Theoretical Study on Optimum Depth of Ground Improvement for Laterally Loaded Piles Embedded in Clayey Deposit-Paper

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This study estimates the optimum depth of ground improvement needed to increase the lateral capacity of piles embedded in soft or medium-stiff clay. The optimum depth is defined as the point below which further increases in soil stiffness produce little additional benefit.<br /><br />The authors use a finite-element Beam on Nonlinear Winkler Foundation (BNWF) model, with soil response represented by clay p–y curves. The model is checked against published results and then applied to concrete piles with different slenderness ratios, soil stiffnesses, and free or fixed heads. Soil stiffness is increased within an improved layer extending down from the ground surface.<br /><br />The results show that improving the soil increases pile-head stiffness and lateral load capacity, but the gains diminish beyond an optimum improvement depth. This optimum depends mainly on pile slenderness and head restraint: it generally decreases as pile slenderness increases, and fixed-head piles require a deeper improved layer than free-head piles. For long piles, the recommended improvement-length-to-pile-length ratio is about 0.05–0.10 for free-head piles and 0.10–0.20 for fixed-head piles. For short piles, the corresponding ratios are about 0.20 and 0.30.<br /><br />Increasing the stiffness of the improved soil raises pile-head stiffness, but has little effect on the optimum depth itself. The findings provide preliminary design guidance and may help avoid unnecessary ground treatment. The proposed approach can be implemented in standard computational software; however, the presented design charts apply to the concrete piles analyzed and should be used accordingly.

Keywords

ground improvement

pile lateral capacity

soft clay

medium-stiff clay

optimum depth

finite-element model

BNWF model

clay p-y curves

pile slenderness ratio

fixed-head piles