Determination And Prediction Of Limit Displacements Of Primary Support System In Highway Mountain Tunnel

The limit displacement of the primary support system is of great significance for the construction safety of the tunnel,and the existing norms of the limit displacement value is divided relatively rough,division condition is relatively small,and the limit state method is relatively difficult in practical engineering application.Therefore,in order to take into account the spatial effect of tunnel excavation,and change the parameters of the tunnel in convenience.Benching-tunnelling method,Reserving-core-soil method,Single-side-drift method,Both-side-drift method,which,are the main four mountain tunnel excavation methods,are used to construct single hole three-dimension numerical model by FLAC~3DD finite difference software.The critical state of surrounding rock deformation is judged by cusp catastrophe model,and the limit displacement value of each model is obtained.The mathematical prediction models of primary support limit displacement under the different conditions can be decided by using support vector regression algorithm which is using genetic algorithm to optimize the parameters of the model.By writing the data of the bolt diamater,elastic modulus,Poisson’s ratio,unit weight,shotcrete thickness,bolt spacing and the buried depth of tunnel into the formula,the value of the ultimate displacement can be obtained and the references for actual construction can be provided.According to the statistical analysis on the calculation results of all models,the influence on different monitoring points of initial support limit displacement value,such as vault settlement,horizontal convergence of arch waist and arch foot,which causing by the different excavation methods,excavation spans,tunnel depths,degrees of surrounding rock,supporting parameters and other factors can be draw.Under the condition of considering the excavation span and height,the limit relative displacement values of different measuring points are obviously different,and the limit relative displacement ranges of different excavation methods are also different;With the increase of the buried depth,the nonlinear characteristics of deformation at each measuring point are more obvious;It can be said that the worse conditions of the rock,the greater deformation of tunnel face in advance,but the proportion is on the decrease;The displacement of the benching-tunneling method is smaller and the deformation is relatively concentrated;The range of deformation range in partial-excavation method is larger,and the timing of the occurrence that mutation happens is relatively far from the excavation face.According to the comparison of simulated data and specifications,it is proposed that the control points of hole deformation can be clearly defined and the buried depth interval should be refined,the allowable displacement range can be determined for different excavation spans and excavation methods respectively.