Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Comparison of Pile-type and Gravity-type Coastal Levees in Terms of Resilienceto Tsunami
Yukihiro ISHIHARAManager, Press-in Technologies Support Department, Giken, Tokyo, Japan
Email: ishihara@giken.com

Koichi OKADA
Construction Solutions Development Department, Giken, Kochi, Japan

Masanori HAMADA
Professor emeritus, Waseda University, Tokyo, JapanABSTRACT
In the Great East Japan Earthquake in 2011, many tide barriers and breakwaters collapsed due to the tsunami. On theother hand, one structure consisting of double sheet pile walls remained almost undamaged. This paper introduces a
newly developed experimental apparatus called ‘Tsunami Simulator’, in which two types of tsunami, surge and reflux,
can be simulated to investigate into the behavior of structures a part of which is embedded in the ground. It also
summarizes the result of the adequately scaled model test on the behaviors of gravity-type and pile-type levees, when
they are hit by surge-type tsunami. The test results showed that the pile-type levee remained resilient to tsunami while
the gravity-type levee failed by sliding. This result demonstrates the effectiveness of the use of piles to improve theresilience of structures. On the other hand, the specification of piles should be adequately determined in design to assure
the resilience of the system. It would be reasonable to allow some plastic deformation of piles, which cannot be adequately
scaled in the model test, especially when a huge tsunami has to be considered.
Key words: Coastal levee, Pile, Tsunami, Model test, SPT

1. Introduction
The Press-in Method is one of the piling techniques

associated with the Great East Japan Earthquake in 2011.

that use a static jacking force to install prefabricated piles.

demonstrated in these incidents had already been noticed

In this method, a piling machine gains a reaction force

before 2011, and efforts have been made to promote thesefrom the previously installed piles, and therefore, a wall ofstructures by naming them ‘implant structures’ (Kitamura,piles is efficiently constructed without bulky weights that2017). A typical example of the results of these efforts, aswill occupy a large space, as shown in Fig. 1.

reported by Ishihara et al. (2018), is the construction ofThe structures with piles, where appropriateTheeffectivenessofstructures

withpiles

coastal levees with piles along the Kochi Coast which isembedment depths are assured to make use of the strengthand stiffness of the soil, are expected to show a greater
resistance and higher resilience to the external loads onthemselves. As shown in Fig. 2, gravity-type coastal
levees without an adequate embedment depth wereobserved to have collapsed, while a pile-type structure wasfound to have remained without significant damages(Furuichi et al., 2015), after the overflowing tsunami- 251 -Fig. 1 Press-in piling system saving temporary works.

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi(a)

Gravity-type levee.
Fig. 3General view of ‘Tsunami Simulator’

respectively. The maximum flowrate of the pumps is 40
cubic meters per minute.
2.2. Types of model tsunami
In this test apparatus, two types of model tsunami can(b) Pile-type structure.
Fig. 2

be generated, as shown in Fig. 4. One is a surge-type

Different resilience to tsunami observed in 2011.

model tsunami, simulating the front part of tsunamis. Thiscan be generated by accumulating water in the surface

expected to be hit by a huge tsunami associated with antank and opening the gate instantaneously. The length ofearthquake with a moment magnitude of 9.0 in the Nankai

the channel is designed so that appropriate values of

Trough (Cabinet Office, Government of Japan, 2012).

Froude Number can be attained at the position of the soil

Further activities have been carried out to investigate thetank. The other is a reflux-type model tsunami, simulating

effect on mitigating tsunami load of a breakwater withthe continuous flow in the body of tsunamis. This can be

piles and porous fibers (Suzuki et al., 2016a) and agenerated by leaving the gate open, accumulating water inbreakwater with arrays of piles (Suzuki et al., 2016b),

the underground tank and running the pumps continuously

using a model test apparatus that can simulate tsunamis.

to maintain a constant flowrate.

This paper introduces the specification of the model
test apparatus to simulate tsunamis, and reports the result
and interpretation of a model test to compare the resilienceagainst tsunami of gravity-type and pile-type coastal(a)levees.

Fig. 4

Surge-type(b) Reflux-type

Model tsunami simulated in Tsunami Simulator.

