The Effects Of Activity Rate On Propagation Of Buried Ground Fissures

The ground fissure is another natural disaster which is independent of landslide,debris flow,collapse,land subsidence and land subsidence in the development of human society.It is a kind of shallow rupture phenomenon on the earth's surface and a geological hazard phenomenon restricted and influenced by many complicated factors as well.At present,most of the ground fissures that have been identified or exposed are surface cracks.In fact,the number and scope of buried ground fissures are more and wider.The disaster characteristics,economic losses and social effects caused by the different activity rates on the fracture propagation of these buried ground fissures may also be quite different.In this paper,the general situation and the activity law of the Xi'an ground fissures have been collected and sorted out;Two sets of physical model tests of the fissure propagation of the buried ground fissures with different activity rates have been carried out.By using PFC~2DD discrete element numerical simulation software,six groups of numerical simulation tests of different activity rates under different overlying soil thickness are carried out.The concrete ideas and results are as follows:1.By collecting and sorting out the characteristics of the Xi'an ground fissure movement,the relationship diagram between the activity rate and the width of the influence zone in Xi'an ground fissures has been drawn out,and the research topic of this paper is introduced.2.Two sets of physical model tests for the propagation of buried ground fissures at different activity rates have been carried out.When the buried fissures move rapidly(collapsing):in planar and profile features,the reverse dip fracture is expanded by large dip angle,the width of influence zone and fracture zone are narrow,the degree of crushing is small;In the stress state,the vertical stress and horizontal stress in the hanging wall and footwall are declined slightly,and then slowly increased,and finally reached to the state of self-gravity stress.On the surface displacement,the displacement include vertical subsidence and horizontal displacement,and the horizontal displacement accounts for about 2/3 of the vertical subsidence,the absorption on hanging wall bottom movement effect is more obvious than the slow rate.When the buried fissures move slowly,in planar and profile features,reverse dip fracture is expanded by small dip angle,the width of influence zone and fracture zone are wide,the degree of crushing is big,and the development of vertical fissures is deep;In the stress state,with the beginning of buried ground fissures,the vertical stress and horizontal stress of hanging wall increase slowly and the footwall is on the contrary,the vertical stress and horizontal stress reaches the state of the self-gravity stress after buried ground fissures stop movement.In the surface displacement,the vertical subsidence is the vast majority,and the horizontal displacement is little.3.Four groups of numerical simulation tests of different activity rate and overlying soil thickness were carried out.The results show that:When the buried ground fissure active rapidly,the propagation angle of the reverse dip fissure is larger,and when the buried ground fissure moves slowly,the propagation angle of reverse dip fissure is smaller,which is consistent with the result of the physical model test,and when the overlying layer is thick,the result is similar to the physical model test,the reverse dip fracture does not extend to the surface of the model,and stop to propagate with the vertical fracture appears.When the"wedge body"collapses,the reverse dip fracture is closed gradually.When buried ground fissures move fast,the vertical stress of the hanging wall of the model changes from compressive stress to tensile stress and compressive stress when the overlying soil is thin,but the vertical stress of the hanging wall is always the compressive stress when buried ground fissures moves slowly.4.The relationship between the crack initiation angle and the activity rate of the buried ground fissures have been analyzed based on the theory of maximum circumferential tensile stress intensity factor,that is,when the buried ground fissures move fast(collapsing),the reverse dip crack has a large crack rise angle.When moving slowly,the angle of initiation is small.