Continuous Limit Analysis Of The Stability Mechanism Of Shallow Shield Tunneling Construction

Shallow shield tunneling in the construction process, the void between the segment lining structure and the formation need to fill through grouting. If the grouting pressure is too small might cause the displacement of the formation is too large, resulting in adverse effects on adjacent structures and even cause the formation of collapse. For the excavation face of shield, also need the right support force to maintain stability. In this paper, clay soil in shallow tunnels, using upper bound limit analysis theory, consider the formation of a continuous displacement mode and the local construction load of the stability mechanism of shield construction, contents and results are as follows:(1) For the mechanism of the stability of surrounding rock of shallow shield tunnel construction, using Osman transverse plane strain failure mode, the ground displacement caused by tunnel excavation is subject to Parker type empirical distribution patterns. The Peck formula is modified and the range of plastic deformation zone are controlled by parameters which are introduced. Supposing that the displacement field is proportional to velocity field, the upper limit theorem is applied and the lower bound value of the support force is obtained by the Matlab program. The effect of parameters on the ground overloading coefficient is analyzed. By changing the tunnel depth, weight of the soil and soil shear strength heterogeneity, the effect of various parameters on the ground overload coefficient are studied.(2) For Osman plane strain model, (box foundation and pile foundation), the effect of local loads on supporting force that is maintaining the stability of tunnel is studied when local loads are existed. For box foundation load, the effect of the size and location of box foundation load on the lower bound of supporting force that is maintaining the stability of tunnel is analyzed. The effect of the size, location and length of pile foundation load on the lower bound of supporting force is researched.(3) The mechanism of the stability of tunnel face, the distribution of the displacement field of Parker empirical formula is described. The volume strain is zero when the plastic deformation is occurred is verified and a proper failure mode is established. The lower bound of support force of maintaining the stability of shield tunnel face is solved by Matlab program, and the buried depth on the lower bound of support force is researched.(4) When the local loads are existed near the tunnel face(box foundation and pile foundation), the effect of local loads on supporting force that is maintaining the stability of tunnel face is studied when local loads are existed. For box foundation load, the effect of the size and location of box foundation load on the lower bound of supporting force that is maintaining the stability of tunnel face is analyzed. The influence of the size, location and length of pile foundation load of the lower bound of supporting force is researched.