Failure Mode And Stability Of The Thick Dangerious Rocks With Soft Foundation

Rockfall is one of the main geological disasters in Southwest China.It has the characteristics of widely distribution,sudden occurring and unpredictable outbursts,which brings serious economic loss and safety hidden trouble to our country.In this paper,Zengziyan dangerous rock mass in Nanchuan district,Chongqing,as an example,combined with on-site investigation,three-dimensional laser scanning technology,detailed investigation of the basic structure of the dangerous rock and its development conditions.It is found that the dangerous rock mass not only has the characteristics of thick soft ground,but also has a deep tension crack on its back wall,and it is affected by several weak structural planes.In this paper,the destabilization failure mode of this dangerous rock mass is analyzed,and two kinds of destabilization failure modes are suggested,namely,slip destabilization failure and dumping instability failure.In addition,the two failure modes of unstable rock mass are calculated.At the same time,the discrete element numerical simulation method is used to study the stability of the dangerous rock mass in the natural state and the base softening condition.The simulation results show that the dangerous rock mass is not deformed and remains stable as a whole.While in the base softening condition,the whole unstable rock mass is destabilized,and the failure mode is destabilization failure.Finally,the rockfall movement path and the influence range of the unstable rock mass are predicted.This study complements the instability and failure modes of thick rock soft rock mass and karst area,and evaluates the stability and danger range of the Zengziyan dangerous rock mass,which provides a basis for further study.(1)Through the field investigation,and combined with three-dimensional laser scanning technology,the basic structural characteristics of Zengziyan dangerous rock mass are revealed in detail.The main body of Zengziyan dangerous rock body is composed of hard and thick layer of limestone,and the lower part is weak shale base.The weak base may be softened easily under weathering or rainfall,resulting in low strength.,Which forms the trailing edge boundary of the dangerous rock mass.The stability of the dangerous rock mass is greatly affected by the cutting of the multiple structural planes.(2)Based on the investigation and study on the basic structural characteristics of dangerous rock mass,the destructive failure mode of dangerous rock body is identified.Because of the different weathering and groundwater softening,the dangerous rock mass may be destabilized along the weak base.In the natural state,because the dangerous rock mass is steep and the trailing edge And it is likely that the unstable failure of the dumped rock may occur.The analysis shows that there are two kinds of destabilization failure modes,namely,dumping and sliding.(3)Based on the identification of the unstable rock mass instability model,the stability of the unstable rock in two failure modes of dumping and sliding is preliminary evaluated based on the limit equilibrium theory.It is shown that the stability coefficient of Zengziyan dangerous rock mass is 1.35 when it collapses in the natural state,which is basically stable state,and the stability coefficient is 1.50 in the case of landslide collapse.Therefore,it is judged that in the natural state The dangerous rock mass is in a state of steady state.Under the rainfall condition,the unstable rock mass in the two kinds of unstable modes is in the unstable state,and the probability of slip instability is greater.Due to the development of fractures in the posterior margin of the perilous rock,a lot of rainwater is poured into the rock mass along the crack in the rainfall condition.The soft base of the weak rock base is soaked and softened,which leads to the decrease of the strength of the base rock mass and the increase of the crack head pressure.But also produce a horizontal thrust of dangerous rock,eventually leading to slippery slope instability.In the seismic condition,the two unstable rock mass under unstable mode are in the unstable state,and the stability coefficient is equal,the dangerous rock under this condition is the most likely to be unstable.(4)The model and process of destabilization failure of Zengziyan dangerous rock mass are studied by using discrete element UDEC software.According to the simulation results,it can be seen that there is no obvious destabilization failure of the dangerous rock mass under natural conditions,and the dumping failure occurred under the softening of the foundation,which indicates that the stability of weak base The fracture is initiated by the collapse of the fulcrum of the fulcrum,leading to the gradual opening of the trailing edge of the fracture,and ultimately from the back of the mother rock,the occurrence of the overall dumping damage.(5)Finally,the trajectory and threat range of the dangerous rock mass are predicted.The movement distance,bounce height and instantaneous velocity of the block are calculated,which provide a reliable basis for rockfall control and avoidance.