Study On Instability Mechanism And Risk Assessment Of Slurry Shield Tunneling Face In Sands Under High Hydraulic Pressure

With the large-scale development of underground space in China, The environmental conditions of shield tunneling is increasingly complex. Many problems appear during the construction process and one of the more prominent problems is face instability and large ground deformation, which is caused by lack of face supporting pressure. Especially with the rapid development of underwater shield tunnel project at home and abroad in recent years, face stability is difficult to control due to shallow depth and high hydraulic pressure. Therefore, the research on instability risk of shield tunneling face subject to high hydraulic pressure has practical significance,In this dissertation, combined with the cross-river shield tunneling project of Jiangxinzhou sation～Zhongjianfengjing interval in Nanjing Metro Line10, face stability during the construction process of slurry shield tunnel in sands under high hydraulic pressure were studied by theoretical analysis, numerical simulation and on-site survey. The main work and achievements are as follows.(1) According to the Mohr-Coulomb failure criterion, the Handy soil arching theory was used to determine the position and shape of unstable soil. The shape of unstable soil is similar to the shell-shaped. Based on the theory of limit equilibrium analysis, face stability analysis model of shield tunnel was established. The calculation results of this model were compared with the results of other theoretical analysis method, the centrifuge test and numerical simulation analysis. The engineering reference of face limit supporting pressure was proposed, which could be used to guide the construction.(2) Based on FLAC3D finite difference calculated program, face limit supporting pressure of large diameter slurry shield tunnel in sands under high hydraulic pressure was studied. The influences of soil internal friction angle, cohesion, elastic modulus, tunnel depth, diameter and water level on face supporting pressure were analyzed. The failure mode of tunnel face and the variation of earth pressure in the instability process were researched. The results show that, compared to the value under no hydraulic pressure, the limit supporting pressure under high hydraulic pressure increases by about an order of magnitude.(3) Learned from the basic idea of the Kent Method, which was proposed by W. Kent Muhlbauer and was widely used in the field of pipeline risk management, the instability risk of slurry shield tunneling face was analyzed. Risk factors included geological index, tunnel index, design index and construction index. A risk assessment model was established and the scoring criteria of risk index in the Kent Method were modified. The relative risk assessment value was calculated by using the algorithm that risk index adding together, and then multiplying with the consequences coefficient. The risk is proportional to the assessment value.(4) Combined with the cross-river shield tunneling project of Jiangxinzhou station～Zhongjianfengjing interval in Nanjing Metro Line10, two typical sections were selected to assess the face instability risk by the risk assessment model which was established in this dissertation.