$4\mathrm{.}7$ Figure P$4\mathrm{.}7$ shows a wall in two layers of clay with $H_1=4\mathrm{.}0m$ and $H_2=4\mathrm{.}0m.$ The groundwater level is on the surface. The undrained shear strengths and saturated unit weight are $s_{u1}=20k\frac{N}{m^2}$ and ${}_{\text{s}\text{a}\text{t}1}=16k\frac{N}{m^3},$ respectively, for the upper clay and $s_{u2}=25k\frac{N}{m^2}$ and ${}_{\text{s}\text{a}\text{t}2}=18k\frac{N}{m^3},$ respectively, for the lower clay. Determine the following:

a$.$ The depth of the tension cracks

b$.$ The variation in Rankine's active pressure with depth

c$.$ The magnitude and location of the total active thrust per unit length of the wall, assuming tension cracks do not occur

Figure P$4\mathrm{.}7$