Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 207

Rock socket shaft friction

Unconfined compression strength-quc (MN/m^2 )

=fs=abquc

Williams & Pells

1.0

0.8

0.6

0.4

0.2

(^00010100)
Horvath
Rock socket reduction factor
a
Rosenberg & Journeaux
Figure 4.33Reduction factors for rock socket shaft friction.
1.0
0.8
0.6
0.4
0.2
0
0 0.2 0.4
Mass factor j
Rock socket correction factor
b
0.6 0.8 1.0
Figure 4.34Reduction factors for discontinuities in rock mass (after Williams and Pells(4.52)).
where
ucwis the unconfined compression strength of the weaker material (concrete or
rock), andfsand are expressed in MN/m^2. The factor bis given as 0.2 to 0.3.
The shaft friction can be increased in weak or friable rocks by grooving the socket.
Horvath et al.(4.54)described experiments in mudstones using a toothed attachment to a
rotary auger. They showed that fswas related to the depth of the groove by the equation:

(4.46)

fs

ucw

0.8(RF)0.45

ucw