Abstract: A novel index named particle relative motion (RPM) is proposed to characterize the progressive failure behavior of materials.The calculation formula of RPM is deriveed by considering both translational and rotational movements of particles.Based on the PFC2D platform, the calculation of RPM field is realized,then a new analysis method for granular material failure is established. Further,the validity of this method in describing the failure process of geotechnical materials is verified by the results of the trapdoor test.Finally,the progressive failure processes and instability modes of different strata induced by tunnelling are analyzed by establishing PFC models taking a tunnel in the Beijing Metro Line 12 as the engineering background.In both homogeneous sand and clay strata,the collapsed areas are presented in an elliptical shape,but it exhibits overal collapse in the sand stratum and gradually falling with blocks from the tunnel crown to surface in the clay stratum.For clay-sand composite strata,the thickness of the upper clay layer is a key factor in determining whether the failure zone develops to the surface and the ultimate instability mode.When the clay layer is thicker,the instability mode is similar to that of a homogeneous clay layer,while when it is thinner,the collapse zone is more likely to develop to the surface and presents a U-shaped instability mode.For sand-clay composite strata,the thickness of the upper strata has a relatively small impact on whether the collapse zone develops to the surface and the final instability mode.The development of collapse in the lower clay layer is similar to that in the homogeneous clay stratum,and the upper sand layer mainly exhibits overall collapse.