Piled Foundations in Liquefiable Soils: accounting for axial loads

Jonathan Adam Knappett, Cambridge University
Geotechnical Engineering Group


This thesis examines the influence of axial load on the behaviour of piled foundations in liquefiable soils, and presents a framework for determining safe axial pile loads which meet both serviceability and ultimate limiting states. Consideration is given both to effects of axial load on the lateral behaviour (amplification and instability effects) and vertical behaviour (liquefaction-induced settlement and bearing capacity failure). In both cases, dynamic centrifuge testing was used as the primary method of investigation.

For pile groups which are subjected to lateral displacement during earthquake shaking, axial loads amplify lateral displacements following a reduction in soil stiffness due to liquefaction. If lateral deflections and/or axial loads are large enough, unstable collapse (bifurcation) can occur, with collapse loads being heavily dependent on dilative soil behaviour at large strain. Pseudo-static BNWF models have been developed which can determine the complete post-buckling response of the system and have been fully validated against centrifuge test data.

For piles bearing in dense or stiff layers, high excess pore pressures in the bearing layer lead to a drop in bearing capacity (modelled using spherical cavity expansion), and consequently large pile group settlement. Shaft capacity in liquefied soil was found to be heavily dependent on the degree of lateral displacement and can increase dramatically due to additional horizontal total stress on the pile from lateral soil-pile (p-y) interaction effects. The loss of pile capacity is compensated for by increased bearing pressure beneath the pile cap due to settlement-induced dilation in the underlying soil. An empirical equation has been developed enabling selection of a suitable static safety factor (SSF) when designing to displacement-based performance criteria.

The interaction of these two behavioural modes was investigated for pile groups in laterally-spreading soils. The BNWF analyses and empirical settlement equations were combined to map liquefaction, spreading and load criteria for different possible modes of failure, and this was validated against additional centrifuge test data. For stiff steel piles, settlement failure is the dominant criterion in determining safe axial loads at both serviceability and ultimate limit states. For more flexible reinforced-concrete piles, both settlement and instability effects can limit axial loads for both limit states, depending on the amount of liquefaction and the degree of pile group lateral displacement.