Precedent long-term gravitational deformation of large scale landslides in the Three Gorges reservoir area, China

Accepted Manuscript Precedent long-term gravitational deformation of large scale landslides in the Three Gorges reservoir area, China

Qinglu Deng, Fu Min, Xingwei Ren, Fangzhou Liu, Huiming Tang PII: DOI: Reference:

S0013-7952(16)30265-4 doi: 10.1016/j.enggeo.2017.02.017 ENGEO 4497

To appear in:

Engineering Geology

Received date: Revised date: Accepted date:

28 August 2016 16 February 2017 16 February 2017

Please cite this article as: Qinglu Deng, Fu Min, Xingwei Ren, Fangzhou Liu, Huiming Tang , Precedent long-term gravitational deformation of large scale landslides in the Three Gorges reservoir area, China. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Engeo(2017), doi: 10.1016/ j.enggeo.2017.02.017

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ACCEPTED MANUSCRIPT Precedent long-term gravitational deformation of large scale landslides in the Three Gorges reservoir area, China a

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a

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Qinglu DENG , Min FU , Xingwei REN , Fangzhou LIU , Huiming TANG

a Faculty of

Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta GA

author. email: [email protected]

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*Corresponding

Engineering, China University of Geosciences, Wuhan, 430074 China

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b School of

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ACCEPTED MANUSCRIPT Abstract: Large scale landslides are common in the Three Gorges reservoir region due to steep slopes and complex geological conditions, fluctuations of reservoir level, etc. Many phenomena and formation mechanisms related to these landslides are not well understood and need more research. In this study, large scale landslide evolution and formation mechanisms are thoroughly investigated in terms of precedent long-term gravitational deformation (PLTGD). Three large scale landslides in the Three Gorges reservoir region, Huangtupo landslide (HTPL), Huanglashi landslide (HLSL), and Baotaping landslide(BTPL),

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are taken for example to discuss their phenomena and features of PLTGD, and to investigate the correlation between the PLTGDs and landslide. Many features associated with landslides

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are caused by slow, downslope gravitational processes before landslide failure, and three

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criteria for recognition of the PLTGD features are summarized. The investigation results reveal that the PLTGDs play an important role in landslide evolution and formation mechanism. Overall, this paper presents evidences for PLTGDs of large scale landslides and

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their correlation with landslide evolution, which is helpful to understand the formation mechanism of large scale landslides in the Three Gorges reservoir region.

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Keywords: Precedent long-term gravitational deformation; Large scale landslides;

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Landslide evolution process; Three Gorges reservoir region.

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ACCEPTED MANUSCRIPT 1. Introduction An inventory of 5300 geohazards, including landslides, debris flows, rockfalls, etc., was completed in 2009 for the Three Gorges reservoir area. Among these geohazards there are more than 3700 landslides with individual volumes of > 1×10 5 m3, and 117 landslides with volumes > 1×10 7 m3 (Fig. 1). Geohazards in the Three Gorges reservoir area attract much attention due to their abundance and complex formation mechanisms. Many phenomena

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and formation mechanisms related to these geohazards are not well understood , and need more research.

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Previous work show that landslides in the Three Gorges reservoir area have several important characteristics: (1) a common association with slide-prone strata of the Badong

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Formation (Yin et al., 2004; Li et al., 2006; Wang, 2007), particularly the argillaceous limestone members (Ke et al., 2007); (2) landslides can be huge, such as the Huangtupo

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landslide (6.9×10 7m3) (Tang et al., 2014), the Baotaping landslide (3.5×10 7m3) (Li et al., 2015), or the Huanglashi landslide (4.0×10 7m3) (Feng, 2009); (3) many huge landslides are

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covered by thick debris, but most lack a well-defined master sliding surface between bedrock and debris (Cui, 1998; Liu, 2005); (4) huge landslides can occur over bedrock with consequent, obsequent, or horizontal slopes, or even on slopes where the strata have

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reversed inclines.

Disagreements exist on the formation mechanisms of these huge landslides. Their occurrence has been attributed to 1) “downward overlapping” (Cui, 1998), where slope

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toppling is followed by snowslide-like failure; 2) multiple, large-scale composite slides or multiple-stage sliding (He et al., 1998; Zhang, 2000; Tang et al., 2015a); 3) debris accumulation with complex origin, including rock avalanche, landslide, and karstification,

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etc. (Yin et al., 2000; Zhang, 2001; Liu2005). Perhaps these disagreements arise because many large-scale landslides are formed during a long process of evolution. During this evolution, slow deformation and gradual down-slope creep precedes catastrophic failure; this protracted process is defined here as “precedent, long-term gravitational deformation” (PLTGD). Extremely slow, deep reaching gravity driven movements of the creep type have been investigated by many researchers since long time. These phenomena, commonly defined “deep-seated gravitational slope deformations”, are large-scale slope movements of different types, such as sagging, deep-seated rock sliding, lateral spreading, etc. (Ter-Stepanian, 1966; Zischinsky, 1966; Varnes et al., 1989; Chigira, 1992; Dramis and Sorriso-Valvo, 1994; Deng et al., 2000; 3

ACCEPTED MANUSCRIPT Agliardi et al., 2001; Esposito et al.,2007; Chigira et al., 2013; Gori et al., 2014; Longoni et al., 2016). Their evolution is controlled by a number of factors, including geological structure, tectonic activity, seismicity, hydraulic gradient changes along slope (Ter-Stepanian, 1977; Radbruch-Hall, 1978; Crosta, 1996; Deng et al., 2000; Galadini, 2006;Moro et al., 2007; McCalpin, 2009; Audemard et al., 2010; Agliardi et al., 2013; Crosta et al., 2013; Di Maggio et al., 2014;Tang et al., 2015a,b; Tsou et al., 2015; Sacchini et al., 2016). Complex deformation phenomena were noticed in or near some landslides in the Three Gorges reservoir area

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(BGCWRC, 1994; Cui, 1998; Zhang, 2000), but little attention has been paid to the relationship between landsliding and the PLTGDs.

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This paper discusses the formation mechanism and evolution characteristics of

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landslides in the Three Gorges reservoir area and their relation to PLTGDs. Three large-scale landslides developed in the Badong Formation are taken for examples, the Huangtupo (HTPL), Huanglashi (HLSL), and the Baotaping (BTPL) landslides. These represent three

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distinct cases in terms of the angular relation between the bedding plane and the topographic slope, specifically being consequent, reversed incline, and horizontal slope,

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respectively. We first discuss the PLTGD phenomena for each of these landslides, then their geological features and the criteria for PLTGD recognition; and finally the correlation of the

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PLTGDs with the formation and structural and geomorphic evolution of these landslides.

