Question

A stratum of 3.5 m thick fine sand has a void ratio of 0.7 and G of 2.7. For a quicksand condition to develop in this strata, the water flowing in an upward direction would require a head of:
1. 7 m
2. 5.56 m
3. 5 m
4. 3.5 m

Answer 1

Correct Answer - Option 4 : 3.5 m
Explanation

Critical hydraulic gradient (i_c): 

The quick condition occurs at a critical upward hydraulic gradient ic when the seepage force just balances the buoyant weight of an element of soil. The critical hydraulic gradient is typically around 1.0 for many soils.

At the critical conditions, the effective stress is equal to zero.

we know,

The critical hydraulic gradient is given by

\({{\rm{i}}_{\rm{c}}} = \frac{{{\rm{G}} - 1}}{{1 + e}} \)

Given, G = 2.7, e = 0.7, Stratum thickness (z) = 3.5 m

\({{\rm{i}}_{\rm{c}}} = \frac{{{\rm{2.7}} - 1}}{{1 +0.7}} =1\)

we know i = h/z

Head required (h)

= ic × thickness of the sand stratum (z)

= 1 × 3.5 = 3.5 m