Section 4: Soil and Water Conservation Engineering
Soil Permeability, Seepage & Flow Nets
Subtopic: Critical hydraulic gradient and quicksand condition
Question Statement:
A sandy foundation layer has a solid specific gravity $G_s = 2.65$ and void ratio $e = 0.85$. The critical hydraulic gradient $i_c$ at which quicksand condition (boiling) occurs is ________ (round off to two decimal places).
Answer Key:
0.89
Step-by-Step Solution & Explanation
Critical hydraulic gradient occurs when upward seepage pressure balances submerged weight of soil:
$i_c = \frac{\gamma'}{\gamma_w} = \frac{G_s - 1}{1 + e}$
Given $G_s = 2.65$ and $e = 0.85$:
$i_c = \frac{2.65 - 1}{1 + 0.85} = \frac{1.65}{1.85} = 0.892$
Rounding to two decimal places: $0.89$.