G-1 Piles in Granular Soils

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 595

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The ultimate load capacity $(Q_{\mathrm{u}})$ of piles, in kN, in granular soils is given by the following formula:

Q _ {\mathrm{u}} = A _ {\mathrm{p}} \left(\frac {1}{2} D \gamma N _ {\gamma} + P _ {\mathrm{D}} N _ {\mathrm{q}}\right) + \sum_ {i = 1} ^ {n} K _ {\mathrm{i}} P _ {\mathrm{Di}} \tan \delta_ {\mathrm{r}} A _ {\mathrm{si}} \dots (1)

The first term gives end bearing resistance and the second term gives skin friction resistance.

where

$A_{p}=$ cross-sectional area of pile tip, in m $^{2}$ ;

$D =$ diameter of pile shaft, in m;

$\gamma =$ effective unit weight of the soil at pile tip, in $\mathrm{kN / m^3}$

$N_{\gamma}$ and $N_{q}$ = bearing capacity factors depending upon the angle of internal friction, $\phi$ at pile tip;

$P_{D}$ = effective overburden pressure at pile tip, in kN/m $^{2}$ (see Note 5);

$\sum_{i=1}^{n} =$ summation for layers 1 to $n$ in which pile is installed and which contribute to positive skin friction;

$K_{i}=$ coefficient of earth pressure applicable for the ith layer (see Note 3);

$P_{Di}$ = effective overburden pressure for the ith layer, in kN/m $^{2}$ ;

$\delta_{\mathrm{i}} =$ angle of wall friction between pile and soil for the ith layer; and

$A_{si}$ = surface area of pile shaft in the ith layer, in m $^{2}$ .

NOTES

1 $N_{\gamma}$ factor can be taken for general shear failure according to good practice [6-2(11)].

2 $N_{q}$ factor will depend on the nature of soil, type of pile, the L/B ratio and its method of construction. The values applicable for driven and bored piles are given in Fig. 11A and Fig. 11B, respectively.

3 $K_{\mathrm{i}}$ , the earth pressure coefficient depends on the nature of soil strata, type of pile, spacing of piles and its method

of construction. For bored piles in loose to dense sand with $\phi$ varying between $30^{\circ}$ and $40^{\circ}$ , $K_{\mathrm{i}}$ values in the range of 1 to 2 may be used for driven piles and in case of bored piles, $K_{\mathrm{i}}$ values in the range of 1 to 1.5 may be used.

4 δ, the angle of wall friction may be taken equal to the friction angle of the soil around the pile shaft.

5 In working out pile capacity by static formula, the maximum effective overburden at the pile tip should correspond to the critical depth, which may be taken as 15 times the diameter of the pile shaft for $\phi \leq 30^{\circ}$ and increasing to 20 times for $\phi \geq 40^{\circ}$ .

6 For piles passing through cohesive strata and terminating in a granular stratum, a penetration of at least twice the diameter of the pile shaft should be given into the granular stratum.