Pore Water Pressures and Ground Motions Monitored During the 1985 Chiba-Ibaragi Earthquake

Abstract

Following the successful measurements of accelerations and pore water pressure rise during the 1980 September 25 earthquake, a set of piezometers embedded in the sand deposit on the Owi No. 1 Island in Tokyo Bay became out of order. A new set of piezometers was again installed at the same site in a bid to monitor pore water pressures awaiting a reasonably large earthquake to come. On October 4, 1985, an earthquake of magnitude of 6.2 shook the area of Tokyo Bay and pore water pressure rise was again monitored, together with the accelerations on the ground surface and in the deposit at a depth of 10 m. The maximum ground surface acceleration recorded was 71 gal and the pore water pressure increase at depths of 6 m and 14 m was 4.5% and 5.3% of the mean effective confining pressure, respectively. On the basis of the laboratory test results performed previously on undisturbed soil specimens, an estimate was made of the pore water pressures that might be produced in the deposit by the seismic shaking with a maximum acceleration of 71 gal. The estimated values of pore water pressure at the depths of 6 m and 14 m were shown to be somewhat larger than that registered during the earthquake.