Example $7\mathrm{.}6$: There is a bed of compressible clay of $4$ m thickness with pervious sand on top

and impervious rock at the bottom. In a consolidation test on an undisturbed specimen of clay

from this deposit $90\%$ settlement was reached in $4$ hours. The specimen was $20$ mm thick.

Estimate the time in years for the building founded over this deposit to reach $90\%$ of its final

settlement.