$(25$ points$)$ In a laboratory consolidation test, $12\mathrm{.}7mm$ thick clay specimen was tested with top and bottom drained conditions and $90\%$ consolidation was observed in $15\mathrm{.}8$ minutes $(t90=15\mathrm{.}8$min $).$ In the field, the same clay material with a thickness of $6\mathrm{.}5m$ is in between sand $($top$)$ and gravel $($bottom$)$ layers. How long will the field clay layer take to achieve $50\%$ and $90\%$ of consolidation settlement?