A $2\mathrm{.}0-$m$-$thick loose sand layer with $15\%$ fines exists at the depth starting

from $3\mathrm{.}0$ m $.$ The groundwater table is at $0\mathrm{.}5$ m from the ground surface. The unit weights of the sand above and below the groundwater are $17\mathrm{.}4$ and $18\mathrm{.}1$

kN$/$m$3,$ respectively. The SPT N$60$ values at the depths of $3\mathrm{.}5$ and $4\mathrm{.}5$ m are $10$

and $11\mathrm{.}5,$ respectively. The site is located at a seismically active area with a

possible maximum magnitude earthquake of $7\mathrm{.}8$ and a peak ground

acceleration of $0\mathrm{.}35$ g $.$ Calculate the factor of safety of this sand against liquefaction.