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An infinite natural slope with angle of slope $15($inclination measured from horizontal$)$ has a saturated

unit weight of $18$ kN$/$m$^3$ and an effective angle of internal friction, $ $=35.$ Determine the factor of safety against failure of the slope $($i$)$ when the slope is completely dry or submerged but without seepage, $($ii$)$ when seepage occurs at a parallel to the surface of slope.