Abstract: At present, the current code for checking the basal heave stability of excavation uses the Prandtl bearing capacity formula, which assumes that the foundation undergoes bilateral sliding failure and does not consider the limiting effect of the soil inside the enclosure on the heave failure of the excavation. This assumption is inconsistent with the actual conditions. Considering the contribution of the shear strength of soils inside the excavation to the basal stability at the bottom of excavation, the minimum width of the foundation for the one-way sliding uplift failure of the excavation enclosure wall is obtained based on the formula of the ultimate bearing capacity of the Terzaghi soils by utilizing optimization method. The formula of the ultimate bearing capacity for the basal stability at the bottom of the excavation is also derived. The results of this formula indicate that the factor of safety of the unilateral uplift sliding is smaller than the estimated value of the Prandtl bearing capacity formula in the current code.

Key words: Deep Excavation, Basal Stability, Ultimate Bearing Capacity of Soils, Terzaghi, Optimized Width