Similarity-Based Nonlinear Settlement Predictions of Circular Surface Footings on Clay

Abstract

The similarity method, employed to obtain nonlinear settlement predictions in undrained conditions for rigid circular footings on deep clay deposits, was introduced more than 70 years ago. This approach is based on the premise that the pressure–settlement curve of the footing and a stress–strain curve from a characteristic point in the soil can be linearly scaled to collapse into a single master curve. The method has been extended to predict deflections of axially and laterally loaded piles and is widely used in the offshore industry. Despite the theoretical and practical appeal of the method as well as its wide application in a range of geotechnical problems, limited investigation and validation exists in the literature. In this work existing classical similarity methods are reviewed, including a Boussinesq solution for elastic soil and the mobilizable strength design (MSD) method. The similarity factors derived from these methods are compared with those obtained from a novel nonlinear cone model solution, and the resulting expressions are evaluated against rigorous numerical analyses undertaken by the authors in FLAC. These are based on two different nonlinear constitutive models (hyperbolic and tanh) calibrated against triaxial tests from three clay deposits. Two alternative families of similarity methods are also compared with classical similarity, namely, a two-part similarity technique (based on separate scaling factors for elastic and plastic strains) and a stiffness similarity approach (based on secant stiffness degradation). Finally, three field test results are evaluated as case studies to demonstrate the applicability of the method in real-life problems. It is concluded that similarity approaches offer a rational yet approximate tool for nonlinear settlement analysis of footings.