Geoscience Reference
In-Depth Information
CHAPTER 11
LIQUEFACTION PERFORMANCE OF SHALLOW FOUNDATIONS
IN PRESENCE OF A SOIL CRUST
George Bouckovalas
1
and Panos Dakoulas
2
1
Geotechnical Engineering, National Technical University of Athens, Greece
gbouck@central.ntua.gr (www.georgebouckovalas.com)
2
Department of Civil Engineering, University of Thessaly, Volos, Greece
dakoulas@uth.gr
Abstract.
Liquefiable soils are currently categorized by all seismic codes as extreme ground
conditions where, following a positive identification of this hazard, the construction of surface
foundations is essentially allowed only after proper treatment soil. This article examines to what
extent this situation may change in presence of a non-liquefiable soil crust, between the founda-
tion and the liquefiable soil. Means are provided for analytical evaluation of the degraded bear-
ing capacity and the associated seismic settlements for the specific case of strip foundations on a
cohesive(clay)crust.Furthermore,theconditionsareexploredwhichensureaviableperformance-
based design, and the issue of a critical soil crust thickness, beyond which liquefaction effects are
minimal, is addressed.
1. Introduction
Building a well engineered surface foundation directly upon the surface of a liquefiable
soil layer, without prior improvement or reinforcement, is clearly out of the question,
sincesettlementswillbeexcessiveanduneven,leadingtostructural,aswellasoperational
failure. The foundation failures shown in Figure 11.1 are merely some typical examples
from the ensuing hazard. However, it is possible that such a solution becomes feasible in
thepresenceofasufficientlythickandshearresistantnon-liquefiablesoilcrust(e.g.clay,
dense or dry sand and gravel, improved soil) between the foundation and the liquefiable
subsoil. The reason is simple: as the thickness of the non-liquefiable soil crust increases
gradually,beyondthemaximumdepthofaPrandtltypefailuremechanism,failureislikely
to develop exclusively into that layer and consequently any liquefaction of the subsoil
will have a minor effect on the post-shaking failure load and the associated settlements.
Thus, the question is not whether there is a beneficial effect of the non-liquefiable soil
crust, but what this effect is and whether it is of engineering interest. In providing a
satisfactoryoverallanswertothesequestions,onemustfirstresolvethefollowingdesign
issues:
(a) What is the bearing capacity of surface foundations on a liquefied subsoil, in the
presence ofa non-liquefied soilcrust?
(b) What arethe liquefaction-induced settlements of thefooting in theabove case?