2. Mid-Triassic strata of the Badong Formation

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Most landslides in this area are associated with Jurassic strata or with mid -Triassic strata of the Badong Formation (Chen et al., 2005; Ding et al., 2007), as shown in Fig.1. In this study, we mainly focus on the landslides developed in the Badong Formation.

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The Badong Formation, which hosts all the landslides considered in detail here, has 4 lithologic members (Li et al., 2006). In ascending order these members are 1) gray argillaceous limestone, 30～90 m thick, denoted T2b1; 2) purplish red pelite alternating with pelitic siltstone, 215～550 m thick, T2b2; 3) gray argillaceous limestone interlayered with grayish green calcareous pelite, 205 ～342 m thick, T2b3; and 4) purplish red pelite alternating with grayish green argillaceous limestone, 266～590 m thick, T2b4. The first (T2b1 ) and third (T2b3) lithologic members of Badong Formation are limestone, whose mineral composition is similar including 55-80% of carbonate minerals (calcite and dolomite), 15-35% of argillaceous minerals and, 5-10% of quartz. Fresh rock of these two members is usually green gray, but it will change to yellow after weathering due to the

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ACCEPTED MANUSCRIPT formation of argillaceous minerals. The second (T2b2) and fourth (T2b4 ) are argillaceous siltstone, whose mineral composition includes about 80% of argillaceous minerals, 10% of calcite, and approximately 5% of Quartz and feldspar. Physical and mechanical parameters of typical rocks of Badong formation are shown in table 1.

3. Huangtupo landslide

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Huangtupo landslide is the largest, most complex landslide in the Three Gorges reservoir region, with an area of 1.35 km 2 and a volume of 69 million m 3 (Fig. 2). This huge

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mass is situated on the right bank of the Yangtze River in Badong County, where the seat of Badong County was formerly located. When it was recognized that the Huangtupo slope was

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a large-scale landslide, the county-seat town of the Badong was moved to Xirangpo, about 10 kilometers to the west.

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The HTPL developed in the south wing of Guandukou Syncline，occupies a consequent slope whose rocks consist of the second and the third members of the Badong Formation (Fig. 3a). The bedding dips downslope at a moderate angle in the middle and upper parts of

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the slope, but is much gentler (~ 10º) lower down and closer to the axis of the Guandukou Syncline (shown in Fig. 3b). The lithology of the third member (T2b3) of the Badong Formation in and near the Huangtupo slope is medium to thick-bedded, gray argillaceous

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limestone with a khaki weathered surface. Cleavages with a typical spacing of several centimeters trending E–W with a steep dip angle are intensively developed in the stratum of

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T2b3. The HTPL is covered by loose debris, whose thickness could reach 90 meters at some points. It might result from previous Huangtupo landslides (BGCWRC, 1994; Deng, 2000). This landslide is 1200 m wide, 1600 m long, and about 6×10 7 m3 in volume with elevations

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of 640 m a.s.l. at the top and 80 m a.s.l. at the bottom (BGCWRC, 1994). However, disagreements persist regarding the scope and boundary of this landslide, and on the nature and origin of the surrounding deformation phenomena (Deng, 2000; Tang et al., 2015b).

3.1 PLTGD phenomena Some typical deformation of rock mass can be observed at the east and west sides of the Huangtupo landslide and the front edge of the riverbank. 3.1.1Toppling A deformed zone that is several hundred meters wide along the eastern edge of the HTPL is characterized by toppling of the rock with intense axial cleavage (Fig. 4). The 5

ACCEPTED MANUSCRIPT cleavage surfaces are fissured, and cleavage slices are rotated and toppled downslope. The intensity of toppling gradually decreases inward and downward, while the rotation angle of the rock slices is also gradually reduced. Though intensive toppling deformation occurred in the strata, the bedding of the rock mass was preserved. In addition, a similar phenomenon is observed at the west side of the HTPL (Fig. 5). From the internal to external slope, the toppling rock mass changed from successive layered rock to fractured rock mass, which was similar to landslide debris. Toppled rock slices are

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accompanied by bedding sliding surfaces, and there is a transitional r elation of deformation between the outer and the inner parts of the toppling slope.

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The intensity of the deformation gradually increases from the internal to the external

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slope. The stratification inside the slope was not destroyed, suggesting that the deformation predates the landslide event.

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3.1.2 Fold and fault

Another complicated deformation phenomenon involves gravity-driven folds and faults

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along the bottom of HTPL, near the bank of Yangtze River. On an outcrop scale these folds are overturned in an asymmetric, knee-like shape. The axial surfaces are sub-horizontal and dip into the slope (Fig. 6). There are many openings with different size in these folds,

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especially in the axial parts. Openings are common in the axial parts of folds, which is distinguished with the tectonic folds.

Though the displacement is small, low-angle shear faults are common in this part. The

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shear faults exhibit wavy surfaces with gentle dips to the south, and are mostly accompanied by crush zones and, in some cases, crumple structures. The asymmetry of folds in the shear zones suggests that the overburden blocks move to the north of the slope (Fig. 7). These

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deformation phenomena only occur at the front edge of HTPL, whereas the east and west sides display a normal sequence of rock. The bedding surfaces of the deformed rock mass are relatively continuous and integrated, which indicates that the rock mass might not belong to landslide debris accumulation. In addition, the slip direction is toward the river, in contrast with what would result from interlayer gliding induced by the Guandukou Syncline in this area (Fig. 8). These features indicate that the folds and faults are neither of tectonic origin nor caused by slope catastrophic sliding. Instead, they were likely generated by long-term, downslope creep deformation.

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ACCEPTED MANUSCRIPT 3.2 Correlation between PLTGDs and evolution process of the HTPL Figure 9 shows a scheme of evolution process of the HTPL in terms of PLTGD phenomena detailed above. Along the axes of the Guandukou Syncline (Fig. 9a), the dip-slope of the HTPL emerges and evolves into a gravitationally unstable state. The slope will keep stable to avoid fully sliding until it is deformed by the PLTGD processes of toppling and deep-seated creep (Fig. 9b). The PLTGD gradually reduces the integrity and strength of the

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rock mass, and this reduction process will continue and deteriorate under the role of the river cut. When the integrity and strength of the rock mass are reduced to a certain extent

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that insufficiently maintains the slope stability, large-scale landslides occur, which we call pre-landsides as shown in Fig.9c. Subsequently, the HTPL undergoes long-term dismantling

landslide to adapt the local change of slope gradient.

