Study Of The Influence Of Parallel Pilots On The Excavation Of High Geostress Soft Rock Tunnels

With the rapid development of national economy and infrastructure, our country has increasingly invested in the areas like energy, transportation, communication and water conservancy, which has enlarged the construction of tunnel engineering day by day. The construction of long and deep buried large tunnels is increasing, and because of its crossing of the high initial geostress areas and encountering of the soft surrounding rocks, the related geological disasters, such as deformation of soft surrounding rocks, often happen. This thesis is based on the Lanjiayan Tunnel(the area of Mao County) combining with the site construction of it, and analyzes the deformation rule and failure mechanism on high geostress soft rock tunnels, as well as the influence of early excavation of parallel pilot on main caves via the comprehensive methods of field research, theory analyzing, site testing and monitoring measurement, numerical simulation analyzing and summarizing and generalizing. Meanwhile,this thesis also analyzes the distribution of plastic zones, the deformation of surrounding rocks and initial stress led by the deformation feature of soft rock tunnels, the different clear distances between parallel pilot and main caves and the different distances of early excavation of parallel pilots, and finally discusses the controlling method of tunnel deformation, choosing a proper excavation proposition best suited for Lanjiayan Tunnel, and proposes a controlling construction technique of deformation of high geostress soft rock tunnels. The conclusions are as follows:(1)The deformation features of soft rock tunnels are: the amount of deformation and the rate of initial deformation as well as the range of surrounding rock constructions are big. Plus,the deformation is obviously affected by the time, and most tunnel soft surrounding rocks are circular stressed and asymmetric. Meanwhile, the deformation of tunnel soft surrounding rocks becomes bigger as the burial depth becomes deeper.(2)The long and deep buried large tunnels, on the condition of high initial geostress,should take proper measures to eliminate or weaken the high geostress, thus decreasing the influence of high geostress on tunnel excavation. The available method is to excavate parallel pilot in the area of high initial geostress with the purpose of using the deformation caused by parallel pilot to eliminate or decrease the high geostress. Furthermore, the excavation of main cave also lowers the stress field of parallel pilot surrounding rocks. During the process of mutual influence, it turns out a condition of relatively low stress field on the whole.(3)There should be a reasonable clear distance between parallel pilots and main caves.When the clear distance is a bit short, in the two tunnels the cylinders of mid-adjacent rock will have a cut-through in plastic zones, thus they are easy to get a shear failure with the action of vertical load; the shorter the clear distance is, the bigger the vault settling and thehorizontal convergence of the main cave are, which is disadvantageous to the stability of the main cave. The proper clear distance between the two tunnels should be among the numbers from 2 times of the main cave diameter to 2.5 times of it.(4)The distance between parallel pilots and the tunnel face of the main caves should not be too short. When the distance of the tunnel face is too short, the vault settling and horizontal convergence of the main cave will be rather big, which is disadvantageous to the stability of the main cave construction; however, when the distance is too long, the vault settling and the horizontal convergence of the parallel pilot will increase, which is disadvantageous to the stability of the parallel pilot construction. Therefore, the thesis thinks that under the construction, the controlling of the distance between tunnel faces of the two tunnels is of great importance, and based on the comparative analysis of surrounding rocks deformation and initial stress, the thesis finds that it is reasonable that the two tunnels are at a distance of 20 meters.(5)Based on the energy conservation theory, this thesis comprehensively discusses the principle of surrounding rocks deformation, and proves that the usage of a scientific releasing method of advance stress can effectively decrease the re-accumulative strain energy of surrounding rocks after the excavation, which will further control the surrounding rocks deformation around the tunnels.