Research On Mechanical Theory Of Adjacent Construction For Tunnels And Its Application In Engineering

With the development of economy in recent years, the government’s efforts of traffic infrastructure construction has been increasing. The improvement of the traffic network and the speed up plan has brought a lot adjacent constructions in engineering. The size is lager and lager and the space between the newly built tunnel and the existing tunnel is closer and closer. In addition, rapid development of urbanization also brought lots of the subway construction of underground transportation and comprehensive utilization of underground space development. All of above will certainly cause a lot of newly built structures construction close to the existing structures. In course of adjacent constructions, the newly built tunnels will change the mechanical cases of the existing tunnels and the initial geostress field will be disturbed time and time again. The stress evolution led to the stress variation of both the newly built tunnels and the existing tunnels which not only brought the safe problem to the existing problem and also the comlexity of the newly built tunnels, such as reduction of bearing capacity and too much deformation and so on. As a result, the research of mechanics of underground engineering and countermeasure has become a top priority.This paper does research on the elastic and plastic stress state during the adjacent construction based on the practical engineering with the method of both theoretical analysis and numerical simulation analysis. This paper also carries out a study on the influence of different construction method for the newly built tunnels to the existing tunnels. The damage location and extent identification of the existing tunnel caused by the newly built tunnel is also researched in detail. At last, the stress and strain rule of the existing tunnel is obtained based on the Da Wangshan-Tunnel project. The main contents and results are shown as the following:(1) The secondary and tertiary stress state is researched during the underground excavation based on the elastic-plastic mechanical theory. The conclusion shows that in condition of the elastic surrounding rock, the stress concentration factor of the parallel or overlapping tunnels is lager than that of the single tunnel the main damager factor of the surrounding rock is the shearing stress. In the condition of the elastic-plastic surrounding rock, the plastic region of the parallel or overlapping tunnels is lager than that of the single tunnel and depth expansion mode is toward the adjacent tunnels. It is suggested to strengthen the supporting for the complexity of the surrounding rock.(2) The influent study with different construction methods such as full-face tunnelling method, benching method, CD excavation method, CRD excavation method and pilot-tunneling method for the newly built tunnels to the existing tunnels is researched based on the theory of new austrian tunneling method. The conclusion shows the selection principle with the result of stress-strain rule and plastic deformation region. This paper also does research on the deformation rule of the key points under the condition of different spacing distances and different buried depth. The influence regularity for tunnel deformation is achieved.(3) The damage location identification and extent identification of the existing tunnel caused by the newly built tunnel is also researched in detail based on the elastic beam and structural mechanics. The modal curvature difference method and neural network method are used to simulate the damage of lining concrete. The thirty-third numerical examples are chosed to verify the method and the result shows that the modal curvature difference method and the RBF neural network method can identify both the location and the extent of the damage effectively.(4) The stress-strain analysis of the existing tunnels is studied under the condition of different construction order, different construction method and different consolidation grouting condition of the newly built tunnels. It is suggested that consolidation grouting should be carried out in order to reinforce the traffic tunnel surrounding rock and small footage should be adopted during the excavation of the tunnels. The footage should be controlled within1.5m or even1.0m at the crossing of the adjacent tunnels. At the same time, strengthen the reinforcement of the supporting and reduce the blasting dosage can also reduce the influence of the newly built tunnels.(5) The impact of the blasting conventional or accidental load during the adjacent construction is discussed with the software LS-DYNA. The conclusion shows that the peak value of the vertical vibration velocity focus on the arch crown and damping down gradually, the peak value of the horizontal vibration velocity first increases and then decreases, the peak value of the vertical vibration velocity is greater than that of the horizontal vibration velocity in the whole section. The research also shows that the arch crown and haunch have obvious response of the blasting shock wave under the accidental explosion load. It is suggested that the vertical vibration velocity and the stress of the arch crown should be controlled to ensure the safety of the tunnels.