The critical hydraulic gradient ($i_c$) is determined by the following formula: \[ i_c = \frac{G_s - 1}{1 + e} \] In this context: $G_s = 2.7$ represents the specific gravity of solids, and $e = 0.7$ denotes the void ratio.
Substituting these values: \[ i_c = \frac{2.7 - 1}{1 + 0.7} = \frac{1.7}{1.7} = 1.0 \]