A clay soil sample is tested in a triaxial apparatus in

A clay soil sample is tested in a triaxial apparatus in consolidated-drained conditions at a cell pressure of 100 kN/m². What will be the pore water pressure at a deviator stress of 40 kN/m²?

σ₁ = 100 kN/m².
deviator stress, = σ₁ – σ₃. = 40
Φ= 0 (clay)
σ₁ = σ₃. tan² α + 2c tan α.

α₂ = 45 +	Φ	= 45°
	2

σ₁ = σ₃ tan²45° + 2c tan 45°.
∴ σ₁ – σ₃ + 2C (C = pore water pressure)

C =	σ₁ – σ₃	=	40	= 20 kN/m²
	2		2