The figure below shows a retaining wall $6$m high resting on an impermeable horizontal

boundary. The back of the wall is vertical, and is provided with a vertical filter drain to intercept

the flow of water from the fill. The vertical filter drain discharges through drainage holes $($weep

holes$),$ spaced at $1$m intervals in the base of the wall.

a$)$ Draw a flow net for the fill, assuming $($i$)$ that sufficient rain falls to maintain the

ground$-$water level at the upper surface $($CBD$)$ of the fill, and $($ii$)$ that any water in the

vertical filter drain is at atmospheric pressure.

b$)$ Determine from the flow net the water pressure distribution along a surface AB that is

inclined at $60$o to the horizontal and passes through the toe of the fill.

c$)$ Calculate the discharge from each drainage hole in m$3/$day$,$ assuming the permeability

of the fill to be k $10-7$m$/$sec$.$

$($Ans$.$: $($c$)$ discharge $5$ x $10-2$ m$3/$day$.)$