Question

An undrained triaxial compression test is carried out on saturated clay sample under a cell pressure of 100 kN/m². The sample failed at a deviator stress of 200 kN/m². The cohesion of the given sample of clay is
1. 100 kN/m²
2. 150 kN/m²
3. 200 kN/m²
4. 50 kN/m²

Answer 1

Correct Answer - Option 1 : 100 kN/m²

Concept:

Unconfined compression test is the unique case of triaxial test in which no cell pressure is applied to the soil sample. It is an undrained test or quick test.

Since the test produces only one Mohr circle (corresponds to σ₃ = 0), the test applies only to soils for which Φu = 0, i.e saturated clays.

The unconfined compression test is often used to determine in situ soft saturated fine-grained soil.

\({\rm{C}} = \frac{{{\rm{Deviation\;stress}}}}{2} = \frac{{{{\rm{\sigma }}_{\rm{a}}}}}{2}\)

Calculation:

∴ \({\rm{C}} = \frac{{200}}{2} = 100\;{\bf{kN}}/{{\bf{m}}^2}\)