the bit. Drilling was continued into rockhead, and followed by the separate operation of
drilling-in a 560 mm steel tubular dead anchor to a depth of 15 m into the rock to provide
an uplift resistance of 7.4 MN. It is evident from Figure 8.16 that the numbers of heavy
components required to be assembled for drilling out the soil from within and below large
diameter piles are considerably greater than those required for the simple operation of
driving the pile by hammer. Space is limited on the deck of a construction barge, and where
floating vessels are used the heavy equipment may need to be secured to the deck by bolting
or chain tackle. Hence, the successive operations of driving the pile to refusal, removing the
hammer, assembling the drilling gear, then drilling, and removing the equipment can be very
protracted. Therefore, if ‘drill and drive’operations are required the aim should be to restrict
the drilling phase to only one operation.
Insert piles can be used where piles driven to their full design penetration fail to attain a
satisfactory resistance, or where ‘drilling and driving’techniques are unable to achieve the
required penetration.
Where insert piles are used, or where single piles are driven within the tubular guides of
a jacket, the transfer of load from the insert pile to the main pile, and from the main pile to
the leg, is made by welded joints at the pile heads or by grouting the annular space between
the members. Both methods can be used together. The grout is prevented from flowing out
from between the bottom of the jacket leg and the pile by means of inflatable packers or
wiper sleeves built into the bottom of the jacket legs. The design of the grout bond from
the pile to the jacket or between piles is described in Section 6.2.5. The need for terminat-
ing an insert pile at the head of the exterior pile or jacket guide can be avoided, in the case
of large-diameter sections by driving the insert pile with a ‘slim-line’underwater hydraulic
hammer.
Instead of relying on the bond stress between pile and grout, mechanical keying devices
of the type described in Section 6.2.5 can be used. They may be essential to transfer the load
from the legs of deep-water platforms to large-diameter piles where the large thickness of
the annulus is of some significance concerning the development of sufficient bond strength
between grout and steel. The shrinkage of a grout rich in cement can be quite significant
within an annulus that is, say, 75 to 100 mm thick, and it has a weakening effect on the grout
bond. In these conditions the development of an allowable bond stress even in the lower
range recommended by the American Petroleum Institute may be impossible to achieve, and
shear keys on both pile and sleeve are necessary to provide the means of transferring
the load through a grouted annulus. Shear keys on the inner surface of a raking sleeve
may prevent the pile from being lowered through the sleeve but they are unlikely to cause
an obstruction when used in a vertical sleeve and pile.
The alternative to adopting insert piles or ‘drilling and driving’techniques to mobilize
compressive or uplift resistance in stiff to hard clays, is to provide an enlarged base to the
piles. This can be achieved by using a rotary under-reaming tool operating below the toe of
an open-ended steel tubular pile. The enlarged base provides both increased resistance to
compressive loads and a positive anchorage against uplift. The uncertainty concerning the
ability of available hammers to drive straight-sided piles to a deep penetration is avoided.
However, there can be difficult problems when the arms of the expanding cutter fail to retract.
When open-end piles are driven into deep granular soil deposits the driving resistance
may be very low for the reasons described in Section 4.3.3. As a result, calculations of
resistance to axial compression loads based on dynamic testing are correspondingly low,
indicating very deep penetration of the pile to achieve the required resistance. These
Piling for marine structures 421