2103
Effects of plane shapes of a cofferdam on 3D seepage failure stability and
axisymmetric approximation
Effets des formes planes d’un batardeau sur la stabilité après une rupture par infiltration
tridimensionnelle et sur l’approximation axisymétrique
Tanaka T., Kusumi S., Inoue K.
Department of Agricultural and Environmental Engineering, Kobe University, JAPAN
ABSTRACT: In the excavation of soil with a high ground water level, seepage failure is often a problem. For excavations over a large
area, seepage failure is a problem in two dimensions. In contrast, the more the region of a cofferdam is restricted, the greater the
seepage flow concentrates three-dimensionally. This three-dimensionally concentrated flow lowers the safety factor for seepage
failure more than under the two-dimensional condition. In this paper, seepage failure experiments were conducted under three-
dimensional flow conditions for various cases of penetration ratios of sheet piles and analyses of FEM seepage flow and stability
against the seepage failure of soil were carried out using the Prismatic failure concept 3D. The critical hydraulic head differences
obtained by experiments and the theoretical values are examined for several cases. Effects of plane shapes of a cofferdam on the
theoretical critical hydraulic head differences and axisymmetric modeling of three-dimensional seepage failure are also discussed.
RÉSUMÉ : Dans le cas de l’excavation d’un sol où le niveau des eaux souterraines est élevé, la rupture par infiltration constitue
souvent un problème. Lorsque l’excavation est effectuée sur une grande surface, la rupture par infiltration devient alors un problème
en deux dimensions. En revanche, plus la zone du batardeau est limitée, plus le flux d’infiltration se concentre en trois dimensions.
Or, comparé à ce qui se passe dans un contexte bidimensionnel, ce flux concentré de manière tridimensionnelle réduit davantage le
facteur sécurité lié à une rupture par infiltration. Dans cet article, nous décrivons les expériences sur la rupture par infiltration menées
dans les conditions d’un flux tridimensionnel pour divers ratios de pénétration dans des palplanches. Nous y rapportons aussi les
analyses du flux d’infiltration et de la stabilité suivant le modèle FEM par rapport à la rupture par infiltration du sol, menées à l’aide
d’un concept prismatique de rupture en trois dimensions. Nous avons examiné, pour plusieurs cas, les rapports de niveau hydraulique
obtenus dans les expériences et la théorie. Les effets des formes planes d’un batardeau sur les rapports de niveau hydraulique critiques
d’ordre théorique et la modélisation axisymétrique de la rupture par infiltration tridimensionnelle y sont également évoqués.
KEYWORDS: three dimensional seepage failure (3DSF), surface shape of a cofferdam, axisymmetric modeling of 3DSF
1 INTRODUCTION
In the excavation of soil with a high ground water level, sheet
piles or diaphragm walls are often used to retain soil and water.
Under such conditions, seepage flow occurs through the soil,
and seepage failure is often a problem. For excavations over a
large area, seepage failure is a problem in two dimensions. In
contrast, the more the region of a cofferdam is restricted and the
deeper the penetration of the sheet piles, the greater the seepage
flow concentrates three-dimensionally within it. The three-
dimensionally concentrated flow lowers the safety factor for
seepage failure more than under the two-dimensional condition
(Nikkei construction, 2001). Such a case must be treated in
three dimensions.
In this paper, seepage failure experiments were conducted
under three-dimensional flow conditions for various cases of
total depths of soil,
T
, and penetrated depths of sheet piles,
D
,
with a plane shape of a cofferdam of 1:2. Analyses of FEM
seepage flow and stability against the seepage failure of soil
were carried out using the Prismatic failure concept 3D. The
hydraulic head differences at deformation in the experiment,
H
y
,
and theoretical critical hydraulic head differences based on the
Prismatic failure concept 3D,
H
c
, are examined for the same
cases. The theoretical critical hydraulic head differences for
various plane shapes of a cofferdam, e.g., short to long length of
1:1, 1:2, 1:3, and 1:4, were calculated, and the effects of plane
shapes of a cofferdam on the theoretical critical hydraulic head
differences are discussed. The axisymmetric modeling of three-
dimensional seepage failure is also discussed.
2 THREE DIMENSIONAL EXPERIMENTS
2.1
Test apparatus
A test apparatus was designed to study 3D seepage failure of
soil within a cofferdam as shown in Figure 1. In the experiment,
one quarter of the three dimensional region is examined. The
seepage tank is made of stainless steel, 1,000mm wide,
1,300mm high and 1,000mm deep. The front of the tank is
made of transparent glass for observation of the behavior of soil
particles inside and the right side of the tank is equipped with
283 piezometer holes for the measurement of pore water
pressures. A cofferdam is mounted on the right/front side with
surface size 200mm
400mm. Seepage water flows through a
sand model under the difference in water head
H
between the
downstream water level at the top of the right-hand-side
drainage hole and the upstream water level kept constant by the
constant-head device.
2.2
Test material and test cases
In seepage failure experiments, uniform fine sand (Lake Biwa
Sand 3: under 850
m mesh, 50% grain size
D
50
=0.283 mm,
specific gravity
G
s
=2.67 and uniformity coefficient
U
c
=1.40)
was used. Seventeen tests E0301 to E0317 were conducted. The
following notation is used:
T
1
and
D
1
are the total depth of soil
and penetration depth of sheet piles on the upstream side,
T
and
D
are those on the downstream side,
d
(=
D
1
D
) is the
excavation depth for the excavation model, and
D
r
is the
relative density of soil. For a no-excavation model,
T
1
=
T
,
D
1
=
D
nd
d
=0 are applied.
a
