Abstract: Examining the impact of spatial effects on the design of small-sized working shafts in deep soft soil regions is crucial for improving the rationality of design and calculation of foundation pit support structures. Herein, case studies are conducted on 31 deep foundation pits of small-sized working shafts along the Tongtu road shield-bored utility tunnels in Ningbo, China. First, based on detailed statistical data including geometric dimensions of the foundation pits and parameters of the support structures, the current design status of such foundation pits is systematically analyzed. Subsequently, a combined approach of theoretical analysis and three-dimensional finite element simulation is employed to thoroughly investigate the deformation characteristics throughout the entire construction process of typical small-sized working shaft foundation pits in deep soft soil strata. A series of calculation models incorporating different long-side dimensions of the foundation pits are established, quantitatively revealing the distribution pattern and influential scope of the spatial effects. Finally, optimization strategies for the support structure design are proposed. The research results indicate that: (1) The working shafts for utility tunnels represent typical deep foundation pits with small dimensions, exhibiting significant spatial effects in deep soft soil regions, suggesting substantial optimization potential in the existing support structure design. (2) The range for optimizing support parameters depends on the long-side dimension: when the long side is ＜25 m, optimization can be applied over the entire length of both the long and short sides; when the long side is ≥25 m, the optimization scope can be effectively reduced to l/3 sections at each end of the long side plus the entire length of the short side （l is side length of foundation pit）. (3) The key factor for deformation control of the support structures is the rational design of the diaphragm wall thickness, while the crucial factor for stability control is ensuring an adequate diaphragm wall embedment ratio.

Key words: deep soft soil region, small-sized deep foundation pit, support structure, spatial effect, numerical analysis