Effect of Liquefaction on Damage to Water-supply Pipelines during the 2011 Tohoku Earthquake

Abstract

The damage caused by the 2011 Tohoku earthquake (Mw = 9.0) to water transmission and distribution pipelines in Japan is investigated using the database on pipeline repairs provided by the Ministry of Health, Labour and Welfare. The numbers of repair are summarized according to areas of water supply corporations and the presence or absence of liquefaction during the Tohoku earthquake. The locations of the pipeline repairs are discussed with respect to the earthquake intensity and geomorphologic conditions. The following findings were obtained: (1) Approximately 25% of the pipeline repairs, with a pipe diameter of more than 50 mm, occurred at liquefied areas and in the pipelines installed in the liquefiable geomorphological units, such as artificial fills. However, 75 % of the repairs occurred outside the liquefied areas and in the pipelines installed in the unliquefiable higher lands, such as the hill and terrace areas comprising hard soils; (2) There was a distinct difference between the Tohoku and Kanto regions in terms of the numbers of repair in the liquefied areas, and the pipelines in the Kanto region were strongly influenced by liquefaction; (3) The number of repairs were counted according to the earthquake intensity and depending on the presence or absence of liquefaction. Pipe repairs were required in the areas where the I_JMA exceeded 5-upper, the PGAs exceeded 150 cm/s², or where the PGVs exceeded 20 cm/s in the liquefied areas; (4) However, the weaker the earthquake intensity, the greater will be the numbers of repair in the liquefied areas. This implies that the pipelines were damaged not by the shaking of the ground, but by the liquefaction that occurred in the areas where the earthquake intensity was not very strong; (5) Counting the numbers of repair with respect to the pipe material and coupling in the area where severe liquefaction occurred, no tendency was found to indicate that the specific material or coupling was selectively damaged; (6) The damage ratio relative to the pipe materials varied considerably according to the area of the water-supply corporation where severe liquefaction occurred; (7) The likelihood of liquefaction-induced damage was evaluated at 6 sites in the liquefied area where many pipeline repairs occurred based on the combination of liquefaction potential index P_L and thickness of surface un-liquefiable layer, H₁ overlying liquefiable layers, which were determined from the result of the F_L analysis. From the result of the analysis, it was found that all of the sites were evaluated as “the highest likelihood of liquefaction-induced damage”.