The Study On The Fracture Propagation Of Co-sedimentary Buried Ground Fissures Under Different Activity Rates

As a new direction of ground fissure research,compared with ordinary ground fissures,the development depth of the co-sedimentary ground fissures is greater,and their deep fracture patterns,cross-sectional structural characteristics and influence ranges are still unkown,which brought huge unknwns for the development of the deep underground space in cities.In the in-depth investigation and research of co-sedimentary ground fissures in Xi'an area,it is found that the co-sedimentary ground fissures in different activity rate regions show different development characteristics on the section.Therefore,based on the Natural Science Foundation project"Study on the mechanism and effect of ground fissure propagation under syn-sedimentation",taking co-sedimentary concealed ground fissures in loess as the research object,Conducted a set of physical model tests on the rupture and propagation of co-sedimentary concealed ground fissures under rapid activity of 2.25m/hour,and PFC^2D discrete element simulation software were used to study the rupture and propagation of co-sedimentary concealed ground fissures under three different activity rates,and provide relevant theories for the development of underground space in the section where co-sedimentary concealed ground fissures are developed under different activity rates.The main conclusions of this article are as follows:(1)The development characteristics of co-sedimentary ground fissures and the activity characteristics of syn-sedimentary ground fissures in various structural ground fissures were summarized.Co-sedimentary ground fissures usually develop in Quaternary sediments,the amount of stratum dislocation increases with the increase of depth,and the thickness of the descending plate is thicker.Its activity modes include continuous activity under syn-sedimentation,intermittent activity under syn-sedimentation,and intermittent activity of syn-sedimentary ground fissures during earthquakes.(2)A physical model test study on the rupture and propagation of syn-sedimentary concealed ground fissures under an activity rate of 2.25m/hour was carried out.Tests have found that under rapid activity,The vertical fractures and anti-dip fractures appear almost simultaneously.The rupture and propagation of anti-dip fractures is usually caused by the falling off of the upper soil body after an anti-dip fractures stops developing,and another anti-dip fracture appears.It has strong penetration and the section after excavation shows the integrity of the fracture zone is better.Co-sedimentation has a greater impact on the rupture and propagation of anti-dip fractures,which are mainly manifested in the healing of the original anti-dip cracks and the generation of new anti-dip fractures.The healed fractures will not appear new ruptures in the subsequent settlement activities.However,the later anti-dip fractures mostly extend upward along the newly-born anti-dip fractures generated under syn-sedimentary action,and the dip angle of the anti-dip fractures becomes slower toward the upper part.The vertical fractures spreads downward from the top of the footwall,with the thickening of the sedimentary caprock,the it tend to shift upward.In the fourth stage of this model test,the anti-dip fractures began to rupture and propagate along the third-stage vertical fracture.The influence range of the main rupture sphere is-0.4-0.4m,of which the hanging-wall part is 1-4times that of the foot-wall part.The proportion of the foot-wall part increases with the thickness of the deposited cap layer,and the deformation angle of the lower wall is Between 75°-82°.(3)Combining with the co-sedimentary ground fracture rupture propagation test under the slow activity of the previous research group,it can be seen that as the activity rate increases,the correlation between the fractures in each period decreases,and the co-sedimentary effect has an increasing effect on the fracture rupture and propagation.The section after excavation shows that the integrity of the soil in the deformation zone is also better,while the inclination angle,the location of the rupture and the width of the rupture zone,etc.of vertical cracks and anti-dip cracks have small changes.(4)Three groups of PFC^2D discrete element numerical simulations of the rupture and propagation of co-sedimentary buried ground fissures under different activity rates were carried out.The numerical simulation results are basically consistent with the model test results.Regardless of the speed of the activity,with the increase of settlement,two types of cracks appear in the model soil,vertical and anti-dip.The faster the activity rate,the greater the influence of syn-sedimentation on the fractures,and the smaller the correlation between fractures in each period.However,when the activity rate exceeds a certain value,the crack propagation phenomenon tends to be consistent.