Volume 6, Issue 1 March 2020

Case-History

Usa Coast Seawall Upgrade Project, Kochi, Japan- Disaster Mitigation Projects at Coastlines in Kochi, Japan Ryo KamiokaManager, Construction Methodology Promotion Section
GIKEN LTD.

INTRODUCTION
Every 100-150 years, Nankaido in Southwest Japan is periodically struck by a large earthquake which is known as the“Nankai Earthquake”. Its most recent occurrence was on the 21st of December 1946, known as the “Showa Nankai
Earthquake”. The earthquake measured between 8.1 and 8.4 on the moment magnitude scale and resulted in at least1362 dead, 2600 injured and 100 missing. During the earthquake, serious damage to levee embankments occurred in
Kochi Prefecture (Fig. 1) due to the tremors, liquefaction and tsunamis, resulting in widespread Inundation in the region.

Kochi
EpicenterFig. 1. The epicenter of the Earthquake (Depth of 30km)

Fig. 2. Pacific Ring of Fire

Japan is located on what is called the Ring of Fire (Fig. 2). This is a giant circle around the Pacific Ocean where edges oftectonic plates are slowly moving. This can result in sudden displacement of the ocean floor, which causes earthquakes
and tsunamis. Beneath the Pacific Ocean along the Southwest coast of Japan, the Philippine Sea Plate is slowly being
pushed under the Eurasian Plate. It is said that within the next 30 years, there is a 70% chance of the next Nankai megaearthquake (Fig. 3).

Fig. 3. Distribution of Maximum Seismic Intensity (Shindo)
in the event of maximum possible Nankai Mega Earthquake4Volume 6, Issue 1 March 2020
PROJECT’S SCOPE OF WORK AND SELECTION OF PRESSED-IN PILE WALLSThe Usa area is located approximately 10km west of Kochi city and at the mouth of the 12km deep Uranouchi Bay, which
is surrounded by the mainland of Shikoku Island and the Yokonami Peninsula. The Usa area is blessed with warm climate,a ria coast and the Kuroshio Current. It is a thriving distribution hub and is rich in fisheries resources. Nowadays, the area
is famous for fish farming, leisure fishing and marine sports and recreation. Due to its calm water, the area is also used as
an important refuge harbor. In the narrow hinterland of the Usa coast, there is an emergency transportation route andthere are approximately 600 households with a population of approximately 4,000. The area was one of the hardest hit
by the previous “Showa Nankai Earthquake” in 1946. Therefore, in order to minimize damage, it is preparing itself for thenext Nankai mega earthquake.

PROJECT’S SCOPE OF WORK AND SELECTION OF PRESSED-IN PILE WALLS
In order to protect the coastlines from the Nankai mega earthquake, coastal infrastructures are being upgraded with highresiliency. This is in line with the National Resilience Plan, which is one of Japan’s national policies. In the Usa area, coastal
protection projects and levee reinforcement projects were carried out to increase resiliency against earthquakes,
tsunamis and storm surges. This document specifically features these coastal and levee seismic reinforcement projects,
which utilize Implant Structures.
*Implant StructureTM: Embedded piled foundation installed by the press-in piling method.
The Usa coast seawall upgrade project stretches approximately 5,800m (Fig. 4). The following seismic design criteria wereapplied on the project.

Usa Area

Kochi

Fig. 4. Coastal Protection Scheme in Kochi Prefecture
1.

Seismic movement level1) Level 1 earthquake ground motions
Moment magnitude scale of earthquake: 8.6
Acceleration of earthquake: 250gal
Horizontal seismic coefficient: Kh=0.19*Level 1 earthquake ground motions: Earthquake ground motions which have a high possibility ofoccurrence, during the service life of proposed structures (every tens or hundreds of years).
2) Level 2 earthquake ground motions
Acceleration of earthquake: 400gal
Horizontal seismic coefficient: Kh=Not specified (Analyzed by utilizing a deformation dynamic analysissoftware)*Level 2 earthquake ground motions: Earthquake ground motions which have an intensity of the maximum
scale, among those expected to occur on proposed structures. Although it has an extremely low possibility5Volume 6, Issue 1 March 2020

2.

