Study On Pre-tightening Force Of Roadway Surrounding Rock Support Based On Stress Gradient Theory

With the increase of coal mining depth,the mining geological conditions become more and more complex,and the total length of roadway tunneling also increases day by day,making the support under complex geological conditions become the primary consideration in the use of roadway.Especially,it is difficult to judge the stability of surrounding rock under geological conditions such as high ground stress and soft rock,high failure rate of bolt support and low maintenance efficiency in later period.Based on the 13803 working face gateway of YuShujing coal mine,ShangHaimiao mining area,this paper analyzes the stress gradient distribution rule near the tunneling section,and designs the bolt preload based on the stress gradient theory,and conducts the key research in the following aspects:(1)In different shapes of underground engineering structures,the stress distribution on a certain section can be mapped from an infinite range outside the plane to a plane inside the unit circle by combining the geometric mapping theory in the complex variable function.Taking the rectangular roadway as an example,the analytical solution of the stress component and stress gradient of the surrounding rock is solved under the background of the plane strain problem.The variation trend of stress gradient obtained by comparative analysis is consistent with that obtained by finite element method.And the maximum error of stress gradient is controlled within 4.5%,which verifies the correctness and reliability of the theoretical calculation method.(2)Through the comprehensive analysis of finite element software FLAC3D and theoretical calculations,it is concluded that the region with higher stress gradient is bound to increase the confining pressure and stress concentration.And when the shape of the roadway excavation is determined,the stress gradient of the surrounding rock of the roadway decreases with the distance from the surrounding rock surface of the roadway.When the monitoring radius is determined,the stress gradient of the monitoring point increases as the tunneling space increases,that is,the equivalent radius increases.(3)Taking the Mohr-Coulomb criterion as an example,in the solution,the compressive strength and basic variables of the material are first calculated by using the yield condition as the basic formula.Then,the interface between the plastic zone and the original rock stress zone is taken as the entry point,and the surrounding rock stress and failure range of the plastic zone are first solved,and then the interface stress of the elastoplastic zone is taken as the basic point to solve the elastic zone stress expression.The polar coordinate expressions of the two are converted into Cartesian coordinate system expressions,and the stress gradient expression is solved according to the micro-element method.Using the same idea and theory to calculate the surrounding rock stress gradient solution model under the Drucker-Prager criterion and the Hoek-Brown criterion.(4)Based on the Mohr-Coulomb criterion,the stress gradient values under theoretical calculation,numerical simulation and field measurement are obtained,and the measured values are correctly evaluated according to the theoretical values.After obtaining the stress gradient of the surrounding rock under unsupported conditions,the simulation results show that when the bolt preload is 150 kN,the deformation of the surrounding rock in the support form is reduced by 40%compared with the unsupported condition,that is,the roadway deformation compensation ratio is 40%and section plastic zone range reduced by 60.4%.Based on this,the corresponding relationship between the stress gradient compensation value and the bolt preload is obtained.The stress gradient compensation coefficient is proposed and used to optimize the bolt pre-tightening force application value.According to the calculation method of stress gradient compensation coefficient,the optimum stress gradient compensation coefficient and the optimum pre-tightening force under this geological condition are obtained.The stress gradient calculation and overall analysis ideas are given for different geological conditions.(5)Finally,relying on the engineering practice,the analysis of the surrounding rock stress and displacement monitoring data of the roadway under the support of the traditional support theory design method and the stress gradient compensation theory design method.When adopting the traditional support theory,only the elastic support zone of the anchor support point in the surrounding rock can be considered.The shortcoming of this design scheme is that the pre-tightening force cannot be changed according to the actual situation of the broken rock of the surrounding rock.The support scheme designed according to the theory of stress gradient compensation not only considers the range of the plastic zone,but also converts the stress state of the bolt to the stress state of the surrounding rock.It can more clearly understand the deformation and stress of surrounding rock under different pre-tightening forces.