2. Text apparatus – ‘Tsunami Simulator’

3. Method of model test

2.1. Specification of the test apparatus
The general view of the ‘Tsunami Simulator’ test

3.1. Specifications of models of coastal levees
Two models of coastal levees were prepared, with aapparatus is shown in Fig. 3. The apparatus consists of ascale of 1/33. One is a gravity-type coastal levee as shown

surface tank for storing water, a gate, a channel, a soil tankin Fig. 5, with a height of 11m and a weight of 980kN/m

for placing a model ground with model structures, anin a prototype scale. The other is a pile-type coastal levee

underground tank for storing water, pumps, a control

as shown in Fig. 6, with an embedment depth of 16m, aboard for operating the pumps and a measurement roomheight of 11m, outside diameter of 1.0m and thickness offor placing devices such as PCs and data loggers. The

22mm at prototype scale.

channel has a length of 19.5m, a width of 1.5m and a depth

The two models were placed side by side, with anof 0.8m. The soil tank has a length of 1.5m, the same width

acrylic plate sandwiched between the two models toas the channel and an additional depth of 0.5m below theseparate the flow of the tsunami around the two levees.

bottom of the channel. The capacities of the surface tankand the underground tank are 40 and 110 cubic meters- 252 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Fig. 5

Gravity-type model coastal levee.

Fig. 7

Estimated SPT N of the Model ground.

Fig. 6 Pile-type model coastal levee.

3.2. Properties of model tsunami and model groundA surge-type model tsunami was adopted in theFig. 8

model test. The flow velocity and the wave height wereHorizontal forces acting on a gravity-type levee.

controlled to be from 10 to 15m/s and around 4m in a
prototype scale, respectively.

acceleration, h is the height of the levee, μ is the coefficient

The model ground was made of saturated crushedof friction at the bottom of the levee, ρc is the density of astones (#7), with the particle size being approximately

concrete and Vc is the volume per unit horizontal depth of

2.5mm to 5mm. A Cone Penetration Test (CPT) was

the body of the levee. Taking the scale as λ, both of Q andconducted in the model ground, and SPT N values, which

F are proportional to λ2, since g, ρw and ρc are identical inare more familiar to the Japanese engineers, werea prototype scale and in the model test. Therefore, theestimated from the CPT results based on the method

equilibrium of horizontal forces can be said to have been

proposed by Jefferies & Davies (1993), as shown in Fig.

adequately scaled in this model test.

7.

Fig. 9 shows the horizontal forces acting on the piletype levee. The passive and active earth pressure acting on

3.3. Scaling laws

the pile wall (Pp and Pa) are expressed as:

Fig. 8 shows the horizontal forces acting on thegravity-type levee. The load of tsunami Q and thefrictional force at the bottom of the levee F are expressed

𝑃𝑝 =1𝐾 ρ gl 2
2 p s(3)𝑃𝑎 =1𝐾 ρ gl 2
2 a s(4)as:1ρ gh2
2 w
F = μρc Vc
Q=(1)where Kp and Ka are the coefficients of passive and active(2)

earth pressure, ρs is the density of the ground and l is theembedment depth of the pile wall. Taking the scale as λ,where ρw is the density of water, g is the gravitational- 253 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

and assuming that Kp and Ka are identical in a prototype
and in the model test, the three forces (Q, Pp, Pa) areproportional to λ2, considering that g, ρw and ρs areidentical in a prototype scale and in the model test. Strictly
speaking, however, Kp and Ka are dependent on the stressthrough the internal friction angle (φ). At smaller stresslevel, φ is larger, and therefore Kp is larger and Ka issmaller. Therfore, the small scale model will be morestable than the prototype.
Regarding the deformation of the pile wall, it isconvenient to separate the pile wall into two, at the level
Fig. 9of the ground surface. Fig. 10 (a) shows the deformation

Horizontal forces acting on a pile-type levee.

of the pile wall above the ground surface, assuming it to
be a cantilever fixed to a rigid ground. The horizontal
displacement at the level of loading (δ1) is expressed by
the horizontal load (P), wall height (h), the Young’s
modulus and the second moment of area of the pile wall
(E and I) as:δ1 =Ph33EI(5)Introducing subscripts ‘p’ and ‘m’ to represent theprototype and the model respectively,
δ1pPp (EI)m hp= ( )()( )δ1mPm (EI)p hm(a)Above the ground surface.