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4. Huanglashi landslide

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(Fig. 9d). A typical process in this stage is the surficial and partial reactivation of the main

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The Huanglashi landslide body is situated at the south limb of the Guandukou Syncline about 2 km downstream of Huangtupo (Fig. 10). This deep-seated landslide is mainly composed of argillaceous limestone and pelitic siltstone of the Badong Formation, covered

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by loose debris deposits (Xu et al., 1991; Yang et al, 1991; Fu et al, 2007). An important distinction between the HLSL and HTPL is that the slope of the former is consequent but that

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of the latter is obsequent.

4.1 PLTGD phenomenon

Engineering geological investigations including drilling and adit exploration have been

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conducted at HLSL since the 1980s. The adit exploration (Fig. 11) clearly show s that the rock structure of the Huanglashi slope changes gradually from the surface to the underground as follows: deposits containing rock and soil, rock blocks, rock mass segmentalized by faults and fractures, downslope dipping strata modified by toppling, rock mass sparsely segmentalized by faults and fractures due to slight topping, and th e undisturbed bedrock. The adit exploration revealed many details including PLTGD phenomena. In the debris accumulation zone of the HLSL (zoneⅠshown in figure 11), from outside to inside, the volume of rock block increases while the soil component decreases, which implies a decrease in weathering intensity.

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ACCEPTED MANUSCRIPT In the toppling rock mass zone (zoneⅡshown in figure 11), the bedding of the rock mass remains intact, although it has been cut by some faults and fractures. The bedding attitude of rock in this zone changes from dipping toward the slope surface to subhorizontal. The density of faults and fractures decreases from about 1 per meter on the outside, to about 1 per dozens of meters on the inside of the HLSL. Most of the faults are normal with medium dips regardless of orientation. The faults associated breccia or fault gouge zones are only centimeters to tens of centimeters wide, and the lithology and bedding attitude of the both

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sides of faults are similar, indicating that displacement is small. Most of these faults were likely not generated by tectonic processes but instead are due to slope process in

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Huanglashi, because (1) many faults occur within the HLSL area, but few exist outside; and

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(2) the density of faults decreases with increasing depth of the slope.

The density of faults and fractures in the bedrock zone (zone Ⅲ of Fig. 11) is less than that in the toppled rock mass zone. The bedding attitude dips inside, and changes gradually

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from low angle to the original bedding attitude of the bedrock.

Although the debris accumulation in the outer layer of the HLSL was is generally

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considered to be a classical product of slope sliding, there is not enough evidences to identify a master sliding surface running through the whole landslide body being found beca use the structure of accumulation changes little by little from the outer layer to the inner one, till to

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bedrock. Therefore, there is reason to believe that the debris accumulation of HLSL was may be produced by the gravity-driven deformation, which was probably formed by the

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superposition of multiple slides.

4.2 Correlation between PLTGDs and evolution process of the HLSL In the HLSL area, the bedrock consists of alternated pelite and pelitic siltstone that dips

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upslope, toward the north at an angle of ~ 30° (Fig. 12a). The long-term gravitational deformation is a type of toppling controlled by the layers. Under the incision of the Yangtze River, the anti-dip layers had been creeping bended by the force of gravity from the external to the internal, by which a gravity anticline gradually occurred. Some tensional fractures in the core and outside limb of the anticline were gradually formed and eventually evolved into normal faults (Fig. 12b). These fractures and faults would expand the toppling zone into deeper levels, while segmenting the rock mass, and promoting weathering and downslope creep, resulting in a loose debris accumulation in the upper layer of the slope (Fig. 12c). Compared to the inner toppled rock mass, the strength of the loose debris accu mulation is

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ACCEPTED MANUSCRIPT weak, rendering it prone to local sliding under various hydrologic, geomorphological and /or external dynamic forces (Fig. 12d).

5. Baotaping Landslide The Baotaping landslide is located near Fengjie, about 162 km upstream of the Three Gorges dam. The landslide is 700 m wide and 900 m long and is mainly covered by a thick

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debris accumulation with a volume of over 30 million m3 (Fig. 13). The bedrock is similar to the HTPL and HLSL areas, but the BTPL has its own characteristics and unique formation

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mechanism. The outcrops are mostly gently-dipping, gray argillaceous limestone that has undergone some deep-seated slope deformation.

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5.1 PLTGD phenomena

A 5~10 m arc scarp can be seen at Zaozibao of 350-400m a.s.l, which probably

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represents the back edge of the BTPL. This arcuate scarp is also a boundary bet ween bedrock and accumulated debris. A road-cut that crosses this scarp reveals the progression

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of deformation from bedrock to fractured rock mass, as well as the gradual deflection of the bedding attitude (Fig. 14). The dip of the layers is 10~20 °, but the dip direction changes from north-west in stable bedrock to south-west, the sliding direction, in the >30m-wide

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deformed zone, which also includes the fracturing of cleavage surfaces. Cleavages are intensively developed in the bedrock along the Chengjiagou road-cut

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slope, located about 350 m a.s.l. Cleavages are seamless and lined with grayish green, phyllitic films with thicknesses of 0.5~2 mm. The cleavages tend to be fissured and opened near the landslide, especially close to its boundary (Fig. 15). Some openings of the cleavage

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surfaces are several centimeters wide. In addition, layers are bent downward as the slide surface is approached. The width of the deformed zone is more than 60 m. The southeastern boundary of the BTPL is stretched along the Xiongjia-ravine. From stationary bedrock to the boundary, the layered rock mass becomes segmented and fragmented, and the bedding deflects to the parallel direction of the boundary (Fig. 16). The inclination direction of the strata gradually changes from SW to NW. The bedding dip becomes gradually steeper, from 10~20° to 40~60°, as deformation intensifies. In places the deformation zone is 50 m wide. In summary, the main characteristics of the Baotaping landslide are: 1) the deformation is limited to a restricted zone, typically a few tens of meters wide, around the landslide. 2) the bedding surface gradually deflects to become parallel to the landslide boundary. 3) the 9

ACCEPTED MANUSCRIPT tensile cracking along cleavage surfaces and the segmentation of the rock mass increase toward the landslide. The transitional relationship between the inner and the outer deformed zone indicates that this zone resulted from pre-landslide, downslope processes, and was subsequently utilized and modified by the landslide.

5.2 Correlation between PLTGDs and evolution process of the BTPL The Baotaping landslide occurs in gently dipping strata of the Badong Formation. The

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formation of the landslide is closely related to unloading of gravity, toppling, and creeping.

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5.2.1 Unloading-toppling-creeping-sliding model

An unloading-toppling-creeping-sliding model was proposed on the basis of PLTGD

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phenomena that are clearly exposed near the Yufu gas station, about 1 km northwest of the BTPL, and situated at the left side of the exit of the Meixi River, a tributary of the Yangtze

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River (Fig. 17).

