Abstract: Project practices show that the narrow foundation pits, behaving a significant size effect, have better stability and smaller deformation than wide ones; so the embedded depth of envelope structure for narrow pits can be properly reduced at same excavation depth. However, most present stability analysis method of foundation pits design did not consider the influence of the excavation width on the safety factors, which leads to conservative design for a narrow pit. In order to make slip surface come into being for narrow pits, by moving the center of the slip arc, let the slip arc through the bottom of the enclosure wall and the intersection of excavation bottom and contralateral enclosure wall. The safety factor of stability calculated based on this slip surface can consider influence of excavation width. According to this idea, foundation pits are divided into different types by different widths, corresponding to the different calculation methods of This method can be transformed to the calculation method in the existing norms at wide pit, which can ensure the its unity with the existing specifications; for narrow pit, . The analysis shows that ignoring the size effect in the excavation design and one-sided emphasis on the insertion ratio is unscientific. Because applying traditional slip surface form of safety factor of heave-resistant stability and no excessive change on the calculation method; the calculation method are suitable for use in the design and construction of foundation pits. This calculation method of provides a theoretical basis for the narrow pit to reduce insertion depth.

Key words: foundation pit, heave-resistant stability, size effect, excavation width, embedded depth