Genetic Mechanism Of Fault-Controlled Ground Fissures Under Extensional Action

Fault-controlled ground fissure s are typical fissures in which fault structure is the dominant factor affecting the formation and development of fissures.They are formed under the influence of fault activities and develop with fault activities,and there is a certain corresponding relationship with them in terms of temporal and spatial distribution,basic characteristics and formation reasons.Taking Handan ground fissures as an example,they are distributed in the hanging wall side of Handan fault,and their formation is affected by NNE-trending tectonic stress,so they are defined as associated ground fissures.It is the most urgent problem to solve the geological disaster of ground fissures to carry out the research on the cause and disaster mechanism of ground fissures.In this context,this thesis studies the causes of ground fissures and the propagation mechanism of fault-controlled ground fissures in Handan City through physical model tests and numerical simulation,and draws the following conclusions:(1)The distribution and development characteristics of ground fissures in Handan were investigated.Handan ground fissures are linear,most of them strike north-south,the rest along the east-west distribution.In the plane,there are 5 parallel fissure zones with equal spacing in the north and south,and their strike is the same as that of Handan fault.The height of the fissure on both sides of the section is different,and the west is high and the east is low.(2)The fault-controlled extension process of Handan fault is simulated.Fissure development is divided into three stages,namely initial fissure,fissure emergence and fissure development.The deep soil layer cracks first at the main fault,and three secondary faults are formed in the hanging wall.The secondary faults extend upward along the opposite direction of the fissure tendency.Finally,the secondary faults run through the soil layer and connect with the surface fissures.The surface soil cracks in the process of secondary fissure propagation,forming five small cracks parallel to the main fault,and forming two parallel fissure zones on the plane.The surface cracks increase with the opening degree of active extension,which is defined as tensile cracks.The surface soil layer in the fissure zone has obvious subsidence,and the subsidence difference is large.The soil layer in the hanging wall and footwall near the fault is in a state of tension,and the transverse strain near the fault increases.(3)The differential settlement of Handan ground fissures is simulated numerically.When the deep fault is not active,the overlying strata will also be affected by the phenomenon of differential settlement.The differential settlement is more obvious with the fault activity,and shear fissures in the same direction as the fault are prone to appear in the strata,which may present a "stepped" landform on the surface.Fault activities will produce horizontal displacement on the surface,and the contour lines of the strata near the fault are densely distributed.The horizontal displacement changes greatly,and the variation increases with fault activities.The horizontal displacement difference on the surface is the greatest,and tension cracks may appear.(4)The extensional associated mechanism of Handan ground fissure and Handan Ground fault is analyzed comprehensively.Handan ground fissure is formed by the Handan ground fissure as the main control factor,seismic action and excessive exploitation of groundwater.The creep of hanging wall strata is controlled by the geostress field.Under the influence of seismic action and excessive exploitation of groundwater and other factors,the soil layer properties change and further accelerate the fault activity.Handan ground fissures are associated with Handan faults.It is inferred that there are secondary faults corresponding to Handan ground fissures in the soil layer below five fissure zones.These secondary faults appear and extend on the main faults,and their tendency is opposite to that of the main faults.There are 79 figures,6 tables and 72 references in this thesis.