STABILITY STUDY OF LARGE UNDERGROUND CAVERNS UNDER HIGH GEOSTRESS

The Laxiwa Hydropower Station,constructed in high mountain with steep slope and deep gorge,is the largest hydropower station in Yellow River area. Its underground powerhouse consists mainly of main powerhouse,auxiliary powerhouse,main transformation cavern,pressure adjustment well,caudal shiplock cavern, and caudal water tunnel,which has vertical depth of 282-429 m. The granite is weakly weathered in the area of the underground powerhouse,with grey-grey white,middle coarse size,block structure and mineral component of mainly feldspar. The underground powerhouse cavern group is in high stress field with the maximal principal stress of -22--29 MPa. In order to evaluate the stability of hard rock cavern group under high geostress,a new method,named PFP(preparation,feedback and prediction),is presented. It is carried out by three phases along with the progress of the excavation of the underground caverns in Laxiwa Hydropower Station. The results show that the deformations of rock wall are different in different spaces and increase with the step of excavation;the sequential tunnelling layer makes the stress undulation and transfer;the stress looseness zone is greatly different from the normal tunnel because of cross-caverns;the EDZ has different failure models and depths in different positions,and so on. The study proposes scientific suggestion for excavation and supporting of the caverns,which is also identified by the mechanical behaviour of rock in practice. The method and conclusions can offer a useful reference for other similar underground projects.