The high, steep slope is underlain by nearly horizontal argillaceous limestone of T 2b3, cut by a well-developed set of NW cleavages. The cleavages are widely fissured and opened.

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Toppling and rotation also occur in the segmented rock blocks. Cleavage fissuring and related rock-block toppling extend to depths of several tens of meters, and become more obvious toward the slope surface, where the successive layered rock is changed to fractured

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rock mass. The loose upper slopes are conducive to sliding. The deformation and failure of the slope can be explained as follows. The cleavages

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were fissured and opened due to unloading during the long term slope evolution , and then the segmentation, disintegration and rock-block rotation took place in the rock mass from the external to the internal slope because of toppling. This long-term gravitational

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deformation described as a type of mass rock creep (Deng et al. 2000), leaded to the gradually deterioration of the rock mass. Landslide movement occurred after the rock strength was sufficiently reduced by this process. In summary, the deformation and failure of the slope appear to be part of a long process that includes unloading, toppling, creeping, and sliding. The process might represent the major formation model of the slope failure in hard rock regions with horizontal bedding. In this model, a set of structural planes that dip steeply into the slope play an important role in unloading and toppling.

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ACCEPTED MANUSCRIPT 5.2.2 Formation process of the BTPL Inspired by the deformation and failure model of the Yufu gas station slope, a four-step development scheme is presented to summarize the evolution process of the Baotaping landslide (Fig. 18). During Stage A, the T2b3 was exposed to the bank slope due to the incision of Yangtze River. Though the bedding attitude of the Baotaping slope is gentle, a set of cleavages with

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high angle is intensively developed, which were formed by tectonics before the slope occurrence. The strike of the cleavages is nearly parallel to the slope direction. These

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cleavages were fissured and opened under slope unloading, and cleavage slices toppled towards down slope due to gravity (Fig. 18a). The action would penetrate downward over a

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long interval, until a thick and loose rock mass is formed. This process significantly reduces the quality and strength of the rock mass, forming a potentially unstable slope (as shown in

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Fig. 19).

In Stage B, if the quality and strength of the rock mass becomes sufficiently reduced by river downcutting, the unstable slope begins to slide (Fig. 18b), perhaps when triggered by

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rain storm, a flood or earthquake. A new scarp, which would be propitious to the formation and development of a new unloading zone, would form after the primary landslide. A new cycle of unloading and sliding begins.

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In Stage C, the stability of the primary landslide is gradually reduced by continued river incision. The newly developed, unstable rock mass would slide along with accumulated

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debris (figure 18c).

In Stage D, a new cycle of unloading and deformation would begin at the steep back scarp of the landslide (Fig. 18d). In member T 2b4 of the Badong Formation, evidence for

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three elevations was found in the BTPL. This indicates that at least three-stages of sliding occurred in the BTPL. In each landslide, the basement rocks might not have slid along the bottom of the unloading deformation zone. Therefore, more or less deformed rock material produced by unloading may still be left in the bedrock beneath the sliding zone and the periphery of the landslide (Fig. 19).

6. Summaries and conclusions Remnants of relevant processes remain after large landslides occur. We show how precedent, long-term gravitational deformations (PLTGDs) affected the formation and evolution of three huge landslides (HTPL, HLSL and BTPL) in the Three Gorges reservoir region. The difference and similarity of the PLTGD phenomena in these three cases are 11

ACCEPTED MANUSCRIPT summarized in table 2. We can see that there are three main similarities: being of large scale (feature 1), all of them developed in the Badong formation (feature 2) and, the existence of deformation transition zone between the main sliding surface and intact bedrock (feature 4). Meanwhile, these three landslides belong to different slope types (feature 3), and their deformations are controlled by different structure planes (feature 5), which leads to different deformation models (feature 6). In particular, we show that PLTGD features are produced regardless of bedding attitude,

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and for different styles and behaviors. The HTPL occurred on a consequent slope, starting from toppling along cleavages at a shallow slope, followed by shearing creep at deeper

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levels. The HLSL occurred at an obsequent slope starting from toppling of rock layer, with

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which fractures, small faults and gravity anticline were developed. The BTPL occurred in hard rock with a gentle dip, and the toppling of cleavages caused by unloading probably played an important role in the early stage of the landslide process. It is worth noting that

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the phenomena of PLTGD is found not only in these landslides, but also in other large scale landslides in the Three Gorges reservoir region, such as the Wushan new county landslide

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(Ke and Guo, 2007), Xicheng landslide in Yuyang county (Fang et al., 2008). The recognition of the PLTGD features is the key to understanding and reconstructing landslide mechanisms. Three criteria are summarized for effectively distinguishing PLTGDs:

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(1) the existence of relatively continuous and integrated beddings in the rock mass. Compared to the landsliding product (loose debris accumulation) in which bedding can not

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be kept after long distance sliding and moving, the product of PLTGD usually has relatively continuous and integrated beddings; (2) the existence of deformation zones that encompass the structural transition from bedrock to accumulated debris ubiquitously occurring in the

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surrounding rock mass or in the basement rock of these large landslides. These zones were not the product of tectonic movement, nor the product of slope sliding, but instead are PLTGD relicts that deteriorated the strength of the rock mass prior landsliding; (3) the different scope and features of PLTGD comparing with the deformation caused by tectonic movement. The scope of PLTGD is generally limited to the landslide area and its surrounding area. The PLTGD shows brittle deformation features, and its kinematic information always point to downward of slope. We conclude that a huge landslide is usually not formed by once time sliding, but may have undergone precedent long-term slope deformation. This PLTGD process can weaken the rock mass, and ultimately result in catastrophic sliding. Thus, the PLTGD process can be considered as an early, recognizable stage that may be a prerequisite to a secondary stage of 12

ACCEPTED MANUSCRIPT catastrophic failure. This geologic recognition should assist in the delineation of landslide susceptible areas.

Acknowledgment This research was financially supported by the Major State Basic Research Development Program of China (No. 2011CB710601) and the National High Technology Research and

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Development Program (No. 2012AA121303). The Project was also supported by the Fundamental Research Funds for the Central Universities (No. CUG160701), China University

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of Geosciences (Wuhan). The authors wish to thank Professor R Criss for his earnest checking and revising for the manuscript.

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Reservoir region and their prevention and control. Resources Environmental Engineering 21(3): 265-268.

