Seismic behaviour of shallow foundations on layered liquefiable soils

Abstract

Earthquakes have been historically perceived as one of the most damaging
natural hazards. Seismic soil liquefaction is often one of the major sources of
damage and disruptions, and has been observed to severely affect key lifelines.
Settlement and tilting of shallow foundations resting on saturated sandy/silty
soils has been repeatedly observed throughout the world as a consequence of
liquefaction or softening of the foundation soil. Such settlements and tilts can
render structures unusable, and homes uninhabitable, causing significant economic losses. Despite the undoubted relevance of this phenomenon, field data on the liquefaction induced settlement of shallow foundations are scarce. New data from 24 buildings that suffered settlement and tilting as a consequences of soil liquefaction during the February 27th 2010 Maule earthquake in Chile, are presented in this work to supplement the existing field cases database. Due to the complexity of this phenomenon, field data are not suffcient to fully understand the mechanisms controlling the settlement of structures resting on liquefied or softened ground.In this framework, centrifuge modelling provides a valuable tool for research by reproducing field conditions in a controlled environment.
A series of 10 dynamic centrifuge tests were performed as part of this work.
Thanks to the University of Dundee newly installed centrifuge-mounted servohydraulic earthquake simulator, scaled version of field earthquake motions were reproduced in the models tested, enhancing the reliability of experimental
results. Particular attention was given to the effect of key parameters on the
observed foundation settlement. These parameters are the bearing pressure of
the foundation, the thickness of the liquefied soil layer and the soil's relative
density. The effect of the soil layering pattern was also investigated, with
particular attention to the effect of a low permeability soil crust overlying the
liquefied soil. Results suggest that the excess pore pressure generation in the
foundation soil is significantly influenced by the stress distribution due to the
presence of the foundation itself. In particular, lower excess pore pressure
where measured in soil subjected to high static shear stresses (i.e. below the
edge of a footing). The soil stratification pattern, and the relative thicknesses
of the liquefied and un-liquefied portions of the soil profile, were also found to
play a crucial role in determining the seismic demand at foundation level and
the type of failure mechanism leading to foundation settlement.
Observed differences between centrifuge (i.e. field) and element testing soil
response are also discussed. Experimental results are compared to field observations, with the aim of improving the current understanding of the behaviour of structures built on shallow foundations in the eventuality of seismic induced liquefaction of their foundation soil.

Date of Award	2013
Original language	English
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Andrew Brennan (Supervisor)