A cut slope is made in a silty clay soil for a new road project, as shown in the figure. The locations of ground water table (GWT) and potential failure surface are shown in the figure. After the cut is made, the excess pore water pressure is fully dissipated, and the shear stress at the point A is $60 \mathrm{kN} / \mathrm{m}^2$. The factor of safety at the point A for longterm stability is __________ (rounded off to 2 decimal places).
Note:
Shear strength properties of silty clay: $c^{\prime}=15 \mathrm{kN} / \mathrm{m}^2, \phi^{\prime}=15^{\circ}$, and $c_u=75 \mathrm{kN} / \mathrm{m}^2$ Unit weight of soil above the GWT $(\gamma)=19 \mathrm{kN} / \mathrm{m}^3$
Unit weight of soil below the GWT $\left(\gamma_{\text {sat }}\right)=20 \mathrm{kN} / \mathrm{m}^3$
Unit weight of water $\left(\gamma_w\right)=9.81 \mathrm{kN} / \mathrm{m}^3$
