Viscoelastic Mechanics Analysis Of Surrounding Rock And Rock Bolt In Underground Tunnel

In recent years,industries such as mining,transportation,and hydroelectricity have been involved in a large number of tunneling projects,and improving the safety of tunneling projects has become increasingly important.During tunnel excavation,stress concentration occurs at the excavation boundary,and when the surrounding rock has low strength,damage can easily occur,requiring certain support measures to be taken for the tunnel.Rock bolt support is a cost-effective and highly applicable means of providing support resistance quickly and has been widely used for reinforcing surrounding rock and stabilizing underground chambers.However,the surrounding rock often exhibits timedependent rheological characteristics,which increase deformation over time and cause damage to rock bolt support structures,eventually leading to disasters such as collapse and tunnel instability.Therefore,rheological mechanics analysis of the surrounding rock and rock bolt support structures has important theoretical significance and engineering reference value.This paper considers the rheological characteristics of rock mass,uses the generalized Kelvin model to simulate the surrounding rock,and based on the complex variable function method and the corresponding principle,studies the deep circular and elliptical tunnels with point-anchored rock bolt support.The main contents and results are as follows:(1)An analytical method for solving the axial force viscoelastic solution of pointanchored rock bolt support in deep tunnels is established.Considering the two-way unequal original ground stress and the support lagging effect,the action of each rock bolt on the surrounding rock is simplified into a pair of equal and opposite concentrated forces.Based on the complex variable method,the equation for solving the elastic solution of the axial force of the rock bolt is derived.According to the corresponding principle,the Laplace transform is applied to the equation to obtain the expression for solving the axial force in the Laplace domain.Using the numerical method of Laplace inverse transform,the viscoelastic solution of the axial force of the rock bolt is obtained by inverse transformation of the value of the axial force in the Laplace domain.When solving the elliptical tunnel,the conformal transformation is used to map the domain outside the circular boundary of the elliptical tunnel to the domain outside the unit circle,greatly simplifying the problem-solving process.The obtained analytical solution is verified using ANSYS software for numerical simulation.In line with the assumptions in the analytical solution,spring elements are used to simulate the point-anchored rock bolt in the numerical method,and numerical models of circular and elliptical tunnels under rock bolt support are established respectively.The results obtained from the analytical and numerical methods are compared,and it is found that the two are in good agreement,verifying the correctness and applicability of the analytical method.(2)The effects of lateral pressure coefficient,Young’s modulus,rock bolt length,viscosity coefficient,preload force,and semi-axis ratio on the axial force variation with time are discussed.The results show that the axial force of the rock bolt gradually increases with time,changing rapidly in a short time and more slowly over a long time.The axial force of the rock bolt arranged along the direction of larger ground stress is the largest,and the axial force of the rock bolt arranged along the direction of smaller ground stress is the smallest.The softer the surrounding rock,the greater the axial force borne by the rock bolt.After the rock bolt’s length reaches a certain value,the axial force decreases with the length’s increase,this provides a theoretical basis for the reasonable selection of rock bolt length.The smaller the viscosity coefficient,the larger the axial force and the rate of axial force change on the rock bolt,and the shorter the time to reach a stable value.The larger the initial preload of rock bolt,the larger the final axial force on it,and the two are linearly related.Under the elliptical tunnel,the semi-axis ratio corresponding to the maximum axial force of the rock bolt at different angles is different.The axial force of the horizontal direction rock bolt is greater under the transverse ellipse,while the axial force of the vertical direction rock bolt is greater under the vertical ellipse.