Analytical and Numerical Approaches to Attain the Optimum Tapering Angle for Axially-Loaded Bored Piles in Sandy Soils

Abstract

This study aims to establish an equation to obtain the optimum tapering angle (αopt) for bored tapered piles that is correlated with pile geometry and sand properties that vary with the relative density. This αopt corresponds with the maximum axial bearing capacity when the volume of material in the tapered pile is maintained identical to the counterpart straight cylindrical pile. First, analytical formulations will be developed to estimate the axial bearing capacity of bored tapered piles that are embedded in sand. The proposed governing equations capture the shaft vertical bearing component of the tapered pile, which is unique to tapered piles and varies nonlinearly with the tapering angle (α). By differentiating the obtained bearing capacity equation for α, an αopt is achieved. The finite element method (FEM) will be adopted to conduct the numerical modeling and to calibrate the model parameters of the proposed analytical equation, which considers soil nonlinearities and interactions between the tapered pile and the surrounding soil that is subjected to axial loading. The UBCSAND constitutive model will be used to simulate the soil response near the tapered pile and the model parameters will be calibrated against laboratory test results for sandy soils with different relative densities. However, due to the complexity of the proposed differentiation and inverse calculation, a numerical solution will be used to obtain the results. Then, the load–displacement curves of the tapered piles will be attained numerically and αopt, which results in the maximum axial capacity of the pile, will be determined. The results exhibit good agreement between the analytically determined axial bearing capacity for the tapered pile and the corresponding numerical modeling predictions. Furthermore, a simplified empirical equation will be established to select αopt, which could be used by practicing engineers.