Experimental And Numerical Simulation Study On Sand Gravel Layer Collapse Induced By Tunnel Lining Damage

With the acceleration of the development of underground space in China,the following problems emerge one after another.Water leakage and sand leakage is a typical disease in underground projects such as tunnels.The water and soil loss caused by water leakage and sand leakage will further lead to sand bursting disaster,resulting in surface settlement and even formation collapse.As a geological disaster,the formation collapse accidents caused by sand burst are often sudden,unpredictable and the consequences are serious.Due to the complexity of on-site hydrogeological conditions and the constraints of on-site environmental conditions,we are unable to carry out relevant experimental research on sand burst disaster on site.In order to deeply understand the sand burst disaster,this paper explores the main influencing factors and formation mechanism of collapse and sand burst through indoor model test and numerical simulation.The main research results are as follows:(1)When the thickness span ratio is greater than 11.1,the collapse is mainly controlled by the thickness span ratio,and the soil layer does not collapse at this time;When the thickness span ratio is greater than 4 and less than 11.1,the collapse is controlled by the thickness span ratio and the gravel particle content.When the gravel particle content is greater than the critical gravel particle content,the soil layer only collapses but does not break sand.When the gravel particle content is less than the critical gravel particle content,the soil layer breaks sand;When the thickness span ratio is less than 4,no matter how large the gravel particle content is,the soil layer will collapse.(2)The sand breaking process can be roughly divided into four stages: initial stage,penetration stage,diameter expansion stage and stability stage;With the increase of gravel particle content,the collapse range of soil decreases first,then increases,and then decreases;The collapse depth and collapse influence range under the critical gravel particle content are the minimum under the same thickness span ratio;The depth of collapse center and the influence range of collapse increase with the increase of soil thickness.(3)The collapse induced by circular leakage is circular on the plane,approximately conic on the section and funnel-shaped on the facade;The collapse range of soil after sand bursting is related to the shape failure angle,which is 18 ° larger than the natural repose angle of soil.(4)In the process of sand bursting,the internal compactness of soil decreases first and then increases;The displacement and deformation of soil are inversely proportional to the thickness span ratio.With the increase of the thickness span ratio,the skeleton effect of gravel particles becomes more and more obvious.If the thickness span ratio continues to increase after it is greater than the critical thickness span ratio,the number of calculation sub steps required to form soil arch will gradually decrease with the increase of the thickness span ratio.(5)The change of load on the floor and the change of coordination number between particles can be used as an index to reflect the formation of earth arch.The fluctuation of floor load and coordination number shows that the formation process of force arch is a continuous cycle process of formation failure re formation re destruction.(6)The content and shape of coarse particles jointly determine whether the soil can form soil arching effect.The higher the content of gravel particles in the soil,the easier it is to form soil arching at the leakage,and when the ratio of long and short axes of gravel particles l > 2.0,it is easy to form soil arching.