Pile Design and Construction Practice, Fifth edition

where

c undrained shearing resistance
B base width
D base depth below the rock surface
 effective density of the rock mass
Nc, Nq, and N bearing capacity factors related to as shown in Figure 4.30.
The above equation represents wedge failure conditions beneath a strip foundation and
should not be confused with Terzaghi’s equation for spread foundations. Because equation 4.40
is for strip loading the value of cNcshould be multiplied by a factor of 1.25 for a square pile
or 1.2 for a circular pile base. Also the term BN/2 should be corrected by the factors
0.8 or 0.7 for square or circular bases respectively. The term BN/2 is small compared with
cNcand is often neglected.
Where it is difficult to obtain satisfactory samples for laboratory testing to determine
c or  the relationship of these parameters to the uniaxial compression strength



200 Resistance of piles to compressive loads

Table 4.14Ultimate base resistance of piles

related to the uniaxial compression

strength of the intact rock and the

RQD of the rock mass

RQD (%) qub c

0 – 70 0.33quc 0.1quc 30

70 – 100 0.33–0.8quc 0.1quc 30 – 60

 ( )

1 000

100

Bearing capacity factor^10

Nc

Ng

Nq

1

01020

Friction angle, f

30 40 50 60 70

Figure 4.30Wedge bearing capacity factors for foundations on rock (reprinted from Pells and

Turner(4.42)).