A Simplified Calculation Method for Stress Concentration Ratio of Composite Foundation with Rigid Piles

Piles in a composite foundation share loads with the surrounding soil instead of carrying all loads alone as a pile foundation. Stress concentration ratio, defined as the ratio of the stress on the top of piles to the stress on the surrounding soil, is essential for the design of piles in the composite foundation. An equal strain condition is widely accepted and adopted in the existing methods of calculating the stress concentration ratio. In other words, the piles should have identical relative displacement with the surrounding soil in an equal strain condition. This assumption may work for flexible and semi-rigid piles; however, it may not be suitable for rigid piles considering the significant stiffness difference of pile and soil. The relative displacement of rigid piles and surrounding soil can significantly change the stress distribution on piles and soils due to the pile-soil interaction (i.e., pile side friction). Therefore, the pile-soil interaction should be considered in the design of the rigid piles in the composite foundation. In this study, a simplified theoretical solution of the stress concentration ratio was derived based on the deformation coordination between rigid pile and soil to consider the pile-soil relative displacement and thus the pile-soil interaction. The pile side friction was assumed as linearly increased with the pile-soil relative displacement and then the stress concentration ratio was derived based on the force and deformation equilibrium in the composite foundation. A neutral point (i.e., location with zero pile-soil relative displacement) was included in the derivation to take the negative skin friction along the pile into account. Lab-scale and full-scale field tests were conducted to verify the effectiveness of the proposed method. Comparison of measured stress concentration ratios with calculated ones using the proposed method demonstrates the effectiveness of the proposed method.