$2-$ A $4m$ deep clay layer is underlain by impermeable shale. A sample of clay from the layer is subjected to an appropriate increment of stress in a consolidation test giving the following results:

Sample height at the start of increment: $55mm.$

Sample height $2$ minutes after the start of increment: $53mm.$

Sample height at the end of increment: $48mm.$

If gravel fill is placed over the clay layer how long would it take for $80\%$ consolidation of the clay to occur?