Pre-boring is a commonly referenced
method for easing the passage of some driven piles into the ground.
However, its use can also be misunderstood or misguided. It is not a
satisfactory way of overcoming significant obstructions to enable
piles to be driven because that which impedes the driven pile will
also in general impede progress of the pre-boring tool.
Pre-boring in sand and gravel presents
a problem because of the inherent instability of the soil through
which the pre-bore passes. When such soil is dense, pre-bores may
stand open temporarily because of arching and the influence of
temporary pore water suction.
However, as soon as a piling tube or
pile enters the bore and the hammer begins striking, the upper
granular soil collapses into the lower part of the bore. The lower
section of the bore will possibly not collapse in this circumstance
at the initial driving strokes because the soil is relatively more
dense and the hammer influence more remote.
The result is frequently that because
of re-compacted debris in the lower bore, piles will not drive back
to the same depth as originally bored. Only if the bore is
temporarily cased to prevent collapse, and if the casing is of large
enough diameter to allow access for the final pile, can a
satisfactory load bearing unit be inserted, albeit with loss of
potential friction resulting from loss of displacement effects and
the need for in-filling around the pile.
As an alternative to trying to form an
open hole in sand soils, the pre-boring tool is sometimes used simply
to stir up the ground, leaving disturbed soil in position. This may
be sufficient to deal with dense soil near ground level.
However, if deep bores are attempted
after this manner, again when a piling tube or pile is entered and
driving begins, the loosened material is compacted down into the
lower part of the bore and becomes virtually indistinguishable from
the original natural soil. Piles will frequently not drive back to
the depth of the pre-bore or may behave inconsistently under applied
load.
It is therefore not generally
satisfactory to use deep pre-bore methods in sands, for example, for
the purpose of ensuring that piles reach a deeper stratum such as
rock unless special temporary casing methods are adopted.
Pre-boring sockets into rock or very
hard soils for the supposed purpose of enhancing end bearing or
reaching strong soil, where there are overlying fill, sand or clay
layers, is also generally futile. For the same reasons as stated
above, it will be found that without guaranteed bore stability and
measures to prevent soil from collapsing into the socket, a
satisfactory load bearing and consistent unit cannot be formed
because of debris falling before the pile arrives.
Pre-bores are satisfactory only under
specific circumstances:
1 To loosen dense upper crust soils and
enable long piles to be driven without breakage. Long piles struck at
the head are really slender columns and so the possibilities of
buckling failure can be very real.
2 To make an open hole in stiff clays
or similar cohesive soils into which a pile is pre-entered. The
purpose in this instance is to avoid or diminish soil heave. If using
the method for the purpose of eliminating ground heave, it is
generally legitimate to choose the area of the bore so that the pile
cross-sectional area is just slightly larger.
Jobs with pre-boring are frequently
associated with claims and cost overruns, partly because it is
difficult to synchronize the activities of boring and driving
machines with consequent delay, and partly because, where the
motivation is to achieve stringent ‘sets’ this may be a major
source of damage to equipment.
