Study On The Kinematic Characteristics Of Loess Seismic Landslide With Different Structures Via Discrete Element Method

The Loess Plateau is the most developed area of loess in China,and it is also an earthquake-prone area.The unique hollow structure and weak cementation properties of loess make it highly susceptible to seismic damage.Successive strong earthquakes in the Loess Plateau region have caused a large number of geological disasters such as landslides.A large number of investigations and studies have shown that complex structures such as tectonic joint surfaces,stratigraphic interfaces and softened liquefaction layers,which are widely developed inside loess slopes,are often the key factors leading to loess seismic landslides.In this paper,based on a large number of field investigations of slope structures in the Loess Plateau area,the damage processes of three different types of structures slopes under earthquake action are simulated by the particle discrete element software,using the Linfen Xiangning landslide,the Jingning Sunjiagou landslide and the Guyuan Shibeiyuan landslide as typical cases,respectively.Based on the analysis of the sliding process,damage mechanism and kinematic accumulation characteristics of slides,a method based on the kinematic principle is proposed to calculate the slip distance applicable to different structural landslides under seismic effects.The main conclusions are as follows.(1)The complex structure developed in the slope of loess area mainly includes loess joints and fissures,loess waterfall holes and traps,loess-mudstone bedrock and other stratigraphic interfaces,loess sandy and powdery and other water-saturated softening layer and liquefied layer.Among them,the partitioning effect of the loess joint surface on the slope as a whole,the double-layer heterogeneous characteristics at the loess-mudstone interface,and the flow-slip characteristics of the softened and liquefied layer of loess all determine the process and mechanism of slope damage and the kinematic characteristics of the sliding body to a certain extent.(2)The damage process of tectonic joint slopes can be summarized as follows:crack expansion along both ends of the joint – sliding surface penetration within the slope – sliding body sliding down along the joint surface.The damage process of slopes with the stratigraphic interface can be summarized as follows: the first collapse damage at the top of the slope by seismic amplification – cracks continuously expand downward along with the interface – cracks extend to the foot of the slope – sliding surface penetration.The damage process of the slope with soft liquefied layer can be summarized as follows: seismic liquefaction of the soft layer in the slope – shear-out damage of the soil body at the foot of the slope – tensile fracture breakage of the overlying loess layer of the slope – wave-like flow slip of the fractured loess body under the action of the liquefied layer dragging and floating.(3)The tectonic joint surface destroys the overall structure of the slope.The very low cohesion of the soil at the joint surface and the flat and smooth characteristics of the tectonic joint surface accelerates the sliding of the slope under seismic action.The damage mechanism of stratigraphic interface slopes can be summarized as the sequential effects of seismic amplification effect,locking segment effect and startingup acceleration effect,which make such stratigraphic interface slopes exhibit the characteristics of rapid and long run-out.The main reason for the long-distance sliding of the landslide of the softened liquefied layer can be attributed to that the liquefaction layer tow floats the overlying fractured loess layer to produce long-distance flow slip.(4)The accumulation of slides of the joint slope is mainly characterized by the process of fragmentation and disintegration of large block slides under the action of frictional tension and oscillation at the horizontal surface in front of the foot of the slope.The stratigraphic interface slope slide eventually accumulates in the area to the left and right of the foot of the slope,and the thickness of the accumulation at the foot of the slope is up to 18 m.The accumulation characteristics of the slope slide of the softened liquefied layer are characterized by the overlying fractured and broken loess layer of the slope,which keeps flowing forward and accumulating by the dragging and floating action of the liquefied layer.The velocity and displacement characteristics of different types of slides are well verified by the damage mechanism and motion characteristics of their corresponding slopes.(5)The slip distance calculation model derived based on energy conservation and Newton's second law of motion shows that the height of the back edge of the landslide,the inclination of the sliding surface,the strength of the seismic action,and the strength value of the soil in the slip zone are the main controlling factors of the slip distance of structural landslides under seismic action.The proposed model of slip distance calculation method was verified by taking Jingning Sunjiagou landslide and Shibeiyuan liquefaction slip as examples.Based on the problems exhibited in the verified results,the method of taking the friction coefficient in the sliding distance model was optimized.For a long-term,the research on seismic landslides of complex slope structures in loess has been more fragmented,and relatively few systematic studies have been reported.In this study,the typical three types of slope structural factors in loess areas are considered,and the damage process and the motion characteristics of slides under the seismic action of different structural slopes are innovatively compared and analyzed.Meanwhile,there is a breakthrough in the derivation of the kinematic slip distance model for structural loess landslides.The research results fill the gap of less research on the kinematic characteristics and slip distance model of complex slope structure loess slopes,and provide a theoretical basis and scientific guidance for the prevention and management of complex slope structure landslide disasters in the earthquake-prone areas of Loess Plateau.