Analysis On The Formation Mechanism Of Ground Fissure In Zhuozhou Country Of Hebei Province

In order to reveal the genesis and development mechanism of ground fissures in Zhuozhou City,based on the field investigation,trench excavation and drilling engineering,this paper analyzes and summarizes the plane development and distribution characteristics,profile structure characteristics and periodic activity characteristics of ground fissures in the study area,and analyzes the change rules of the shape and displacement of the overlying soil under the tensile stress environment of the anatomic crust basement through the physical simulation test Fracture development and fracture process,as well as structural characteristics of the section.The main conclusions are as follows:1.The study area is located in the northern part of jiduan depression.Quaternary strata are widely distributed in the area.The strata are mainly fault block structures with strong neotectonic activity.The main faults in the area are Babaoshan fault,Huangzhuang Gaoliying fault,Shunyi Liangxiang fault,Daxing fault and dongleizi Laishui fault.Since Neogene,these faults are mainly strike slip extensional movement,and cause NE-NEE tensile stress on shallow surface.The study area is located at the intersection of Yanshan seismic belt and Taihang Mountain front seismic belt,which is currently in the fourth period of seismic activity-high tide period.With the continuous enhancement of human engineering activities,groundwater overexploitation is serious,which causes ground subsidence and accelerates the development of ground fissures.2.Field investigation shows that the ground fissures in the study area are mainly distributed in diaoweo Township and quay town.As of January 2019,65 dew points of ground fissures have been found in the area,and they are mainly distributed in three sections:(1)diaoweo village 1 ground fissure zone(Group),which is in the shape of"Y"and strikes near East-West;(2)diaoweo village 4 ground fissure zone(Group),which is in the shape of straight line and strikes nw285 degrees;(3)Fuluo village Yingcun ground fracture zone(Group),in a straight line,strike nw330 degrees.In terms of time,the activities of ground fissures in the study area have a certain periodicity,about 3-4 years as a cycle,and the occurrence time of ground fissures on the surface is concentrated in July,August and September every year.According to the trench exploration in the typical area,the ground fractures in the study area are tensile fractures and have the characteristics of multiple periodic activities.There is no vertical dislocation and strike slip along the fracture surface.According to the drilling data,it is found that the upper stratum of the study area is silty soil with poor physical and mechanical properties,while the lower stratum is silty sand and fine sand.The stratum is continuously flat,and there is no trace of fault extending upward and penetrating shallow surface activity.3.There are two kinds of geometric movement characteristics of ground fissures in the study area,one is horizontal tension movement which is perpendicular to both sides of the fissure face,the other is horizontal tension movement which is perpendicular to the plane of the fissure face,at the same time,some settlement occurs in the section where the ground fissures develop.The characteristics of ground fissure activity intensity are as follows:the maximum horizontal tension rate of the wall crack in diaowo village 1 is about 0.19cm/a,the expansion rate of the length of the ground collapse pit is about 0.06m/a,the expansion rate of the width is about 0.03m/a,and the expansion rate at the depth of 0.6m is about 0.01m/a.The maximum horizontal tension rate of the wall crack in fuluoying village is about 0.19cm/a,the expansion rate of the length of the ground collapse pit is about 0.13m/a,the expansion rate of the width is about 0.11m/a,and the expansion rate at the depth of 0.6m is about 0.04m/a.4.The formation of the ground fissures in the study area is closely related to the regional tensile stress field and hydrodynamic(groundwater,surface precipitation)activities.The basement tensile stress field is the main reason.The hydrodynamic force accelerates the development of the fissures and makes the ground fissures exposed to the surface.The genetic model of ground fissures is:basement tension(Foundation)-surface precipitation and groundwater level decline(accelerated development).5.The results of horizontal tension test of physical model show that:1)There are three types of cracks in the soil:(1)the main crack(f₁)which starts from the preset crack at the bottom of the model and starts to zigzag upward and runs through the whole soil longitudinally,and tends to the fixed plate at the depth of 15-55cm,tends to the moving plate at the depth of 0-15,the width of the lower part of the crack is 3-5cm,and the width of the upper part is 1-3cm;(2)the secondary crack formed by the unloading tension of the soil caused by the shallow(f₁)crack(f₁₂,f₁₃)is vertically expanded from the top to the bottom of the model surface.(3)the secondary cracks(f₂-f₁₁)formed by tensile stress in the middle of the mobile disc are located at the bottom boundary of the soft layer(silica gel),and extend upward,downward or bidirectional.The inclination is larger or even erect,and generally tends to be fixed.2)The process of soil extension and deformation can be divided into five stages:the main crack(f₁)initiation,the main crack rapid expansion and the secondary crack(f₂,f₃)initiation;the main crack(f₁)accelerated development,the secondary crack(f₂,f₃)continued expansion and the secondary crack(f₄-f₇)initiation;the main crack(f₁)fracture extended to the surface and the secondary crack(f₂-f₇)continued development and expansion;the early stage The fracture continued to expand,unloading tension secondary fracture(f₁₂,f₁₃)and secondary fracture(f₈-f₁₁)started to crack;the early fracture continued to develop,unloading tension secondary fracture(f₁₂,f₁₃)and secondary fracture(f₂-f₁₁)accelerated to develop.3)There is obvious settlement phenomenon in the soil mass corresponding to the main crack(f₁),and the distance from the main ground crack f₁ is different,so is the settlement and thinning amount of the soil mass:the settlement and thinning amount in the soil mass close to the ground crack is large,and the settlement and thinning amount in the soil mass far away from the ground crack is small.4)The total width of the impact zone of the main crack(f₁)is 155cm,of which the width of the serious impact zone is 75cm,and the width of the general impact zone is 80cm.The total width of the impact zone corresponding to the actual distance is 46.5m.The width of severe impact zone is 22.5m,and that of general impact zone is 24m.