御前崎地盤上下変動への地下水位の影響評価のためのボーリングコア収縮・膨張実験

Abstract

The change of pore water pressure in sedimentary layers is often a major cause of subsidence and re-upheaval of ground surface. In this paper, the compaction and expansion of sedimentary rocks are measured while the pore water pressure fluctuates under constant lithostatic load.The samples used for the tests were cored from the six depths of a borehole up to the maximum depth of 250 meters at Hamaoka, Shizuoka prefecture. A sample was set within a vessel and weights were place on the sample to produce compression equal to the overburden pressure at the sampled depth. Pore water pressure was then introduced from one side into the sample. The pore pressure was reduced and increased several times. The change in the height of sample was measured. It was observed that as the pore pressure increased, the sample expanded, and as the pore pressure decreased, the sample shrinked. We calculated the coefficient of expansion and compaction due to changes of pore water pressure.The results of the experiments allow us estimation of amounts of ground subsidence or re-upheaval which would result from changes in the water level 1 meter, the ground level subsides 5 millimeters. When the underground water level rises 1 meter, the ground level uplifts 2 millimeters.The estimated change of ground level due to the pore water pressure change is only about one tenth of the observed seasonal fluctuation of the ground level of the tip of Omaezaki Peninsula, Shizuoka prefecture, but it is enough to explain the gradual re-upheaval at the Kawasaki City, Kanagawa prefecture. The compaction experiment with the control of pore water pressure provides a valuable tool for estimation of subsidence and re-upheaval due to the change of water level of underground aquifer.