A sand deposit has a porosity of \(\frac{1}{3}\) and its specific gravity is 2.5. The critical hydraulic gradient to cause sand boiling in the stratum will be

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BPSC AE Paper 5 (Civil) 14 Oct 2022 Official Paper
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  1. 0.75
  2. 1.0
  3. 1.25
  4. 1.5

Answer (Detailed Solution Below)

Option 2 : 1.0
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Concept:

Critical hydraulic gradient (ic): The quicksand / boiling condition occurs at a critical upward hydraulic gradient typically around 1.0 for many soils, when the seepage force just balances the buoyant weight of an element of soil.

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

\({i_c} = \frac{{G - 1}}{{1 + e}}\;{\rm{OR\;}}\left( {{\rm{G}} - 1} \right) \times \left( {1 - {\rm{n}}} \right)\)

Calculations:

Critical hydraulic gradient

 n = 1/3, G = 2.5

 G - 1 = 1.5

\(\begin{array}{*{20}{c}} {{i_c} = \left( {{\rm{G}} - 1} \right) \times \left( {1 - {\rm{n}}} \right) = 1.5 \times \left( {1 - \frac{1}{3}} \right) = 1} \end{array}\)

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