1623
Shaking model tests on mitigation of liquefaction-induced ground flow by new
configuration of embedded columns
Essais sur table vibrante pour une attenuation de l'écoulement des sols du a la liquefaction par une
nouvelle configuration de colonnes enterrees
Takahashi N.
Sumitomo Mitsui Construction Co., Ltd. Chiba
Derakhshani A., Rasouli R., Towhata I.
The University of Tokyo
Yamada S.
Osaka City University
ABSTRACT: Traditionally, countermeasures against seismic liquefaction aimed to prevent significant development of excess pore
water pressure. Although this aim was achieved by a variety of measures, the limitation has been understood as well in the recent
times. The limitation is typically found in lifelines and transportation lines (road embankment) together with river levees for which
financial requirement is more strict and also residual deformation is allowed to occur to a certain extent. In this regard, the present
study addresses installation of stable cement-mixed soil columns in liquefaction-prone subsoil so that ground deformation may be
constrained during earthquakes and lateral flow of liquefied sand may be reduced. For its validation, two types of shaking-table model
tests have been conducted in 1-G environments.
RÉSUMÉ : Traditionnellement, les mesures de lutte contre la liquéfaction sismique visaient à empêcher un développement significatif
de la pression interstitielle de l'eau en excès. Bien que cet objectif ait été atteint par une variété de mesures, leurs limites ont
également été perçues récemment. Les limitations concernent typiquement les infrastructures critiques et les lignes de transport
(remblai routier) ainsi que les digues fluviales pour lesquelles les besoins financiers sont plus stricts et les déformations résiduelles
sont tolérées dans une certaine mesure. À cet égard, la présente étude porte sur l'installation de colonnes de mélanges sol-ciment
stables dans des sous-sols susceptibles de liquéfaction afin que les déformations du sol puissent être limitées durant les séismes et que
l’écoulement latéral du sable liquéfié puisse être réduit. Pour la validation de l’étude, deux types d'essais sur table vibrante ont été
effectués dans des environnements 1-G.
KEYWORDS: liquefaction, deformation, mitigation, model test.
1 INTRODUCTION
Recent developments of deep soil mixing for ground
improvement have produced different variations in geometry of
solidified soil. For example, uniform mixing of the entire soil
with grouting agent is the original idea and, because sand grains
are thereby bonded with each other, it is very reliable. However,
the construction cost for this is the highest among all other
options. Accordingly, improvements of grid type, wall type, and
columnar type have been attempted.
For the aim of liquefaction mitigation, either grid or wall
types have been used in practice. It is expected therein that the
rigidity of the improved grid or wall constrains the shear
deformation of sand inside the grid (or between parallel walls),
and hence pore pressure development during strong shaking is
significantly reduced (Suzuki et al., 1989). The effect of the
grid-type mitigation was validated during the 1995 Kobe
earthquake in which subsoil liquefaction was prevented and an
overlying water-front building was completely protected from
seismic damage (Suzuki et al., 1995).
In contrast to grid- or wall-type grouting, the columnar type
was considered less effective than others in liquefaction
mitigation in spite of its lower installation cost (Koga et al.
1986). However, more recent shaking model tests by Yasuda et
al. (2003) and Tanaka et al. (2003) indicate that columns are
able to constrain shear deformation of soil and hence mitigate
the onset of liquefaction. Moreover, Yamamoto et al. (2006)
carried out numerical analyses to indicate that columnar
improvement develops similar prevention of pore pressure
development as grid type does if the improvement ratio exceeds
35%, although the grid type is better in performance if the
improvement ratio is less. In this regard, the present study
conducted 1-G shaking model tests on liquefaction mitigation
achieved by assembling four columns into one (CDM-Land4
method; CDM Association, 2002) so that the rigidity is
significantly increased. Furthermore, attention was paid to the
geometry of columns, supposing that an irregular installation of
columns may be able to achieve better mitigation than
conventional regular (square or triangular) configuration of
columns. (Towhata et al. (2010) and Takahashi et al. (2010))
The following text addresses first more details of irregular
installation of columns, followed by shaking model tests to
examine the effects of irregular configuration of columns in 1-G
environment. The first series of shaking model tests were
conducted in a rigid soil box in which a sloping and liquefiable
sandy layer was placed at the top. The second series of tests
were performed on sheet-pile quay wall models with liquefiable
backfill sand.
2 IRREGULAR CONFIGURATION OF UNDERGROUND
COLUMNS
Figure 1 illustrates three kinds of column configurations that are
addressed in this paper. The irregular configuration in Fig. 1(a)
shows that a 2*2 square grid with a spacing of "d" is shifted
either by 2d or d/2 distance in X and Y directions, respectively.
This configuration does not allow free passing of liquefied
subsoil through column spaces in contrast with the square and
triangular configurations (Figs.1(b) and (c)) where a straight
flow passage is available; see red arrows in these figures. This
lack of free and open space in the irregular configuration is
further expected to restrain cyclic shear straining during shaking
and reduce the probability of liquefaction.