Dramis, F., Sorriso-Valvo, M., 1994. Deep-seated gravitational slope deformations, related

MA

landslides and tectonics. Engineering Geology 38, 231–243. Esposito, C., Martino, S., Scarascia-Mugnoza, G., 2007. Mountain slope deformations along thrust fronts in jointed limestone: an equivalent continuum modelling approach. Geomorphology

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90, 55–72.

Fang, Y., Zhang, J., Yang, J., Deng, Q., 2008. The research of prophase slope creeping motion of

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Xicheng landslide in Yunyang County of Three Gorges project area. The Chinese Journal of Geological Hazard and Control19(1), 32-35 Feng, L.B., 2009. Study on quantitative relation between load-unload response ratio and stability

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factor & its optimized draining control of Huanglashi landslide. Master thesis, Qingdao Technological University. Fu, Y.P., Pan, W., Zhao, X., 2007. Engineering geological characteristics, formation mechanism and forecast of deformation of slack bedrock zone under Huanglashi landslide. Journal of Geological Hazards and Environment Preservation 18, 61-64. Galadini, F., 2006. Quaternary tectonics and large-scale gravitational deformations with evidence of rock-slide displacements in the Central Apennines (central Italy). Geomorphology 82, 201–228. Gori, S., Falcucci, E., Dramis, F., Galadini, F., Galli, P., Giaccio, B., Giaccio, P., Messina, A., Pizzi, A., Sposato, D., Cosentino, 2014. Deep-seated gravitational slope deformation, large-scale rock failure, and active normal faulting along mt. Morrone (Sulmona basin, 14

ACCEPTED MANUSCRIPT central italy): geomorphological and paleoseismological analyses. Geomorphology 208, 88– 101. He, M.C., Cui, Z.Q., Chen, H.H., Lu,C., Wu, X.,1998. Study on tectonic deformation field for Wushan Paleo-landslide system

in the Three Gorges Reservoir area.

Journal of

Engineering Geology 6(2): 97-102 Ke, Y. Y., Guo, F., 2007. Research on ancient landslide of deep bedrock in Wushan County of the

PT

Three Gorges Reservoir region. Industrial Construction 37 (Supplement): 898-902. Li, H.Z., Deng, Q.L., Wen, B.P., Zhang, G.C., 2015. Study of formation mechanism of Baotaping landslide in Three Gorges Reservoir region. Yangtze River 46(9): 46-50.

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Li, H.L., Yi, S.H., Deng, Q.L., 2006. Development characteristics and their spatial variations of

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Badong Formation in the Three Gorges Reservoir region. Journal of Engineering Geology 14(05): 577-581.

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Liu, C.Z., 2005. On the evolution of complex slopes in the Three Gorges of Changjiang River. Hydrogeology & Engineering Geology 32(1):1-6.

Longoni, L., Papini, M., Brambilla, D., Arosio, D., Zanzi, L., 2016. The role of the spatial scale

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and data accuracy on deep-seated gravitational slope deformation modeling: the Ronco landslide, Italy. Geomorphology 253, 74-82.

McCalpin, J., 2009. Paleoseismology, 2nd edition. Academic Press, Amsterdam 848.

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Moro, M., Saroli, M., Salvi, S., Stramondo, S., Doumaz, F. 2007. The relationship between seismic deformation and deep-seated gravitational movements during the 1997 Umbria–

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Marche (central Italy) earthquakes. Geomorphology 89, 297–307. Radbruch-Hall, D., 1978. Gravitational creep of rock masses on slopes. In: Voight, B. (Ed.), Rockslides and Avalanches; Natural Phenomena. Elsevier, Amsterdam 607–657.

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Sacchini, A., Faccini, F., Ferraris, F., Firpo, M., Angelini, S., 2016. Large-scale landslide and deep-seated gravitational slope deformation of the Upper Scrivia Valley (Northern Apennine, Italy). Journal of Maps 12, 344-358. Tang, H.M., Li, C.D., Hu, X.L., Wang, L.Q., Criss, R., Su, A., Wu, Y.P., Xiong, C.R., 2014. Deformation response of the Huangtupo landslide to rainfall and the changing levels of the Three Gorges reservoir. Bulletin of Engineering Geology & the Environment 74, 1-10. Tang, H.M., Li, C.D., Hu, X.L., Su A.J., Wang L.Q., Wu Y.P., Criss, R., Xiong, C.R., Li, Y.A., 2015 a. Evolution characteristics of the Huangtupo landslide based on in situ tunneling and monitoring. Landslides 12, 1-11.

15

ACCEPTED MANUSCRIPT Tang, H.M., Liu X., Hu X.L., Griffiths, D.V., 2015 b. Evaluation of landslide mechanisms characterized by high-speed mass ejection and long-run-out based on events following the Wenchuan earthquake. Engineering Geology 194, 12-24. Ter-Stepanian, G., 1966. Types of depth creep of slopes in rock masses. Rock Mechanics 2, 71–79 Ter-Stepanian, G., 1977. Deep-reaching gravitational deformation of mountain s lopes. Gen. rep. Sec., 2, IAEG Symposium of Prague, 1977, Bulletin of the International Association.

PT

Engineering Geology 16, 87–94. Tsou, C.Y., Chigira, M., Matsushi, Y., Chen, S.C., 2015. Deep-seated gravitational deformation of mountain slopes caused by river incision in the Central Range, Taiwan: Spatial distribution

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and geological characteristics. Engineering Geology 196, 126-138.

SC

Varnes, D.J., Radbruch Hall, D., Savage, W.Z., 1989. Topographic and structural conditions in areas of gravitational spreading of ridges in the Western United States. US Geological

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Survey, Professional Papers 1496 1–28.

Wang, Z.H., 2007. Landslide distribution and development in the towns of the Three Gorges Reservoir area. The Chinese Journal of Geotechnical Hazard and Control 2007, 18(01):

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33-38.

Xu, F.X., Cai Y.J., Tian C.J., Yang T.M., 1991. The study on crooked and slack bedrock zone under Huanglashi lands lide. The Chinese Journal of Geological Hazard and Control 2(3):

ED

54-62.

Yang, T.M., Yan Y.Z., Sun Y.Z., Xu F.X., 1991. Analysis of stability and deformation

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characteristic of Huanglashi lands lide. The Chinese Journal of Geological Hazard and Control 2(3), 31-41.

Yin, Y.P., Hu, R.L., 2004. Engineering geological characteristics of purplish-red mudstone of

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Middle Tertiary Formation at the Three Gorges Reservoir. Journal of Engineering Geology, 12(02):124-135.