(b)

Below the ground surface.

3(6)Considering Pp/Pm = λ2 and hp/hm = λ, the scaling law canbe satisfied if (EI)m/(EI)p = λ-4. In this model test, thematerial for the model levee was chosen so that (EI)m/(EI)p= λ-3.65, approximately satisfying the scaling law.
Fig. 10 (b) shows the deformation of the pile wall
below the ground surface. If the embedment depth is great
enough, the horizontal displacement of the pile wall at theFig. 10ground surface (δ2) can be expressed as (JGS, 2010):δ2 =h0 =1+βh02EIβ3MPkH
β=√4EI4P(7)

Deformation of the pile wall.

where kH is the horizontal subgrade reaction of soil.
Therefore,

(8)

(9)

(1+βp h0p ) βm 3 (EI)m Ppδ2p= ()()( )(EI)p Pmδ2m
(1+βm h0m )βp 3- 254 -(10)Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Considering Pp/Pm = λ2, the scaling law is satisfied under
the following condition:((1+βp h0p ) βm 3(1+βm h0m )βp3)((EI)m) = λ-1(EI)p(11)In this model test, the value of the left term of Eq. 11 wasadjusted as λ-0.85, by assuming kHm = kHp, approximately
satisfying the scaling law. Strictly speaking, kHm will be(a)When tsunami hit the model levees.

smaller than kHp due to the smaller stress level, and thedeformation of the model levee will be larger than that ofthe prototype levee. Therefore, the model levee in this test
can be said to have been safely scaled.
On the other hand, as it is difficult to scale the failureor the plastic deformation of piles, the specification ofpiles was determined so that the deformation of piles did
not exceed their yield points both at model scale andprototype scale.
4. Results and interpretations(b)

Sliding of the gravity-type model levee.

(c)

Rotation of the gravity-type model levee.

Fig. 11 shows the behaviors of the two model levees
when they were hit by the model tsunami. Just after thegravity-type model levee was hit by the model tsunami(Fig. 11 (a)), it slid (Fig. 11 (b)) until arriving at the endof the soil tank, and then rotated around the edge of thesoil tank (Fig. 11 (c)). On the other hand, the pile-typemodel levee deformed slightly but retained its position andheight (Fig. 11 (d)).
For the gravity-type model levee, the sliding failure
was caused by the greater horizontal load acting on themodel levee compared with the frictional resistance at thebottom of the model levee. This failure has an engineering
sense in this test, since the scaling laws are adequately
satisfied as discussed in the previous section. Othermechanisms causing additional modes of failure, such as
the insufficient bearing capacity of the model ground toresist to the moment load associated with the height of theacting point of the tsunami load, and the reduction of the
bearing capacity of the model ground due to theliquefaction of itself, might have existed as well. These
two mechanisms have less engineering sense in this test,
as the scaling laws are not strictly satisfied. Consideringthat the sliding failure occurred before the failure of themodel ground, the behavior of the gravity-type model- 255 -(d) Elastic deformation of the pile-type levee.
Fig. 11

Behavior of the model levees.

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

levee in this test can be said to have had an engineering

effectiveness of the use of piles to improve the resilience

sense in the early stage of its behavior before its rotation.

of the structure. On the other hand, the specification ofFor the pile-type model levee, it is supposed that thepiles should be adequately determined in design to assure

earth pressure around the pile wall was effectively utilized

the resilience of the system. It would be reasonable toto resist the horizontal and moment load due to the modelallow some plastic deformation of piles, which cannot betsunami. This point was adequately scaled in the test andadequately scaled in the model test, especially when a

can be expected for the pile-type levee in a prototype scale.

huge tsunami has to be considered.

Also, the elastic deformation of the pile wall in this test
can also be scaled up to consider the elastic deformationof the pile wall in a prototype scale, as the bending

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