Study On Mechanical Characteristics And Stability Of Tunnel Under Overbreak

Overbreak is a common problem in tunnel drilling and blasting construction.At present,domestic and foreign scholars have little research on the three-dimensional stability of tunnel overbreak,and there is little research on the mechanism of tunnel collapse after overbreak.Based on the overbreak phenomenon in practical engineering,this dissertation studies the stability of surrounding rock and supporting structure during overbreak by using the method of numerical simulation,and focuses on its three-dimensional mechanical characteristics.At the same time,based on the upper bound method of limit analysis,the failure mode of tunnel collapse surface during overbreak in arch is established,and the change of collapse surface shape during overbreak is studied.The main research results are as follows:(1)The stability of Maodong surrounding rock caused by over and under excavation is studied from five aspects: surrounding rock grade,excavation method,over and under excavation position,over and under excavation height and angle.(2)The mechanical characteristics of tunnel support structure under overbreak are systematically studied,and the influence of overbreak of vault,spandrel and side wall on tunnel is analyzed from two aspects of overbreak height and overbreak angle;The deformation,stress and internal force of the initial support of the tunnel after overbreak catchment are obtained.(3)Using the method of numerical simulation,a three-dimensional finite element model is established to simulate the overbreak in front of the tunnel during tunnel excavation,and the changes of surrounding rock displacement,stress and internal force of the supporting structure are obtained.Its influence on the three-dimensional spatial stability of the tunnel is studied from three aspects: overbreak height,overbreak angle and overbreak footage.(4)Based on Hoek Brown strength criterion,the failure mode of tunnel collapse shape is established by using the upper bound theorem of limit analysis.Using the principle of variational method and a series of formulas,the shape function of tunnel collapse surface under overbreak is obtained,and the sensitivity of relevant parameters is analyzed.