slurrying of the walls of the borehole and, in the most adverse case, the shaft friction
corresponds to that typical of a smooth-bore hole in a soft clay. In stronger and fragmented
rocks the slurrying does not take place to the same extent, and there is a tendency towards the
enlargement of the drill hole, resulting in better keying of the concrete to the rock. If the pile
borehole is drilled through soft clay this soil may be carried down by the drilling tools to fill the
cavities and smear the sides of the rock socket. This behaviour can be avoided to some extent by
inserting a casing and sealing it into the rock-head before continuing the drilling to form the
rock socket, but the interior of the casing is likely to be heavily smeared with clay which will
be carried down by the drilling tools into the rock socket. Wyllie(4.39)suggests that if bentonite
is used as a drilling fluid the rock socket shaft friction should be reduced to 25% of that of a
clean socket unless tests can be made to verify the actual friction which is developed.
It is evident that the keying of the shaft concrete to the rock and hence the strength of the
concrete to rock bond is dependent on the strength of the rock. Correlations between the
unconfined compression strength of the rock and rock socket bond stress have been estab-
lished by Horvarth(4.50), Rosenberg and Journeaux(4.51), and Williams and Pells(4.52). The
ultimate bond stress, fs, is related to the average unconfined compression strength, ,by
the equation:
(4.44)where
reduction factor relating to as shown in Figure 4.33
correction factor related to the discontinuity spacing in the rock mass as shown in
Figure 4.34.
The curve of Williams and Pells in Figure 4.33 is higher than the other two, but the factor
is unity in all cases for the Horvarth and the Rosenberg and Journeaux curves. It should also
be noted that the factors for all three curves do not allow for smearing of the rock socket
caused by dragdown of clay overburden or degradation of the rock.
The factor is related to the mass factor, j, which is the ratio of the elastic modulus of
the rock mass to that of the intact rock as shown in Figure 4.35. If the mass factor is not
known from loading tests or seismic velocity measurements, it can be obtained approxi-
mately from the relationships with the rock quality designation (RQD) or the discontinuity
spacing quoted by Hobbs(4.53)as follows:
qucfs qucquc206 Resistance of piles to compressive loads
RQD (%) Fracture frequency per metre Mass factorj0 – 25 15 0.2
25 – 50 15 – 8 0.2
50 – 75 8 – 5 0.2–0.5
75 – 90 5 – 1 0.5–0.8
90 – 100 1 0.8– 1As a result of later research Horvath et al.(4.54)derived the following equation for
calculating the socket shaft friction of large diameter piles in mudstones and shales:
Unit shaft friction fs b (^) ucw (4.45)