Q$2.(5$P$)$ The following standard penetration tests results were obtained in a uniform silty sand:

$\backslash$table$[[$Depth $($m$),1\mathrm{.}0,2\mathrm{.}0,3\mathrm{.}0,5\mathrm{.}0],[N_{60},12,13,18,15]]$

The groundwater table is at a depth of $2\mathrm{.}5$ m $.$ Assume a reasonable value for $,$ then determine ${}^{\text{'}}$ for each test. Finally, determine a single design ${}^{\text{'}}$ value for this stratum.