The discontinues deformation analysis (DDA) is a powerful tool to predict and describe the slope failure of fractured rock. An important feature in DDA is the contact detection between different blocks. In this paper first of all a brief introduction on the basic principles of contact detection is presented. Then a new method is proposed that consider the influence of the total forces direction acting on rock. Finally the slope failure is simulated using this method. The results imply the advantages of applying this new method to present the mechanism of rock slop failure.
Many materials and structures have a blocky nature, being formed by components that may undergo different movements. The blocks, interact through the joints or interfaces between them. For example, rock masses are divided into discrete units by joint and faults. Soils are composed of small particles. Stones and bricks form the fabric of masonry structures. A contact of blocks may consist of a material, such as mortar in masonry, or it may be plain interactions of solid objects, such as joints in rock. The explicit modeling of these contacts, represented as structural discontinuities, is outside the capability of continuum idealization, is outside the capability of continuum idealizations, which generally underlie standard finite element models . Discrete element models are very appropriate tools to represent blocky structures. In rock slope engineering, it is well known that the behavior of a slope failure is strongly affected by its discontinuities (Goodman 1976 ). Ohnishi et al. (1995)  demonstrated the applicability of the original 2D discontinuous deformation analysis (Shi and Goodman 1988 ; Shi 1989 ; Jing 1998 ) to a slope failure simulation. However, the DEM has to use contact damping to diminish the unexpected forces and vibrations when blocks are in contact with one another, but this contact damping lacks physical meaning in the computations (Chen et al. 2002 ). In addition, the input value of the contact damping affects the correct contact forces and the contact period during computation. However, DDA does not need damping to keep the computation convergent in this situation and as a result can simulate the contact with more physical meaning than the DEM. DDA has the following major characteristics (Sasaki et al. 1994 ): 1.The principle of minimum total potential energy is used to calculate an approximate solution similar to FEM. 2. Dynamic and static problems can be solved by applying the same formulations. 3. Any constitutive law can be incorporated. 4. Any contact criterion (i.e., Mohr-Coulomb criterion), boundary conditions (i.e., constraint displacement), and loading conditions (i.e., initial stress, inertia force, volume force, etc.) can be modeled. In view of these characteristics, this study 584 aims at extending contact detection algorithm and developing necessary implementation. The developed code is then applied to the slop as a validation of the present approach.