ABSTRACT
Generally, there exist mostly convex blocks in the walls, roof and floor of underground chambers. However, in the corners of intersecting chambers, there are sometimes non-convex blocks, where a typical peculiarity is that the free planes contain concave combinations. If joints are densely distributed in these areas, there exist probably key blocks, and may cause hidden troubles to the safety of projects. Therefore, the complex key blocks in the corners of intersecting chambers are studied by using a symbolic method based on the theory of united blocks, which not only has an important significance on the stability
1 INTRODUCTION
In the various kinds of underground engineering, such as conservancy and electric power, transportation, mining and military engineering, a large amount of huge underground chambers are built. Many structures with different sizes and different shapes can be formed by the combination of these chambers. Obviously, a number of complicated problems of engineering geology can be bound to be encountered because of these huge underground spaces. One of the most concerned problems is whether the surrounding rock will produce shear slip or other forms of instability under the condition of in situ stress and engineering loads.
