Geoscience Reference
In-Depth Information
CHAPTER 15
DEVELOPMENTS OF SOIL IMPROVEMENT TECHNOLOGIES
FOR MITIGATION OF LIQUEFACTION RISK
Ikuo Towhata
Department of Civil Engineering, University of Tokyo, Japan
towhata@geot.t.u-tokyo.ac.jp
Abstract.
Studies on liquefaction have a history of more than 40 years since Niigata and Anchor-
age were attacked by disastrous earthquakes. The topics of interest have changed with time and
recent interest is focused on advanced mitigation which can achieve cost-effectiveness and/or
is useful for existing structures. The present text describes recent experimental studies in both
laboratory and field. The concerned mitigation technologies consist of blasting for low cost,
grouting of colloidal silica for existing and sensitive structures, and drain pipes which are feasible
inasmallspaceunderanexistingstructure.Theconductedtestsshowedhowtheyhaveadvantages
over othermethods ifthey are used in appropriate conditions.
1. Introduction
Theaimoftechnologicaldevelopmentsformitigationofliquefactionriskhasbeenchang-
ing in the past in the following manners. Until 1970, the major interest lay in the identi-
fication of the causative mechanism of liquefaction. Example of this type may be found
in Yoshimi (1967). Studies in those days helped development of mitigation technologies
such as densification and drainage. At the same time, Koizumi (1966) compared SPT-N
in Niigata City before and after the 1964 quake, showing that liquefaction potential is
closely related with density of sand. This study resulted thereinafter in design formula
for evaluation of liquefaction potential based onpenetration resistance insoils.
While the above studies continued in 1980s in a more detailed and precise manner, a
differentviewpointemerged.Thenewpointwastwofold,consistingofinterestsinmech-
anismofpermanentdisplacementofgroundandperformance-basedmitigation.Thelatter
impliesthatinexpensivemitigationshouldbeallowediftheextentofgrounddeformation
is sufficiently small. This idea is considered important nowadays in design of lifelines
and other structures for which costly mitigation measures are not appropriate. Hence,
predictionoftheperformanceofsubsoilwithandwithoutmitigationagainstliquefaction
attractsengineeringconcern.Inotherwords,thereisastrongtrendtowardsperformance-
based design.
Technological development is also focused on existing structures. Being overlain by a
structure in use, the subsoil cannot be easily improved. In particular, powerful tamping
byabigmachinewithgroundvibrationand/ornoiseisextremelydifficult.Anotherissue
