The rotational-translational loess landslides are widely distributed in northwest China, usually posing threats to the surrounding residents and infrastructure. These loess landslides are characterized by the formation of multiple slip surfaces during the run-out process, and the mechanisms of this phenomenon in loess landslides have not been sufficiently investigated. Therefore, in this paper, we integrated the elastic-plastic strain softening constitutive law into the original DualSPHysics code to extend its application in simulating rotational-translational loess landslides. Two benchmark cases are studied to validate the model, the failure process of a cohesive soil slope without strain softening and that of a sensitive clay slope with strain softening. The results illustrate that our model can effectively predict large deformation. Then, the run-out process of the Caijiapo landslide in northwest China is analyzed by the modified model to investigate its failure mechanism. The results illustrate that the failure pattern of the Caijiapo loess landslide is very different from the typical retrogressive failure of clay landslides. The main slip surface of the Caijiapo landslide is controlled by the pre-existing structural plane. The second and third slip surfaces of this landslide are formed inside the sliding mass due to stress redistribution during the run-out process. Three scarps are formed in the landslide deposit because of the formation of multiple slip surfaces. This deposition morphology can be well reproduced by the SPH model taking strain softening into account, while the results using an SPH model without considering strain softening cannot capture this essential deformation characteristic.

Relying on the tunnel engineering crossing large active fault fracture zone in high intensity seismic area in Western China, a large-scale model test of tunnel through multiple slip surfaces under strike slip motion was carried out. The deformation patterns and damage characteristics of tunnel structure and surrounding rock were studied based on displacement, strain and internal force response, as well as crack morphology. The results revealed that microscopically the model soil experienced the process of contacting, compaction and relative movement under fault dislocation, and was macroscopically accompanied with fracture occurring, and further expansion. Compared with fault movement form containing single slip surface, the active wall produced nonlinear linkage displacement to fault fracture zone when there existed multiple slip surfaces. The model soil exhibited a multistage dislocation along longitudinal direction after test. The dislocation of model soil on both sides of major slip surfaces reached to 25.9 mm and 18.8 mm, which was about 2.2-3.4 times of the dislocation on both sides of the minor slip surfaces in fault fracture zone. The overall deflection of lining segments and the torsion of flexible joints corporately undertook the fault dislocation. Lining segments near the major slip surfaces had opposite trends of tensile and compressive deformation, where the failure of tensile bending damage was dominant. Tunnel segments near minor slip surfaces underwent integral linear deflection along with surrounding rock, and were less affected by fault dislocation. The fragile sections of tunnel structure were located near main slip surfaces, the fragile parts were invert, arch springing and arch spandrel, which were mainly damaged by tensile, compressive and shear affection. Based on the deflection corner beta of each lining segment, combined with damage pattern and internal force distribution trend, it is suggested that 2d similar to 3d (d represents the span of tunnel) in range near major slip surfaces is the main affected zone, while the range of 4d in the middle of fault fracture zone is the minor affected zone. The partitioned fortification needs to be adopted when tunnel is to cross fault fracture zone.