Research On Engineering Characteristics Of Surrounding Rock And Initial Supporting Optimization Of Mountain Tunnel In Fractured Rock Mass

With the implementation of China’ s “traffic power” strategy,the rapid development of tunnel construction has provided a solid foundation for the progress of national economy and science and technology in recent years.The tunnel is a complex system composed of surrounding rock and supporting structure,and its mechanical response characteristics and environmental action mechanism possess the characteristics of changeable,dynamic and uncertain.Consequently,the support design and the tunnel construction are mainly referring to engineering experience,lacking scientific planning and theoretical data support,which leads to the insufficient rationality of support design.Based on the investigation and study of relevant research results at home and abroad,the article takes the tunnel construction project as the engineering support,and uses the methods of statistical investigation,theoretical analysis,numerical calculation and field test to systematically study the engineering characteristics of surrounding rock and the optimization of initial support of mountain tunnel in fractured rock mass.The spatial and temporal laws of surrounding rock deformation and mechanical behavior of supporting structure are analyzed,moreover,the three-dimensional discrete element numerical model of the fractured rock tunnel was established by integrating the Discrete Fracture Network(DFN)simplification technology,and the engineering response characteristics of the surrounding rock under the probability distribution of structural plane parameters were revealed.The optimization model of initial support parameters with high performance and low carbon emission is developed,and a set of optimization strategies are proposed to achieve the synthetic optimization of engineering safety,performance efficiency and low carbon emissions,which provides a scientific basis and engineering practice method for the optimization of support parameters of mountain tunnels in China.The main research contents and conclusions of this paper are as follows:(1)Taking the typical sections with different fracture degrees as the research object,the monitoring data of tunnel vault subsidence and surrounding displacement,contact pressure between surrounding rock and primary support,axial force of anchor bolt,steel support stress and shotcrete stress are analyzed based on the multi-section field monitoring of tunnel engineering,illustrating the spatiotemporal evolution characteristics of surrounding rock deformation and support structure stress.(2)A numerical model of tunnel excavation in fractured rock mass considering the characteristics of the structural plane is established.The structural plane data of the typical section is investigated and counted,and the probability distribution function of the structural plane parameters is fitted.The DFN conforming to the engineering geological situation is generated by Monte Carlo method,and the applicability of the simplified DFN technology is analyzed and verified.The numerical model of tunnel excavation in fractured rock mass was established by the three-dimensional Distinct Element Code(3DEC),and the monitoring data are utilized to verify the applicability of the proposed numerical model.(3)The study reveals the influence of the probability distribution of structural plane parameters on the engineering response of surrounding rock.Three groups of numerical simulation tests(containing 46 kinds of working conditions)are completed to reveal the influence of geometric parameters such as crack inclination,inclination,trace length and spacing,and mechanical parameters such as strength and stiffness on the vertical displacement and failure of surrounding rock under the condition of multiple sets of structural planes.Meanwhile,three kinds of critical strain warning horizontal lines are clarified,and the probability of surrounding rock stability under the condition of probability distribution of structural plane parameters is obtained.(4)The prediction model of initial support performance is established.Taking the control rate of surrounding rock deformation and the utilization rate of supporting material performance as the response indexes of supporting performance,a two-level full factor test was carried out to quantify the main effect and interaction effect of design parameters on the response indexes,identify the design parameters that have significant influence on the initial support performance,and establish the prediction model of initial support performance.The predicted value is consistent with the numerical simulation and field monitoring results,which verifies the effectiveness of the prediction model.(5)The optimization model of initial support parameters considering high performance and low carbon emission are proposed.Based on the carbon emission coefficient method,the carbon emission analysis method for the initial support construction of tunnels is clarified.The optimization model of initial support parameters is constructed which develops the explicit design parameters as the design variables,and constructs the range of standard values as the constraint conditions,and sets the initial support performance and carbon emissions as the objective functions.A set of optimization strategies is proposed based on the constrained multi-objective evolutionary algorithm.The workable optimization effects are achieved according to the analysis of engineering case,and the construction objectives of engineering safety,high performance and low carbon emissions are realized.