Analaysis on deformation mechanism of landslide at tunnel entrance of expressway in loess area

When constructing expressway tunnels in loess area, creep of overlying loess caused by tunnel opening construction often occurred, which caused deformation and cracking of tunnels due to landslide sliding, and made the tunnels unable to operate normally. Based on the above problems, collected the data of tunnel entrance landslide in Loess area, selected six typical cases to analyze the inducing factors and forming conditions of this type of landslide, combined with geological structure model to analyze the deformation development process and movement characteristics of tunnel entrance landslide. The influence law of tunnel opening excavation on overlying loess was studied by numerical simulation method, and the stress mode of tunnel in landslide was determined, which was verified by typical engineering examples. The results show that the landslides at the tunnel entrance in the loess area mostly occurred in the Loess Plateau Beam area and the gullies often existed at the foot of the slope. Landslide body was mainly Quaternary Aeolian Loess and Malan loess. Artificial disturbance and rainfall infiltration were the main inducing factors of landslide. The inducing mechanism was that there was an empty surface at the foot of the slope in tunnel construction, and the excavation inside the slope caused the redistribution of stress in the slope body. At the same time, the mechanical properties of rock and soil mass were reduced under the condition of rainfall. According to the induced causes, the failure mechanism and formation process of landslide can be divided into four stages: stress adjustment stage of tunnel excavation, creep shear and tension crack stage, deformation acceleration stage and slope instability stage. The case of finite element numerical simulation well reflects the distribution of deformation field, stress field and failure area in landslide body, and the spatial relationship between tunnel and landslide controls the nature and scale of landslide. The research results can provide practical experience and theoretical guidance for similar tunnel entrance landslide.