Study On Installation Time Of Initial Support System With FLAC3D

With the increasing investment in infrastructure in our country,the development and utilization of underground space become increasingly important.However,due to the complexity of the underground environment,the problem of destruction and large deformation of surrounding rock is frequently encountered in the process of excavation of underground engineering.So it is necessary to support the surrounding rock.As an important part of surrounding rock support,initial support has a direct impact on the safety of underground engineering.The effect of support is influenced by the support type,the support way,the support time and so on.Since the support type and the support way have been confirmed in the actual construction process,the selection of the supporting time is particularly important.For deep buried tunnel and underground hydropower house which differ in excavation ways,the installation time of initial support system in the process of excavation is studied,and the main research content is stated as follows:(1)Aiming at the problem of numerical simulation of deep tunnel excavation,the longitudinal and transverse range of model are investigated with the finite difference software FLAC3D and the compound failure criterion.The influence of the proximal boundary,the remote boundary,the upper range,the lower range and the width of the model on the excavation displacement of tunnel is analyzed,then the suggestions of the optimum model range are given.The results indicate that when the longitudinal range of model is not less than 7.5 times tunnel diameter and the upper range,as well as the half width of the model,is no less than 7 times tunnel diameter and the lower range is no less than 6 times tunnel diameter,the accuracy of general numerical simulation will be satisfied.(2)As for the problem of optimally installation time of initial support system in deep tunnel excavation,a 3-D numerical model is established.The relationship among the excavation unloading rate,the displacement completion rate and the distance between the tunnel face and the monitoring section is studied under different rock types,different lateral pressure coefficients and different burial depths.Finally,the optimal installation time of initial support system is discussed with the distance between the tunnel face and the monitoring section as the index.The results indicate that with the properties of surrounding rock becoming poor,the optimal installation time of initial support system gradually moves up;with the increase of the lateral pressure coefficient,the optimal supporting time delays when the lateral pressure coefficient is less than 1,and advances when the lateral pressure coefficient is more than 1;with the increase of buried depth,the optimal installation time of initial support system gradually moves up.(3)Regarding to the problem of optimally installation time of initial support system in underground hydropower house excavation,a 3-D numerical model of typical unit block is established.Considering that the excavation of underground powerhouse is carried out by stages,therefore the concept of total length of excavation is put forward to determine the position of the excavation face.The relationship among the excavation unloading rate,the displacement completion rate and the total length of excavation is studied under different rock types.Finally the optimal installation time of initial support system expressed by the total length of excavation is given.The results indicate that the later the surrounding rock exposes,the later the optimal installation time of initial support system will be.The optimal installation time of initial support system for ?-class surrounding rock needs to move up,compared to?-class surrounding rock,especially for the first phase and the second phase.(4)Based on the theory of fluid-solid coupling,the effect of seepage on excavation is considered.Considering that the flow time needs to be decided in seepage calculation,the time required for a one-time excavation is set equal to the time required for step-by-step excavation.The results indicate that for deep buried tunnels,the optimal installation time of initial support system under seepage conditions moves up than that without seepage conditions.For the underground hydropower house,the optimal installation time of initial support system will move up under seepage conditions for the surrounding rock exposing early,and changes little for the surrounding rock which exposes later.