The Evolution Properties Of Deep-Incised Valley Side-Slope And Slope Stability Analysis

In order to investigate the influence on the planed Sichuan-Tibet railway of exogenetic processes of deeper river cutting with high geostress and violent weathering of rock mass, numerical simulations of the deformation and stress field characteristics of deep-incised valley under high geostress were implemented by FEM softerware ABAQUS. Field Variable method was considered to change mechanical parameters of rock mass corresponding to the influence of rebound deformation. Furthermore, deeper river cutting under special different geostress boundary conditions in each side(attributed to Indian plate extrude to Eurasian plate) was simulated. Moreover, took Layue Landslide of318national highway in Dongjiu River Valley for a case, by comparing of considering unloading process of deeper river cutting or not, the unloading effect on slope stability was discussed.The main conclusions are as follows:(1) The effect of unloading is more intensive in valley under high geostress, and rock mass is more vulnerable subjected to effect by unloading process with high tectonic stress. The rebounding displacement of rock mass under40MPa tectonic stress is about3-6times than that just considering self-weight stress field, and the conjugated structural planes composed of steep and gently inclined planes appeared in valley side-slope induced by the stress deflection effect, which may correspond to potential instability mode of collapse and slip by engineering excavation. Introducing a parameter k of ratio of deviator stress before and after unloading to evaluate the unloading effect degree of rock mass, the result of k under40MPa tectonic stress is1.5±2.5in valley side-slope (220m higher than valley floor) and extended hundreds of meters horizontally. While without tectonic stress, k keep about1.The higher geostress values, the larger rebounding deformation, and the area of stress concentration extend deeper, deviator stress ratio k accelerated growth non-linearly. The values of tectonic stress have great influence on unloading degree of rock mass.(2) Considered the strength degradation of rock mass when unloading under40MPa tectonic stress with Field Variable method, stability of valley side-slope further decreased, such as the height range of plastic yield area extended36%and the deep range extended18%. The fracture extended to the upper of slope, more potential slip surfaces appeared and larger rebounding displacement.(3) High geostress was applied on one side of valley due to extrusion of Indian plate, the rebounding deformation on the bank under30MPa tectonic stress is bigger than the opposite bank which without tectonic stress, and the range of deformation more than100mm on the bank under30MPa tectonic stress is1.1times higher and1.5times deeper than the opposite bank. Conjugated structural planes mean that slope destabilization may superpose slip and collapse along adverse inclined planes. While the opposite bank only has gently inclined planes, the structural damage of rock mass is relatively small.According to deviator stress ratio value, the effect degree of unloading is biggest when both two sides have high tectonic stress, and the situation of only one side has high tectonic stress is lesser, and condition of without high tectonic stress is the least effect.(4) Disaster prevention suggestions were put forward to offer reference for regional route selection in deep-incised valley under high geostress. For lager rebounding deformation and deeper plastic yielded zone, excavation should be strictly controlled with careful study, and support measures and pre-reinforcement technology should be strengthened. The conjugated structural planes and deep unloaded band bring disaster risk to route engineering. For instance, lead to the stability of tunnel slope decreased, stress focus on entrance section of tunnel, and the side walls and bottom of bridge foundation pit may collapse or slip due to the superposition of steep and gently inclined planes. In order to prevent rock burst phenomenon in stress concentration area, many measures were taken to minimize excavation disturbance, such as timing of excavation should be paid more attention to release stress smothly, water injection and other measures should be taken to reduce rock brittleness.(5) The stability of Layue slope considering unloading process was analysised by strength reduction method of FEM. Compared with not considering unloading process, the stress of rock mass and deformation value increased, and the range of deformation enlarged and safety factor decreased. It indicated that, the stability analysis of strong unloading valley area in southwest China should take the unloading process influended by the tectonic movement induced high geostress boundary conditions into account, and provides reasonable measures for prevention and treatment of geologic disaster.