Abstract：Discontinuous deformation analysis(DDA) and discrete element method(DEM) are two important numerical methods that analyze large-scale opening，sliding，and complete detachment of elements or blocks of discontinuous media such as fractured blocky rock masses. These methods have been widely used in codes and applied to two-dimensional analysis of discontinuous deformation. However，the methods have scarcely ever been applied to three-dimensional analysis of practical rock engineering problems. Until recently，spheres，bonded spheres and ellipsoids are used as elements or blocks in 3D codes，but they can not reveal the real behavior of more angular block system such as fractured rock masses in which joints，fissures and faults are intersected. During modeling the 3D rock block system，DDA and DEM confront a great difficulty in contact detecting between two blocks because the existing detection methods cannot get the exact contact relationship sometimes between two 3D blocks that contact each other. The common-plane(C-P) method introduced by Cundall can detect the correct contact relationship between two 3D blocks provided that the common-plane position could be located correctly. Unfortunately，the algorithm to position the common-plane can¢t always get the correct position because it can¢t reveal exactly the influence of variation of a block geometrical shape upon the common-plane position. The limitations and advantages of three contact detection schemes，especially two main schemes used in codes，are discussed. The principle only according to which the correct detection method can be brought out is represented. Based on advantages of the existing methods，a new approach called incision body is put forward. The simulation examples of block movement are also given. The simulation result shows that the incision body method is capable of overcoming indeed limitations of the existing methods，detecting 3D block contacts correctly and hence simulating the interactive detachment and sliding between two blocks.
