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

Evaluating the Efficiency of Jacked-in Piles as Tsunami Defences
A. DOBRISAN
BA MEng, Department of Engineering, University of Cambridge, Cambridge, UK
Email: ad622@cam.ac.uk

S.K. HAIGH
MA MEng PhD, Department of Engineering, University of Cambridge, Cambridge, UK

Y. ISHIHARA
MA, Construction Solutions Development Department, Giken Ltd., Kochi, Japan

ABSTRACTThe 2011 Tōhoku earthquake and tsunami brought into question the adequacy of the pre-existing shallow foundation
seawall design due to the large number of failures recorded. A new type of seawall made of adjoining large, jacked-in,
steel pipe piles embedded 10-15m into the ground is currently installed along the Kochi coastline. Seawall design against
tsunamis is a unique, true ULS design, which presently is carried out using codes of practice not specifically tailored forthis scenario. Through small scale wave flume experiments and lateral loading tests of single piles at full-scale theadequacy of current design methodology as well as the efficiency of the new steel pile seawall design is assessed. It isfound that current Japanese codes, although designed for tsunami shelters, provide appropriate predictions of wave force
for the case of seawalls needing to withstand overtopping waves. The lateral pile tests highlight the current codes’
accuracy in predicting pile stiffness and bending moment profile. Remaining shortcomings, such as no provision in current
seawall design for the effects of soil softening during tsunamis, are evinced. Even so, steel pile walls seem an effective
seawall solution due to their high embedment and capacity to dissipate wave energy through yielding.
Key words: Seawall Design, Tsunami, Lateral Resistance, Wave Flume, Steel Piles

1. Introduction
In the aftermath of the devastating 2011 Tōhoku
earthquake and tsunami the large number of seawall
failures, exemplified by Fig. 1 and Fig. 2 from Kato et al.
(2012), evinced a need for a redesign. Instead of theshallow foundation concrete caisson design a new solution
currently being installed along the coastline of Kochi
prefecture is a seawall made from adjoining large, jackedin steel pipe piles embedded 10-15m into the ground. An
immediate advantage of the new design is its beneficial
Fig. 1role even in the case of extremely large waves. This results

Seawall overturning (Kato et al., 2012)

from the piles’ capacity to yield and dissipate wave energy

to safely withstand overtopping waves (Suppasri et al.,as well as the unlikeliness of piles becoming debris due to2012).

the large embedment. These advantages align well withthe post-2011 Japanese requirement of seawalls needing- 289 -

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

Even so seawall design against tsunamis is a unique,true ULS design, which currently is carried out using

3. Experimental Setup

codes of practice not specifically designed for this3.1. Wave flume experiments
The experiments were carried out in the Giken

scenario.

Tsunami Simulator, a 14m long by 2m wide wave flume(Fig. 3). Two steel plate model walls, 10cm and 15cm highrespectively, were used in these tests. Pressure transducers
were fitted along the height of the walls to quantify thewave-wall interaction. Four uniformly spaced transducers

Fig. 2

Scour seawall failure (Kato et al., 2012)

2. Research aims
Through the use of small scale wave flume modelexperiments and lateral loading tests of piles at full-scale,Fig. 3

10cm model wall and wave sensors

this paper aims to investigate the adequacy of current

were installed on the 15cm high model wall. A denser

design methodology as well as assess the efficiency of thearray of sensors was employed for the 10cm high wall,

new steel pile seawall design. In designing a seawall

shown in Fig. 4.

against tsunamis the major challenges are estimating thewave force and quantifying the effect of the dynamic
nature of the load as well as assessing whether the seawall
has the capacity to carry the total loading.
The wave flume experiments are employed to obtaintime histories of wave force across multiple scenarios.
These forces are compared with predictions from theJapanese code (Okada et al., 2006). Since the code waswritten with tsunami shelters in mind and the assumptionthat waves are much smaller than the building size, it does
Fig. 4

not presently take dynamic amplification into account.

