Abstract: To clarify the spatial effect of the smallscale deep foundation pit of utility tunnels using shield method during construction process, a case study is conducted on nine working shaft foundation pits along the Tongtu road utility tunnel project in the central area of Ningbo, China, and the design of supporting structure for deep foundation pit of working shaft is introduced. Then the threedimensional numerical calculation model of #1 working shaft foundation pit is established using finite element analysis method, and the deformation characteristics of the diaphragm wall structure and the surrounding soil during the excavation of foundation pit are analyzed by introducing the scale coefficient 〖WT５《TNR#I》〗R〖WT５《TNR》〗. The results reveal the following: (1) In the deep soft soil layer, the influencing ranges of spatial effect at different depths are almost the same, whereas the influence of spatial effect on deep soil is relatively larger than shallow soil. (2) The critical dimention value of the spatial effect can be obtained based on numerical calculation. Within the influence range of spatial effect, the increase of 〖WT５《TNR#I》〗R〖WT５《TNR》〗 value is nonlinear, otherwise, the 〖WT５《TNR#I》〗R〖WT５《TNR》〗 value is stable at 1.0, which agrees plane strain assumption. (3) The calculation method of standard method is conservative if the dimension of the long side of the foundation pit is less than the critical value. The influence range of spatial effect can be judged in combination with threedimensional numerical calculation, and the support structure design can be optimized.

Key words: soft soil area, utility tunnels using shield method, working shaft, smallscale deep foundation pit, spatial effect, numerical simulation