Geotechnical Engineering

DHARM

682 GEOTECHNICAL ENGINEERING

guess. These formulae yield efficiency values less than unity, and as such, will not be applica-

ble to closely spaced friction piles in cohesionless soils and to piles through soft material rest-

ing on a firm stratum.

The Converse-Labarre formula, the Feld’s rule and the Seiler-Keeny formula are given

here:

Converse-Labarre formula

ηg = 1 – φ

90

mn 11 nm

mn

L ()( )−+ −

NM

O

QP

...(Eq. 16.47)

where ηg = efficiency of pile group,

φ = tan–1

d

s

in degrees, d and s being the diameter and spacing of piles,

m = number of rows of piles, and

n = number of piles in a row (interchangeable)

Feld’s rule

According to ‘‘Feld’s rule’’, the value of each pile is reduced by one-sixteenth owing to the effect
of the nearest pile in each diagonal or straight row of which the particular pile is a member.
This is illustrated in Fig. 16.16.

2 piles

@ 15/16

hg= 94%

3 piles

@ 14/16

hg= 97%

4 piles

@ 13/16

hg= 82%

5 piles

4 @ 13/16

1 @ 12/16

hg= 88%

9 piles

4 @ 13/16

4 @ 11/16

1 @ 8/16

hg= 72%

Fig. 16.16 Efficiencies of pile groups using Feld’s rule

Seiler-Keeney formula

The efficiency of a pile group, ηg, is given by

ηg = 1 0 479

0 093

2

1

03

− (^2) −
F
HG
I
KJ
+−
+−
F
HG
I
KJ
L
N
M
M
O
Q
P
P

• 
  


• 
  

  .
  .
  .
  ()
  s
  s
  mn
  mn mn
  ...(Eq. 16.48)
  Here m, n and s stand for the number of rows of piles, number of piles in a row and pile
  spacing, respectively.
  UV
  W