10cm model wall cross section

The validity of this premise in the case of seawalls can be

A wave height sensor and 2 velocity sensors wereassessed through the use of the wave force time histories

fitted in the flume just in front of the test wall. One of thecoupled with insight of the pile-soil system stiffness

velocity sensor was lowered to the base of the wall, while

gained from full-scale lateral pile tests.

the other measured water velocities just above the wall as

The full-scale tests on single piles identical to those

the water passed over. Another height and velocity sensor

installed on the Japanese coast also serve to check how

pair was positioned 4m ahead of the wall to measure thewell design codes (DNV, 1992) scale to tsunami seawalls.

incoming wave characteristics (seen in Fig. 3). A total of

These codes are mostly based on the works of Reese et al.

47 experiments were carried out in the Tsunami Simulator,(1974) which analysed data from lateral tests on 0.5-0.6m

with wave heights both greater and smaller than the

diameter flexible piles, unlike the much stiffer, 1m

heights of the walls. A typical initial impact between adiameter piles used in the construction of the new seawalls.

wave and model seawall is shown in Fig. 5.

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

Table 2.
Dia-

Test Piles Key Data

SectionPile

meter Thickness Length

Embed- Yield Tensile
ment

Stress Stress

m

mm

m

m

MPa

MPa

1.0

22

15

10

315

490

1.0

12

15

10

235

400

1.0

12

15

10

395

504

Pile 1(SKK490)
specif.
Pile 2
Fig. 5(SKK400)

Model tsunami wave hitting wall

specif.

3.2. Lateral Pile Tests
Two full-scale lateral pile tests were designed to(SKK400)

investigate the performance of a 5m seawall subjected tomeasuredPile 2an overtopping tsunami wave. While a typical seawall isgenerally comprised of a contiguous row of piles, thesufficiently stiff and was not affecting the experiment. The

behaviour of singular piles is investigated to provide

configuration is shown in Fig. 6. The position of therelevant insight. The tests were carried out at thehydraulic jack (2.4m above ground) was chosen as to

Nidahama test site in Kochi, Japan. A Giken Ltd trial site,

coincide with the point of application of the equivalentnext to the Pacific, with sandy soil up to 5m depth andstatic wave force as described by Okada et al. (2006). An

sandy gravel below it, Nidahama closely mimics theassortment of sensors was used to monitor the pile

ground conditions the pile seawalls were installed in along
the Kochi coastline. Three SPTs and two triaxial
consolidated drained tests were used to assess groundproperties, summarized in Table 1. Further information on
the test site can be found in Gillow et al. (2018).
Table 1.
Water

Derived soil properties at Nidahama test site

t

’

Rd

 crit peak

m

kN/m3

kN/m3

%

Deg

Deg

7.45

20

12.4

70

32

40

table depth

Fig. 6Lateral pile loading system

response during testing: 10 strain gauge pairs wereTable 2 highlights the key properties of the two steel

positioned on the pile at 1m intervals below ground,tubular piles. The yield stress and the ultimate tensile

inclinometers were placed at the same sections as thestress capacities were taken from the manufacturer’s

gauges and LVDTs and wire displacement sensors were

specification sheet. However further data from mechanicalused to measure accurately the pile displacement at thetesting was made available for Pile 2. As can be seen there

ground level and at the jack position.
The two tested piles were subjected to a small

is a significant factor of safety included in the material

number of unload reload cycles, each cycle having

properties as quoted by the manufacturer.
The piles were loaded horizontally through aincreasing maximum loading. Pile 1 was subjected to 2

powerful hydraulic jack. To provide reaction a system ofcycles, the maximum force overall being 1836kN

4 steel piles was employed. These had similar properties(corresponding to the observed onset of yielding). Pile 2

to the tested piles and their deflection was monitored

was tested up to failure, over 6 cycles and with an input

throughout the test to ensure that the reaction system wasforce smaller or equal to 1314kN.

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

4. Results. Discussions.

acting on the wall, of height 3xhwave . Since for most of the

4.1. Wave Force Estimation

wave flume tests hwall < 3xhwave the trapezoidal component

Fig. 7 shows typical excess water pressures (above
baseline hydrostatic) recorded on the model wall during aof hydrostatic pressure highlighted in Fig. 8 was
integrated to yield maximum predicted surge force.

simulated tsunami wave. To be noticed are the highAs exemplified in Fig. 9 there was large variation inmaxima recorded when the bore hits the wall (highlighted

the accuracy of the code surge force prediction across theby horizontal bars in Fig. 7). These high pressures

wave flume tests. To potentially explain the significant
scatter, it was observed that the relationship between themeasured incoming wave height and velocity in the wave
flume did not follow the approximation introduced by
Camfield (1980) and further refined by Palermo et al.
(2009) of what is characteristic of a tsunami breaking into
a turbulent bore:
𝑣𝑤𝑎𝑣𝑒 = 2 √𝑔 ℎ𝑤𝑎𝑣𝑒(1)

where hwave and vwave are the incoming wave properties, farfrom the model seawall.