Yin, Y.P., Zhang J.G., Chen B.S., Kang H.D., 2000. Formation mechanism of large-scale loose sediment at the relocation sites of Wushan County on the Three Gorges. Journal of Engineering Geology 8(03):265-271. Zhang J.G., 2000. Features and occurrence mechanism of the large complex lands lides in Baotaping, Fengjie county in the Three Gorges reservoir region. Geological Comment 46, 431-436. Zhang, J.G., 2001. Features and Genesis of Large Complex Landslide Developed from 3rd Member of Badong Formation in the Newly Built Wushan County Seat, Three Gorges Reservoir Region. Acta Geoscientica Sinica 22(2): 145-148. 16

ACCEPTED MANUSCRIPT Zischinsky, U., 1966. On the deformation of high slopes. Proc. 1st Congr. Int. Soc. Rock

AC C

EP T

ED

MA

NU

SC

RI

PT

Mechanics 2, 179–185.

17

ACCEPTED MANUSCRIPT

N

HLSL

Beijing

BTPL

HTPL

40

80km

PT

0

AC C

EP T

ED

MA

NU

SC

RI

Fig.1. Map showing large landslides (volume> 5×10 6 m3 ) in the Three Gorges reservoir area

18

ACCEPTED MANUSCRIPT Fig.6a

15°

gtz e

100

Fig.5

25 0

Ya n

150

20 0

Ri ve r

15 0 10 0

Ravine 3

Ravine 4

Huangtupo la ndslide

Fig.6b 200

250 300

Fig.7

I PLTGD

II

38°

T2b3

Qdel(T2b2)

T2b2

550

47°

RI

T2b2

PT

450

N

600

200

0

debris originated from T2b2

Qdel(T2b3)

boundary of landslide debris accumulations

I

boundary of PLTGD

debris originated from T2b3 terrace

400m

SC

650 Qdel(T2b2)

Fig.4

Ravin e1

Ravine 2

Qdel(T2b3)

500

30 0

100

elevation contours (m)

NU

landslide boundary (①the main landslide of the Huangtupo; ②Erdaogou landslide;③Sandaogou landslide)

MA

Fig.2. Engineering geological map of the Huangtupo landslide, showing locations discussed in the

AC C

EP T

ED

text

19

ACCEPTED MANUSCRIPT HLSL

BTPL

Fengjie

J

N

er Yangtze Riv

T2b 4 Gezhouba Dam Three Gorge Dam 4

2

32°

Guandukou Syncline

Badong

T1 j

Yangtz e River

Jurassic

f ault

Middle Triassic Badong Formation Member IV

anticline

Yichang

Badong HTPL

T2b 1

A’ T2b T2b3 46° T2b 2 A

3

T2

b

T2

HTPL

J

b

T2b 4 T2b 3

T 2b 3

Middle Triassic Badong Formation Member Ⅲ

T 2b 2

Middle Triassic Badong Formation Member II

T2b 1

Middle Triassic Badong Formation Member I

T1 j

Lower Triassic Jialingjiang Formation

HLSL

syncline

46°

bedding attitude

boundary between Member of the Badong Formation

T2b 2

landslide boundary sinistral strike-slip f aults

4 km

A

（a） N

T2b1

1000

A'

500

Guandukou Syncline

T1j

500 fault

250

（m）

750

750

EW/40N

Yangtze River

T2b2

250 T2b3 bedding

0

bedding N70E/22SE

N70W/35NE

1000(m)

0

SC

500

PT

（m）

2

RI

0

（b）

NU

Fig.3. Sketch map showing (a) schematic geological map of Badong (b) A-A’ profile map of

AC C

EP T

ED

MA

Guandukou Syncline

20

ACCEPTED MANUSCRIPT N road

T2b3

T2b3

fault N78W/52NE

bedding N78W/54NE

f ault Third Member of Badong Formation( T2b3) 5

10 m

PT

0

RI

Fig.4. Cross-section showing toppling deformation in the gray argillaceous limestone (T 2 b3 ) near

AC C

EP T

ED

MA

NU

SC

the east edge of the Huangtupo landslide. See Fig. 2 for area of detail.

21

ACCEPTED MANUSCRIPT

N

fault

fault

fault

N88W/30N

N88W/40N

bedding bedding N88W/48N N82W/32N

N85W/40N

PT

bedding cleavage N80W/45N N20W/30NE

RI

Third Member of Badong Formation( T2b3)

40 m

NU

20

EP T

ED

MA

Fig.5. Toppling deformation at the west side of HTPL. See Fig. 2 for area of detail.

AC C

0

SC

f ault

22

road

ACCEPTED MANUSCRIPT

Third Member of Badong Formation( T2b3) fault

N Qdel

PT

fault N88W/30S

1m

RI

0

Yangtze River

SC

a Third Member of Badong Formation( T2b3)

NU

f ault

MA

NW

10

20 m

EP T

0

ED

Qdel

fault

fault bedding N75W/55SW N35E/12NW

N75W/45SW

bedding N55E/12NW

b

AC C

Fig. 6. Sliding zones and intensive deformation at the front edge of the Huangtupo landslide. See Fig. 2 for areas of detail.

23

ACCEPTED MANUSCRIPT bedding N80W/10S

bedding N60E/23NW

N30W

fault N75E/10SE

bedding N40E/17SE

fault N75E/28SE

fault N75E/24SE

Third Member of Badong Formation( T2b3) f ault 1.5

3（m）

PT

0

RI

Fig.7. Low-angle shear faults and accompanied asymmetric folds at the front edge of the HTPL.

AC C

EP T

ED

MA

NU

SC

See Fig. 2 for area of detail. Movement directions of the overburden blocks are to the north.

24

ACCEPTED MANUSCRIPT

N Guandukou syncline low-angle Yangtze River shear fault

T2b 2

(a)

axial surface of asymmetric folds

SC

RI

syncline

PT

T2b 3

(b)

T2b 3

NU

member of Badong Formation

member of Badong Formation

shear/ displacement direction

MA

T2b 2

axial surface of interlayer folds

Fig. 8 Sketch map showing (a) the deformation style of the Huangtupo slope, and (b) the possible

ED

deformation style induced by the interlayer gliding due to the folding of the Guandukou syncline.

AC C

EP T

The square in figure (b) is the corresponding position to the slope toe of the Huangtupo mass.

25

ACCEPTED MANUSCRIPT N a

Yangtze River

c Yangtze River

d Yangtze River

Yangtze River

pelitic limestone (T2b3) with intense cleavage

debris from T2b3

debris originated from T2b2

pelite alternating with pelitic siltstone (T2b2)

deep-seated creep zone

sliding surface

toppled pelitic limestone

PT

b

RI

Fig. 9. Evolution process of the HTPL in terms of PLTGDs. (a) Pre-deformation stage of the

AC C

EP T

ED

MA

NU

SC

Huangtupo slope; (b) PLTGDs; (c) preliminary landslide; (d ) secondary landslide.

