Pile Design and Construction Practice, Fifth edition

where is the rock strength defined as above and RF is a roughness factor given as

(4.47)

where

average height of asperities
rs nominal socket radius
Lt the total travel distance along the grooved profile
Ls the nominal socket depth
is further defined as the radial distance from a socket profile to the surface of an imaginary
cylinder which would fit into the grooved socket. A factor of safety of 2 is recommended to
be applied to fscalculated from equations 4.46 and 4.47. There may be practical difficulties
in measuring the depth of the groove achieved by the rotary tool, particularly where direct
visual or underwater television methods of inspection are used in muddy water.
CIRIA Report 570(4.55)recommends that the shaft friction of bored piles in very weak
mudstones can be calculated in the same way as piles in stiff clay using either effective stress
methods (equation 4.6) or undrained shear strengths (equation 4.7). However, the report
points out the difficulty in obtaining satisfactory samples in weak weathered mudstones
with the result that the cuvalues are likely to be low and hence the calculated shaft friction
will be over-conservative. When effective stress methods are used should be taken as zero
to allow for softening and a remoulded value of of 36 should be assumed. Laboratory
tests gave Kovalues of 1.5 to 1.6. Report 570 relates in equation 4.7 and (Kin equation
4.6) to the weathering grades of mudstone shown in Table 4.17.
The allowable bearing pressure on the base of bored piles in weak rocks depends to a
great extent on drilling techniques. The use of percussive drilling tools can result in the



c

r

r

RF

r

rs^ ^

Lt

Ls

ucw

208 Resistance of piles to compressive loads

25

20

15

10

5

0

0 0.2 0.4 0.6 0.8 1.0

Upper limit

Fracture frequency per metre

Lower limit Strong rocks

Mass factor j

Figure 4.35Mass factor value (after Hobbs(4.53)).