Fig. 7

Excess water pressure vs Time Test 45(hwave / hwall = 1.45)

presented large variability, with durations between 5100ms.

Furthermore,thepeaks did

not happen

concurrently for all sensors. Lugni et al. (2006) and Kihara
et al. (2015) encountered the same phenomena in theirwave flume experiments and attributed the irregularbehaviour to the adiabatic process of a large amount of air
bubbles being mixed in the turbulent bore.
A different way to plot the results is to look at the
total force acting on the wall by integrating the pressure
values. Since the pressure peaks are asynchronous thisreduces their perceived overall effect on the structure.

Fig. 9Furthermore, standards of practice generally deal withforce prediction for two tests on the 15 cm model wall

total wave force acting
on structures and assuch we can compare
our force results to thecode values.
Okada

et

al.

(2006) consider peak
dynamic wave loading
Fig. 8Trapezium cutout

asequivalenttoahydrostatic distribution

Measured force against code maximum surge

To further investigate the issue, the hydrostatic andhydrodynamic wave forces were independently derived
from the height and velocity sensors at the wall andcompared with the total wave force as computed from thepressure transducers. As seen in Fig. 10 both thehydrostatic and upper hydrodynamic forces predict wellby themselves the maximum surge force. Across the 47
wave flume experiments, the hydrostatic force was moreconsistently a good prediction of maximum surge force
than the hydrodynamic force, potentially owing to the fact

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

Fig. 10

Fig. 11

Wave Force vs Time Test 38

Bernoulli Estimate vs Time Test 38(hwave / hwall = 2.11)(hwave / hwall = 2.11)

that the hydrodynamic result was computed from wave

This upper bound result, further named the Bernoulli

velocity above the wall, assumption which might require

estimate, can be used to compute not just the maximum

adjustment.

surge force prediction, but to estimate the wave force

The fact that the hydrostatic force at the wall is byprofile across the duration of the whole wave flume

itself a good estimate of wave surge force is anexperiment. As shown in Fig. 11 the Bernoulli estimate

encouraging result. A possible interpretation would be thatyields encouraging results. The method provides similarly

by diverting the wave momentum upwards, the wall

good estimates across all tests, for both model sea walls.

experiences the maximum force when the water reaches

Fig. 12 shows a comparison between the code

maximum height and the wave is about to collapse back(Okada et al., 2006) and the Bernoulli estimate in

down on itself.

predicting maximum surge force. The comparison

However it would be beneficial if the water height inincludes data from all tests and for both walls by using

front of the model seawall could be inferred from hwave andnon-dimensional expressions for force. Even though on

vwave exclusively as these represent the input parameters,

average both methods predict the experimental force well,

the wave characteristics far from the wall.

there is significantly less scatter in the Bernoulli results.

Even though the generated wave in the flume is a

Thus the Bernoulli estimate significantly improves theturbulent bore, if assuming only a small amount of its
energy is lost due to turbulent mixing until the wave
reaches the wall, Bernoulli’s energy conservation along a
streamline can be written as:
𝜌𝑣 2= 𝑐𝑜𝑛𝑠𝑡.
(2)
2
As described above the maximum hydrostatic force
𝜌𝑔ℎ +on the wall occurs when wave velocity is lowest(corresponding to maximum potential energy). In thisassumption, an upper bound for the highest water level infront of the wall can be found by considering the watervelocity just in front of the model wall as zero:ℎ𝑚𝑎𝑥 = ℎ𝑤𝑎𝑣𝑒 +

2
𝑣𝑤𝑎𝑣𝑒
2𝑔

(3)

