$7\mathrm{.}48$ A $22m$ thick normally consolidated clay layer has a load of $150$kPa applied to it over a large areal extent. The clay layer is located below a granular fill $)=(1800\frac{kg}{m^3}$ thick. A dense sandy gravel is found below the clay. The groundwater table is located at the top of the clay layer, and the submerged density of the soil is $950k\frac{g}{m^3}.$ Consolidation tests performed on $2\mathrm{.}20cm$ thick doubly drained samples indicate $t_{50}=10\mathrm{.}5$min for a load increment close to that of the loaded clay layer.

a$.$ Compute the effective stress in the clay layer at a depth of $16m$ below the ground surface $3\mathrm{.}5yr$ after application of the load.

b$.$ At $t=4yr,$ what is the average degree of consolidation for the clay layer?