Abstract
Fig. 3 shows the relation between groundwater head H and cumulative compaction s which were recorded by the Matsubara observation well set in Hiratsuka City, Kanagawa Prefecture. The relation between H and s can be read as a stress-strain curve (MIYABE, 1972). The following three patterns have been distinguished from H-s curve (Research Group for Water Balance, SHIBASAKI 1975). I: compaction stage with declining groundwater head II: rebounding or stopping compaction stage with rising groundwater head III: compaction stage with stable groundwater head (residual compaction stage) We consider that if residual compaction rate ds/dt (mm/month) is nearly zero at the third patern stage, it is possible to prevent land subsidence at field site. As shown in Fig. 3, each ds/dt corre sponding to mean stable groundwater head Hm can be required. Fig. 4 shows the liner relation of these values of Hm and ds/dt that are plotted on logarithm graph. We can see groundwater head of 35.2 m below the ground surface corresponding to negligible residual compaction rate of 0.1 mm/month. The groundwater head obtained by the mentioned method is persumed as the permissive critical groundwater head for the prevention of land subsidence. Closed relation is also observed between puming yield Q from the whole basin and groundwater head H at the Matsubara station, and the fluctuation of groundwater head is directly affected by pumping yield. We can read off Fig. 7-(b) permissive yield of 100,000 m^3/day corresponding to permissive groundwater head of 35.2 m.
