Study On The Effect Of Fracture Propagation And Compaction Of Syndepositional Ground Fissures

As a typical geological disaster,ground fissures are particularly serious in Fenwei Basin,North China Plain and Suzhou-Wuxi-Changzhou area of China,among which Xi’an ground fissures are the most famous urban geological disasters in the world.Based on the field geological survey and collected data,it is found that most of the tectonic ground fissures show obvious syndepositional characteristics.In view of the fact that the ground fissures have different activity rates and show different compaction States of the overlying soil under the action of syndeposition,the rupture propagation of syndepositional ground fissures under different compaction conditions is studied.Through physical simulation and numerical simulation,the whole process of rupture propagation of syndepositional ground fractures with an inclination of 75 ° is simulated and reproduced.The compaction effect of syndepositional ground fissures is revealed,and the corresponding results lay a theoretical foundation and basis for the development,utilization and comprehensive control of deep underground space in the area of ground fissures,which has important scientific and practical significance.The main achievements of this paper are as follows:(1)Syndepositional ground fissures have the characteristics that the thickness of the hanging wall is larger than that of the footwall in the same sedimentary layer,and the dislocation distance gradually increases with the increase of depth.(2)The results of physical simulation tests under compaction conditions show that there are mainly two types of ground fissures in the profile,namely,nearly vertical fissures and anti-tilt fissures.Most of the fractures in the hanging wall section are antidip tensile fractures with a dip angle of 60 ° ～ 85 °,which are combined with the preset fractures in a "y" shape;most of the fractures in the footwall section are nearly vertical horizontal tensile fractures.There is a through surface crack on the upper and lower walls respectively,and the width of the crack gradually widens with the decline of the upper wall,and the width is concentrated between 5 and 10 mm.The width range of the surface fracture zone of the hanging wall and footwall has a small difference,and the single shape is arc.(3)The test results under non-compaction conditions show that there are nearly vertical fractures,anti-dip fractures and dip fractures in the profile.The three types of cracks are mainly developed in the projection of the preset cracks and the superficial part of the model footwall,and with the increase of the number of test periods,the number of anti-dip cracks and down-dip cracks developed upward on both sides of the preset cracks decreases,and the extension length becomes shorter.The subvertical fractures are only developed in the footwall and gradually develop to the edge of the model,and the number of developed fractures is large,but the extension is short and the width change is small.The width of footwall fracture zone increases with the increase of soil thickness,and the shape of single surface crack is arc.(4)The numerical simulation reveals the fracture propagation process of syndepositional ground fissures under compaction and non-compaction conditions.The simulation results show that at the beginning of the hanging wall activity,less secondary small fissures are developed at the pre-set fractures,and with the continuation of the decline,these small fissures extend and connect with each other,forming anti-dip and dip cracks,and nearly vertical cracks are developed in the shallow part of the soil layer;With the further increase of activity,the original fractures extend and develop along the direction of dominant fracture,and some of them are connected with each other,eventually forming a certain scale of fracture concentration zone.The difference is that in the compaction condition,the along-dip and anti-dip cracks extend to the surface with the descending of the hanging wall,and local particle falling occurs in the upper part of the model,resulting in the widening of the nearly vertical cracks,while in the non-compaction condition,the along-dip cracks do not penetrate to the surface with the descending of the hanging wall.The numerical simulation results are consistent with the physical simulation results.(5)The development of the cracks in the compaction test follows the law of "local new growth→multiple development→extension and development→collapse and fall→expansion and formation",and the generation of the non-compaction cracks follows the law of "local new growth→development in the zone→penetration and extension→expansion and formation".(6)The width of the influence zone increases with the increase of soil thickness in both cases,and the width of the influence zone in the non-compaction case is significantly larger than that in the compaction case.