BACK ANALYSIS OF SURROUNDING ROCK STABILITY BASED ON EXCAVATION PROCESS OF UNDERGROUND POWERHOUSE AT LAXIWA HYDROPOWER STATION

The excavation size of Laxiwa underground powerhouse is huge;and its surrounding rock is mainly brittle hard granite rock.Moreover,the mountains are high;the slopes are steep;and the valleys areas are narrow in the area plant.Except that,the regional stress field values are high and the high stress fields distributions are complex,which are extremely detrimental to the stability of the cavern.In order to estimate the stability of surrounding rock and supporting long-term safety of underground plant after excavation,based on back analysis model of excavation and the change laws of displacement measured by multipoint extensometers at different depths from the tunnel wall,the tunnel surrounding rock is divided into loose rock zone,transition zone and stability zone.And then,rock mass mechanical parameters were back analysed and calculated by using the measured displacement increment on each layer caused by excavation and the objective function of its mean square error.When excavating underground powerhouse each layer,the mechanical parameters and ground stress parameters near rock mass were obtained.Combination of ground stress parameters and mechanical parameters,the follow-up analysis of the stability of surrounding rock of excavation plant was evaluated and predicted.Also a standard is presented to determine the stability of surrounding rock of Laxiwa underground plant.The standard for subsequent control of underground plant monitoring deformation provides a basis for effective guidance of the plant supporting design and excavation construction.