# A moist sand was tested in an unconfined compression test and failed at an axial stress of 150 kPa. The failure plane was found to make an angle of 60

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A moist sand was tested in an unconfined compression test and failed at an axial stress of 150 kPa. The failure plane was found to make an angle of 60° with the horizontal. Then the apparent cohesion contributed by capillary moisture will be:
1. 43.3 kPa
2. 129.9 kPa
3. 144.9 kPa
4. 38.82 kPa

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Correct Answer - Option 1 : 43.3 kPa

Concept:

Unconfined Compression test:

(i) It is a special case of the triaxial test in which confining pressure is zero. It means only deviator or shear stress is applied.

(ii) The axial stress at failure is called unconfined compressive strength since confining pressure σ3 = 0

hence, σ1 = σ3 + σd = σd

Using,

σ1 = σ3 tan2(45° + ϕ/2) + 2c tan(45° + ϕ/2)

σ1 = 2c tan(45° + ϕ/2)

Calculation:

Given,

axial stress (σd) = σ1 = 150 kPa

θ = 45° + ϕ/2 = 60°

∵ σ1 = 2c tan(45° + ϕ/2)

150 = 2c tan 60°

c = 43.33 kPa