Analysis On The Affecting Law Of Water On The Stability Of Surrounding Rock Of Super-large Karst Cave

Karst is widely distributed in southwest China because of the special lithology,geological structure,characteristics of neotectonic movement,climate,exogenous water and landform.With the continuous development of railway construction in China,many super-large karst caves have been exposed.In order to deal with the harm of these karst caves to tunnel construction,it is necessary to evaluate the stability of the caves.And among the various factors affecting the stability of caves,the effect of water must be considered.In this paper,the influence of water on the stability of super-large karst caves is analyzed qualitatively and quantitatively through field investigation cases,discrete element numerical simulation software and formula deduction.(1)Based on the field investigation cases of three super-large karst caves exposed in Chengdu-Guizhou Railway,the qualitative analysis process of the stability of karst caves is summarized.The area with abundant underground water and water passageway is the area with poor stability of surrounding rock.The mechanical action of groundwater seepage may cause some problems,such as falling block,instability of the side wall around the cave,and landslide of accumulation at the bottom of the cave.(2)The stability changes of karst caves under different hydraulic pressures were simulated by 3DEC.And the results are similar to those of field case studies.After calculation,the seepage field distribution law of groundwater under different hydraulic pressures is the same.With the increase of hydraulic pressure,the normal stress on the joint surface of the rock body decreases continuously,leading to the increasing displacement and the risk of collapsing blocks.At the same time,the shear stress in the rock around the cave increases with the increase of hydraulic pressure,which makes the volume of the plastic zone of the side wall increase continuously,And the distribution of plastic zone is controlled by joint planes.(3)In order to better understand the stability of cave-wall rock mass under the action of hydraulic pressure,the 3DEC was used to simulate the triaxial compression test of fractured rock mass under the action of different hydraulic pressure on joint planes.The strength of rock mass decreases gradually because of hydraulic pressure.The reason for the decrease is that the hydraulic pressure cancels part of the normal stress on the joint planes and weakens the internal friction angle and cohesion of rock mass.The internal friction Angle is not sensitive to the hydraulic pressure,while the cohesion decreases obviously with the increase of hydraulic pressure.(4)Based on the influence of water on the stability of karst caves and considering the factor of hydraulic pressure,the formulas for calculating the local stability of the roof and the stability of the joint plane of the cave wall were deduced.And the stability of the roof and the wall of the karst cave in the Shanggao mountain were calculated.