Example $9\mathrm{.}8$: A canal is to be excavated through a soil with $c=15k\frac{N}{m^2},=20,e=0\mathrm{.}9$ and

$G=2\mathrm{.}67.$ The side slope is $1$ in $1.$ The depth of the canal is $6$ m $.$ Determine the factor of safety

with respect to cohesion when the canal runs full. What will be the factor of safety if the canal

is rapidly emptied?