and Zanchi, 2000). It is well known that both a rock slide and tectonic faulting can form an important weak zone (Illies, 1981; Chang et al., 1981; Crosta, 1996; Crosta and Zanchi, 2000; Cruden and Martin, 2007; John and Douglas, 2012). However it is often not easy to decide which process has taken place. Michael et al. (2012) found that a landslide scarp can easily be misinterpreted as a fault. However, tectonic activity can play a key role in the development of a large-scale rock slide (Brideau et al., 2009). In this paper we describe the study of a very large, interesting deep-seated rock slide, called the Sanmasan landslide, in the new Fengjie town of the Three Gorges Reservoir area, Chongqing, China (Figs. 1 and 2).
Before the start of our research there were two viewpoints or hypotheses about the origin of the weak zone: a huge ancient slide or a fault graben. Some authors (BIS-CWRC, 1997, 1999) suggested that it was caused by geological tectonic activity, as the slope area is surrounded by four normal faults (see Supplement 1, from the previous study), due to the formation of a local fault graben, and this viewpoint was quoted in later research papers (Li et al., 2002; Wang et al., 2006; Qi et al., 2012). Other authors (Nanjiang Geological Team, 1999) speculated that the weak zone was the result of a very large ancient rock slide. These two different viewpoints confused the land use planners and those responsible for geological hazard prevention in the new Fengjie town. A large number of resettlemed residents and buildings on the Sanmashan area were located in the new Fengjie town (Fig. 3), and filling–drawdown cycles of the reservoir water table and rainfall may affect the stability of the landslide with disastrous consequences (Alonso and Pinyol, 2010; Barla et al., 2010; Paronuzzi et al., 2013; Tang et al., 2015a). Therefore, the main question to be answered by our research is as follows: was this large-scale discontinuous weak zone caused by a huge ancient rock slide or by a local fault graben structure? If it was concluded that it is a huge landslide, how did that ancient slide happen? Is it stable and safe under the filling–drawdown cycles of the Three Gorges Reservoir?
To solve these questions, the authors have carried out geological investigations and mapping, performed a deep exploration borehole, and carried out rock and soil mechanics tests, a large centrifuge model experiment, and a stability analysis.
Panoramic photograph of the Sanmashan landslide before
the Three Gorges Reservoir filled with water and before the relocation of
the new Fengjie town (photograph was taken by Nanjiang Geological Team in 1998).
The attitude of the bedrock bedding is 335–350
The residential buildings on the Sanmashan landslide in the new Fengjie town after the Three Gorges Reservoir filled with water (photograph taken 6 July 2013). The elevation of the Yangtze river water level is about 147 m a.s.l.
There are three major geomorphological levels in China (Li et al., 2001). The new Fengjie town in the Three Gorges Reservoir area is close to the eastern margin of the second level. This region is part of the Yangtze platform, a tectonic unit in which the basement rocks are mainly composed of early Proterozoic metamorphic volcaniclastic rocks and intrusive magmatic rocks. The overlying sedimentary rocks, deposited during the Triassic, were folded and uplifted during the Yanshan phase at the end of the Jurassic. The upper Yangtze platform fold belt, the marginal depression of the Sichuan Basin, and the Dabashan platform fold belt converge in this region (No. 107 Geological Team of Sichuan Province, 1980) (Figs. 4 and 5).
The rocks in the new Fengjie town area are of Triassic age (No. 107 Geological
Team of Sichuan Province, 1980; Li et al., 2006).
The Early Triassic Jialingjiang group ( The Middle Triassic Badong group ( The overburden soils contain landslide deposit (
After reviewing the available geological maps, aerial photographs, previous papers, and reports on the area (No. 107 Geological Team of Sichuan Province, 1980; Chen and Zhang, 1998; Nanjiang Geological Team, 1999), we conducted field work and geological mapping and collected the following information: the stratigraphic distribution in the Sanmashan area (shown in Fig. 7); the outcrop features of bedding and rock types in the Sanmashan area (shown in Fig. 6); and the cross section of the key geological structure and lithologies, the Zhuyi anticline in the new Fengjie town area (shown in Fig. 8). The palaeostress reconstruction of the tectonic stress field in the Sanmashan area, the attitude of the two limbs of the Zhuyi anticline (Fig. 9) and its conjugate shear joints (Fig. 10) are analysed in Fig. 11.
Regional tectonic domains and geostress fields (image from Google earth).
Schematic N–S geological cross section of background with regional tectonic domains. The location of the A–B profile is shown in Fig. 4.
