Section 4: Soil and Water Conservation Engineering
Soil Mechanics
Subtopic: Active and passive earth pressures
Question Statement:
A retaining wall of height 5 m retains cohesionless dry soil (density 1.9 Mg/m³, angle of internal friction 28°). Backfill surface is horizontal. Find the active and passive earth pressures per meter length of the wall (kN). [g=9.81 m/s²]
Options:
(A) 84.11, 645.38
(B) 645.38, 84.11
(C) 142.12, 381.63
(D) 381.63, 142.12
Official Answer Key:
A
Step-by-Step Solution & Explanation
γ = 1900×9.81 = 18,639 N/m³. Ka = (1−sin28°)/(1+sin28°) = 0.3610; Kp = 1/Ka = 2.770. Active force = ½KaγH² = 0.5×0.3610×18.639×25 ≈ 84.11 kN. Passive force = ½KpγH² = 0.5×2.770×18.639×25 ≈ 645.38 kN.
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