Analysis Of The Causes Of Ground Fissures In Xiong'an New Area Based On The Characteristics Of Rock And Soil Mass Changes

Xiong'an New District is located in the hinterland of Beijing-Tianjin-Baodin,one of the regions with densely developed fissures.Studying the development mechanism of ground fissures and the deformation characteristics of the surface soil layer in the new district has great significance to local basic planning and construction and disaster prevention.Based on the field investigation of ground fissure,the geotechnical test and physical simulation test,the variation characteristics of rock and soil mass in the ground fissure zone in the new district are analyzed and studied.Mainly achieved conclusions are following:1.The Xiong'an New Area is located in the northern part of the Jizhong Depression,and there are many tertiary structural unit faults.After the Neogene,all the faults,except the Niudong Fault,have ceased to operate,and the activity intensity of the Niudong Fault is gradually weaken.The area is covered by huge Cenozoic strata,and it is difficult to see the bedrock exposed.2.The groundwater in the study area mainly exists in the aquifer composed of silty soil,and the thickness of the aquifer varies greatly.There are four aquifer groups from top to bottom;groundwater level monitoring data shows that in recent years,due to overexploitation of groundwater The groundwater level in the area is decreasing year by year,which causes large-scale ground subsidence.About 72%of the area in the study area has a land subsidence rate exceeding 10 mm/a.The severe and subsidence areas are concentrated in the north of Xiong County,and the general trend of the land subsidence rate change in the study area has been increasing year by year.3.The investigation shows that the ground fissures in the study area are concentrated in the area of Daying Town-Zhugewangzhuang-Xiongzhou Town in Xiong County,and the surrounding areas centered on Dahe Town and Rongcheng County in Rongcheng County.As of January 2019,a total of 54 ground fissures were found in the area,42 of that appeared after January 2009.The ground fissures in the study area are mainly exposed in the form of irregular,circular,elliptical and strip-shaped collapse pits.The development scale of each crack is unequal and the difference in strike direction is significant.Shallow,with a trumpet shape that is wide at the top,narrow at the bottom,and nearly upright,with a width of about 5m at the widest point.The gap is loosely filled by late-stage mixed soil.Tensile fractures have the same stratum thickness on both sides of the fracture at the same time.There are no signs of relative displacement in the direction of the vertical fracture surface,and no signs of relative strike-slip movement along the fracture surface.4.The geotechnical test results of the ground fissure zone in the study area show that its physical and mechanical properties show different characteristics at different depths.1)The shallow surface soil of the ground fissure zone has lower water content and density,higher porosity,larger average particle size and lower shear strength than the undisturbed soil at the same depth away from the ground fissure;2)Compared with the undisturbed soil at the same depth,the soil layer below the fracture zone has larger water content and density,lower porosity,smaller average particle size and higher shear strength.5.The physical model test of ground fissure development shows that under horizontal tension:1)Three types of cracks occur in the soil:primary tensile cracks,secondary tensile cracks and unloading tensile cracks.Among them,the main tensile cracks(f₁)cracked from the prefabricated cracks in the bottom plate,developed in a zigzag upward and finally cut off the entire soil body,and were divided into upper and lower sections according to the difference in tendency:the upper section tends to move the disk,the lower section tends to the fixed plate as a whole;secondary tensile cracks(f₂?f₁₁)are from the inside of the soil,and it develops upward,downward or bidirectionally along with the stretching process.It is concentrated on the moving disk.The appearance is steep or even nearly upright.The overall tendency is to fix the disk,and it can be changed according to the shape.It is divided into two types:linear type and(reverse)S type;unloading tensile cracks(f₁₂,f₁₃)extend downward from the surface of the fixed disk soil body,and are generally upright.2)There is obvious uneven settlement of the soil near the main crack,and the settlement zone of the soil layer is different at different depths.3)The density of the soil near the main crack is greatly reduced,with a decrease of 9%relative to the background value.The density of the soil on the side of the moving disk is slightly reduced,and the reduction of the soil is 3%relative to the background value.The density has not changed basically;combined with the development of cracks and the location of the density zone,the influence of the extension deformation on the width of the belt is 130cm.Among them,the general influence is 55cm and the serious influence is 75cm.In practice,the ground fissure influence zone may be 39m wide,which is generally Affects the bandwidth of 16.5m and seriously affects the bandwidth of 22.5m.6.The observation results of the microstructure of the soil show that:1)The simulated soil has the following microstructural characteristics under tensile deformation:Compared with the undisturbed soil at the same depth,the cracks at each depth affect the pore size of the soil,the compactness and the particle orientation The degree of arrangement is high,and the particle size is unchanged.2)The microstructure of the ground fissure zone soil in the study area has different characteristics at different depths.Among them,the shallow surface soil of the fissure zone has a larger pore size and particle size than the undisturbed soil of the same depth,and the degree of compaction and particle orientation Compared with the undisturbed soil at the same depth,the lower part of the soil in the ground fissure zone of the new area has a smaller pore size and particle size,and a higher degree of compaction and orientation of particles.3)Comparing the microstructure characteristics of the ground fissure zone soil and the simulated extensional deformation soil in the study area,it is found that the overall microstructure changes of the two are similar,indicating that the physical simulation of the extensional deformation test of the soil reveals the soil in the study area to a certain extent.Microstructure characteristics of the body after deformation and cracking in the horizontal tensile stress field.7.According to the characteristics of the geological environment in the study area,the development and distribution characteristics of the ground fissures and the characteristics of the profile structure,combined with the comparison results of the macro-and micro-structure characteristics of the ground fracture zone and the physical simulation of the tensile deformation fracture zone soil,the analysis believes that:disorder development Geothermal resources and excessive drainage of groundwater triggered ground subsidence in the study area,and uneven ground subsidence induced differential compaction deformation and destruction of shallow soil,which promoted the development of ground fissures.The development process of ground fissures in the study area is summarized as"uneven ground settlement-ground fissure expansion and development-later hydrodynamic transformation"...