Rock outcrops in the new Fengjie town area. D01: the
first section of the Middle Triassic Badong group (
Geological map at 1 : 10 000 scale of the new Fengjie town area. The location is the area of the yellow box in Figs. 1 and 4. The topographic map was prepared in 1990 before the relocation of the new Fengjie town. The area of the green box in this map is an area of major site investigations (see Fig. 12).
It is well known that the formation, orientation, and evolution of faults
and joints in rock are controlled by the palaeotectonic stress field.
Conversely, the tectonic stress field can be deducted from the mechanical
properties of faults and joints and their structural geological orientation.
The Zhuyi anticline at the Sanmashan area is a double inverted anticline (an
overturned and closed fold with a dipping axial plane and overturned
strata) with strong crushing of strata in the core of the Zhuyi
anticline (Figs. 7–10). This shows that compressive stresses have
resulted in closed folding and compressional thrust structures. With the help of
the stereographic projection analysis of the conjugate shear jointing and
the overturned bedding, the regional maximum compressive stress direction in
the new Fengjie town can be determined as 344–352.5
We especially investigated the four normal faults in the field,
These data provide rather a large amount of evidence for the assumption that this area is a huge deep-seated ancient rock slide.
Geological cross section of the Zhuyi anticline. The location of the C–D cross section is shown in Fig. 7. It is a double overturned anticline, see also Figs. 9 and 10.
Detailed geological mapping, the investigation of an exploratory tunnel and a foundation pit, and the logging of 21 deep bore holes were carried out at the site. The results are shown in a detailed geological map of the new Fengjie town area at a scale of 1 : 10 000 (Fig. 7), and an engineering geological map of the Sanmashan landslide at a scale of 1 : 1000 (Fig. 12). The outline of the landslide, the geomorphological features and the sliding traces and structural geological features of the Sanmashan landslide are shown in Figs. 12–14. Eighteen samples were collected from the sliding plane, the slide mass and bedrock for the geotechnical laboratory tests.
A steep (70
During the construction of the new Fengjie town in 2000, the sliding plane in the upper part of the landslide and the scarp were exposed in a foundation excavation (Fig. 14c). The sliding surface is very obvious and very smooth, showing the characteristics of squeezing and sliding.
The sliding plane at the eastern lateral boundary has a thickness of 1.1 m
(Fig. 14e). It is exposed in the exploratory tunnel near the eastern
lateral boundary, it is very smooth, and also shows the characteristics of
squeezing and sliding (Fig. 14f). These sliding traces are very clear,
but have been misidentified as faults
The western lateral boundary, called Longzibaogou, leads into the Yangjiaping gully (Figs. 12 and 14). The sliding surface there was found and observed in a horizontal borehole (Fig. 14g); the slickensides are clear and smooth.
The elevation above sea level of the toe of the landslides about 90 m. There are three secondary (partial) landslides at the foot of the Sanmashan landslide called the Houzishi, Zhiwuyou, and Laofangzi landslides (Figs. 12–14).
Physical, mechanical, and hydraulic properties for the five main geologic units.
The bedding plane on the south limb of the Zhuyi anticline. The ripple marks show that the sequence is overturned.
Conjugate shear joints of in rock outcrop on the south limb of the Zhuyi anticline.
Stereographic projection of the orientation of the two
limbs of the Zhuyi anticline and the conjugate shear joints.
Geological map at a scale of 1 : 1000 of the Sanmashan landslide (the topographic base map is surveyed in 2004). The location of map is the area of the green box in Fig. 7. The area of the Sanmashan landslide is subdivided into four domains (I–IV). The three secondary partial landslides are outlined at the foot of the Sanmashan landslide.
Geological cross sections through the Sanmashan
landslide. The location of the cross sections is shown in Fig. 12.
Various views of the Sanmashan landslide.
The Sanmashan landslide can be subdivided into four domains (I–IV) based on
differences in morphology, geological structure, and inferred failure
mechanics (Figs. 12 and 14). Seven engineering geological units were
identified in these four domains (Fig. 13).
Units 1 and 2 are the landslide deposits ( Units 3, 4, and 5 are the bedrock ( Unit 6 is alluvium and proluvium ( Unit 7 is artificial fill soil (
Eighteen samples were collected from the sliding zone, the slide mass, and the bedrock below the Sanmashan landslide. The physical and mechanical parameters of the main six geologic units were determined by laboratory testing. Cohesion and internal friction angle were obtained from triaxial saturated consolidated undrained shear tests. The saturated permeability coefficient of the slide mass was acquired by double-ring infiltration testing in the field. The saturated permeability coefficients of the bedrock are estimated values. The test results are shown in Table 1 and were used for the following analysis.
