Question

A singly-reamed, $8\text{m}$ long, RCC pile (as shown in the below figure) weighing $20\text{kN}$ with $350\text{mm}$ shaft diameter and $750\text{mm}$ under-ream diameter is installed within stiff, saturated silty clay (undrained shear strength is $50\text{kPa}$, adhesion factor is 0.3, and the applicable bearing capacity factor is 9) to counteract the impact of soil swelling on a structure constructed above. Neglecting suction and the contribution of the under-ream to the adhesive shaft capacity, what would be the estimated ultimate tensile capacity (rounded off to the nearest integer value in $\text{kN}$) of the pile?

• A. $132\text{kN}$
• B. $156\text{kN}$
• C. $287\text{kN}$
• D. $301\text{kN}$

Answer 1

The correct option is D $301\text{kN}$


Weight of pile,
$P_s=20\text{kN}$
Shaft dia, $D_0=350\text{mm}$
Under ream dia,
$D_u=750\text{mm}$
Undrained shear strength,
$C=50\text{kPa}$
Adhesion factor,
$\alpha =0.3$
$N_c=9$
Ultimate Tensile capacity will be due to
1. Friction along the length of pile $\left(P_1\right)$
2. Bearing action caused by underreamed portion $\left(P_2\right)$
3. Dead weight of pile $\left(P_3\right)$
Tensile capacity due to friction,
$P_1=f_s\times A_s$
$=\alpha .C.\pi D_0\times L$
$=0.3\times 50\times \pi \times 0.35\times 7.6$
$=125.286\text{kN}$
Tensile capacity due to bearing action
$P_2=C{N}_c.A$
$=50\times 9\frac{\pi (D_u^2-D_0^2)}{4}$
$=50\times 9\times \frac{\pi \times ({0.75}^2-{0.35}^2)}{4}$
$=155.51\text{kN}$
$P=P_1+P_2+P_3$
$=125.286+155.51+20$
$=300.796\text{kN}$