A clay stratum of 18 metres thickness was found above a sand stratum when a boring was made. The clay was consolidated under the present overburden pressure. The hydrostatic pressure at top of the clay stratum was found to be 54 kN/m². Due to pumping of water from the sand stratum, the pressure in the pore water below the clay layer was decreased permanently by 54 kN/m². If the void ratios of clay before and after pumping were 0.93 and 0.90, respectively, measure the ultimate settlement because of pumping.