The flow net around a sheet pile wall is shown in the sketch. The properties of the soil are: permeability coefficient $0.09 \: m/day$ (isotropic), specific gravity $=2.70$ and void ratio $=0.85$. The sheet pile wall and the bottom of the soil are impermeable.

The factor of safety against the occurrence of piping failure is

1. $3.55$
2. $2.93$
3. $2.60$
4. $0.39$