A rock mass stability analysis method coupling block theory and three-dimensional discontinuous deformation analysis (3D DDA) has been developed. In this paper, two case histories are presented in which the coupled method developed was used to analyze the stability of underground openings. One case history involved a proposed new highway tunnel, and the other involved an underground chamber for a hydroelectric power plant. The stability analyses allowed the design of stabilization using rock reinforcement. It was found that for these two case histories, the stabilization designs from the coupled method gave stabilization designs that were less conservative than the actual designs.
1. INTRODUCTION
Block theory, developed by Goodman and Shi [1], has some limitations. The mode and stability analyses consider only sliding modes and some special rotational modes and cannot handle general modes of simultaneous sliding and rotation. Furthermore, they do not consider dynamic equilibrium. To overcome these limitations, block theory?s removability analysis is coupled with three-dimensional discontinuous deformation analysis (3D DDA) single block analysis. 3D DDA single block analysis is used to perform the mode and stability analyses, because DDA considers dynamic equilibrium and can handle general modes of failure including simultaneous sliding and rotation. In this way, the advantages of both block theory and DDA can be combined. A design method coupling block theory and 3D DDA has been developed [2]. To test the coupled method, the coupled method has been applied to several case histories. Two of these case histories involved rock slopes, one involving a rock slope in the Three Gorges Reservoir area in China [2, 3], and one involving some rock slopes in Macau, China [3]. This paper presents two other case histories in which the coupled method was applied to analyze underground openings, one involving a tunnel and one involving an underground chamber.
2. CASE 1: THE PROPOSED ELK CREEK TUNNEL REPLACEMENT
The Elk Creek Tunnel Project involved replacing an existing highway tunnel and two adjoining bridges near Elkton, Oregon [4]. The project was designed but has not been constructed. The new tunnel would be approximately 366 m long. The design called for a horseshoe-shaped tunnel, with a width of 13.1 m, a maximum height of 8.2 m, and a tunnel roof arch height of 4.5 m. The tunnel is horizontal and has a curved alignment in plan, with an average tunnel axis orientation of N56°E.
2.1. Rock Joint Properties
The rock mass at the tunnel site consisted of sandstone and siltstone/shale. The structural features of the rock mass included bedding planes, joints, and a fault near the east portal of the existing tunnel. Four discontinuity sets were identified through statistical analysis of stereographic projections of joints and bedding planes measured in the field. The orientations of the discontinuity sets and the fault are given in Table 1 [4]. Based on laboratory direct shear and triaxial shear tests on discontinuities, the friction angles of joints in sandstone, joints in siltstone/shale, and bedding planes were estimated to be 42°, 35°, and 24°, respectively. The friction angle of the fault plane was estimated to be 35° [4].
