Figure $5\mathrm{.}35$ shows the cross$-$section of a long coffer dam formed by parallel rows of sheet piles driven into an isotropic layer of sand.

$($a$)$ Sketch a flow net $($half only required$)$ for the seepage and estimate the seepage quantity in $\frac{m^3}{hr}$ per metre run if $k=6\mathrm{.}5{10}^{-4}\frac{m}{s}.$

$($b$)$ Calculate the factor of safety against there being a base failure in the excavation due to heaving. $($Saturated unit weight of soil $=20\mathrm{.}4\frac{kN}{m^3}.)$