Question

A rectangular footing $L\times B$ is to be placed at a depth D below ground level such that $\frac{D}{B}<2.5$ . The factor $N_c$ to be used in deciding of the allowable bearing capacity for the footing as given by Skempton is calculated using the equation
(where, $N_{CR}=N_C$ for rectangular footing,
$N_{CS}=N_C$ at surface )

• A. $N_{CR}=1.4N_{CS}$
• B. $N_{CR}=(1+0.2\frac{D}{B})N_{Cs}$
• C. $N_{CR}=(1+0.2\frac{B}{L})N_{Cs}$
• D. $N_{CR}=(1+0.2\frac{B}{L})(1+0.2\frac{D}{B})N_{Cs}$

Answer 1

The correct option is D $N_{CR}=(1+0.2\frac{B}{L})(1+0.2\frac{D}{B})N_{Cs}$

According to Skempton's analysis of cohesive soils, it has been shown that the bearing capacity factors in Terzaghi's equation tends to increase with depth for a cohesive soil.
$For\left(\frac{D_f}{B}\right)<2.5,(where{D}_{f}isthedepthfooting$ and B is the base width)

$\left(N_c\right)$ for rectangular footing

$=5(1+\frac{0.2D_f}{B})(1+\frac{0.2B}{L})\le 9$

$\left(N_c\right)$ for circular and rectangular footing

$=6(1+\frac{0.2D_f}{B})(1+\frac{0.2B}{L})\le 9$

$For\left(\frac{D_f}{B}\right)<2.5,\left(N_c\right)forrectangularfooting$

$=7.5(1+\frac{0.2B}{L})\le 9$