Section 4: Soil and Water Conservation Engineering
Soil Mechanics
Subtopic: Shear strength
Question Statement:
A cohesive soil has an angle of shearing of 15° and a cohesion of 35 kPa. The value of lateral pressure in the cell for failure to occur at a total stress of 300 kPa during the triaxial test is
Options:
(A) 59.58 kPa
(B) 122.92 kPa
(C) 140.41 kPa
(D) 230.34 kPa
Official Answer Key:
B
Step-by-Step Solution & Explanation
Mohr-Coulomb failure criterion: σ₁ = σ₃·N_φ + 2c√N_φ, where N_φ = tan²(45°+φ/2). φ=15° ⟹ N_φ = tan²(52.5°) = 1.698. With σ₁ (axial, failure) = 300 kPa (the cell/confining pressure σ₃ is being sought): σ₃ = (σ₁ − 2c√N_φ)/N_φ = (300 − 2×35×1.303)/1.698 = (300−91.2)/1.698 = 122.92 kPa.
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