$4\mathrm{.}6$ An open clay layer is $6$ m thick, the value of $c_v$ being $1\mathrm{.}0\frac{m^2}{?}$ year. The initial distribution of excess

pore water pressure varies linearly from $60$ kPa at the top of the layer to zero at the bottom. Using

the finite difference approximation of the one$-$dimensional consolidation equation, plot the iso$-$

chrone after consolidation has been in progress for a period of three years, and from the isochrone

determine the average degree of consolidation in the layer.