Base stability of deep excavation in clay reinforced with grout piles

For clay reinforced with grout piles, its shear strength is not easy to determine because the overall strength characteristic of the reinforced soil is the combined result of the shear strengths of the clay and grout piles,  improvement ratio, stress path, boundary condition and so on. The purpose of this study is to develop an undrained stability analysis for the base stability of an excavation when grout piles are installed. This analysis is derived by combining the circular slip mechanism and two equivalent shear strengths: (a) the axial compression undrained shear strength of clay for the slip surface outside the excavation; (b) the axial extension undrained shear strength of clay reinforced with grout piles for the slip surface inside the excavation. This study indicates that the difference between the factor of safety calculated from the proposed method and that predicted from the anisotropic bearing capacity method is limited. However, the applicability of the proposed method to excavations with a longer depth of retaining wall (D > 50·0 m) and a narrower width of excavation (B < 0·80D) is still subject to further investigation.