According to residents, a small shallow soil slide and the formation of cracks had already occurred at the front of the secondary (partial) landslide during rainfall before 2003.
The filling–drawdown cycles of the Three Gorges Reservoir from June 2003 to December 2015 are shown in Fig. 16. (i) On 1 June 2003, the first reservoir filling phase started. On 10 June 2003, the reservoir level reached an elevation of 135 m. (ii) On 20 September 2006, the Three Gorges Reservoir second phase of the reservoir filling started. On 27 October 2006, the reservoir level reached an elevation of 156 m. (iii) On 28 September 2008, the third phase of the reservoir filling started. On 14 November 2008, the reservoir level reached an elevation of 171.81 m. On 26 October 2010, the reservoir level reached its maximum design elevation level of 175 m.
During the first phase of the reservoir filling, several cracks were found in the surface of the Houzishi landslide in December 2004. The first crack occurred at an elevation of 150 m, had a length of 17 m and a width varying from 5 to 28 mm. GPS monitoring of the deformations was carried out by the local government at locations shown in Fig. 12. GPS1, GPS2, and GPS3 were located near the central axis of the Sanmashan landslide, GPS4 and GPS5 on the Houzishi landslide, GPS6 on the Zhiuwuyou landslide, and GPS7 on the Laofangzi landslide. The monitoring data were collected twice a month starting on 5 July 2006, but this frequency was increased in the rainy season. The results of the GPS monitoring are shown in Fig. 17.
The displacement curves show that the total displacements of GPS1, GPS2, and GPS3 remain near zero, which show that the middle and back parts of the Sanmashan landslide are stable. However, the total displacements of GPS4 and GPS5 were 19.3 and 16.4 mm from 5 July 2006 to 31 December 2011 on the Houzishi landslide. The total displacements of GPS6 and GPS7 were only 6.7 and 9.8 mm for the Zhiwuyou and Laofangzi landslides respectively. Analysis of the relation with reservoir level and rainfall, the displacements began to increase after the filling phases of 20 September 2006 and 28 September 2008. Since May 2009, however, none of the GPS monitored displacements have increased further. We assume that the Sanmashan landslide as a whole is inactive nowadays, but a partial reactivation of the landslide can be induced by reservoir water fluctuations or rainfall.
Figure 13 shows that the original surface of the ancient slide was a gently
inclined slope consisting of limestone of
Filling-drawdown cycles of the reservoir from 2003 to 2015.
The similarity coefficients for the main physical quantities in the centrifuge model.
Physical and mechanical parameters of the materials in the centrifuge model test.
The geotechnical centrifuge used for the model test.
The assumed geological structure of the original river bank slope.
The centrifuge test model.
The centrifuge technology is widely applied in geotechnical and geological engineering, especially in the simulation of slope deformation and failure (Taylor, 1995; Nakajima and Stadler, 2006). A centrifuge is a piece of equipment that rotates an object around a fixed axis (it spins in a circle), applying a potentially strong centrifugal (outward) force perpendicular to the axis of spin. Geotechnical centrifuge modelling is used for physically testing soils and rock models. The centrifugal acceleration is applied to models to scale the gravitational acceleration and enable prototype scale stress in scale models. Problems can be studied such as occur in the design of foundations for buildings and bridges, the design of earth dams, tunnels, and analysis of slope stability, including effects such as blast loading and earthquake shaking. Large centrifuges are used to simulate high gravity or acceleration environments. For our model tests we used the centrifuge at the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection in Chengdu, China (Fig. 18). With a maximum acceleration of 500 gt it is the largest geotechnical centrifuge in Asia.
The geological model of the original slope (Fig. 19) was generalized into a geotechnical test model (Fig. 20). Based on the previous research (Yang, 1988; Li et al., 2001), three periods of incision of the Yangtze River are assumed. According to the similarity theory (Taylor, 1995) and the dimension relationship between geological model and experimental model we determined the similarity coefficient (Table 2). From the physical and mechanical parameters of the geological units of the Sanmashan landslide (Table 1), we determined the physical and mechanical parameters of the model materials with the similarity coefficient (Table 3).
The model after slide. The rupture surface of the slide is roughly curved, successive rotational slides occurred around one axis.
