Granite rocks are widely distributed in the territories of Hong Kong, where the fresh
rock is blanketed by varying thicknesses of weathered rock in the form of a porous
mass of quartz particles in a clayey matrix of decomposed feldspar and biotite(4.44).
The Geotechnical Office (GEO) of the Hong Kong Government(4.45)recommends that
the end-bearing resistance of piles should be expressed in terms of a saferather than an
ultimate value. They recommend that piles should be driven to refusal in a fresh to
moderately decomposed or partially weathered granite having a rock content greater than
50%. For these conditions the allowable load on the pile is governed by the permissible
stresses on the material forming the pile. This recommendation assumes that the rock joints
are widely spaced and closed. In the case of open or clay-filled joints, the yielding of the
pile at the toe should be calculated using the drained elastic modulus of the rock. In Hong
Kong the modulus is related to the standard penetration test N-value. The GEO publication
gives an Evvalue of 3.5 to 5.5N(MN/m^2 ). It is pointed out that Nmay be increased by
compaction during pile-driving.
The shaft friction developed on piles driven into weak weathered rocks cannot
always be calculated from the results of laboratory tests on rock cores. It depends on
such factors as the formation of an enlarged hole around the pile, the slurrying and
degradation of rocks, the reduction in friction due to shattering of the rock by driving
adjacent piles, and the presence of groundwater. Some observed values are shown in
Table 4.16. In the case of brittle coarse-grained rocks such as sandstones, igneous
rocks and some limestones, it can be assumed that pile driving shatters the rock around
the pile shaft to the texture of a loose to medium-dense sand. The ultimate shaft friction
can then be calculated from the second term in equation 4.16 using the appropriate
values of Ksand. Where rocks such as mudstones and siltstones weather to a clayey
consistency making it possible to obtain undisturbed samples from boreholes, the weath-
ered rock can be treated as a clay and the shaft friction calculated from the methods
described in Section 4.2.1.
The effects of degradation of weakly cemented carbonate soils caused by pile driving
have been discussed in Section 4.3.3. Similar effects occur in carbonate rocks such as
detrital coral limestones, resulting in very deep penetration of piles without any significant
increase in driving resistance. An example of the low driving resistance provided by weak
coral limestone to the penetration of closed-end tubular steel piles at a coastal site in Saudi
Arabia is shown in Figure 4.31.
Beake and Sutcliffe(4.46)observed ultimate unit shaft resistances of 170 and 300 kN/m^2
from tension tests on 1067 and 914 mm ODtubular steel piles driven with open ends
into weak carbonate siltstones and sandstones in the Arabian Gulf. The mean compression
strengths of the rocks were 3.2 and 4.7 mN/m^2. The two test piles were 4.2 and 4.55 m
into the rocks. The above shaft resistances were 0.04 to 0.10 of the mean unconfined
compression strength of the rock.
Although a relationship was established between the base resistance and SPT N-values
of piles driven into chalk as noted above, no meaningful relationship could be found
with shaft resistance. The CIRIA recommendations(4.43)in Table 4.15 are the best
possible estimates derived from pile loading tests. The CIRIA report recommends that
whenever possible a preliminary trial pile should be tested to verify the design. It
should be noted that dissipation of excess pore pressure caused by pile driving can
increase the shaft resistance of piles in chalk. Therefore, as long a delay as possible
202 Resistance of piles to compressive loads