Volume 1, Issue 2 December 2016

Case History

Seismic and Liquefaction Countermeasuresin Kochi Coast, JapanMr. Sachio Shintaku
Kochi Office of River and National Highway,
Ministry of Land, Infrastructure, Transport and Tourism (MLIT)
Kochi Office of River and National Highway has mainly three objectives to conduct countermeasures for river, coast androad. The ‘river’ project includes River improvement and maintenance of Monobe and Niyodo Rivers, control of thelandside water along Uji and Kusaka Rivers, survey of the hydrology and water quality, and making flood warning andflood forecast. In ‘coast’ project, we construct coast protection facilities and make flood warning in Kochi Coast. In ‘road’project, we construct sidewalks in Tosa City.
Kochi Coast is about 30km long, from Konan City to Tosa City in the central part of Kochi Prefecture. Coast section under
the jurisdiction of MLIT are 13.3 km long. It is sandy coasts, lying from Nankoku City to Tosa City. As Kochi Coast is shaped
like a fan and is opened toward the south, it is susceptible to a typhoon, and regardless of the course of typhoon, waves
tend to converge and their height increases. Sources of sand in the coasts are Monobe and Niyodo River, located in theeast and west of the coast.
In the past, beautiful sand beaches spread in Kochi Coast. However, costal erosion has progressed due to a massive
consumption of sea sand and gravels. Due to the disappearance of sand beach and the high waves, the frequency ofoccurrence of disaster has increased. Collapse of coastal levee and the closed traffic of the prefectural road have occurred
frequently in recent years.

Figure 13

Volume 1, Issue 2 December 2016
There are several types of countermeasures against storm surge and erosion in Kochi Coast: the offshore breakwater, theheadland defense, the artificial reef and the artificial nourishment.
The estimation of damage due to tsunami in Kochi Prefecture, published by the Cabinet Office of the Government ofJapan in 2012, is shown in Figure 1. The inundation areas are painted with colors. For examples, red means 15 to 20
meters height, orange 5 to 10, yellow 3 to 5 and green 2 to 3. Seismic and tsunami countermeasures in Niyodo River andMonobe River were completed in 2012 respectively. Along the coast, there are five sections, and two of them in Nii andNino were completed in 2014 and 2012.
In designing the structures in Kochi Coast, the earthquake and tsunami are assumed to be the ones determined by ‘Kochi
Prefectural Committee of Earthquake and Tsunami Disaster Prevention Technologies’. The assumed earthquake isTonankai and Nankai Earthquakes with the magnitude of 8.6. The planned height is determined as the highest of theheights assumed in each coastal region. It is 8.0m in Kochi Central Coast and in Nankoku Konan Coast. For tsunami withrelatively high frequency of occurrence, the basic policy is to construct the coastal protection facilities such as coastallevees. For tsunami with largest scale, the basic policy is to integrate the structural and non-structural countermeasures,and the main option will be the evacuation.
The examples of seismic and tsunami countermeasures in Kochi Coast are shown in Figure 2. The construction methods
were selected in terms of the workability and economic efficiency, taking into account the ground conditions in each area.
In Nii section, tubular pile wall and double sheet pile walls were adopted, taking into consideration the shape of the leveeand the ground conditions. In Nagahama and Tobara sections, a tubular pile wall was adopted, as the construction space
was limited due to the adjacent houses. In Nankoku section, a tubular pile wall was adopted, as the construction space
was limited due to the adjacent houses and plastic greenhouses.
There are two advantages in the tubular pile method. Firstly, the space required for construction is smaller, comparedwith other methods. Secondly, smaller construction space reduces the influence on adjacent structures and traffics. The
construction procedure of the tubular pile method is as follows. Firstly, the construction of temporary works. Secondly,
earthwork and construction of retaining wall. Thirdly, emplacement of materials. Fourthly, installation of tubular piles bythe Gyropress Method.

【Nii section】
Tubular pile wall
Prefectural road
Parapet【Nagahama section】

Double sheet pile walls
Wale and tie rod
Wave-dissipatingconcrete block

FencesExisting
revetment

Existing
wave dissipating
Prefectural concrete blocks

Fill

Existing
revetment

L shaped
retaining wallAs(花)Prefecturalroadroad
Tubularpile

Tubular pile
Sheet pile

Tubular pile wall was adopted, as theconstruction space was limited due to theadjacent houses.

Tubular pile wall or double sheet pile walls were adopted, taking into consideration the shape ofthe levee and the ground conditions.

【Tobara section】【Nankoku section】

Prefectural
Pavement road
Superstructure concrete

Existing revetment
Existing
wave dissipatingconcrete blocks

Construction space

三連ブロックSand beach

L shaped retaining wallAs

Plastic greenhouse

テトラポットTubular pile （L=12.0m）
Existing
revetment

Tubular pile

Tubular pile wall was adopted, as theconstruction space was limited due to theadjacent houses.

Tubular pile wall was adopted, as theconstruction space was limited due to theadjacent houses and plastic greenhouses.

Figure 24Volume 1, Issue 2 December 2016
Finally, construction of superstructure. There were three difficulties in this construction. Firstly, consideration of safety oftraffic was necessary because of the limited construction space. Secondly, it was necessary to cope with the obstacles
such as the existing levees. Thirdly, it was a hard work to secure the press-in machines, as they were frequently occupiedin other projects.

Picture 1Picture 2

The situation after the completion of construction is shown in Picture 1. The left one is the double sheet pile wall methodin Nii section. The right one is the tubular pile method in Tobara section.
The situation during the construction in Nagahama section is shown in Picture 2. The left one is the view of Nagahama
section. The right one is GRB Non-staging Method.
Finally, the seismic and tsunami countermeasures in Kochi Port is based on the plans of ‘triple protection’ with three linesof protection, which has been shown by the country and the prefecture. The first protection line is the breakwaters which
are expected to reduce the energy of tsunami and conserve the function of Kochi New Port. The second protection lineis the outer edge and mouth of Urado Bay, which is expected to prevent or reduce the intrusion of tsunami. The thirdprotection line is the revetments in Urado Bay, which is expected to prevent the collapse of revetments or the inundationof inland. We are going to push these plans forward in the future.
※ This case history is based on the presentation at “9th IPA press-in Engineering Seminar in Kochi 2016”, which washeld in July 2016.
5