Given cover soil thickness $H_{cs}=3\mathrm{.}0ft,$ and cover soil unit weight ${}_s=120\frac{lb}{f{t}^3}.$ A local subsidence occurs which is estimated to be $1\mathrm{.}0ft$ depth by $3\mathrm{.}0ft$ radius. The geomembrane is $40$mils thick and has a ${}_{\text{a}\text{l}\text{l}\text{o}\text{w}}=1000l\frac{b}{i}{n}^2.$ Determine the factor of safety of the geomembrane against the mobilized tensile stresses.