For coarse sand with void ratio 0.68 and specific gravity 2.68, critical hydraulic gradient is

Critical hydraulic gradient i_c = (G - 1) / (1 + e), where G = specific gravity of soil solids (typically 2.65–2.68) and e = void ratio. At i = i_c, effective stress = zero (quick condition/sand boil/piping failure). Submerged unit weight = (G-1)*gamma_w/(1+e), so i_c = gamma_sub / gamma_w. Safety factor against piping = i_c / i_actual. i_c=(G-1)/(1+e)=(2.68-1)/(1+0.68)=1.00

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