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Fig. 12Non-dimensional Measured vs Predicted
Maximum Wave ForceProceedings of the First International Conference on Press-in Engineering 2018, Kochi

precision of the force estimates over the code (Okada etal., 2006) in the case of the wave flume experiments.
However as stated prior, the waves generated in the
wave flume do not have the same link between wave
velocity and height as a tsunami. If the Bernoulli estimate
is applied for a tsunami, substituting the result of (1) into(3) yields hmax = 3xhwave . Thus the Bernoulli estimate
generally becomes equivalent to the code (Okada et al.,2006) for tsunami waves. This would suggest that
although the code was originally designed for the scenario
of waves significantly smaller than buildings/seawalls, themethodology still applies for larger tsunami waves, aslong as the seawall is able to divert most of the wave
Fig. 14 Pile 1 Horizontal Load vs Displacement

momentum upwards. However, if incoming waves aremany times greater than the seawall the Bernoulli

Fig. 15 shows the force-displacement curve for Pile 2,

Estimate would suggest the surge height (and force) willagain with the DNV (1992) prediction on top. Stiffness is

be diminished on account of a significant amount of wave

well predicted in the case of the second pile too, however

momentum passing over the wall (Fig. 13).

failure load is again under-predicted if taking into accountThus, from the wave flume experiments applying

the manufacturer-specified yield and ultimate tensile

current codes will result in a good approximation of actual

stresses. If correcting with the inspection values, as seen

wave surge force, without however any safety factor built

in Fig. 16, DNV (1992) predicts both stiffness andin by default.

ultimate strength with good accuracy.
As such an intrinsic safety factor of 2 in the lateralresistance of the piles seems to have been placed inthrough the steel manufacturer reporting conservative
material properties. This cannot be taken for granted as
there is variability in the quality of batches even from thesame manufacturer. However to note is that the DNV(1992) calculation in itself does not appear to include
safety margins.

Fig. 13

Tsunami surge height – predicted behaviour

4.2. Lateral resistance of single piles
Fig. 14 shows the force-displacement curve for Pile
1 at the hydraulic jack loading position (2.4m above
ground). Pile 1 was tested up to yield. Overlaid on top of
the experimental data is the DNV (1992) prediction of thecurve given the parameters in Table 1 and Table 2. Even
though the standard was designed around smaller andmore flexible piles, it applies well for the static loading of
the larger diameter piles installed in tsunami seawalls. For
the case of initial loading the stiffness is correctlypredicted, however the failure load is under-predicted.
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Fig. 15 Pile 2 Horizontal Load vs Displacement(manufacturer specified yield strength)

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

upper half of the embedded length of the pile. The same
was found to be true for Pile 2. Hence it would seem thatthe piles need not be embedded to 10m to withstand thesame amount of force. However, a tsunami wave likely
induces significant excess pore pressure in the ground as
well as high hydraulic gradients between the sides of aseawall. These would lead to reduced effective stress andpotentially scouring which would diminish significantly
the strength of the soil, especially in the upper layers of
the ground, warranting a higher embedment than would
result from the static lateral tests.
4.3. Dynamic amplification
To assess dynamic amplification due to the tsunami

Fig. 16 Pile 2 Horizontal Load vs Displacement(measured yield strength)loading it is first observed that the wave force in the case
From the recorded strain gauge data bending momentof a seawall is driving the wall-soil system, much like thecan be derived at the sensor locations along the pile for

input acceleration from an earthquake drives soil-

each load increment. Bending moment results provide

structure systems into vibrating. Thus, response spectra

insight into the way lateral load is carried by the soil-pilecan be derived in similar fashion for seawalls as for

system. Fig.17 shows Pile 1 bending moment data for thebuildings to investigate which structures are most

instances in which the hydraulic jack applied 673kN andsusceptible to damage based on their natural period.

1238 kN of lateral force respectively. For comparison theFig. 18 shows a typical Dynamic Amplification

DNV (1992) bending moment prediction is also shown in

Factor (DAF) spectrum from a wave flume experiment.

