$5.($a$)$ You are working on a site where the forthcoming closure of an industrial plant

and the cessation of water extraction is predicted to lead to a local rise in ground

water level. At the site a layer of clay, $6$m thick, is located beneath a layer of

gravel $8$ m in thickness. An impermeable rock strata underlies the clay. The

water table is currently located at $6$ m below ground surface. However, it is

predicted that the water table will ultimately rise by $3$ m$.$ Estimate the ultimate

heave associated with the rise in ground water table that will occur, assuming

the soil properties listed below:

Bulk unit weight of gravel above the water table $=18\mathrm{.}5$ kN$/$m$3$

Bulk unit weight of gravel below the water table $=19\mathrm{.}5$ kN$/$m$3$

Bulk unit weight of clay $=21\mathrm{.}0$ kN$/$m$3$

Initial void ratio for the clay, eo $=0\mathrm{.}675$

Swelling index for the clay, cs $=0\mathrm{.}54$

$(8$ Marks$)$

$($b$)$ Comment on why in part $($a$)$ the swelling index is used rather than the

compression index.

$(4$ Marks$)$

$($c$)$ To estimate the time taken for this heave to occur what additional information

would you need?

$(4$ Marks$)$