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.　©　2024　Editorial　Board　of　Journal　of　Basic　Science　and　Engineering.　All　rights　reserved.