Abstract:

 The existing theoretical studies on the stability of shield tunnel excavation face have not taken into account the effects of rainfall environment and groundwater level, especially the geotechnical effects induced by heavy rainfall infiltration. Firstly, a layered hypothetical Green-Ampt model is used to simulate the heavy rainfall infiltration process. Secondly, on the basis of the shear strength theory and the soil effective stress theory, the modified Mohr-Coulomb criterion and the Darcy’s law are combined to derive the apparent cohesion expression in segmental form related to rainfall intensity and duration, and then, based on the upper bound theory of limit analysis, a three-dimensional (3D) logarithmic spiral failure mechanism is constructed, and the work done by apparent cohesion is introduced into the virtual power equation of upper bound theorem, the upper limit solution of the support force of shield tunnel excavation face is obtained under the conditions of heavy rainfall infiltration and groundwater level through optimization calculation. Finally, the theoretical upper bound solution is compared with the 3D numerical results and existing experimental results, and a good agreement is obtained. In addition, the key parameters such as rainfall intensity and water table depth are analyzed. The results show that when the rainfall intensity is small (F/k_s≤1, where F is the mean rainfall intensity, and k_s is the saturated infiltration coefficient), the ultimate supporting force of tunnel excavation face accelerates with the increase of rainfall intensity, but has no obvious change with rainfall duration; the ultimate supporting force increases slowly with the increase of rainfall intensity, but significantly with the increase of rainfall duration when the rainfall intensity is large (F/k_s>1); the ultimate supporting force of tunnel excavation face decreases linearly when the normalized groundwater table depth is small (Z_w/D≤3, where Z_w is the depth of groundwater level, and D is the excavation diameter); and the rate of change gradually decreases and finally becomes stable when the normalized groundwater table depth is large (Z_w/D>3).

Key words: heavy rainfall, shield tunnel excavation surface, Green-Ampt model, 3D logarithmic spiral, upper bound theorem