Geoscience Reference
In-Depth Information
of 10m and a depth of 24m to stabilize the foundation ground against external seis-
micforce.Usingexternalforces,astabilityanalysiswasconductedthatincludedsliding,
overturning, bearing capacity of the stratum,and circular slipfailure.
6.2.4. Hachirogata Polder dike (partially quoted from Civil Eng. Dept. Akita
Prefection , 1990)
Polder dikes in Hachirogata in Japan have been damaged by several earthquakes since
their construction. During the restoration work after the 1983 Nihonkai-chube earth-
quake, an investigation was conducted to make the dikes resist earthquakes because they
had been damaged repeatedly, as shown in Figure16.6(3). Counterweight fills, founda-
tion improvement, dewatering, etc. were investigated as possible liquefaction counter-
measures. Improving the foundation of the loose sand layer widely covering this area
would have cost an enormous amount. Therefore, the basic idea for the design of coun-
termeasures was to increase the overburden pressure by dewatering. Sheet piles were
driven into the riverside toes, drains were installed and drainage trenches were dug.
6.2.5. Tokachi River dike (partially quoted from Hokkaido Development, 1994)
TheTokachiRiverdikesufferedseveredamageduringthe1993Kushiro-okiearthquake.
During the restoration work, gabions and drains were applied at the toe of a dike. The
waterlevelinsidethedikewasmeasuredaftertherestorationwork.Itwasconfirmedthat
the water level was lowered withthetreatments.
7. Remediation methods for existing sea walls
7.1. PRINCIPLE OFREMEDIATION
Liquefactionofthegroundbehindaseawallincreasestheearthpressureontheseawall.
Liquefactionofthegroundunderaseawalldecreasesthebearingstrengthoftheground.
These two triggers cause large movement, tilting, and settlement of the sea wall. There-
fore, the best measure is to prevent liquefaction of the ground behind a sea wall and/or
thefoundationgroundunderaseawall.Theothermeasureistoincreasetheresistanceto
movement of a sea wall bysome restraining work.
7.2. EXAMPLE OF TREATED STRUCTURES
7.2.1. Kushiro Port (partially quoted from TC4, 2001)
Kushiro Port is located on the island of Hokkaido, Japan's northern most island. At
this port, the seabed deposit, on which quay walls were constructed and backfilled with
dredged sand, is relatively dense. The only liquefiable soils are the loosely deposited
backfill sand. Following the lessons learned from the damage to Akita Port during the
1983 Nihonkai-chubu earthquake, remediation measures were applied to some quay
walls. At the No. 1 wharf, in the east district, backfill sand just behind the wall was
treated with gravel drains, and the ground behind the gravel drains was treated with sand
compaction piles. After these treatments, the 1993 Kushiro-oki earthquake hit Kushiro
Port. The treated quay wall suffered no damage though not-treated quay walls suf-
fered severe damage. The damaged quay walls were treated during the restoration work.
