Downdrag Analysis on Piles by Cyclic Hyperbolic t-z Curves

When soils surrounding a deep foundation settle, a reversal of the direction of interface skin friction occurs over some upper section of the piles, thereby increasing both the axial stresses in the piles and the pile settlement, also known as downdrag. Limit analysis is generally ineffective at predicting pile settlements as it does not account for the dependency of mobilized friction on the relative displacement between the soil and pile surface. In earlier works, a hyperbolic equation was used to model the relationship between mobilized friction and the relative displacement between pile and soil. In this paper, the hyperbolic equation is extended to model the friction reversal. The hyperbolic cyclic equations are experimentally verified then incorporated into a spreadsheet program “t-zMich” with a new downdrag module. The iterative analysis accounts for both rigid translation and elastic compression of the pile to predict the mobilized side friction along the length of the pile and the overall downdrag.