Pile Design and Construction Practice, Fifth edition

198 Resistance of piles to compressive loads
the rock mass below the pile toe but the amount of movement will not necessarily be large
since the zone of rock influenced by a pile of slender cross-section does not extend very
deeply below toe level. However, the temptation to continue the hard driving of slendersection
piles to ensure full refusal conditions must be avoided. This is because brittle rocks
may be split by the toe of the pile, thus considerably reducing the base resistance. The splitting
may continue as the pile is driven down, thus requiring very deep penetration to regain the
original resistance.
Where bedding planes are steeply inclined with open transverse joints there is little
resistance to the downward sliding of a block of rock beneath the toe and the movement will
continue until the open joints have become closed, or until the rock mass becomes crushed
and locked together. This movement and crushing will take place as the pile is driven down,
as indicated by a progressive tightening-up in driving resistance. Thus there should be no
appreciable additional settlement when the working load is applied. However, there may be
some deterioration in the end-bearing value if the piles are driven in closely spaced groups
at varying toe levels. For this reason it is desirable to undertake re-driving tests whenever
piles are driven to an end bearing into a heavily jointed or steeply dipping rock formation.
If the re-driving tests indicate a deterioration in resistance, then loading tests must be made
to ensure that the settlement under the working load is not excessive. Soil heave may also
lift piles off their end bearing on a hard rock, particularly if there has been little
penetration to anchor the pile into the rock stratum. Observations of the movement of the
heads of piles driven in groups, together with re-driving tests indicate the occurrence of
pile lifting due to soil heave. Methods of eliminating or minimizing the heave are described
in Section 5.7.
Steel tubes driven with open ends, or H-section piles are helpful in achieving the
penetration of layers of weak or broken rock to reach virtual refusal on a hard unweathered
stratum. However, the penetration of such piles causes shattering and disruption of
the weak layers to the extent that the shaft friction may be seriously reduced or virtually
eliminated. This causes a high concentration of load on the relatively small area of
rock beneath the steel cross-section. While the concentration of load may be satisfactory
for a strong intact rock it may be excessive for a strong but closely jointed rock mass. The
concentration of load can be reduced by welding stiffening rings or plates to the pile toe
or, in the case of weak and heavily broken rocks, by adopting winged piles (Figure 2.19).
The H-section pile is particularly economical for structures on land where the shaft is
wholly buried in the soil and thus not susceptible to significant loss of cross-sectional
area due to corrosion. To achieve the maximum potential bearing capacity it is desirable
to drive the H-pile in conjunction with a pile driving analyser (Section 7.3) to determine
its ultimate resistance and hence the design working load, verified if necessary by pile
loading tests.
The methods given below for calculating the ultimate bearing capacity assume that this is
the sum of the shaft and base resistance. Both of these components are based on correlations
between pile loading tests and the results of field tests in rock formations or laboratory tests
on core specimens.
Where the joints are spaced widely, that is at 600 mm or more apart, or where the joints
are tightly closed and remain closed after pile driving, the ultimate base resistance may be
calculated from the equation:
q b � 2N �q uc
(4.39)