Schematic cross section of the Sanmashan ancient slide.
The Chana landslide, Longyangxia, China.
The model test results were shown in an earlier study (Figs. 97.8–97.10, Tang et al., 2015b) (see Supplement 2). The deformation and failure process during the centrifuge test has the following characteristics. First, the soft rock in the lower part of the model slide body is deformed in shear. Then, tension cracking occurs in the hard rock in the upper part. Finally, the rupture plane is formed when the tension cracks extend downwards towards the zone of shear deformation and the slide body moves. The deformation and failure process can be characterized as creep–crack–cut. The sliding surface is curved, but non-circular. A steep main scarp cuts through the hard rock in the upper part of the slope and forms the proximal part of the rupture surface, daylighting at the crown. The slide movement is roughly rotational around an axis that is parallel to the ground surface (Fig. 21).
According to international standards (WP/WLI, 1993), the type of the Sanmashan ancient slide should be classified as a successive rotational rock slide (Fig. 22), as we already concluded after our field survey (Fig. 13). This type of slide generally occurs as a result of river incision, in subhorizontal layered rock masses on a gently inclined slope with hard rock overlying soft rock. Other examples are the Chana landslide, Longyangxia along the Yellow River (Fig. 23) and the Yanchihe landslide in Hubei Province, China (Zhang and Huang, 1990).
On the basis of the results of the research described above, we analysed the influence of seepage on the stability of the Sanmashan landslide.
We used the software Geo-Slope 2007 (Geo-Slope International Ltd., 2007). The variation of the groundwater level induced by the reservoir filling or rainfall and the related change in pore water pressure are simulated using the code SEEP/W. We carried out the limit equilibrium slope stability analysis with the software SLOPE/W. During the analysis, the output of the seepage simulation was used as input for the stability analysis, for which we adopted the Morgenstern and Price (1965) method to calculate the factor of safety (FOS).
Three cross sections (Fig. 13) were chosen to conduct the two-dimensional combined seepage–slope stability analysis. For each cross section, five main hydrogeological–geomechanical units are identified.
(1) The upper unit of the slide mass is dense to very dense and contains
grey or dark grey crushed stone in a matrix of silty clay and sand. The
crushed stone is derived from limestone (
The physical and mechanical parameters of the analytical model were determined according to the material testing results. The permeability of the different units is an important parameter affecting the stability analysis. We have carried out an inversion analysis on the data used in a previous study (Wang et al., 2013). The parameters of the model materials of stability analysis are listed in Table 1. The hydraulic conductivity functions that we adopted for the main hydrogeological units are shown in Fig. 25.
Model of the two-dimensional numerical stability analysis of the Sanmashan landslide (cross section I–I' in Fig. 12).
Hydraulic conductivity functions adopted for the main
hydrogeological units: (1)
The water level fluctuation rates chosen for the filling–drawdown cycles of the reservoir level (Stages A–H see Fig. 27).
Annual maximum rainfalls in the period 1964–2013.
The maximum rainfall intensity for different return periods.
The reservoir level data from 2011 to 2015.
The mean values of the reservoir level during the filling–drawdown cycles in 2011–2015. The chosen values for the analysis (see also Table 5).
The average monthly rainfall from 1964 to 2013 and the chosen value of the filling–drawdown cycles of the reservoir level for the numerical stability analysis.
The repeated filling–drawdown cycles and rainfall are the main factors that affect the stability of the Sanmshan landslide (Zangerl et al., 2010; Paronuzzi et al., 2013; Galeandro et al., 2013). Based on a statistical analysis of the historical data, the reservoir level and rainfall values are determined, as follows.
Figure 17 shows that the reservoir level has varied annually between the elevations of 175 and 145 m since January 2011. Through the statistical analysis of the historical data from 2011 to 2015 of the reservoir level (Fig. 26), the annual fluctuation rate of the cycles is determined (Fig. 27 and Table 4). It was used for the boundary conditions in the numerical modelling.
The historical rainfall records of 1964–2013 for the new Fengjie town have been collected and analysed. The variation of average monthly rainfall during the years 1964–2013 shows that heavy rainfall is mainly concentrated in June and July (Fig. 28). Based on the probability statistics of the maximum daily rainfall (Table 5), the maximum rainfall intensity for different return periods has been calculated (Fig. 29 and Table 6). A daily rainfall of 140 mm in return periods of 50 years (2 % probability) has been adopted as the boundary condition in numerical modelling.
