Abstract: To determine the stress release rate of shield tunneling face in uppersoft and lowerhard strata, the variation rules of the stress release rate of surrounding rock under various proportions of hard and soft rocks on the tunnel face and buried depths are studied via numerical simulations using an existing stress release rate calculation method by volume loss rate iteration. The results are compared with the field data collected for Guangzhou metro line 21. The results indicate the following: (1) For shield tunneling in the interface between hard and soft rocks, the initial stress release rate of surrounding rock ranges from 24% to 36%, and it linearly increases with  the proportion of hard rocks on tunnel face. (2) Compared with the strata condition, the influence of the buried depth on the stress release rate is less. In addition, in the simulation of shield tunnel passing through uppersoft and lowerhard strata, dynamically adjusting the stress release rate under different excavation steps according to the changes in surrounding rocks is feasible.

Key words: shield tunnel, numerical simulation, stress release rate, uppersoft and lowerhard strata