Geotechnical Engineering

DHARM

PILE FOUNDATIONS 665

where s = final penetration (set) per blow. It is taken as average penetration per blow for
the last 5 blows or 20 blows depending on whether the hammer is a drop hammer
or steam hammer,
C = empirical constant (representing the temporary elastic compression of the hel-
met, pile and soil)
This is a dimensionally correct equation—H, s and C should be in the same units.
Then the units of Qup will be those of Wh.
A factor of safety of 6 was introduced to make up for any inaccuracies arising from the
use of arbitrary values for the constant, while arriving at the allowable load on the pile.

Qap =

WH

sC

h.

6 ()+

...(Eq. 16.20)

The value of C (in cm) is taken as 2.5 for drop hammer, and 0.25 for steam hammer.
For convenience in practical use, Eq. 16.20 may be transformed into mixed units as
follows:

Qap =

500

325

WH

s

h.

()+

for drop hammer ...(Eq. 16.21a)

Qap =

500

325

WH

s

h.
(.)+ for steam hammer ...(Eq. 16.21b)
where H and s are respectively in metres and millimetres, and Qap in the same units as Wh.

For double acting steam hammers:

Qap =

().

(.)

WapH

s

h+

625 +

...(Eq. 16.22)

where Wh = weight of hammer (newtons),

a = effective area of piston (mm^2 ),
p = mean effective steam pressure (N/mm^2 ),
H = height of fall of hammer (metres)
s = final penetration of pile per blow (mm), and
Qap = allowable load on the pile (kN).
(Note: This equation has mixed units).
Numerous load tests on piles indicate that the real safety factor of this formula aver-
ages 2 instead of the assumed value of 6; it can be as low as 2/3 and as high as 20.

For wood piles driven with drop hammers and for lightly loaded short piles driven with
steam hammers, the Engineering News formula gives a crude indication of pile-capacity. For
other conditions, it can be very misleading.

Hiley’s formula

This is considered to be the most complete one, as described by Chellis (1961). Hiley (1930)
takes into account the energy losses in Eq. 16.18 as follows:

1. Elastic compression of pile, pile cap and soil:
   1
   2

Qup(c 1 + c 2 + c 3 ) or Qup. C