Research On Mechanism Of Loess Joints And Fissures Under Unloading Of Loess Cut Slopes In NW China

Loess collapse is one of the biggest harmful geological hazard in loess area.Practices indicate that artificially unloading by cut slope is one of main factors resulted in loess collapse. With the development of economic speed,the increase of residents,the continuously expanding of the city,in order to expand construction site,there will a large deal of cut slope engineering,more and more potential collapse hazards appear thereupon too.In this dissertation, in order to meet the needs of studying on mechanism of loess collapse under the action of unloading by cut slope and practice projec,a comprehensive study on propagation mechanism of joints and fissures in loess body induced by cut slope was made. First of all,analyzed basic mechanical parameters and physical parameters of Q3 intact loess in Luo Chuan. Secondly,apply three apparatuses in the unsaturated soil(GDS),and has carried on unloading test of loss in different conditions. Using CT scanner to observe meso-structure and observe microcosmic structure of loss with scanning electron microscope before and after unloading test,study on influence factors and propagation mechanism of joints and fissures in loess body.At last,based on the results of physical and mathematical simulation of the vertical fractures propagation process using FLAC3 D, summarized the propagation mechanism of joints and fissures under the action of unloading by cut slope.And give some suggestions for practice project.The result shows,strength of loess under triaxial test on lateral unloading is higher than conventional triaxial test result.With the increase of moisture content and initial confining pressure,the stress-strain curves of specimen gradual transition from softening to hardening,and specimen shows shear dilatation characteristics in the process of unloading. Soil samples have different deformation and failure characteristics under the different conditions ofunloading. Generally,failure mode can be divided into splitting failure,slip failure,and plastic failure. After unloading, most of the CT number in different regions of the soil specimen’s section would be enhanced with the increase of the unloading weight, the unloading velocity, moisture content, and initial confining pressure. If initial confining pressure is high,there would be a big rate of increase amplitude of the CT number.Most of the variance in central area of specimen increase,but most of the variance in macrozone and whole area decrease. unloading conditions have different influence on extending of joints and fissures. Only reach a certain unloading weight,can the unloading crack appear. The higher the unloading velocity is,the lower moisture content is、the lower initial confining pressure is,the more massive fissures soil has. Contrarily,microfissures develope well.