Mechanical Behaviors And Deformation Mechanisms Of A Double-arch Tunnel Reconstructed From An Existing Tunnel

In recent years, due to the significant increase of transportation volume in China, more and more existing highway tunnel can not meet transportation demand. Consequently, reconstructing the existing highway tunnels to expand their cross sections has become an important and effective measure to solve this urgent problem. Until now, however, there are few reported literatures researching about reconstructing an existing shallow tunnel into a large-span double-arch tunnel.Based on the practical reconstructing engineering of Jingya tunnel, this paper mainly studied the mechanical behavior and deformation mechanism of pillar and linings in the process of reconstructing an existing shallow tunnel into a large-span double-arch tunnel.In this study, a large scale model test was carefully designed in the principle of similarity theory. Ground surface settlement, change of stress in the surrounding rocks and stress induced in pillar and linings were detailed investigated in this model test. The results of the test showed that the excavation of the second tunnel induced to higher surface settlement than that of the first tunnel, however, the maximum of surface settlement is above the first tunnel. The excavation of the second tunnel had a significant impact on the mechanical behaviors and deformation of pillar and linings of the first tunnel. Due to the repeated excavations, the stability of the rocks above the pillar were seriously disturbed, which resulted into the maximum stress in the spandrel of the first lining of the second tunnel(left tunnel).In addition, a three dimensional model was established and was used to perform numerical simulations with FLAC3D5.0 software. The results of numerical simulation, to some extent, revealed the load transform mechanism and interactive deformation effect between the two holes of the new double-arch tunnel in the process of reconstruction. The effect of excavating lagging distance between the two holes on the mechanical performances of the structure of double-arch tunnel was also explored. The results showed that the excavation only had significant effects on the stability of the rocks and behaviors of the pillar and linings which had a distance less than 1D(the span of a single tunnel) from the tunnel face. When the lagging distance between the twin tunnels was higher than 2D, the lagging distance only had a slight influence on the mechanical behaviors and deformation mechanisms of pillar and linings.The results of this study not only can give significant directions for the reconstruction engineering of Jingya tunnel, but also can provide valuable references for the subsequent tunnel reconstruction engineerings.