| During the construction of metro tunnel,the surrounding rock is disturbed by excavation,and the internal stress will be redistributed and reach a new equilibrium state.In this process,the strength and stability of surrounding rock close to the tunnel will decline,forming a loose circle of surrounding rock,which is prone to plastic slip,block falling,collapse and other problems.In order to ensure the scientific and effective support of the surrounding rock of metro tunnel and save the construction cost,it is particularly important to accurately obtain the scope of the loose circle of the surrounding rock of the tunnel.Through literature research,theoretical analysis,numerical simulation,field test,mathematical model prediction and other methods,the scope of surrounding rock loose zone of metro tunnel is studied and predicted.The main work is as follows:(1)Investigate the research status of determining the scope of surrounding rock loose zone at home and abroad.The research methods to determine the scope of loose ring are classified,and the problems existing in the research content at this stage are put forward.(2)This paper introduces the mechanical definition of loose circle,summarizes the widely used calculation method of surrounding rock loose circle,measures the support reaction on site,and uses the elastic-plastic analytical solution to calculate the loose circle range of three sections of the supporting project.(3)Summarize and analyze the main methods of using numerical simulation to judge the loose ring,use Midas GTS NX software to conduct numerical simulation on the supporting project,and judge the loose ring range of numerical simulation.This paper analyzes the main factors affecting the loose circle of surrounding rock,selects the influencing factors according to the characteristics of the loose circle of surrounding rock of metro tunnel,uses the control variables for scheme design,and obtains the variation characteristics of the loose circle of each part of the tunnel under each influencing factor.(4)Summarize the commonly used field detection methods and analyze their advantages and disadvantages.In order to avoid the randomness of the detection results,combined with the actual situation,two field detection methods of surrounding rock loose zone are determined.Using the on-site detection data,the range of loose circle of tunnel in three sections is judged respectively.Finally,the results of the two detection methods are compared,and the final result shows that the error of the detection results is small.(5)Based on the analysis of the influencing factors of the loose zone of the surrounding rock of the metro section tunnel,two single mathematical models are combined to form a combined model,which is improved and predicted.Firstly,the traditional grey prediction model is used to predict the loose circle,and then the artificial neural network model is established with MATLAB software to predict the loose circle.The results show that the error of both prediction results is too large.The results show that the improved grey neural network has higher accuracy than the single prediction model and the traditional grey neural network,and can be used to predict the surrounding rock loose zone of metro tunnel. |
