Static Liquefaction Mechanical Behavior And Induced-sliding Mechanism Of Saturated Loess

In the Loess Plateau area(such as South Jingyang Plateau area in Shaanxi Province,Heifangtai area in Gansu Province,etc.),flow slide landslides are widely developed.Such landslides have the characteristics of sudden,high-speed and long-distance.They often affect a large area and are highly destructive,seriously threatening the safety of local people's lives and properties.It is found that the cause of this kind of landslide is closely related to the static liquefaction of the underlying saturated loess.At present,the evolution process,typical characteristics,influencing factors and mechanical behavior mechanism of static liquefaction of saturated loess are not completely clear,so it is urgent to carry out systematic research.Therefore,relying on the typical flow slide high-speed and long-distance landslides widely developed in South Jingyang plateau,Shaanxi Province,this paper carried out the static liquefaction mechanical behavior and induced-sliding mechanism of saturated loess under the support of the major project of National Natural Science Foundation of China,“mechanism and prevention and control of major engineering disasters on the Loess Plateau”(No.41790440),and the key project of National Natural Science Foundation of China,“response mechanism and early warning of human activities on Loess Landslides”(No.41130753).By means of on-site geological survey,micro-meso multi-element test and laboratory test,the research target of this study is: to comprehensively trace back the on-site evidence of the static liquefaction of flow slide landslide in South Jingyang plateau;to explore the internal relationship between the micro-meso structure morphology and potential liquefaction of loess;to reveal the mechanical behavior of static liquefaction of saturated undisturbed loess;to clarify the influencing factors of the static liquefaction capacity of saturated loess;to construct a method suitable for determining the static liquefaction capacity of saturated loess;to make clear the static liquefaction induced-sliding mechanism of saturated loess.The main research results of this paper are as follows:(1)The occurrence of the flow slide landslides in South Jingyang Plateau is closely related to the irrigation of the edge,and is controlled by the high-steep terrain characteristics of the edge,the precipitation and the irrigation of the edge,the crack of the edge of and the development characteristics of the ancient soil-loess interlayer sequence.Among them,the high-steep terrain form of the loess slope provides the initial potential energy for landslide;the precipitation and the irrigation provide a hydraulic source for the saturation process of the loess of the edge;the open penetrating cracks create the basic conditions for the acceleration of the infiltration of surface water;the development of the ancient soil-loess inter-layer sequence produces a typical hysteresis effect,which accelerates rapid water saturation and softening of overlying loose loess and induces static liquefaction.(2)The special physical properties and pore structure of the loess in South Jingyang Plateau are the internal causes of the static liquefaction.Among them,the grading state of the loess shows the characteristics of double peak type,and there is the characteristic of typical grain group dimorphism.The pore structure is dominated by the aerial structure,accompanied by the coexistence of the aerial structure and mosaic structure.The typical two grain structure and poor grading characteristics are the basic material conditions for the static liquefaction of loess.The structural conditions for static liquefaction are provided by using the silt aerial structure as the framework and the clay aggregates inlaid or filled in the pores or contact areas(3)Through the ICU test of saturated undisturbed loess under different confining pressures,it was found that the stress-strain curves of saturated undisturbed loess in South Jingyang plateau show the characteristics of strong strain softening under different confining pressures,and the stress-strain curves have no obvious regularity under different confining pressures.It reflected that the different layers and buried depth of saturated loess in South Jingyang plateau all have potential liquefaction capacity.(4)Based on the ICU test results of saturated undisturbed loess,two new ideas were put forward:(1)structural contact collapse stage;(2)pore throat blocking stage.Among them,structural contact collapse is the necessary prerequisite for strain softening and liquefaction of saturated loess,and it can be divided into two sub stages:(1)contact rapid hardening stage of aerial structure;(2)collapse rapid softening stage of aerial structure;pore throat blocking is an important condition for saturated loess to make pore water pressure rise and induce static liquefaction,and its evolution process embodies two aspects:(1)after structural collapse,the coarse particles arrange,recombine and stabilize;(2)the fine particles move,gather and block under the action of pore water pressure.Based on the mechanical mechanism analysis of the three-stage process of the stress-strain curve of the undisturbed loess,it was clear that the static liquefaction of undisturbed loess is the results of the coordinated coupling evolution of the contact cementation strength and the friction strength of the particles.A three-stage model of strength component deconstruction of saturated undisturbed loess was established.(5)Comparative analysis of static liquefaction tests of saturated loess under different conditions(initial pore structure,gradation,dry density and shear rate)was carried out.The influence of initial pore structure,grading state,dry density and shear rate on the static liquefaction of saturated loess was clarified,and the criterion for evaluating the static liquefaction capacity of saturated loess was proposed.Based on the liquefaction characteristics of saturated loess in South Jingyang plateau,the quantitative classification criteria and ideas for evaluating the static liquefaction capacity of loess in South Jingyang plateau were proposed.(5)Based on the analysis of the induced-sliding process of high-speed and long-distance landslides of South Jingyang plateau in Shaanxi province and Heifangtai in Gansu Province,the induced-sliding mechanism of the static liquefaction of saturated loess was further divided into:(1)the saturation process and evolution mechanism of loess overlying the relative aquiclude;(2)the liquefaction process and occurrence mechanism of loess overlying the relative aquiclude;(3)the critical induced-sliding and criterion of loess overlying the relative aquiclude;(4)the thixotropic liquefaction characteristics of the saturated loess and the mechanism of high-speed and long-distance slide.The occurrence process and internal mechanism were discussed.