DDA Analysis On Instability Mechanism Of Dangerous Rock Mass Under Liquid-solid Coupling

As a kind of sudden geological disasters,dangerous rock collapse has the characteristics of wide distribution and great harm after occurrence,which seriously affects the social and economic development and people ’s property safety in disaster-prone areas.In the existing engineering cases,the instability of dangerous rock mass is often closely related to rainfall.In order to reveal the influence of rainfall on the stability of dangerous rock mass,this paper mainly carries out the secondary development of DDA method to realize the stability analysis of dangerous rock mass under rainfall conditions.The Monte Carlo algorithm is used to establish the randomly distributed two-dimensional fractures and combine them to form a discrete fracture network.According to the seepage theory of fractured rock mass,the secondary development of DDA is carried out.The water load matrix is derived by the principle of minimum potential energy and added to the overall equilibrium equation of DDA to establish the fluid-solid coupling calculation model.On this basis,the Richards equation discretization is introduced into DDA,and the Van Genuchten model is combined to realize the simulation of transient unsaturated seepage.The improved DDA was applied to the Datizi dangerous rock mass in Sanxi Township,Wushan County,Chongqing City,and the instability mechanism under natural conditions and rainfall conditions was analyzed by numerical simulation.The main research contents of this thesis are as follows :(1)Research on DDA pretreatment.The Monte Carlo algorithm is used to generate random numbers that conform to statistical laws to simulate the occurrence of dangerous rock fractures,so that they are more in line with the true distribution of dangerous rock fractures.On this basis,the fracture network is split,and the long fractures are split into multiple short fractures,so that each fracture network can be correctly identified to avoid the problem that the long fractures or concave polygon boundaries cannot be identified.(2)Research on DDA steady seepage calculation method.Based on the single fracture steady seepage model of rock mass,the influence of single fracture seepage on the node of discrete fracture mesh model is considered,and then the discrete fracture mesh seepage matrix under the condition of two-dimensional steady seepage is obtained.According to the basic principle of DDA,the potential energy minimization treatment is added to the load matrix of DDA overall equilibrium equation as water load,and the overall equilibrium equation considering water load is established,so as to realize fluid-solid coupling.The correctness of the program is verified by classical examples.(3)Research on DDA transient seepage calculation method.The two-dimensional Richards equation is introduced into the DDA,and it is discretized in the form of finite difference to establish the transient overall seepage matrix conforming to the DDA calculation format.The volumetric water content on the fracture network nodes was obtained by solving the matrix,and then the pore water pressure corresponding to the volumetric water content was calculated by Van Genuchten model,so as to realize the fluid-solid coupling calculation under the transient seepage state of DDA.The correctness of the program was verified by two examples of one-dimensional infiltration and Vuaclin two-dimensional indoor sand tank infiltration experiments.(4)Apply the improved DDA method to engineering cases.Considering the influence of fluid-solid coupling on the stability of dangerous rock mass under natural and rainfall conditions,the instability mechanism of dangerous rock mass under different conditions is studied.The conclusions are as follows : Under natural conditions,the maximum displacement point of the dangerous rock mass appears at the bottom of the rock mass.Due to differential weathering,there is a concave cavity at the bottom of the rock mass,and the dangerous rock mass slides along the cracks between the rock mass and the trailing edge.Under the rainfall condition,the maximum displacement of the dangerous rock mass appears in the top free block,and its main deformation is the tensile fracture of the rock mass.As the tensile fracture block also has a downward sliding trend,the rainfall type mainly affects the displacement of the dangerous rock mass,while the influence on the instability mechanism of the dangerous rock mass is relatively small.The influence of rainfall on dangerous rock mass is mainly reflected in the increase of pore water pressure in fractures and the weakening of rock strength parameters.Only considering the change of pore water pressure,the displacement of dangerous rock increases,and the overall trend is similar to natural conditions.When only considering the weakening of rock strength parameters,the displacement of dangerous rock mass increases greatly,especially the displacement of block in saturated zone at the top of dangerous rock mass increases obviously,but it is still less than the displacement of dangerous rock mass considering both factors.Therefore,the weakening of rock strength parameters has a greater impact on the stability of dangerous rock mass,but the change of pore water pressure in fractures is also a factor that cannot be ignored.Through numerical simulation,the typical engineering cases are analyzed to reveal the instability mechanism of dangerous rock mass under rainfall conditions,which provides reference for the prevention and control of dangerous rock mass collapse.