43 / 2021-11-14 16:32:52

Characteristics and mechanism of sand-mudstone interbedded bank slope degradation in three Gorges Reservoir area, Hubei Province

Deterioration; Interbedded rock mass; Clastic rock; reservoir bank; Deterioration process; Mechanism

   Since the operation of the Three Gorges Reservoir in Hubei Province, the annual periodic water storage and withdrawal process resulted complex influence in the geological environment. The clastic interbedded bank slopes in the Three Gorges reservoir area of Hubei province are widely distributed and are greatly affected by the change of water level. The typical clastic interbedded slope instability cases, such as Shanshugou landslide and Kamenziwan landslide, showed that the interbedded bank slope was easy to be unstable, and has a great impact on the property security and the economic development of the reservoir area.

    The investigation of clastic bank slope in the Three Gorges reservoir area of Hubei province shows that sand-mudstone interbedded rock mass is the most widely distributed lithologic assemblage and the most unfavorable to bank slope stability. The deterioration of the slope under this lithologic combination can be divided into fissure cutting-spallation and block splitting-erosion. The former is characterized by massive sandstone cut along fractures and mudstone fragmentation and disintegration, while the latter is characterized by protruding sandstone remnant and concave mudstone. The results of spot load test and rock rebound test prove that compared with rock mass in the area above 175 m, the influence of reservoir water level variation on rock mass is greater, especially on mudstone rather than sandstone. The bank slope structure of sand-mudstone interbedded rock mass is an important external factor leading to the difference of bank slope deterioration. The weak zone is easy to form in the unfavorable bank slope structure (downward slope or oblique slope), which makes the condition of shear slip. When the slope toe deformation reaches the critical condition, it is easy to induce the bank slope instability. However, sand-mudstone interbedded rock mass in favorable bank slope structure (reverse or gently inclined transverse slope) usually causes small-scale bank collapse deformation or slow bank reconstruction. The results of laboratory simulation test also show that long-term soaking tends to affect the clay minerals of sand-bearing mudstone, destroying the cluster effect of skeleton particles and clay minerals and leading to disintegration, which is accelerated by the dry-wet cycle process. The influence of the two experimental processes (long-term soaking and dry-wet cycle process) on sandstone is only to enhance the connectivity of sandstone skeleton grains, resulting in the increase of porosity and the expansion of local micro-cracks, which is the fundamental internal cause of the differential deterioration of sand-mudstone interbedded bank slope.

    The characteristics and mechanism of sand-mudstone interbedded bank slope degradation in three Gorges Reservoir area, Hubei Province are revealed in this paper. It provides support for the reasonable evaluation of bank slope instability and deformation monitoring, as well as the early warning and prevention for sand-mudstone interbedded bank slope in the later stage. It’s also of great reference significance for the research on the interbedded clastic rock mass in other reservoir area.