Annex J(Clause 11.2.2)

Load Carrying Capacity of Under-reamed Piles from Soil Properties

J-1 Ultimate Load Capacity

The ultimate load capacity of a pile can be calculated from soil properties. The soil properties required are strength parameters, cohesion, angle of internal friction and soil density.

a) Clayey soils – For clayey soils, the ultimate load carrying capacity of an under-reamed pile may be worked out from the following expression:

where

$Q_{u} =$ ultimate bearing capacity of pile, in kN;

$A_{p} = \text{cross-sectional area of the pile stem at the pile tip, in m}^{2};$

$N_{c} = \text{bearing capacity factor, usually taken as 9;}$

$C_{\mathrm{p}} =$ cohesion of the soil around toe, in $\mathrm{kN / m^2}$;

$A_{a} = (\pi/4)(D_{u}^{2}-D^{2})$ , where and D are the under-reamed and stem diameter, respectively, in m;

$C_{a}$ = average cohesion of the soil along the pile stem, in kN/m $^{2}$ ;

$A_{s}$ = surface area of the stem, in m $^{2}$ ;

$A_{s}^{\prime}=$ surface area of the cylinder circumscribing the under-reamed bulbs, in $m^{2}$;

$C_{\mathrm{a}}^{\prime} =$ average cohesion of the soil around the under-reamed bulbs; and

$\alpha =$ reduction factor (usually taken as 0.5 for clays).

NOTES

1 The above expression holds for the usual spacing of under-reamed bulbs spaced at not more than one and a half times their diameter.

2 If the pile is with one bulb only, the third term will not occur. For calculating uplift load, the first term will not occur in the formula.

3 In case of expansive soil, top 2 m strata should be neglected for computing skin friction.

b) Sandy soils

where

$A_{\mathrm{p}} = \pi D^{2} / 4$ , where $D$ is stem diameter, in m;

$A_{\mathrm{a}} = \pi /4(D_{\mathrm{u}}^{2} - D^{2})$ where is the under- reamed bulb diameter, in m;

$n =$ number of under-reamed bulbs;

$\gamma =$ average unit weight of soil (submerged unit weight in strata below water table), in $\mathrm{kN / m^3}$;

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

$d_{\mathrm{r}} =$ depth of the centre of different underreamed bulbs below ground level, in $\mathrm{m}$;

$d_{f}$ = total depth of pile below ground level, in m;

$K =$ earth pressure coefficient (usually taken as 1.75 for sandy soils);

$\delta =$ angle of wall friction (may be taken as equal to the angle of internal friction $\phi$);

$d_{1} =$ depth of the centre of the first under- reamed bulb, in m; and

$d_{n}$ = depth of the centre of the last under- reamed bulb, in m.

NOTES

1 For uplift bearing on pile tip, $A_{\mathrm{p}}$ will not occur. 2 $N_{\gamma}$ will be as specified in good practice [6-2(11)] and $N_{\mathrm{q}}$ will be taken from Fig. 11B.

c) Soil strata having both cohesion and friction — In soil strata having both cohesion and friction or in layered strata having two types of soil, the bearing capacity may be estimated using both the formulae. However, in such cases load test will be a better guide.

d) Compaction piles in sandy strata — For bored compaction piles in sandy strata, the formula in (b) shall be applied but with the modified value of $\phi_{1}$ as given below:

where

$\phi =$ angle of internal friction of virgin soil.

The values of $N_{\gamma}$, $N_{\mathrm{q}}$ and $d$ are taken corresponding to $\phi_{1}$. The value of the earth pressure coefficient will be 3.

e) Piles resting on rock — For piles resting on rock, the bearing component will be obtained by multiplying the safe bearing capacity of rock with bearing area of the pile stem plus the bearing provided by the bulb portion.

NOTE — To obtain safe load in compression and uplift from ultimate load capacity generally the factors of safety will be 2.5 and 3, respectively.

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS