G-8 Piles in Weathered/soft Rock

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 598

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For bored piles founded in weathered/soft rock different empirical approaches are used to arrive at the socket length necessary for utilizing the full structural capacity of the pile.

Since it is difficult to collect cores in weathered/soft rocks, the method suggested by Cole and Stroud using 'N' values is more widely used. The allowable load on the pile, $Q_{a}$ , in kN, by this approach, is given by:

Q _ {\mathrm{a}} = c _ {\mathrm{u} 1} N _ {\mathrm{c}} \cdot \frac {\pi B ^ {2}}{4 F _ {\mathrm{s}}} + \alpha c _ {\mathrm{u} 2} \cdot \frac {\pi B L}{F _ {\mathrm{s}}}

where

$c_{u1}$ = shear strength of rock below the base of the pile, in kN/m $^{2}$ (see Fig. 13);

$N_{c}$ = bearing capacity factor taken as 9;

$F_{\mathrm{s}} =$ factor of safety usually taken as 3;

$\alpha = 0.9$ (recommended value);

$c_{\mathrm{u2}} =$ average shear strength of rock in the socketed length of pile, in $\mathrm{kN / m^2}$ (see Fig. 13);

B = minimum width of pile shaft (diameter in case of circular piles), in m; and

$L =$ socket length of pile, in m.

NOTE — For $N \geq 60$ , the stratum is to be treated as weathered rock rather than soil.

Shear Strength kN/m2	Approx. N Value	Strength / Consistency	Grade	Breakability	Penetration	Strength
40000		Strong	A	Difficult to break against solid object with hammer		Can not be scratched with knife
20000
10000	600	Moderately strong	B	Broken against solid object with hammer		Can just be scratched with knife
8000
6000	400
4000		Moderately weak	C	Broken in hand by hitting with hammer		Scratched with knife. Can just be scratched with thumb-nail
2000	200		D	Broken by leaning on sample with hammer	No penetration with knife	Scratched with thumb-nail
1000		Weak
800	100		E	Broken by hand	Penetration to about 2 mm with knife
600	80	Hard or very weak	F	Easily broken by hand	Penetration to about 5 mm with knife
400	60
200	40	Very stiff			Penetrated by thumb-nail and to about 1.5 mm with knife
100	20	Stiff			Indented by thumb
80
60		Firm			Penetrated by thumb with effort
40	10
	8	Soft			Easily penetrated by thumb
20	6
10	4
	2	Very Soft

NOTE — Standard penetration test may not be practicable for $N$ values greater than 200. In such cases, design may be done on the basis of shear strength of rock.

FIG. 13 CONSISTENCY AND SHEAR STRENGTH OF WEATHERED ROCK

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

ANNEX H

(Clause 8.3.5.2)

Analysis of Laterally Loaded Piles