Failure Mechanism And Stability Analysis Of Surrounding Rocks Of Underground Caverns Under Excavation In High In-situ Stress Areas

China is a big country in the construction of water conservancy and hydropower projects in the world,especially the scale of hydropower stations represented by the southwest region with complex geological conditions and high in-situ stress levels is in the forefront of the world.The construction of a hydropower station is a complex system engineering,which inevitably requires large-scale rock excavation.Excavation of the rock mass will break the original stress balance and cause stress concentration in the surrounding rock of the cavern,leading to instability of the surrounding rock of the cavern,which will have a serious impact on the construction process and subsequent normal operation of the hydropower station.Therefore,it is of great engineering significance to study the stability of surrounding rock of underground caverns of hydropower stations under constructions in high in-situ stress areas.This paper focuses on the key scientific issue of"instability mechanism of surrounding rock of hydraulic and hydropower underground cavern excavation under high in-situ conditions".Relying on the microseismic monitoring project of tailrace tunnels of Shuangjiangkou Hydropower Station,the characteristics of surrounding rock microseismic activity during excavation were analyzed.And the micro-fracture mechanism was analyzed,and a new evaluation index of surrounding rock stability was proposed.At the same time,using the realistic failure process analysis software based on the strength reduction method-RFPA^2D-SRM,the stress field and displacement field distribution of the surrounding rock of the tailrace tunnels under the excavation disturbance were studied,and the gradual evolution process of the surrounding rock micro-fracture of the tailrace tunnels was reproduced.The damage law and mechanical mechanism of the surrounding rock were revealed.The research results can provide reference for the stability evaluation of surrounding rock and disaster prevention of underground caverns under the excavation in high in-situ stress areas.The main research results of this article are as follows:（1）The tailrace tunnel microseismic monitoring system and a spatial array of recyclable sensors were successfully constructed.Through manual fixed-point percussion test and simplex method,the P wave velocity of surrounding rock was determined to be 5100m/s,and the average positioning error was 7.2m at this time.Combining on-site construction conditions and experience of similar projects,time-frequency analysis technology was used to identify various event signals in the tailrace tunnels of Shuangjiangkou Hydropower Station,and microseismic signals were accurately identified to ensure the accuracy and reliability of monitoring results.The first microseismic event generated after the blasting excavation event was extracted.The distance between the blasting point and the microseismic event caused by blasting ranged from12.1m to 44.1m.And the distance of 50m was determined as the influence distance of the blasting excavation of the tailrace tunnels of Shuangjiangkou Hydropower Station.（2）Through real-time monitoring of the micro-fracture of the surrounding rock of the tailrace tunnels,the temporal and spatial distribution of microseismic events in the surrounding rock of the tailrace tunnels under the excavation disturbance was analyzed,and the response relationship between microseismic activity and excavation construction was determined.Based on the daily accumulated microseismic energy E and b values,a new surrounding rock stability evaluation index lg E/b was proposed,which can better measure the stability of surrounding rock.The evolution characteristics of the cumulative released energy,energy index,and cumulative apparent volume were analyzed.Before the local instability of the surrounding rock of the tailrace tunnels occured,the value of lg E/b showed an increasing trend.When the value of lg E/b reached the peak value,the tailrace tunnels underwent local instability.At the same time,the value of lg E/b had a good response relationship with the construction intensity of the tailrace tunnels.Before the local instability of the surrounding rock of the tailrace tunnel occured,there was a trend of rapid increase in the cumulative released energy and cumulative apparent volume,and a substantial decrease in the energy index.The micro-fracture mechanism of the surrounding rock of the tailrace tunnels was mainly tensile fracture.（3）Using the real failure process analysis software based on the strength reduction method-RFPA^2D-SRM,the whole process of the initiation,development and expansion of the surrounding rock micro-cracks under the excavation of the typical tunnel structure and the influence zone of the structural plane under the condition of high in-situ stress was sequentially simulated.The distribution characteristics and evolution of the surrounding rock stress field and displacement field of tailrace tunnel under the construction disturbance were explored.On this basis,numerical simulation studies on the damage laws of the surrounding rock of the tailrace tunnels under the influence of different deviatoric stress states,different positions and different directions of structural planes were carried out.The horizontal stress,the safety distance of the structural plane and the direction of the structural plane which have the least impact on the stability of the surrounding rock were determined.The research results can provide a scientific basis for site selection,construction progress control and risk assessment of caverns in high in-situ stress areas.