Q$3($a$)$ Differentiate between total stress and effective stress.

$($b$)$ A sediment settling lagoon has a depth of water of $2m$ above the clay base. The clay

layer is $3m$ thick and this overlies $4m$ of medium sand, which in tum overlies

impermeable rock. Given that the unit weight of elay, medium sand and sediment of

silty fine sand is $17\mathrm{.}5k\frac{N}{m^3},19\mathrm{.}3k\frac{N}{m^3}$ and $16k\frac{N}{m^3},$ respectively. Produce the

effective stress pressure diagram at the top of the clay and at the top and bottom of the

second layer under the follor

$($i$)$ Initial, before any sediment is deposited.

$($ii$)$ After a $1m$ layer of sediment of silty fine sand has been deposited

$($iii$)$ After draining the lagoon down to base level, with the same thickness $(2m)$ of

sediment still in place.