Question

For the soil profile shown in figure below, the minimum number of precast concrete piles of $300\text{mm}$ diameter required to safely carry the load fo a given factor of safety of 2.5 (assuming $100\%$ efficiency for the pile group) is equal to

• A. 10
• B. 15
• C. 20
• D. 25

Answer 1

The correct option is C 20

Dia of pile, $d=0.3\text{m}$

Length of pile,
$L=10\text{m}$
Factor of safety,
FOS = 2.5
Efficiency $=100\%$
$C_u=100\text{kPa}$
$\alpha =0.57$
Laod carrying capacity of single pile,
$f=C_u{N}_c{A}_b+\alpha {\overline{C}}_u.A_s$
$=150\times 9\times \left(\frac{\pi \times {0.3}^2}{4}\right)+0.57\times 100\times (\pi \times 0.3\times 10)$
$=632.64\text{kN}$
As group efficiency is $100\%$
Load carrying capacity of pile group
$=F\times \text{number of piles}$
$\Rightarrow F_{\text{group}}=n.F$
$F_{\text{group}}=\text{F.O.S.}\times \text{Applied load}$
$n\times 632.64=2.5\times 5000$
$\Rightarrow n=19.75=20\text{piles}$