Partial reactivation of a huge deep-seated ancient rock slide: recognition, formation mechanism, and stability

Abstract

Abstract. 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.

Shear Resistance Variations in Experimentally Sheared Mudstone Granules: A Possible Shear‐Thinning and Thixotropic Mechanism

“…The absence of the phenomenon in the sand‐sized granules and, conversely, the occurrence of some slip‐weakening behavior—in the investigated displacement rate range and test duration—could be explained by the smaller pore size, which did not allow for sufficient mobility of the viscous mud but allowed the same mud to be sheared over long distances and time, making its thixotropic behavior apparent. Similarities in structure and composition between the experimentally obtained shear zones and those of landslides moving along bedding planes and within weak layers of saturated and weathered mudstone (e.g., Tang et al, ; Zhang et al, ), and the fact that such landslides can suffer dramatic acceleration due to a possible rate‐weakening mechanism (Q. Xu et al, ), suggest that the observed behavior might play a role in the kinematics of some landslides in mudstone layers.…”

“…The mudstone granules were extensively crushed and damaged (Figures S3, S8, and S9 in the supporting information), similarly to what can happen in nonplastic granular soils (e.g., Fukuoka et al, ; Xu et al, ). Furthermore, as the mudstone contains high proportions of clay (Miao et al, ; G. Xu et al, ; Xu et al, ; Tang et al, ), a soft, high‐water content mud was formed in the shear zone during the tests in saturated condition, and filled the pores between the granules (Figure S4b).…”

“…Furthermore, as the mudstone contains high proportions of clay (Miao et al, 2014;G. Xu et al, 2015;Xu et al, 2016;Tang et al, 2016), a soft, high-water content mud was formed in the shear zone during the tests in saturated condition, and filled the pores between the granules ( Figure S4b).…”

“…In this study, we carried out ring shear tests at various strain rates on shear zone material collected in an exploratory tunnel across the upper part of the deposition area of the Sanmashan landslide, a large landslide in a mudstone formation in the Three Gorges area (N31°01′00″, E109°28′30″, new Fengjie Town, Chongqing, China) (Tang et al, ). In this type of landslide, common in the Sichuan basin and in the Three Gorges area, failure occurs within a weak mudstone layer, weathered cracked, crushed, and softened by the actions of shear strains and pore water infiltrating through a well‐developed system of vertical fractures (Xu et al, ).…”

Centrifuge Modeling and the Analysis of Ancient Landslides Subjected to Reservoir Water Level Fluctuation

“…The Three Gorges Reservoir Area (TGRA) has been a landslide-prone area since ancient times due to geological environment and precipitation. In addition, a large number of landslides reactivated after the impoundment of the Three Gorges Reservoir, such as the Qiangjiangping landslide, Shuping landslide and Baishuihe landslide [9][10][11][12]. These landslides are characterized by large areas and damage to property, such as causing cracks in roads and houses.…”