●Research Trends and Results

Study of earthquake and tsunami countermeasuretechnologies to prevent sewage treatment systemdamage
YOKOTA Toshihiro, HeadFUKUYA Wataru, Senior Researcher
MATSUHASHI Manabu, Researcher
Water Quality Control Department, Wastewater System Division(Key words) Great East Japan Earthquake, sewage facility, liquefaction, tsunami
1. Outline of damage to sewage systems by theGreat East Japan Earthquake
The Great East Japan Earthquake severely damaged
many sewage facilities extending from the Tohoku
region to the Kanto Region. In particular, treatment
plants and pumping stations along the Pacific Coast
were severely damaged by tsunami, and along theshore of Tokyo Bay and in the downstream region of
the Tone River in the Kanto, liquefaction throughout
the region obstructed the sewage conveyancefunctions of sewage pipeline facilities (Fig. 1).

Figure 1. View of Damaged Sewage Facilities
2. Questionnaire survey
The National Institute for Land and InfrastructureManagement carried out a questionnaire survey of
local governments in the damaged region (effective
response rate: 69%) in order to clarify the causes ofdamage to sewage treatment plants and sewage
pipeline facilities caused by the Great East Japan
Earthquake. Figure 2 shows the percentages ofdamage by cause to individual facilities (categorized
as 23 facilities) in sewage treatment plants and topipeline facilities.
Among sewage treatment plants, 54% weredamaged by the tsunami, followed by 41% by theearthquake motion, and 4% by liquefaction. Of thedamaged pipes, 90% were damaged by liquefaction,and most of these, at 66%, were damaged byliquefaction of the pipe backfill, which has been aproblem since the Niigata Chuetsu Earthquake. Andunlike past earthquakes, liquefaction of surrounding
ground caused 25% of the damage.

Causes下水処理場の被害要因
of damage to sewage treatment plants

47%

48%

地震
Earthquake
液状化
Liquefaction

4%

Causes of damage to pipelines
管きょ施設の被害被害要因

0.03
0% 4% 5%

1%

津波
Tsunami
その他
Others

25%
66%

地震動
Earthquake
motion
埋戻し土の液Liquefaction
ofbackfill 状化
Liquefactionof
周辺地盤のsurrounding ground
液状化
Tsunami (somelocations 津波（未調査not yetsurveyed)箇所あり）

Others その他

Figure 2. Causes of Damage to Sewage Facilities
3. Problems at sewage facilities caused by the Great
East Japan Earthquake
Sewage treatment plants in cities along the coastline
are natural flow type which must discharge their water
into public bodies of water, so the treatment plants areinevitably constructed relatively close to the ocean.
Therefore, this tsunami caused extensive damage: thedestruction of building structures by its powerful wave
force and damage to electrical and mechanical bysubmersion. But present aseismic design guidelines do
not mention tsunami countermeasures, so concepts ofsewage facility design considering tsunami-resistancecountermeasures based on an early clarification of thestate of damage will be compiled and released in thefuture. And among sewage pipelines, many without
earthquake resistance countermeasures were damaged,
including those where liquefaction of the surrounding
ground caused soil to flow in from the lateral pipes sothat the pipes were plugged with soil, obstructing theirsewage conveyance function, although the damage tothe pipes themselves was minor. Therefore, it isexpected that aseismic design guidelines will beexpanded in order to deal with damaged backfill byencouraging
conventionalliquefaction
countermeasures, and for facilities damaged byliquefaction of the surrounding ground, liquefaction
countermeasures will be taken on lateral pipes, whichhave not been the object of past measures.
4. Utilizing the results
Concepts of sewage facility design considering
tsunami-resistance countermeasures and concepts ofthe expansion of liquefaction countermeasures will be●Research Trends and Results
announced by the Sewage System Earthquake/
TsunamiCountermeasure
TechnologyStudy
Committee, and later will be reflected in aseismic
countermeasure guidelines based on a study by theSewage Facility Aseismic Countermeasure Guideline
Revision Committee.
[Reference]
Fifth
ReportbytheSewage
SystemEarthquake/Tsunami Countermeasure Technology
Study Committee