Experimental Study On Evolution Mechanism Of Landslide With Retaining Wall Locked Segment

China is one of the countries with frequent geological disasters in the world.Geological disasters cause a large number of casualties and economic losses every year.Landslides have the characteristics of high frequency,great harm,and strong suddenness.In the investigation of a series of large-scale landslide cases,the existence of the locked segment was confirmed.The occurrence of large-scale landslides is generally accompanied by the sudden brittle failure of a locked segment on the sliding surface.Landslides can be divided into two types depending on whether there are locked segments that control stability:locked segment type and non-locked segment type.Locked segment type landslides are usually larger in scale and higher energy storage.Once they lose stability and slide,locked segment type landslides often lead to high-speed and long-distance landslides.At present,scholars have carried out a lot of research work and achieved some scientific research results in the classification of locked segment type landslides and the landslide evolution law.However,there are few studies on the evolution mechanism of the landslide with retaining wall locked segment.Therefore,it is of great scientific and engineering significance to carry out research on the evolution mechanism of the landslide with retaining wall locked segment.This paper takes A’hai landslide with retaining wall locked segment as the research object,and systematically analyzes its disaster environment.By means of physical model test and numerical analysis,the stress response and deformation law during the evolution process of landslide with retaining wall locked segment are studied.The acoustic emission characteristics of retaining wall locked segment are summarized,and the evolution mechanism of landslide with retaining wall locked segment is revealed.Based on the deformation and acoustic emission characteristics during the fracture process of retaining wall locked segment,a method for identifying the volume expansion point of locked segment is proposed.Finally,the failure time prediction of the landslide with retaining wall locked segment is studied by inverse velocity method.The main research results are as follows.（1）Taking A’hai landslide with retaining wall locked segment as a typical case,the engineering geological conditions are analyzed,and the deformation characteristics of the landslide are summarized combined with the field monitoring data.On this basis,the disaster-pregnant environment of A’hai landslide with retaining wall locked segment is analyzed from the aspects of topographic conditions,rock lithology and geological structure.The steep trailing edge,the gullies on both sides and the effective free face on the leading edge constitute the topographic conditions for the development of A’hai landslide.The sliding mass has a double-layer structure.The upper layer is colluvial soil with gravel to form an overhead structure,and the lower layer is fully weathered limestone and basalt,which constitutes the rock mass structure formed by the landslide.The F₁ fault in the middle of the landslide and its upper colluvial soil with gravel is retaining wall locked segment,which controls the stability of the landslide.Bounded by the F1 fault,the sliding mass has a hard upper and lower soft structure.The fully weathered basalt at the lower part of the F1 fault is broken,and the mechanical properties are poor.Fully weathered basalt will disintegrate and soften when encountering water,easily forming weak surface and further evolving into potential sliding surface of landslide.Landslide deformation has the characteristics of big two ends and small middle.（2）Based on the geomechanical model of the landslide with retaining wall locked segment,four single retaining wall landslide model tests and one double retaining wall landslide model test were carried out.A series of instruments（3D laser scanner,micro-earth pressure sensors,high-definition camera,resistance strain gauges,acoustic emission system,piezoelectric acceleration sensors,etc.）were used to monitor the slope morphology,stress,displacement,acceleration and acoustic emission information of the landslide.The influence of strength,thickness,location and quantity of retaining wall locked segment on landslide evolution process is studied comprehensively and systematically.This paper summarizes the deformation and acoustic emission characteristics of the retaining wall locked segment.The experimental results show that the increase of the strength and thickness of the retaining wall locked segment can improve the stability of the landslide,and the different locations of the retaining wall locked segment will affect the landslide failure time.（3）The fracture process of retaining wall locked segment is studied by numerical simulation.Combined with the results of the physical model test,the failure mechanism of landslide with retaining wall locked segment is further revealed.During the evolution process of landslide with retaining wall locked segment,after the failure of the sliding mass at the upper part of the locked segment,the retaining wall locked segment will bear most external loads and play a locking role.The upper sliding mass of the locked segment is extruded under external load.With the continuous loading,the micro-cracks in the retaining wall locked segment first appear at the interface between the upper sliding mass,the retaining wall locked segment and the bedrock.As the load increases,the micro-cracks further extend along the sliding surface until the retaining wall locked segment is penetrated.After the fracture of the retaining wall locked segment,the landslide failed as a whole.（4）The number of acoustic emission events and energy increase during the fracture failure of the locked segment.The b value reflects the inhomogeneity of the rock.Based on the physical model tests of the landslide with retaining wall locked segment,the acoustic emission characteristics in the deformation process of retaining wall locked segment are analyzed.This paper introduces the ratio of acoustic emission accumulative ringing count to accumulative energy（ΣN/ΣE）and b value to identify the volume expansion point.The experimental results show that the decrease of b value and the ratio of acoustic emission accumulative ringing count to accumulative energy（ΣN/ΣE）and the occurrence of high energy level acoustic emission event can be used as a criterion for volume expansion point.（5）The inverse velocity method takes the starting point in the tertiary creep stage of the landslide,that is,the onset of acceleration point as the starting point of landslide failure prediction.According to the acoustic emission characteristics in the deformation process of retaining wall locked segment,it is considered that the volume expansion point can be used as the onset of acceleration point of landslide failure prediction by inverse velocity method,and the failure prediction of the landslide with retaining wall locked segment is carried out.The research results show that the predicted failure time is close to the actual failure time using the inverse velocity method with the onset of acceleration point as the starting point,but there is still a certain amount of lag.To study this problem,starting with the onset of acceleration point,the gradual approximation method is used to predict the failure time.Through calculation and analysis,the failure time prediction by inverse velocity method is carried out with the energy accumulation stage as the starting point,and the prediction results are consistent with the actual failure time.（6）Based on the systematic analysis of the engineering geological conditions and deformation characteristics of A’hai landslide with retaining wall locked segment,the influences of thickness,location and inclination angle of retaining wall locked segment on landslide deformation and stability are studied.The failure mechanism of A’hai landslide with retaining wall locked segment is further revealed.The increase in the thickness of retaining wall locked segment can improve the stability of landslide and reduce the deformation.The retaining wall locked segment is located in the middle and front of the landslide mass,which can effectively improve the overall stability of the landslide.The retaining wall locked segment is located in the middle and rear of the landslide mass,and has little effect on the stability of the landslide.For the A’hai landslide with retaining wall locked segment,when the retaining wall locked segment is perpendicular to the sliding surface,the locking effect is the most obvious,and the improvement of the overall stability of the landslide is the most obvious.When the angle between the retaining wall locked segment and the sliding surface is 25°,the sliding mass will slide along the contact surface between the retaining wall locked segment and the sliding mass.