Pile Design and Construction Practice, Fifth edition

Rock socket reduction factor a
1.0
0.8
0.6
0.4
0.2
Resistance of piles to compressive loads 207
Williams & Pells
Horvath
Rosenberg & Journeaux
Rock socket shaft friction
= fs = abquc Unconfined compression strength-quc (MN/m2 0
0 0 10 100
)
Figure 4.33 Reduction factors for rock socket shaft friction.
Rock socket correction factor b
1.0
0.8
0.6
0.4
0.2
0
0 0.2 0.4 0.6 0.8 1.0
Mass factor j
Figure 4.34 Reduction factors for discontinuities in rock mass (after Williams and Pells (4.52) ).
where �� ucw is the unconfined compression strength of the weaker material (concrete or
rock), and fs and are expressed in MN/m2 . The factor b is 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) ��ucw described experiments in mudstones using a toothed attachment to a
rotary auger. They showed that fs was related to the depth of the groove by the equation:
f s
�� ucw
� 0.8(RF) 0.45
(4.46)