This paper describes the rockburst phenomena around circular tunnels excavated in limestone formation; analyses the failure mechanism of surrounding rock of the tunnel caused by rockburst, the stress condition leading to rockburst and the relation between rockburst intensity and stress state; indicates the possibility of predicting the occurrence and intensity of rockbursts using FEM on the basis of the above analysis and gives an engineering case.
The Tianshengqiao two stage hydropower station is situated in the lower reaches of the Nanpanjiang River at the juncture of Guizhou and Guangxi provinces in the Southwest of China. It has a low dam, long headrace tunnels and an installed capacity of 1.32 million kilowatts. The three tunnels are straight, each has a length of 10 km, and run parallel to one another in the river bend area on the right bank (Fig. 1). The tunnels are excavated by TBM and drilling-and-blasting method. Three construction adits are excavated among which the adit no. 2 is located at the transitional part of Nila anticline and Zhongshanbao syncline with its axis subparallel to the fold and has a length of about 1.3 km. The adit no. 2 and a part of the tunnel in limestone formation upstream from it are constructed by TBM while the others by drilling-and- blasting method. The adit no. 2 constructed by TBM is circular in cross section and 10m in diameter, just like the tunnels. The average buried depth is about 400m, with a maximum of 760m. The tunnel line runs through an area of limestone and dolomitic limestone in the upper reaches of Yachagou Brook (Fig. 2). The compressive strength of limestones ranges from 60 to 100 MPa. This segment of tunnel line is about 7 km long and is rockburst prone. In the lower reaches of Yachagou Brook, the formation consists of sandy shale which is fractured and no rockburst occurred there. The main geological structure is the Nila anticline with its trend turning from NWW to NE in this area. Palaeotectonic stress field in this area is perpendicular to the anticline axis while the present one acts in NW-SE direction. In the valley area of Nila anticline and the edit no. 2, geostresses are measured at the points shown in Fig.l.
Rockburst occurred first at the roof and floor of edit no. 2 at a buried depth of about 200–250m, and then in the major tunnel in limestone many times. In edit no. 2, the extent and depth of failure in the surrounding rock and burst intensity increase with increasing buried depth of the edit. The observed data show that the initial stress in rock mass is not necessarily the gravitational stress field. Therefore, the dip angle of principal stress in the rock mass in edit no. 2 can be inferred from the location of surrounding rock failure caused by rcckburst. Moreover, it is known that the rockburst intensity is related to the magnitude of stress.
