Irreversible deformation of a coarse particle assemblage is mainly due to a change m its fabric, and this process is attended by a noticeable dilation which is confirmed through the new visualization technique (Laser-Aided Tomography: LAY). Numerical analysis by means of discrete elliptic element method (DEEM) is conducted to study the dilating process. The effect of both inter-particle slipping and rolling of embedded grains is discussed, and a simple conceptual model of slope failure, m which dilation leads to frequency dependence of the failure acceleration, is presented.