An engineering project requires a temporary excavation through an existing clay

embankment. The proposed excavation will be $5$ m wide and $6\mathrm{.}5$ m below the height of the

embankment crest. A series of sheet piles will be driven into the ground to a depth of $9\mathrm{.}5$ m

below the embankment crest prior to excavation, and will form the side walls. A sketch of the

proposed excavation with relevant depths is shown below:

Sand

Impermeable Mud Stone

Construct a flownet to represent the flow of water towards the excavation, assuming that

the permeability of the ground is isotropic $($i$.$e$.$ horizontal and vertical permeabilities are

identical$)$ and that the clay is relatively impermeable. In your flownet, you should use a

total of $5$ flowlines $($including the boundary flowlines$)$ to represent the flow on one side

$($note the symmetry of the problem!$).$

Using your flownet, estimate the total flow of water $($per metre cross section$)$ into the

excavation assuming that it is possible to pump water out of the excavation quickly

enough to keep the water table at the base of the excavation. From lab tests, the sand was

found to have a permeability of $7{10}^{-4}\frac{m}{s}.$

Calculate the pore water pressures at the points marked A$,$ B$,$ C$,$ D and E$.$