| With the process of development the western region, the focus of the national infrastructure transferred to the west, the highway tunnel construction is increasing. Mountain Highway tunnel entrance construction has been the difficulty. As the tunnel entrance, there is a feature of the most common, such as soft rock mass, weathered broken, terrain bias phenomenon, etc. Number of objective factors is bound to cause accidents, such as serious rock mass deformation, difficult to form a cavity and cave-collapse after tunnel excavation. Therefore, the construction of tunnel entrance is the key factor in tunnel construction.In this paper, Pingzhai tunnel construction project for the background, refer to the features of current entrance with the four basic types of excavation(orthogonal excavation method, oblique excavation method, the lateral excavation method and one-way excavation method),Pingzhai tunnel entrance with the left cave construction program is proposed by considering the actual terrain and geological conditions of Pingzhai tunnel entrance. The dynamic mechanical process of Pingzhai left tunnel tntrance tonstruction is analyzed by combining monitoring site with 3d numerical simulation. It is show that the methods of middle horizontal guide cave can be on construction of left tunnel entrance,when the excavation of the positive direction can not be mplemented because of adverse conditions on tunnel entrance. It is effective that ensuring construction period and the safety of tunnel portal construction, and can provide reference such as tunnel entrance projects.In this paper, the completed work are as follows:â‘ Pingzhai tunnel geological environment conditions and general overview of the whole tunnel design are analyzed, the stability of the tunnel entrance is evaluated , and the construction Pingzhai characteristics of tunnel entrance excavation are summaried. Refer to the current methods of tunnel entrance excavation , through analyzed the actual terrain conditions of Pingzhai tunnel portal, it is put forward a method that excavation the left tunnel portal section by changing proposed construction hole.â‘¡The content of monitoring on construction of tunnel entrance are introduced, the measurement results are on-site detailed analyzed, a study of mechanical characteristics on the tunnel construction of the left tunnel entrance is analyzed, and then pointed out the important parts of Supporting structure during construction of tunnel entrance.â‘¢Two tunnel entrance excavation program are simulation by using FLAC3D numerical simulation software,with comparison between numerical simulation and monitoring results, the numerical simulation is reliable, and the comparison between two programs was analyzed.â‘£With the numerical simulation of program of excavation,the dynamic mechanical construction process of the left tunnel entrance had been analyzed. Then, the stress and displacement characteristics of the surrounding rock and the loads of the support structure had been summerized.
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