Sheet pile is driven into the permeable soil $($see Fig. $1),$ the following are given: $-$ Upstream: depth of water $=6$ m $-$ Downstream: depth of water $=2$m $-$ The coefficient of permeability $($i$.$e$.$ hydraulic conductivity$)$ k$=2\mathrm{.}5\backslash$times $10^-5$m $/$ sec$,-$ Flow net: N$\_$d$=8$ and N$\_$f$=3-$ Depth of embedment of sheet pile into soil: D$=4$m $-$ Saturated unit weight of soil: y$\_$sat $=15$ kN $/$ m$^3$ Fig. $1.$ Sheet pile driven into permeable soil layer Determine: a$.$ Calculate the rate of seepage per unit length. b$.$ Factor of safety against downstream heave, and assess the stability of soil on the downstream side.