Research Trends and Results
Resilient Structure for Coastal Dikes -- Structural Device Considering theEffect of Pressure Increase in Dike Body -SUWA Yoshio, Head; KATO Fuminori, Senior Researcher; HIMENO Kazuki, Researcher
Coast Division, River Department
Keywords: tsunami, coastal dike, resilient structure
should be avoided. It is also effective to provide a device
that controls the movement of the landward slope
armoring, and thus, reduces the force with which thelandward slope armoring pushes the foundation when thepressure increases inside the dike body. It is also
effective to install sheet piles in the foundation to control
its movement, although they are not as strong as soil
improvement.
Dike toparmor
天端被覆工

Rise in saturated surface
浸潤面⑤⑥Landward
slope armoring
裏法被覆工③②④①(i)Increase in lift pressure applied to the landward①堤体内の浸潤面の上昇により裏法被覆
slope armoring due to the rise in saturated surface
工に作用する揚圧力が上昇
in the dike body②裏法被覆工と裏法面との摩擦力が低下(ii)
Decrease in frictional force between the landward
slope armoring and landward slope③裏法被覆工の自重が基礎工に作用(iii)
Dead weight of the landward slope armoring acts
④基礎工が滑動若しくは転倒(iv)⑤裏法被覆工が移動し、天端被覆工
Foundation slides or falls.
(v) Landward slope armoring moves, creatingとの間に隙間
a gap under the top armor.
(vi)⑥フィルター層及び堤体土の流出
Outflow of filter layer and dike body soilon the foundation.

Figure 1: Disaster Mechanism of Landward Slope
Armoring by Rise in Saturated Surface
Reducethe force that
裏法被覆工のpushes the foundation by
移動抑制によりcontrollingthe movement ofthelandward slope armor
基礎工を押す力
armoring.

を低減する

被覆工の下に捨てProvide
concrete
sub-slabs
under theコンクリートを施すarmor(to avoid the（フィルター層を入filterlayer).
れない）
Use material resistant to
浸透しにくい材料をsaturation for the dike
堤体土に用いるbody soil.

Control
the movement
矢板により基of the foundation with
礎工の移動を
sheet pile.

洗掘

Scour

1. Background and objective
Engineers have long sought a coastal dike structurethat can be resilient even in the case of a tsunami
exceeding the designed tsunami height of the dike.
When tsunami waves overtop a three-face-armoredconcrete dike, saturated surface in the dike body increases
as water penetrates the body; the force of the overtopping
water results in scouring on the landward side. To protect
the landward slope toe from scouring, soil improvement
and other remediation methods were found to be effective1), and soil improvement was adopted in the disaster
restoration project for the Southern Coast of Sendai Bay.
This study examined a resilient structure against tsunami
overtopping for coastal dikes with flat laid concrete armor,
for which the pressure rise in the dike body is a concern,by conducting experiments with a large model to detectpressure rises in the dike body in the case of tsunami
overtopping. 2)
2. Results of study
First, pressure gauges were attached under the concretearmoring to measure the pressure inside the dike bodyduring overtopping. It was found that, when thetsunami overtopped the coastal dike, pressure under theconcrete armoring increased and reached a level at which
the concrete armoring on the landward slope could
become unstable. This would depend on conditions, sincea filter layer is provided under the concrete armoring orseaward slope toe to accelerate saturation into the dikebody.
Next, assuming an area where the space on the landside of the dike was limited, an experiment with a large
model was conducted. The purpose of this experiment
was to study the impact of the filter layer under theconcrete armoring and the effect of sheet piles (e.g., sheet
piles installed under the foundation of a coastal dike withconcrete armoring to control scouring). Movement ofthe landward slope armoring was controlled when nofilter layer was provided under the armoring, andmovement of the foundation was controlled by
combining the landward slope armoring with theembankment soil by an iron frame.
The disaster mechanism of coastal dikes with concretearmoring that was confirmed by this experiment isillustrated in Fig. 1. Considering this mechanism, as
shown in Fig. 2, a filter layer that facilitates saturation in
the dike body between the armor and dike body soil

抑制する

Figure 2: Structural Device for Coastal Dike withConcrete Armors in Limited Space[Reference]
1) NILIM: A Study on Resilient Structures for Coastal Dikes,
NILIM Technical Bulletin, 2012
http://www.nilim.go.jp/lab/fcg/labo/02_02.html

2) Kato, Suwa & Hatogai: Structures for Coastal Dikewith Concrete Armors Resilient to Tsunami Overflow,
Journal of JSCE, Series B2 (Coastal Engineering), Vol.69,
No.2, pp.I_1021-I_1025, 2013.