Harry George Poulos

This paper describes an approach for utilizing in-situ measurements of shear wave velocity Vs to carry out preliminary and check design calculations for shallow and deep foundations. For estimates of foundation movements, Vs can be used directly to estimate the small-strain stiffness of the soil or rock strata, while for ultimate capacity calculations, use is made of empirical correlations between Vs and penetration resistance measures, which in turn are correlated to the foundation resistance characteristics. The approach is applied to a series of published tests on shallow footings, and on a series of pile load tests for a very tall building. For these cases, comparisons of the calculated with the measured load – settlement behaviour indicates that the suggested approach provides a reasonable, albeit somewhat conservative, level of agreement.

This paper sets out an approach for utilising the small strain shear modulus of a soil to derive deformation parameters for foundation settlement prediction. This approach is an extension of that described by Mayne and his co-workers, with consideration being given also to the drained behaviour as well as the undrained behaviour of the soils. Both the undrained and drained moduli are related to the small strain shear modulus, with the soil compressibility also being incorporated in the drained case. The deformation parameters derived from this approach have been used to compute the load-settlement relationship for shallow foundations on sand and on clay, and for a single pile in residual soil. In the cases considered, satisfactory agreement is found between measured and calculated load-settlement behavior.

Deep excavations inevitably induce stress relief and soil movement, which may affect the safety and serviceability of nearby existing pile foundations. Although the effects of excavation on adjacent piles have been studied, excavation-induced pile settlement and change of shaft resistance deserve more investigation. In this paper, centrifuge simulation of the effects of multipropped excavation in-flight on vertically loaded floating piles in sand is reported. Piles were restrained in terms of rotation and lateral deflection at or above ground-surface level in three different tests. In addition, three-dimensional numerical analyses using an advanced soil model were carried out to back analyze the centrifuge tests. A pile-raft settled a similar amount due to excavation as a free-head pile and an elevated pile group. During excavation, a downward load-transfer mechanism in the piles can be identified. Shaft resistance in the upper part of the pile decreases due to the downward soil movement and the reduction of normal stress acting on the pile. In order to maintain vertical equilibrium, the pile has to settle to mobilize higher shaft resistance in the lower part of the pile and toe resistance. Lateral restraints imposed on the pile head have a significant influence on induced pile bending moment. Induced bending moment due to excavation can exceed the pile bending capacity.