A $10\ m$ high slope of dry clay soil $\text{( unit weight}\: = 20 kN/m^3$), with a slope angle of $45^{\circ}$and the circular slip surface, is shown in the figure (not drawn to the scale). The weight of the slip wedge is denoted by $W$. The undrained unit cohesion $(c_u)$ is $60\: kPa$.

The factor of safety of the slope against slip failure, is

1. $1.84$
2. $1.57$
3. $0.58$
4. $1.67$