26

ACCEPTED MANUSCRIPT N

HLSL

Q del

Q col+dl Gezhouba Dam Three Gorge Dam

J

T2b4

T2b4

Q

landslide deposits

active landslide boundary

800

elevation contours (m)

del

Q col+dl

Jurassic

Middle Triassic Badong Formation Member IV

T2b

T2b2

Middle Triassic Badong Formation Member II

Taizijiao

T2b3 400

Hengping

28°

attitude

300

22° 2

Q del

stratigraphic boundary

1

200

31° 1 T2b 32°

4

2 Q del

adit exploration

Huanglashi

3

SC

Yangtz colluvial slope deposits

T2b2

Shiliubaoshu

Q del

100

Qcol+dl

4

T2 b 500

26°

31° bedding

4

25°

RI

Middle Triassic Badong Formation Member Ⅲ

600

3

T2 b

T2b3

700

J

5

4

investigation boundry

800

Q col+dl

20°

Zhaojiaoshu T2b

28°

Zhoujiaw an

landslide boundary

Son gjia wu ch

Q del

Dashiban

ver Yangtze Ri

ang

31°

Yichang

Badong HTPL

PT

BTPL

Fengjie

e River

0

100

1

T2b

200 m

NU

Fig.10. Schematic geological map of Huanglashi landslide (slight modification according to

AC C

EP T

ED

MA

BGCWRC, 1991)

27

ACCEPTED MANUSCRIPT

T2b3

T2b4 fault

N87E/20N

F77

F103 F80

F57 F55

F46 F37 F32

F54

Lc

S

PD1(338.68) F73 F70 F58

fault

F96 fault

fault

Ⅲ

fault EW/22S

EW/32S

N85W/60N N84E/75N

250

F47

200

150

100

F52 F48

T2b3

F39

50

II

F21 F12

F23

150

EW/20S 50

II

F8 F7

F9

PD2(244.83) sliding surface

EW/28S N78E/85S 100

Ⅲ

S

PD3(121.38)

EW/72N 100

0

50

I

II

F17

T2b3 F28 F25

F5

T2b2

40 m

150

100

Ⅲ

F24 F21 F19 F18 F14

fault 150

Middle Triassic Badong Formation Member IV

T2b3

tension fracture

I

zone of debris accumulation

PD4(216.30)

II

limestone

I

II

debris F

0

PD5(437.61)

Qdel

50

100

0

I

Qdel landslide deposits

normal fault

S

sliding surface

Ⅲ zone of bed rock

II zone of toppling

serial number of adit PD F31 fault and its serial number (216.30) exploration and its elevation

EW/20N

S

Middle Triassic Badong Middle Triassic Badong T2b2 Formation Member II Formation Member Ⅲ pelite and siltstone

pelitic limestone Lc

F11 F2

sliding surface fault T2b4N80E/40N N85W/25S

N40E/77NW

T2b4

50

PT

10 20 30

Qdel

RI

N80E/22S

F27 F26 0

S

F2 fault

fault 200

0

I

Qdel

fault

fault N85W/35S

F31

0

Q

S

fault

fault

200

I del

SC

4

AC C

EP T

ED

MA

NU

Fig. 11. Geological profile of adit exploration of the HLSL (modified according to Xu et al., 1991)

28

ACCEPTED MANUSCRIPT

Yangtze River

J T3

T2 b4

T2 b3 T2 b

2

T2b1

J

T3 T2 b4 T2 b3 T2 b2 T2 b1 (b)

PT

(a)

Yangtze River

T3 T2 b4

T2 b3

T2 b3 2

T2 b2 T2 b1

T2 b T2 b1 (c) J

RI

J

T3 T2 b4

SC

J

Yangtze River

Late Triassic

T3

T2b 3

Middle Triassic Badong Formation Member Ⅲ

T2b 2 Formation Member II

Qdel

debris accumulation

NU

Jurassic

Middle Triassic Badong

T2b 4 T2b1

Middle Triassic Badong Formation Member I tensional fractures

MA

sliding surface

(d)

Middle Triassic Badong Formation Member IV

Fig. 12 Evolution process of the HLSL in terms of PLTGDs. (a) pre-deformation stage of the

ED

Huanglashi slope; (b) toppling along with the formation of anticline, fractures and faults; (c)

EP T

toppling spreading to the deeper parts, and formation of debris accumulation in the upper part of

AC C

the slope; (d) modification by local sliding in the debris accumulation.

29

ACCEPTED MANUSCRIPT 10°

T 2b 3

15°

200

10°

150 C

11°

T2b 4

23°

T 2b 3

24°

T 2b 3

Zaozibao Q del

14° 20°

vine a Ra henji

7°

10°

Fig.14

27°

4

48°

21°

Q del

250

43°

45° 28° 30°

350

20°

Q del

300

T 2b ide dsl lan ng i p ta Bao

100

19°

Fig.15

5°

250

Qcol

Q edl

T 2b 4

300

T2b3

28°

T 2b 4

14°

Fig.16

g ji on Xi

300

Q edl 250

Q edl

Yaokui Tower 100

N

20 0

PT

Q al

ine av aR

T2 b 3

0

100

RI

150

Yangtze River

T2b 2

200 m

SC

100

T 2b 2 Middle Triassic Badong Formation Member II

T 2b 3 Middle Triassic Badong Formation Member Ⅲ

T2b 4 Middle Triassic Badong Formation Member IV

Q del landslide deposits

Q col rock fall deposits

Q edl

Fig. 14

bedding attitude

landslide boundary

observation location and corresponding figure in the text

ED

MA

Fig.13. Geological map of the Baotaping landslide.

EP T

road

7°

NU

alluvial deposit

AC C

Q al

talus material and residual deposi t

30

ACCEPTED MANUSCRIPT NE

sliding surface

bedding bedding N40W/65SW SN/10W

N45W/80SW

bedding N50E/29NW

landslide surface pelitic limestone with cleavage (T2b3) debris accumulation

2.5

5m

RI

0

PT

modified rockmass keeping laminar appearance

SC

Fig.14. Sketch showing the transition from bedrock to debris accumulation near Zaozibao. See

AC C

EP T

ED

MA

NU

Fig. 13 for location.