3.
4.
5.

of occurrence, it would have a cause catastrophic impact on the area.
Required aseismatic performance of seawalls
1) As for Level 1 earthquake ground motions
Structures maintain their serviceability during and after earthquakes.
2) As for Level 2 earthquake ground motions
Structure damage caused by earthquakes is not critical, but minor and local, and can be repaired in a shortperiod of time without additional reinforcement works.
Wave impact (Impact of tsunamis)
130kN/m (Impact load by floating objects is not considered.)
Required height of seawalls
2.2-2.4m above the existing seawalls
Service life of proposed structures
50 years

The proposed seawalls were installed right in front of the existing seawalls. Behind the proposed seawalls, there was aroad and dense residential area. In addition to this, there was only a restricted narrow working space available alongside
the proposed seawalls. Therefore, the Implant Structure (embedded steel tubular pile wall installed by the press-in piling
method) was chosen for the seawall upgrade work, to satisfy the following criteria.
1) High resiliency and toughness of seawalls against earthquakes and tsunamis, with the above mentioned
aseismatic performance2) Minimum footprint of proposed seawalls
3) No interference to the adjacent road traffic and daily activities in the community during construction4) Construction works within a restricted working space
5) Low noise and vibration
6) Economy7) Minimize construction time

Fig. 5. Typical Cross Section of Proposed Seawall

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Fig. 6. Detail of Seawall (Plan)

Fig. 7. Detail of Seawall (Longitudinal Section)

GROUND CONDITIONIn the project areas, there are liquefiable soil layers (loose sand and silty sand layers) beneath the existing levees, which
are broadly located. 1,000mm diameter steel tubular piles were installed into the non-liquefiable soil layer (silt layer)
underlying the liquefiable layer, to minimize the horizontal deflection of the levee in the event of earthquakes (Fig. 8).

Fig. 8. Soil boring log

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CONSTRUCTION METHODThe Gyropress Method TM (rotary press-in piling method) was utilized to install tubular piles efficiently in the restrictedworking space, with less noise and vibration. In order to overcome local rock and unforeseen obstructions, sacrificial
drilling bits were attached on the pile tip. When encountering major obstacles, a core barrel (reusable steel tubular pies
with larger number of non-sacrificial drilling bits) was utilized to penetrate through the obstacles prior to the pileinstallation.
Where there is enough access available for a normal service crane, the standard Gyropress Method was utilized to install
tubular piles (Photo 1). In contrast, in some areas of the project site, the access for construction machines in front of theexisting sea walls was only a few meters wide. In order to overcome the extremely difficult working conditions, theGyropress method with the GRBTM Non-staging system was utilized (Photo 2).
*GRB Non-staging System: A footprint-free pile installation method, which comprises 1) Silent Piler, 2) Clamp Crane and3) Pile Runner. With the GRB Non-staging System, all the steps including transportation, pitching and installing the piles,
are completed on the piles since each mechanical device travels on top of previously installed piles.

Photo 2. Pile Installation (Gyropress Method)with GRB Non-staging System)

Photo 1. Pile Installation (Standard Gyropress Method)

CONCLUSIONThe Implant Structures comprises high modulus pile materials and they are firmly installed into the ground using thepress-in piling method. Being embedded into the ground, the structure is extremely resilient despite its narrow footprint.
When liquefaction is a concern, in the case of earthquakes, Implant Structures do not normally require solidifying
surrounding ground. On the other hand, gravity structures normally require ground improvement, when being
constructed above a liquefiable soil layer. Also, Implant Structures can be constructed within restricted working space,since the press-in piling method enables prefabricated piles to be installed using a minimum working space.
Coastal infrastructures are critical in the protection of human life and as defenses for other infrastructures in theirhinterland. Urban coastlines are a priority in the importance of protection works. In such places, generally only narrow
spaces are available for construction works. Therefore, gravity structures are not often considered on urban coastlines
due to their need for considerable space. In contrast, the combination of narrow and resilient Implant Structures and thepress-in piling method is a perfect fit for construction on or the rehabilitation of coastlines. Implant Structures can also
be installed rapidly and economically. Therefore, construction costs and time are less compared to large gravity structures.

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