A clay layer, $20$ ft thick is covered by $40$ ft sandy gravel stratum with porosity of $30\%,$ and a dry unit weight of $103$ pcf$.$ Tests on the undrained samples of the clay gave $c=2\mathrm{.}9,{}_{\text{s}\text{a}\text{t}}=112pcf$ and ${}^{\text{'}}=24$ degrees.

a$.()$ Find the soil shear strength at the clay's midlevel $($point A$)$

b$.()$ The effective and total stress acting on the vertical face of a soil element at the clay $($point A$)($principal stresses$)$

$\backslash$table$[[40$ ft$,$grad,$\backslash$table$[[13$ ft$]]],[$Sand$,\backslash$table$[[\backslash$table$[[{}_d=103pcf$