Fig. 17. Similar to the case of the load-displacement

With amplifications of 70% for certain frequencies thecurves, the code (DNV, 1992) provides an accurate

possibility of dynamic amplification for seawalls cannot

prediction of bending moment, at both low and highbe trivially neglected. Generally, the flume tests showloading forces. This holds true for both pile tests.
Moreover from the bending moment profiles in Fig.
17 it is seen that the region of the pile carrying the highest
moment (and thus likely to form plastic hinges first) is theFig. 18

DAF for a Pile-Soil System with ζ = 5% from

Wave Loading in Test 34 (hwave / hwall = 1.95)

dynamic amplification occurring mostly for less stiff
Fig. 17

Bending moment profile with depth at 673 kN

structures, with longer natural periods, above 0.5s.

and 1238 kN lateral load respectively

An upper bound for the pile natural frequency can be- 295 -

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

computed using the stiffness of the force-displacement
curves (Fig. 14 and Fig. 16). It computes to 8Hz, outside

References
Camfield, F.E. 1980. Tsunami Engineering. Special

the amplification region predicted by the flume tests. Even

Report No. 6. US Army Eng. Waterw. Exp. Stn.

so, as discussed in section 4.2, stiffness degradation due toCoast. Eng. Res. Cent. Vicksbg. Miss., pp. 222.

soil softening can reduce

the natural frequency

DNV. 1992. Foundations (No. 30.4). Det Norske Veritas.

considerably and hence make dynamic amplification

Gillow, M., Haigh, S.K., Ishihara, Y. 2018. Water Jetting

relevant and potentially dangerous.

for Sheet Piling, in: The First International
Conference on Press-in Engineering. Kochi,

5. Concluding remarks
Through wave flume and lateral pile tests it has been

Japan.
Kato, F., Suwa, Y., Watanabe, K., Hatogai, S. 2012.

shown current codes of practice generally scale up well to

Mechanisms of coastal dike failure induced by

the case of large tsunami seawalls.

the Great East Japan Earthquake Tsunami.

The wave flume results show current codes in Japan(Okada et al., 2006) designed for tsunami shelters provide

Coastal Engineering Proceedings, 1 (33).
Kihara, N., Niida, Y., Takabatake, D., Kaida, H.,

appropriate predictions of wave force for the case of a

Shibayama, A., Miyagawa, Y. 2015. Large-scaleseawall built to withstand overtopping waves.

experiments on tsunami-induced pressure on a

Full-scale lateral pile tests were carried out to checkvertical tide wall. Coastal Engineering, 99, pp.

whether codes derived from data on smaller, more flexible
piles, are appropriate in the tsunami seawall scenario. The

46–63.
Lugni, C., Brocchini, M., Faltinsen, O.M. 2006. Wave

results show current codes (DNV, 1992) predict stiffness,

impact loads: The role of the flip-through. Phys.

displacement as well as bending moment profile with highFluids, 18 (12), pp. 122101.

accuracy.

Okada, T., Sugano, T., Ishikawa, T., Takai, S., Tateno, T.,

However, there are still shortcomings due to the2006. Tsunami loads and structural design ofcodes not being specifically developed for the tsunami use

tsunami refuge buildings. The Building Centrecase. The effect of soil softening during the tsunami,of Japan.

currently not explicitly taken into account in design, may

Palermo, D., Nistor, I., Nouri, Y., Cornett, A. 2009.

be significant. It leads to reduced seawall resistance andTsunami loading of near-shoreline structures: apotentially to an increase in apparent wave load throughprimer. Canadian Journal of Civil Engineering.

dynamic amplification. As a consequence, more researchis required to tackle the problem of seawall design against

36 (11), pp. 1804–1815.
Reese, L.C., Cox, W.R., Koop, F.D. 1974. Analysis ofhigh tsunami waves specifically.

laterally loaded piles in sand. Offshore

That being said, despite remaining unknowns, steel

Technological Conference, Houston, Texas, pp.

pile walls seem to be an effective seawall solution. This isdue both to the high embedment which mitigates some of473-483.
Suppasri, A., Koshimura, S., Imai, K., Mas, E., Gokon,

the problems associated with soil softening, but also to

H., Muhari, A., Imamura, F. 2012. Damage

their capacity to be useful beyond yield, continuing to

Characteristic and Field Survey of the 2011absorb wave energy by means of plastic deformation.

Great East Japan Tsunami in Miyagi Prefecture.
Coastal Engineering Journal. 54 (1), 1250005.

6. Acknowledgements
The authors would like to thank Giken Seisakusho
Ltd. for funding this work through the Giken-Cambridge
collaboration. Acknowledgements must be paid to
President Kitamura for the continued success of thiscollaboration.

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