The 3D base stability of excavations in soft soils is evaluated using FEM with reduced shear strength. The numerical results indicate that the base stability of excavations is significantly influenced by the ratio of the depth to the width, the thickness of the soft soil layer between the excavation base and hard stratum, the depth of the walls inserted below the excavation base, the stiffness of the walls, and the ratio of the length to the width of excavation. Whereas; the 3D effect can be neglected at a ratio (length/width) of the excavation larger than 5.0.