Abstract: In shield tunneling, it is very critical to estimate the loosening earth pressure reasonably for preventing ground instability and controlling shield’s driving force effectively. However, due to the influence of ground property, overburden depth and geometric construction parameters, the ground behavior is extremely complicated. Comparing with the limit equilibrium method, the theoretical basis of the limit analysis theory is more rigorous. Therefore, the loosening earth pressure of shield-driven circular tunnels is investigated by an upper bound approach in limit analysis, and the Mohr-Coulomb failure criterion is adopted. Furthermore, a multi-blocks failure mechanism is developed for loosening earth pressure in sandy ground. The basic examples are carried out, and the results are examined by comparing with Terzaghi theory, numerical simulation and centrifugal tests. It is confirmed that the proposed model predicts the loosening earth pressure reasonably well at the case of large overburden depth, and shown that the overburden depth and friction angle are the important factors affecting the loosening earth pressure.

Key words: limit analysis theory, limit equilibrium method, loosening earth pressure, shield tunneling, Terzaghi theory