Volume 1, Issue 1 September 2016

Case History-2

Construction in Okitagawa Discharge Channel
Mr. Mikiya Kubo
Toyama Prefecture, Toyama Civil Engineering Centerあい 富2015の 山Okitagawa Discharge Channel L=423m
Revetments
風 地andと 方Water
Gate
L=16m
Complete L=353m
や 鉄

Toyama bay

Figure 1 is the outline of theimprovement construction of Okitagawa
Discharge Channel. By 2014, 87% of thewhole length of the channel has alreadybeen finished, as shown in black. The
procedure of the construction was thatfirstly sheet pile were installed, thenchemical grouting was conducted, andfinally the box culvert was positioned. At
present, the red parts in the river mouth
are under construction.

河口部

市
道
加島
瀬羽町線

（主）
富山
魚津線

市
道
滑
川
富山線

Frequent
Inundation Area

ま 道
鉄 本道 線

都市計画道路加島町下島線
（主）
滑川
上市線

（一）
古鹿
熊
滑
川線

河口部
The height of the existing coastal levee
L=52m
is as high as 3.8m. Initially it was around
2014→2015 2015
1.0m, but was raised every time it2015
2014→2015 2015
Manufacture of Coastal levee Box Culvert Box Culvert Box Culvertexperienced the wave overtopping.
Training levee
15.0m
27.0m
6.0m
When we construct the mouth of thedischarge channel, we need to break
Figure 1
this existing coastal levee. There weremainly three points to consider as thedesign conditions. The first point was the hardness of the ground. The ground contained dense sand and gravel andboulders. SPT N values were more than 50. In addition, we worried about how to deal with the possible undergroundobstacles such as the wave dissipating concrete blocks. The second point was the narrow space. There were wave
dissipating concrete blocks and the sea water itself in the sea side and there was a public road in the land side. The third
point was the short construction period. The construction was not able to be carried out in winter or in the season offishing. We had only four months for construction, from July to October. Other condition to consider was to leave theexisting levee as it was as much as possible. This was because there is a risk of the overtopping of the storm wave, if theexisting levee was removed all at once.

Stop-logNew

Toyama bay

Wave
direction←Okitagawa Discharge Channel
maintenance space
Existing
New
ExistingFigure 26

Volume 1, Issue 1 September 2016

Figure 3

As summarized in Figure 2, at the cross point of the coastal levee and thedischarge channel, the levee normal was determined to be shifted towards thesea side, for the following three reasons. The first reason was to ensure themaintenance space for the discharge channel. This required the gates for heavyequipments to go into the space. Secondly, it was necessary to ensure thestrength of the new levee to be higher than the existing levee, according to theagenda with the coast administrator. If the new levee normal was the same asthat of the existing one, we had to construct the gates on the normal, which
deteriorates the strength of the levee. The third reason was a new road justbehind the existing levee, which was planned to be constructed in the future. It
was necessary to plan the construction not to block the planned road.
Therefore, it was not possible to shift the levee normal towards the land side.
There were 4 alternatives for the new coastal levee. A gravity-type revetment,a counterfort retaining wall, a cantilever wall (Tubular sheet pile, Hard Ground
Press-in Method) and a cantilever wall (Tubular pile, Gyropress Method). These
alternatives were compared in terms of workability, economical efficiency,
flexibility to problems (such as obstacles) and the impact on surroundings(noise and vibration). As a result, Gyropress Method was adopted.
The comparison of the 4 alternatives is shown in Figure 3. The first alternativeis the gravity-type revetment. This plan uses square concrete blocks andtemporary earth retaining walls. As it requires the temporary retaining walls
and the chemical injection as the countermeasure against the ground water,
the construction period is the longest of the four and the cost is not reasonable.
The second alternative is the counterfort retaining wall. Although it can saveCut-off plate

PL-300×9（SS400）

300

91,1801,18000

Seaside海側

Landside

陸地側01,180RC ‘

PileΦ1000
L=17.5m

Figure 47

Volume 1, Issue 1 September 2016

the temporary retaining walls, it requires the counterfort pile walls and tie rods, and the construction period will not beshort enough. The third alternative is the cantilever wall with steel tubular sheet piles, constructed by Hard Ground Pressin Method, where the piles themselves are used as the coastal levee. Although it has a high performance in water cut-off,the workability under the existence of the obstacles and the economic efficiency is low. The fourth alternative is thecantilever wall with steel tubular piles, constructed by Gyropress Method. The piles themselves are used as the coastallevee. As it is better than others both in terms of the workability and the economic efficiency, we determined to adopt it.
Figure 4 is the detail of the structure of the coastal levee, designed on condition that the construction method is theGyropress Method. The length of construction is 25m in the direction of crossing the discharge channel. The diameter ofthe pile was determined to be 1000mm, considering the economic efficiency and the availability of the press-in machine.
The weak point of this construction method is the cut-off work between the piles. There is 18cm gap between two piles.
If nothing is done to close this gap, sand outflow will be caused by the effect of the wave in the future. Therefore, cut-off
steel plates were welded onto the piles, and in addition, grout injection was conducted behind the piles by using boringmachines.
Figure 5 shows the progress of the construction of the discharge channel. The yellow sections are the construction workof the coastal levee, which were completed in March this year. The red sections are the box culverts and were completedin June this year. The green sections are the training levees to prevent the estuary closing. The construction have juststarted in July. The situation of piling construction by Gyropress Method in the yellow section is shown in Picture 1.
PlanA

B-B

1,000

3,000

7,900

5,000
1,000780 600 1,000
620 700 600
新設海岸堤防し角落600
既設海岸堤防撤去+7.10

B+4.303,800④2,800消波ブロック（4t)+3.30
消波ブロック

②③

3,800

B

2,000

1:0.5

①A

1,000

3,850
2,850 1,000

コンクリート

A-A

鋼管杭φ1000(t=12) L=15.5m

28,00025,000導流堤横断図3,000

3,000-2.35

方塊ブロック

鋼管矢板L=17.5m+1.50+0.4451.
1:

被覆石
200～500kg/個程度

500 1,000
500-2.35

1:1.5-11.20

根固方塊6.90t
2.50×1.50×0.80
基礎砕石

Figure 5Coastal levee with steel tubular piles
was the optimal option for this sitewith unfavorable conditions. The
method will be adopted more in thefuture, especially where speed andcertainty is required. Demand for
Gyropress Method will be increased,
if the construction productivity is
standardized and the economic
efficiency is improved further.

Picture 18