G-3 Use of Static Cone Penetration Data · NBC 2016 (SP 7:2016) Volume 1

G-3.1 When static cone penetration data are available for the entire depth, the following correlation may be used as a guide for the determination of ultimate load capacity of a pile.

G-3.2 Ultimate end bearing resistance $(q_{\mathrm{u}})$ , in $\mathrm{kN / m^2}$ , may be obtained as:

q _ {\mathrm{u}} = \frac {\frac {q _ {\mathrm{c0}} + q _ {\mathrm{c1}}}{2} + q _ {\mathrm{c2}}}{2}

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

where
    N = average N values at pile tip;
    L_b = length of penetration of pile in the bearing strata, in m;
    D = diameter or minimum width of pile shaft, in m;
    A_p = cross-sectional area of pile tip, in m^2;
    \overline\{N\} = average N along the pile shaft; and
    A_s = surface area of pile shaft, in m^2.
NOTE — The end bearing resistance should not exceed 400 NA_p

where

$q_{c0} = \text{average static cone resistance over a depth of } 2D \text{ below the pile tip, in kN/m}^{2};$

$q_{c1} = \text{minimum static cone resistance over the same } 2D \text{ below the pile tip, in kN/m}^2;$

$q_{c2} =$ average of the envelope of minimum static cone resistance values over the length of pile of 8D above the pile tip, in kN/m²; and

$D =$ diameter of pile shaft.

G-3.3 Ultimate skin friction resistance can be approximated to local side friction ( $f_{s}$ ), in kN/m $^{2}$ , obtained from static cone resistance as given in Table 8.

G-3.4 The correlation between standard penetration resistance, N (blows/30 cm) and static cone resistance, $q_{c}$ , in kN/m $^{2}$ , as given in Table 9 may be used for working out the end bearing resistance and skin friction resistance of piles. This correlation should only be taken as a guide and should preferably be established for a given site as they can vary substantially with the grain size, Atterberg limits, water table, etc.

Table 8Side Friction for Different Soil Types (Clause G-3.3)

Sl No.(1)	Type of Soil(2)	Local Side Friction, $f_s$ $\text{kN/m}^2$ (3)
i)	$q_c$ less than 1 000 kN/m

Table 9Correlation Between N and q_{\mathrm{c}} for Different Types of Soil (Clause G-3.4)

Sl No.(1)	Type of Soil(2)	$q_e/N$ (3)
i)	Clay	150-200
ii)	Silts, sandy silts and slightly cohesive silt-sand mixtures	200-250
iii)	Clean fine to medium sand and slightly silty sand	300-400
iv)	Coarse sand and sands with little gravel	500-600
v)	Sandy gravel and gravel	800-1 000

G-4 USE OF STANDARD PENETRATION TEST DATA FOR COHESIONLESS SOIL

G-4.1 The correlation suggested by Meyerhof using standard penetration resistance, N in saturated cohesionless soil to estimate the ultimate load capacity of driven pile is given below. The ultimate load capacity of pile ( $Q_{u}$ ), in kN, is given as:

Q _ {\mathrm{u}} = 4 0 N \frac {L _ {\mathrm{b}}}{D} A _ {\mathrm{p}} + \frac {\overline {{N}} A _ {\mathrm{s}}}{0 . 5 0}\tag{... (3}

The first term gives the end bearing resistance and the second term gives the frictional resistance.

G-4.2 For non-plastic silt or very fine sand the equation has been modified as:

Q _ {\mathrm{u}} = 3 0 N \frac {L _ {\mathrm{b}}}{D} A _ {\mathrm{p}} + \frac {\overline {{N}} A _ {\mathrm{s}}}{0 . 6 0} \quad \dots \tag {4}

The meaning of all terms is same as for equation 3.

G-4.3 The correlation suggested by Meyerhof using standard penetration resistance, N in saturated cohesionless soil to estimate the ultimate load capacity of bored pile is given below. The ultimate capacity of pile ( $Q_{u}$ ), in kN, is given as:

Q _ {\mathrm{u}} = 1 3 N \frac {L _ {\mathrm{b}}}{D} \mathrm{A} _ {\mathrm{p}} + \frac {\overline {{N}} A _ {\mathrm{s}}}{0 . 5 0} \dots (5)

The first term gives end bearing resistance and the second term gives frictional resistance.

The meaning of all terms is same as for equation 3.

NOTE — The end bearing resistance should not exceed $130 NA_{p}$ .

G-4.4 For non-plastic silt or very fine sand the equation has been modified as:

Q _ {\mathrm{u}} = 1 0 \frac {L _ {\mathrm{b}}}{D} \mathrm{A} _ {\mathrm{p}} + \frac {\overline {{N}} A _ {\mathrm{s}}}{0 . 6 0}\tag{... (6}

The meaning of all terms is same as for equation 3.

G-5 Factor of Safety

The minimum factor of safety for arriving at the safe pile capacity from the ultimate capacity obtained by using static formulae shall be 2.5.

G-6 Piles in Stratified Soil

In stratified soil/C- $\phi$ soil, the ultimate load capacity of piles should be determined by calculating the skin friction and end bearing in different strata by using appropriate expressions given in G-1 and G-2.

G-7 Piles in Hard Rock

When the crushing strength of the rock is more than characteristic strength of pile concrete, the rock should be deemed as hard rock. Bored piles resting directly on hard rock may be loaded to their safe structural capacity.