With a drawdown of the reservoir level from 175 to 145 m (Fig. 27), the delay in the lowering of the groundwater level after the lowering of the reservoir water level will cause a decrease in the landslide stability. After stabilization of the reservoir water level at 145 m, further lowering of the groundwater level in the sliding body will gradually lead to an increase in the stability of the landslide (Figs. 30 and 31).
During a rise in the reservoir water level from 145 to 175 m (Fig. 27), the stability of the landslide will increase as the rise in the groundwater level in the slide mass lags behind the rise of the reservoir water level. After reaching the reservoir water level of 175 m, the stability of the landslide will again gradually decrease with a further rise of the groundwater level (Figs. 30 and 32).
Due to the filling–drawdown cycles of the Three Gorges Reservoir (Fig. 27), but without the rainfall effects, the minimum stability coefficient (FOS) of the Sanmashan landslide is 1.617, and that of the partial landslide at the front is 1.114.
A daily rainfall of 140 mm in return periods of 50 years (2 % probability), during a period of reservoir water level drawdown from 175 to 145 m, has little effect on the stability of the whole landslide (decrease in FOS of 2.4 %), but has a large influence on the secondary partial landslides at the front (decrease in FOS of 4.2 %) (Fig. 32).
During rainfall and filling–drawdown cycles in the Three Gorges Reservoir, the minimum stability coefficient (FOS) of the Sanmashan landslide is 1.578 and of the partial landslide at the front it is 1.067.
Maximum rainfall intensity in mm day
Ground water level changes during the filling–drawdown cycles of the Three Gorges Reservoir.
Factor of safety (FOS) variation during the filling–drawdown cycles of the Three Gorges Reservoir (Points A–H see Fig. 27).
Factor of safety (FOS) variation due to rainfall and filling–drawdown cycles of the Three Gorges Reservoir (Points A–H see Fig. 27).
Extensive field studies and mapping, deep borehole drilling, investigation of the already existing tunnels and a large foundation excavation pit as well as centrifuge model testing and numerical stability analysis were carried out to investigate the stability of a deep-seated ancient rock slide in the Three Gorges Reservoir area.
Three tectonic units, the upper Yangtze platform fold belt, the marginal depression of the Sichuan Basin, and the Dabashan platform fold belt, converge on this region. In the new Fengjie town, the site of the landslide, a N–S compressive geostress field is the tectonic background. For this reason we discarded the theory that the deformations in the slope were related to a fault graben, as that would have needed a tensional geostress field. Our investigations have also shown that the four normal faults, which in earlier studies were used to support the hypothesis of a fault graben, share some common characteristics, which do not support the local fault graben theory. Thus we come to the conclusion that a local fault graben does not explain the presence of the large-scale weak layer in the Sanmashan area.
Meanwhile, there are numerous unique geological features and interesting slickenside relics of the ancient slide discovered in the field. From the structural geological evidence, geomorphological evidence and slickenside traces, we conclude that this area is a huge deep-seated ancient rock slide, called the Sanmashan landslide.
A steep cliff over 130 m in height forms the back scarp of the Sanmashan
landslide. Two gullies, Baiyangping as a western and Sunjia as an eastern
boundary, as well as the Yangtze River in the front, form the boundaries of the
landslide. The terrain slope angle of the landslide is 15–20
The original surface of the ancient slide was a gently inclined slope
consisting of limestone of
Finally, the combined analysis of seepage and slope stability of the Sanmashan landslide has been carried out, the results of which show that during rainfall and reservoir filling–drawdown cycles the Sanmashan landslide as a whole is stable and dormant, while the secondary (partial) landslides in the toe area are basically stable but can be reactivated. This research provides an important guidance for strict land use planning in the sliding zone.
This research was financially supported by the National Basic Research Program of China (No. 2013CB733202), the Key Project of TGR (No. SXKY3-1-1-200901), and the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (No. SKLGP2015Z008). The authors sincerely thank Xuebin Huang, Wenming Cheng, and Kaixiang Xu for their assistance. Wei Zhang, Ruihua Xiao, Lei Zhang, and others contributed to the investigation and drilling. Guang Zheng and Kai Wang contributed the centrifuge model testing. Fan Yang and Yangjian Cao contributed the stability analysis. The manuscript was directed by Zhuoyuan Zhang and Guangzong Peng. The authors wish to thank the anonymous referees for their helpful suggestions and constructive comments, which have contributed greatly in improving the quality of the manuscript. Edited by: T. Glade Reviewed by: C. Xu and two anonymous referees