Research On Strength Damage,stress Evolution And Fracture Slip Characteristics Of The Fault Zone

The fault zone is a kind of geological tectonic belt with complex structure.Due to compression and shear,the tectonic stress in the fault zone is complex and the primary structure of rock is seriously damaged.The existence of fault zones brings great challenges to the prevention of geological disasters and the development of geological resources in tectonic zone.Therefore,the research of fault zone evolution and deformation-slip mechanism is very important for earthquake science,safety engineering and mining engineering.Based on this,this paper focuses on the fault strength damage,stress evolution and fracture-slip characteristics of fault zone.Using experimental tests and theoretical analysis to study the evolution of rock damage structure and mesoscopic mechanism in the fault zone.In this paper,based on the plane strain assumption of isotropic elastomers,we established a physical model of the fracture zone.We revealed the effect of pore water on stress rotation in the fault damage zone,and revealed the slip characteristics of the fault rock after peak stress.The main research achievements are as follows:(1)We carried out tests to study the mineral composition,mesostructure and pore characteristics of sandstone in Longmenshan fault zone of Sichuan Basin.Results show that the main minerals of the sandstone in the Longmenshan fault zone are: quartz,feldspar and mica.Sandstone mainly contains mesopores and macropores,but no micropores,and the pore size is distributed between 2 nm and 10 μm.This research lays the foundation for the follow-up research on mechanical characteristics and deformation mechanism.(2)According to the growth curve of the normalized porosity,the degradation process of sandstone was divided into three phases.During the first stage,the initiation of new cracks dominates,the length of microcracks is short(≤ 200 μm),and the directions of microcracks are randomly distributed.The growth of intergranular cracks is slow,and the number of transgranular and trans-intergranular cracks does not change significantly.In the second phase,the growth rate of the normalized porosity increased linearly as a response to the percentage of failure.At this stage,although intergranular microcracks still dominate,there was a significant increase in the density of transgranular-intergranular microcracks and transgranular microcracks.Furthermore,microcracks developed in parallel with principal stress direction with the increase of the microcrack number and length.In the third phase,the normalized porosity increases at a large accelerating rate.Microcracks continued to grow along the major principal stress direction and penetrate each other.(3)The increasing-amplitude cyclic tests were carried out to study the influence of damage crack in the damage area of fault zone on rock mechanical properties and tectonic stress distribution.The dissipated energy damage model was used as the medium to connect the rocks with different distances from the fault core with the laboratory tests.Based on the physical model of the fault zone,the evolution of stress is discussed.In the damage zone of the fault,the maximum principal stress increases and the angle between the maximum principal stress and the fault plane decreases with the decrease of the distance from the fault core.In addition,the stress rotation rate in the damaged zone does not change uniformly with the proximity to the fault core.At the boundary between the damaged zone and the original rock zone,the stress rotation rate is very slow.With the approach to the fault core,the stress rotation rate gradually accelerates,and the stress rotation amplitude reaches the maximum at the boundary of the fault core.Pore water affects the stability of the fault zone through two aspects.On the one hand,pore water affects the rotation amplitude of stress in the fault zone and controls the stress distribution around the fault.On the other hand,pore water reduces the strength of local fault rocks through water-rock interaction.(4)The sliding test of intact rock was carried out,and the sliding weakening model based on brittle fracture propagation was established to describe the mechanical behavior of fault sliding under stress.According to the variation trend of shear stress with relative slip distance,the shear fracture process of intact rock was divided into slip strengthening and slip weakening stage.In the slip strengthening stage,the shear stress increases with the increase of relative slip distance.In the slip weakening stage,the shear stress decreases rapidly with the increase of slip distance.When it reaches a certain value,it decreases at a constant speed,and finally takes the stable value-residual friction stress.