Experimental and Numerical Study on Pressure Distribution under Screw and Straight Pile in Dense Sand

Abstract

A similar theoretical approach is generally used to estimate the end-bearing capacity of straight and screw piles. However, under similar ground conditions (stress level and bearing-capacity factor) and pile tip area after the pile is installed, the end-bearing capacity of the screw pile is less than the straight pile. In this study, the experimental tests were modeled in a three-dimensional (3D) finite-element code to better understand the pressure distribution and mobilized shear strength under straight and screw piles embedded in similar ground conditions. Moreover, the effect of helix position on the pile bearing capacity and pressure beneath the helix and shaft of the screw pile was also investigated. A helix-to-shaft diameter ratio (Dh/DSS) of 2.8 was used. A good agreement between experimental and numerical results was found. The results indicated that the pressure distribution under the screw pile with a thick helix (non-deformed helix) and straight pile are similar when the helix is positioned (dh) at the pile tip (dh = 0). As the helix is moved further away from the pile tip (dh > 0), the pressure under the helix gradually decreases, while the pressure under the shaft increases and then decreases within a certain dh/Dh. The mobilized shear strength contribution of the helix decreased with the increase in helix position-to-helix diameter ratio (dh/Dh). Moreover, when dh/Dh ≥ 1.5, both central shaft and helix behaved independently, and their contribution in mobilized shear strength cannot be considered as a group. Regarding the effect of the helix position on the shaft pressure, when dh/Dh < 1.5, the ratio of screw pile to straight pile end-bearing capacity decreases almost linearly with the increase in dh/Dh. A modified equation is proposed that includes the two factors—helix position and shape (helix which differentiates screw pile shape from the straight pile)—to estimate the end-bearing capacity of the strong helix screw pile.