A sheetpile wall is embedded $4$ m into a dense sandy silt having a saturated unit weight of $20$ kN$/$m$3.$ There is a difference in total head of $4\mathrm{.}5$ m between ground level and the excavated side of the sheetpile. A flownet is drawn to evaluate the seepage conditions around the pile $-$ the flownet has a total of $10$ equipotential drops and the mean number of drops that have occured at the mid point of the base of the soil prism subject to heave is $5\mathrm{.}5.$ If a depth of $0\mathrm{.}5$ m of water remains on the excavated side of the sheetpile and taking the unit weight of water equal to $9\mathrm{.}81$ kN$/$m$3$ the factor of safety against piping due to heave is approximately