Base Stability of Grout Pile–Reinforced Excavations in Soft Clay

Grout piles are used to reinforce base soil to increase the base stability of excavations in soft clay and to reduce excavation-induced ground movements. To propose a model for the base undrained stability of deep excavation in clay reinforced with grout piles, this paper presents an anisotropic strength criterion for clay reinforced with grout piles and the upper-bound bearing capacity theory for excavation. The proposed model is capable of determining the factor of safety against base heave for an excavation with or without ground improvement for various-strength anisotropy ratios of clay and clay reinforced with grout piles. The suitability of the model is verified with five field excavation cases. Among them, four were with no ground improvement, and one was with ground improvement. Generally, the larger the ground improvement rate, the more the material strength of a grout pile can be mobilized. The factor of safety also becomes larger when the ground improvement rate is higher. However, the factor of safety becomes insensitive to the undrained shear strength of a grout pile when the improvement ratio is less than 10%. Results from the model found that the increment of factor of safety against base heave and the increment of improvement ratio is generally $1:2$. Finally, because the proposed method does not consider the influence of displacement at the retaining wall bottom and the geometry of excavation, it can be used only in excavations with the width and depth of ground improvement zone larger than $1.41D_p$ and $1D_p$ ($D_p$ is depth of the retaining wall minus depth of excavation), respectively.