Determination Of Surrionding Rock Parameters Of Tunnel Portal And Construction Application Based On Displacement Back Analysis

In recent years, the underground engineering which typical representative is tunnel engineering have get rapid development with the vigorous boom of the transportation. Now the development of underground engineering is steping into a field which is difficult, complex technology and high precision. The development of long tunnel project and development and utilization of underground space have become the main theme of this century.At present, methods for determining the physical and mechanical parameters of rock and soil in geotechnical engineering are commonly used for laboratory test and field test. Because of the disturbance of the soil test, the randomness of the field test, the operation error and the test fund, the parameters can not be accurately represent actual situation, which makes the reference less than satisfactory. Therefore, how to effectively select the parameters of rock and soil parameters for underground engineering design and construction and other work is currently one of the problems faced by geotechnical engineering.The overlying strata of the tunnel portal section is thin and the tunnel is shallow buried, and it is extremely easy to cause the roof collapse, landslides and other engineering accident in construction.This paper takes Sanmen river tunnel in Liuzhou Guangxi for example, based on the displacement back analysis theory of rock and soil engineering, establishing the monitoring system to monitor the displacement of the tunnel construction process, establishing 3D numerical modelby MIDAS/GTS, back analyzed the physical and mechanical parameters of the surrounding rock of tunnel portal, and take the results into practice. The main contents are as follows:1、This paper discusses the classification of displacement back analysis, introduces the principle of positive and negative analysis, introduces how to determine the sensitivity of the parameters and how to determine the parameters, discusses the significance and realization method of the displacement back analysis in the stability analysis of the tunnel portal.2、Based on the basic principle of displacement back analysis, a tunnel monitoring measurement system is established combined with the Sanmen river tunnel engineering to measure the displacement measurement value of tunnel portal. Regression analysis of the common function of the measurement value by using the regression analysis method to obtain regression equation of measurement value, and use the regression equation calculate displacement of the measuring point on the tunnel perimeter when surrounding rock become stable, and provide the results to back analysis.3、Establishing a MIDAS/GTS finite element numerical model. Using golden section optimization method iterative calculate the parameter, then obtain the back analytical solution that close to reality. Taking the back analytical solution into the model and calculate, then predict the displacement of tunnel periphery in later construction. The results show that the results of the back analysis have a good applicability, it provide the basis for stability of tunnel portal and the time of secondary lining construction.