Investigation and Forecast Earthquake Damage for Sewage Systems

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ABSTRACT
Japan is known as one of the areas where earthquake occurs most to be located in border of oceanic plate and continental plate. On this account, earthquake countermeasures are pushed forward triggered by the great Hanshin earthquake (1995), but measures for existing facilities are not yet enough.

In such situation, the mid Niigata prefecture earthquake of October 23, 2004 attacked sewerage facilities and sever damage such as conduit failure and floating of manholes took plaice. Investigation of the cause why such damage occurred, and necessity of earthquake countermeasure was reaffirmed.

On these backgrounds, the factors of sewerage facilities’ failure that was brought by earthquakes of the past were analyzed, and damage estimation method was developed. With this approach, effective countermeasures could be figured out by appraising damage to sewerage facilities due to predicted earthquakes.

INTRODUCTION
Japan is one of the areas where earthquake occurs most to be located in border of oceanic plate and continental plate (See Figure 1-1). Japan had 22% of all the big earthquakes exceeding magnitude 6.0 that occurred in the world after 1995, as shown in Figure 1-2. Sewer system is one of the most important lifelines, because it is indispensable to keep the city clean by removing wastewater even after earthquakes. At the same time, it prevents secondary disaster so that the rainwater may not flood the city.

Japan suffered great damage by the great Hanshin earthquake (1995) in sewerage facilities. On the basis of this experience, technology was developed to prevent damage of sewerage facilities, and earthquake disaster countermeasures were pushed forward. However, measures for enormous existing facilities are still insufficient, therefore it is an urgent issue to promote countermeasures against earthquakes that are thought to occur anytime in Japan. In such situation, the mid Niigata prefecture earthquake of seismic coefficient 7 at maximum occurred in 2004, and damage to sewer network, such as conduit failure and floating of manhole, took place.

The damage of sewer networks makes drainage of sewage difficult, and it affects everyday life directly, for example, it restricts the use of bathroom. In addition, it causes floating of manholes getting in the way of traffic, and giving obstacles to emergency operations. Through such an experience, investigation of damage cause and necessity of countermeasures was recognized again. On this account it is important to take effective countermeasures for sewer networks on the basis of the prediction of facility damage and its influence on the society.

On these backgrounds, the factors of sewerage facilities’ failure which was brought by earthquakes of past were analyzed, and damage estimation method was developed. In other words, what kind of actions should be taken and how the actions should be prioritized were figured out by appraising seismic damage of sewerage facilities.

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