Study On The Dip Effect Of Synsedimentary Ground Fissures

Ground fissures are cracks with a certain length and width formed on the surface under the natural factors such as tectonic movement,and the human factors such as resource exploitation.Based on the investigation of ground fissure disasters in China,it is found that ground fissures in some areas have the characteristics of synsedimentary structures.Moreover,the scope,angle and disaster characteristics of the affected zones are also different if the dip angles of structural ground fissures in deep strata are different in the process of extending to the surface.Combined on the research method of syn-sedimentary structure,and based on the physical model test of fracture propagation of syn-sedimentary ground fissure at 60°dip,the rupture propagation process of synsedimentary ground fissures with different dips and different sedimentary periods is simulated by finite difference software FLAC^3D and the discrete element software PFC^2D,and the basic law of synsedimentary ground fissures and dip effect are revealed.The main results are as follows:(1)Through field investigation and data collation,it is found that synsedimentary ground fissures and synsedimentary structures have many similarities in geometry,kinematics,dynamic genetic mechanism,development factors,etc.The specific performance is that the fault distance of synsedimentary ground fissures in many areas increases with the depth of the stratum.The thickness of the footwall of the same sedimentary layer is greater than that of the footwall.(2)Through the physical model test of 60°dip synsedimentary ground fissure fracture propagation,it is found that after each stage of synsedimentary activity,there is an anti-dip fracture and at least one steep dip fracture;According to the stress and strain monitoring results,both anti-dip cracks and steep dip cracks belong to tensile cracks;The plane characteristics are mainly arc and parallel,and the profile characteristics are mainly reverse"Y"type,compound reverse"Y"type,"arch"type and compound"arch"type;The synsedimentary ground fissure fracture propagation development model is divided into three types:the fracture propagation model in the same sedimentary period,the ground fissure fracture propagation model between different sedimentary periods and the"arch"ground fissure fracture propagation model between different sedimentary periods;Combined with the stress and strain monitoring results,the influence width of crack propagation is determined and divided into two types:main deformation zone and micro deformation zone.(3)Using the finite difference software FLAC^3Dand the discrete element software PFC^2D,three different dip angles(60°,75°and 90°)of the synsedimentary ground fissure fracture propagation tests are simulated.The results show that in synsedimentary activities,the positions of anti-dip fractures and steep dip fractures are the maximum tensile stress and maximum tensile stress in horizontal direction,which are mutually supported by the stress and strain monitoring results in physical model tests.It is verified that steep dip fractures and anti-dip fractures belong to extensional ground fissures;The smaller the inclination angle of cracks,the larger the maximum stress area produced in the newly deposited soil layer,the greater the traction force of ground fissure activity on the soil,the faster the time,and the greater the change of stress area produced on the surface.The development process and position of microfractures are basically consistent with the physical model test.Steep-dip fractures appear at the maximum displacement change in the horizontal direction,and reverse-dip fractures extend upward from the steep-dip fractures produced in the previous sedimentary period.Its position mainly appears at the displacement change in the vertical direction;It is found that the smaller the dip angle of the preset fissures,the larger the angle between the anti-dip fissures produced by the new overburden and the horizontal plane,and the larger the width of the influence zone is.