31

ACCEPTED MANUSCRIPT S

bedding N60E/10SE

fault N70E/65NW

cleavage N55E/66NW

pelitic limestone with cleavage (T2b3)

bedding N86W/21S

Qdel sliding surface EW/26S

debris accumulation modified rockmass keeping laminar appearance landslide surface

25

PT

fault 0

50 m

RI

Fig. 15. Sketch showing the transition from bedrock to deformed, accumulated debris at the

AC C

EP T

ED

MA

NU

SC

Chengjiagou Road cut. Fig. 13 shows location.

32

ACCEPTED MANUSCRIPT N80W

Baotaping landslide

0

10

bedding

20m

N20W/17SW

pelitic limestone

bedding N50W/33SW

debris accumulation

bedding

fault

N25E/43NW

SN/50W

landslide boundary

f ault

PT

Fig. 16. Sketch of the road cut in the north of the Yaokui Tower, showing the gradual change of

AC C

EP T

ED

MA

NU

SC

RI

bedding attitude from bedrock to fractured rock mass. Area of detail in Fig. 13.

33

10 0 m 10 m

N

Qdel Qdel Qdel bedding cleavage bedding beddingbedding cleavagebedding N75E/4SEcleavage N65W/75NE bedding N63W/19SW N75E/4SE N75E/4SE N65W/75NE N65W/75NE N63W/19NW N63W/19NW Middle Triassic Badong del sliding surface landslide deposits T 2b 3 Q Middle Formation TriassicTriassic Badong Badong Member ⅢQdel landslide surfacesurface deposits deposits slidingsliding T2b 3 Middle Qdel landslide 0

10 m

Formation SectionSection Ⅲ Formation Ⅲ

MA

T2b 3

N

NU

T2b 3

T2b 3 T2b 3

Fig.17 Distance view (a) and its sketch (b) of the Yufu gas station slope, showing rock mass

EP T

ED

structure experienced unloading and sliding.

AC C

Baotaping landslide

S35W S35W

SC

0

RI

PT

ACCEPTED MANUSCRIPT

34

ACCEPTED MANUSCRIPT

a

T2b4 Yangtze River

T2b 2 Middle Triassic Badong Formation Member II

T2 b 3 T2 b 2 T2b4

T2b 3 Middle Triassic Badong Formation Member Ⅲ

T2 b 3

T2b4

Rb4

b Yangtze River

Q del

Middle Triassic Badong Formation Member IV

T2b 2 4 Rb 4 rock debris from T2b

T2b4

c

T 2b 3

Q del

Qdel

T2 b 2

PT

Rb4

Yangtze River

landslide deposits

Rb4 Yangtze River

Q del

potential sliding surface

Rb4

T 2b 3

SC

d

RI

sliding surface

T2b4

cleavage

NU

T2 b 2

Fig. 18. Evolutionary sketch of the Baotaping landslide. (a) river downcutting followed by unloading,

MA

cleavage opening, and toppling; (b) preliminary sliding followed by unloading-cleavage openingtoppling process in the rear of the landslide; (c) secondary circle of sliding and long-term slope

AC C

EP T

ED

deformation; (d) the slope appearance at the present time, after multiple stages of development

35

ACCEPTED MANUSCRIPT

a

T2 b4

T2 b3

T2b2

T2b 2 Middle Triassic Badong Formation Member II

T2b 3 Middle Triassic Badong Formation Member Ⅲ

Middle Triassic Badong Formation Member IV cleavage

transition zone

RI

highly deformed area

T2b4

PT

a

AC C

EP T

ED

MA

NU

SC

Fig.19. Sketch map for the special pattern of the pre-landslide deformation of Baotaping slope

36

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

PT

Table 1 Physical and mechanical parameters of typical rocks of Badong formation Density Saturated compressive Shear (g/cm3 ) water strength (MPa) Softening Deformatio strength Voidag coefficien n Lithology Saturate e Saturate Dry absorptio Dry t modulus C(MPa φ(° d n(%) d (ρd ) n (Cd ) ηc E(MPa) ) ) (ρsat ) (Csat ) Wp(%) T 2 b4 Purple-red 2.5 2.55 1.11 2.80 26.5 11.4 0.43 3980 5.3 46 Argillaceou 2 s siltstone T 2 b3 Green-gray 2.7 123. 2.72 0.02 0.05 76.5 0.62 58324 8.9 64 Argillaceou 2 2 s limestone 2 T2b Purple-red 2.5 2.59 0.75 1.93 45.5 33.3 0.73 10.696 4.2 61 Argillaceou 7 s siltstone T 2 b1 2.3 Thin layer 2.50 2.00 4.72 33.3 28.3 0.85 6130 3.9 58 6 marl

37

ACCEPTED MANUSCRIPT Table 2 Summary of the PLTGD phenomena of the three large-scale landslides Features

HTPL

HLSL

BTPL

69

4.0

30

Landslide scale 1

（million m3 ） Strata of

2 landslide body Slope type

2

3

4

3

T 2 b 、T 2 b 、T 2 b

T2b

Consequent

Obsequent

Horizontal

PT

3

3

T2b

In the trailing

RI

edge of the

landslide, rock

SC

Toppling of the rock with intense axial cleavage is

developed along the

of deformed band

deformed zone is about 40m. The

deformed zone along the edge of relationship BTPL is found,

intensity of toppling gradually decreases

between whose rock deformation zone attitude is and bedrock is

from the external

gradually showed by

debris to internal

deflected toward

bedrock. Gentle-dip

AC C

meters wide

gradual transition

EP T

4

retained. Tens of

exploration, and a

ED

characteristics

deformation is

found by adit

whose width of Location and

and dumping

deformed zone is

MA

edge of HTPL,

cleavage cracking

A 40-120m wide

NU

intensively

mass with

deformation the boundary of features in terms of

faults and

the landslide. rock integrity and

asymmetric folds

There is a gradual dip.

which are gravity

transition

driven exist in the

relationship

front zone of HTPL.

between deformation zone and bedrock.

5

Control

Bedding plane and

Bedding plane

38

Cleavage plane

ACCEPTED MANUSCRIPT structure plane

cleavage plane

of deformation Gravity-driven toppling Gravity-driven deformation controlled by

model

cleavage in the

6

toppling

Unloadingtoppl

deformation

ingcreepingsl

controlled by

iding

shallow layer; bedding plane

AC C

EP T

ED

MA

NU

SC

RI

deep-seated creep

PT

Deformation

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ACCEPTED MANUSCRIPT Highlights 

Precedent long-term gravitational slope deformation (PLTGD) has much bearing on landslide evolution

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PLTGD features were investigated by three large landslides in the Three Gorges reservoir area, China. Criteria for recognition of the PLTGD features of large scale landslides are summarized.



The correlation between the PLTGDs and landslide evolution were investigated.

AC C

EP T

ED

MA

NU

SC

RI

PT



40