Types of pile 65permanently in the ground with or without an in-filling of concrete. Soil is removed from within
the tube as it is rotated down, by various methods including grabbing, augering and reverse
circulation, as described in Section 3.3.5. The tube can be continuously rotated by a hydraulically
powered rotary table or be given a semi-rotary motion by means of a casing oscillator.
The drilled-in tubular pile is a useful method for penetrating ground containing boulders
or other obstructions, heavy chisels being used to aid drilling. It is also used for founding in
hard formations, where a ‘rock socket’capable of resisting uplift and lateral forces can be
obtained by drilling and grouting the tubes into the rock, under-reaming as necessary. In this
respect the drilled-in tubular pile is a good type for forming berthing structures for large
ships. These structures have to withstand high lateral and uplift loads for which a thick-
walled tube is advantageous. In rock formations the resistance to these loads is provided by
injecting a cement grout to fill the annulus between the outside of the tube and the rock
forming the socket. Code of practice requirements for these and other forms of drilled-in
tubular piles are as given in Section 2.3.4 for cast-in-place piles.
Where tubular steel piles have to be driven and sealed into a pre-formed hole ready to drill
a rock socket, care must be taken not to over-drive the pile. ‘Curtain folds’and ovality can
occur, potentially compromising the load-bearing capacity and are difficult to rectify to
produce an acceptable pile. It is preferable to use an under-reamer or hole opener to match
the outside diameter of the pile before finally driving to seal the tube.
In the USA steel H-sections are lowered inside the drilled-in tubes and concrete is placed
within the tubes to develop full end bearing on the pile and to ensure full interaction between
tube, H-section ‘core’and concrete. Because of the area of steel provided by the combined
steel and concrete sections, very high loads can be carried by these ‘caisson’piles where
they are end bearing on a hard rock formation.
2.5 Composite piles
Various combinations of materials in driven piles or combinations of bored piles with driven
piles can be used to overcome problems resulting from particular site or ground conditions.
The problem of the decay of timber piles above groundwater level has been mentioned in
Section 2.2.1. This can be overcome by driving a composite pile consisting of a precast concrete
upper section in the zone above the lowest predicted ground-water level, which is joined to a
lower timber section by a sleeved joint of the type shown in Figure 2.3. The same method can
be used to form piles of greater length than can be obtained using locally available timbers.
Alternatively, a cased borehole may be drilled to below water level, a timber pile pitched
in the casing and driven to the required depth, and the borehole then filled with concrete.
Another variation of the precast concrete–timber composite pile consists of driving a hollow
cylindrical precast pile to below water level, followed by cleaning out the soil and driving a
timber pile down the interior.
In marine structures a composite pile can be driven that consists of a precast concrete upper
section in the zone subject to the corrosive influence of sea-water and a steel H-pile below the
soil line. The H-section can be driven deeply to develop the required uplift resistance from shaft
friction. EC4 requirements apply to the design of composite steel and concrete structures.
Generally, composite piles are not economical compared with those of uniform section,
except as a means of increasing the use of timber piles in countries where this material is
readily available. The joints between the different elements must be rigidly constructed to
withstand bending and tensile stresses, and these joints add substantially to the cost of the pile.