Question

In a braced vertical excavation of 5 m height and 2.50 m width in a cohesive soil having undrained cohesion equal to $20kN/m^2$ and bulk unit weight of $20kN/m^3$, what is the factor of safety against heave failure at its base?

• A. 1.0
• B. 1.20
• C. 1.40
• D. 2.0

Answer 1

The correct option is C 1.40

$\begin{array}{cc}\text{Factor of safety,}& \\ F& =\frac{C_u{N}_c}{\gamma H}\\ \text{Given}{C}_u& =20kN/m^2,\gamma =20kN/m^3\\ \text{Bearing capacity factor}{N}_c& \text{from Skempton's}\\ \text{analysis is}& \\ N_c& =5(1+0.2\frac{D_f}{B})\\ \text{for}\frac{D_f}{B}& <2.5\\ & =7.5for\frac{D_f}{B}>2.5\\ \text{Thus}{N}_c& =5\times (1+0.2\times \frac{5}{2.5})=7.0\\ \therefore F& =\frac{20\times 7}{20\times 5}=1.4\end{array}$