where the bearing capacity factor:The variations in caused by joints in the rock mass are demonstrated by the comparisons
in Table 4.13 of observations of the ultimate base resistance of driven and bored piles
terminated in weak mudstones, siltstones and sandstones with the corresponding Nvalues
calculated from equation 4.40. For these rocks the values as recommended by Wyllie(4.39)
are in the range of 27 to 34 giving values from 2.7 to 3.4.
It will be noted that the calculated values in Table 4.13 are considerably lower than
the range of 2.7 to 3.4 established for rocks with widely spaced and tight joints. The reduc-
tion is most probably due to the jointing characteristics of the rock formation in which the
tests were made. A measure of the joint spacing is the rock quality designation (RQD)
determined as described in Section 11.1.4. Kulhawy and Goodman(4.40,4.41)showed that
the ultimate base resistance (qub) can be related to the RQD of the rock mass as shown
in Table 4.14.
Where laboratory tests can be made on undisturbed samples of weak rocks to obtain the
parameters cand , Kulhawy and Goodman(4.40, 4.41)state that the ultimate bearing capacity of
the jointed rock beneath the pile toe can be obtained from the equation:
qub cNcDNq (4.40)BN
2
N
N
N
N tan (^2) 45
2
Resistance of piles to compressive loads 199Table 4.13Observed ultimate base resistance values of piles terminated in weak mudstones,
siltstones, and sandstones
Description of rock Pile type Plate or pile Observed bearing Calculated
diameter (mm) pressure at failure
(MN/m^2 )
Mudstone/siltstone Bored 900 5.6 0.25
moderately weak
Mudstone, highly to Plate test 457 9.2 1.25
moderately weathered
weak
Cretaceous mudstone Bored 670 6.8 3.0
weak, weathered,
clayey
Weak carbonate Driven 762 5.11 1.5
siltstone/sandstone
(coral detrital
limestone)
Calcareous sandstone Driven tube 200 3.0 1.2
weak
Sandstone, weak to Driven 275 19 a 1.75
moderately weak
Note
a From dynamic pile test.
N