A braced cut, 5 m wide and 7.5 m deep is proposed in a cohesion

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A braced cut, 5 m wide and 7.5 m deep is proposed in a cohesion less soil deposit having effective cohesion c’ = 0 and effective friction angle, φ’ = 36°. The first row of struts is to be installed a depth of 0.5 m below ground surface and spacing between the struts should be 1.5 m. If the horizontal spacing of struts is 3 m and unit weight of the deposit is 20kN/m³, the maximum strut load will be

1. 70.87 kN
2. 98.72 kN
3. 113.90 kN
4. 151.86 kN

Correct Option: C

Number of struts

=	75. - 0.5
	1.5

= 4.67 ≈ 5

Maximum force will be at centre.
Active earth pressure,
σ₁ = σ₃ = tan α – 2 c tan α
γz = α₃ tan² α

20 × 7.5 = σ₃ tan²		45 +	Φ
			2

= σ₃ tan²		45 +	36
			2

⇒ 150 = σ₃ × 3.85
∴ σ₃ = 38.96 kN/m²
Uniform pressure = 0.65 × σ₃
= 0.65 × 38.96 = 25.32
∴ Maximum force on a street at middle
= 25.32 × 3 × 1.5 = 113.9 kN