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Ground improvement piles induced shear strength increase in normally consolidated clay

Abstract Ground improvement piles, are usually installed by injection or mixing rather than by driving. So, soil around a ground improvement pile is mainly subjected to lateral displacement and is similar to that of soil around an expanding cavity. A ground improvement pile not only generates excess pore water pressure in the soil but also causes subsequent soil strength change if drainage of excess pore water pressure is possible. This paper presents the results of a large scale laboratory model test on the grout bulb induced undrained shear strength increase in normally consolidated clay. If sufficient time is allowed for pore water pressure dissipation (t > 3tp, tp is the time needed to complete the primary consolidation), the vane shear strength of soil can be increased by more than 170% for the soil right outside the expansion body. This decreases gradually to the initial vane shear strength at a distance of 7rcc (rcc = radius of expansion body). Based on the theory of the cylindrical expansion cavity and the model test results, a semi‐empirical model is developed to estimate the change of undrained shear strength ratio of in‐situ clay before and after pile installation at different distances from an expansion body. For normally consolidated clay, a two‐fold increase in undrained shear strength ratio can result from ground improvement pile use.