Ground water lowering at Minami-Kurihashi in Kuki City, Japan as countermeasure against liquefaction

Extensive liquefaction took place in Minami-Kurihashi, Kuki City, Saitama, Japan during the 2011 Off the Pacific Coast of Tohoku Earthquake, which induced damage to houses and infrastructures. Based on the results of a detailed geotechnical investigation, the ground water lowering method has been adopted and executed as a countermeasure against liquefaction. This report summarizes how this countermeasure could be implemented, including the results from field monitoring and relevant analyses of the local ground water level and surface settlement that were conducted during and after the ground water lowering. By newly installing a network of drainage ditches, cut-off sheet pile walls, and operating manhole pumps, it was possible to lower the ground water level in order to prevent/reduce future liquefaction damage. In the non-execution areas outside the cut-off walls, the ground water level was not affected by this operation. As of the end of the monitoring period, the measured maximum settlement and tilting ratio were 36 mm and 1.03/1000, respectively. All the monitoring data satisfied the pre-assigned control limits of 40 mm and 2/1000, respectively. In order to numerically simulate the monitored behavior, modification of the compression index and coefficient of permeability was required. Based on the results of a long-term behavior analysis using the modified parameters, the final settlement and tilting ratio were predicted to be, at maximum, 54 mm and 1.50/1000, respectively. The latter value satisfied the pre-assigned threshold limit of 3/1000.