The transversal isotropic media model is adopted to multivariate regression analysis of the stress field of strata. The stress field thus obtained is then used to quasi 3D numerical analysis of the stability of a large underground opening complex. In addition, the above analysis takes the earthquake effects into consideration and the useful suggestion is proposed for the anchor reinforcement scheme of the surrounding rocks.
Daganshan hydroelectric power station is situated in the middle reaches of the Dadu River in Shimian county of west part of Sichuan Province. The maximum dam height is 210 meters and the general installed capacity is 2400MW. The dam site is located in complex geological structural and seism geological environments, having fairly intensive geotechnical movements. The seism intensity is Richter VII. The rockmass surrounding the project area consists mainly of granite, however, several diabase dikes are exposed in the underground power house area, thrusting into the granite and several large faults penetrate through the power house area.
During the engineering exploration period of the project, three dimensional measurements were conducted in six boreholes drilled in a testing adit in the power house area using shallow overcoring technique. The fitting analysis for the initial stress field of the area was made.
The scope of the computation for the central cross section of each machine unit are all 350m from the transformer house towards both left and right boundaries and 200m from the chambers bottom towards the upper and lower boundaries. For the rocks in the range from the place 200 meters above the complex's top to ground surface, their computed weight is taken as external loads exerted on the upper boundary. FLAC3D code is used to make computations. The triangular loads applied on the model's outside boundary are used to simulate the initial geostress according to regression analysis results.
The pseudo-static method is adopted to make analysis on the effects of seisms on surrounding rocks stability. In using the pseudo-static method, the seismic actions are first reducible to that of the horizontal or vertical unvaried acceleration, then this acceleration produces an inertia concentrate force upon the mass center of element, and afterwards, the concentrated force is conversed equivalently into the loads on the element nodes to perform calculations. The horizontal peak acceleration of the dam base rocks is 0.251 g. The calculation results show that the seismic loads make almost no effects in the elasto-plastic dimensions of rock surrounding. The maximum displacement increment of high wall for both machine units 2# and 3# is quite small, being about 2%, the probable maximum seismic displacement of the side wall for power house is about 4.3 cm and the maximum seismic displacement of tailrace chamber's side walls is about 2.9 cm. The calculation results are tabulated as in Table 3.