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.
Highlights
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)
We especially investigated the four normal faults in the field, F8, F20, F19, and F26, which form the local fault graben (BIS-CWRC, 1997, 1999)
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
Summary
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). The identification and formation mechanism of deepseated 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 Tang et al.: Partial reactivation of a huge deep-seated ancient rock slide
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