Study On The Mechanism Of Overburden Collapse Induced By Groundwater Fluctuation In Karst Area

In recent years,as the increase of tunnel engineering projects in karst mountain areas,the occurrences of the rapid decline of groundwater level caused by karst water inrush disasters and the further induced surface karst collapse are more frequently.This paper focus on the typical karst collapse in the Guizhou section of the Shanghai Kun high speed railway.Employing physical model test and numerical simulation to study the mechanism of the karst collapse caused by the fluctuation of the groundwater level.The main results are obtained as follows:(1)According to the analysis of the geological survey data in the study area,the geological structure of the study area is mainly dual-layered geological structure which is composed of clay covered layer and soluble rock.Have analysed the feature of the karst collapse in study area,the main factors causing collapse are the seepage potential of groundwater and the vacuum negative pressure after the water level drop in the soil cave.(2)Based on the geological structure of the strata,a device for simulating karst collapse was designed,and the physical model test of karst collapse process was carried out.The results show that the submersible erosion mechanism of karst collapse can be summarized as a cyclic erosion effect that in the order of lateral submersible erosion of arch foot,developing a gap between the bottom of the hole,hole body seperation,Instability and collapse of the cave wall,the expansion of soil cave and the lateral submersible erosion of arch foot.(3)Based on the percolation theory and the principle of effective stress,the numerical simulation of the formation and development of soil cave in the process of karst collapse is studied with the help of the FLAC3D finite difference software.The following conclusions are obtained:when the groundwater level drops,the larger pore water pressure difference and the maximum shear stress concentration above the opening of the karst cave are the main factors causing the soil failure to develop soil caves.The expansion and development of the soil cave is due to the failure of the arch foot of the soil cave caused by the seepage pressure and the shear stress concentration and when the failure is further developed,tensile failure of the soil at the top of the cave would happen;when the water level rises,the seepage has a temporary supporting effect on the wall of the cave.When the water level fluctuates,the soil is subjected to strain softening under the action of cyclic water pressure,which causes the soil cave to lose stability.(4)The orthogonal test with three factors(soil strength,the diameter of the cavity,the permeability of the soil)was conducted to investigate the critical thickness of the overburden under the water table fluctuation by applying numerical analysis.The orderof influence factor effect on the critical thickness is the diameter of the cavity>soil strength>the permeability of the soil.