Civil Engineering Reference
In-Depth Information
may be reinforced and lengthened if necessary to cater for the forces as they occur.
In such circumstances, it is usually better to choose the simple solution of making
piles longer and stronger than to experiment with more fragile slip-coating solutions.
Where the heave is related to deep basement excavation only, these measures may not
be necessary, depending on analysis of the amount of movement expected, since some
small degree of cracking in a finished pile may be acceptable.
The general stability of excavations or sloping ground surrounding a piling site can
lead to slip failures and movements which produce large forces on the piles. These
forces are sometimes difficult to quantify, but must be taken into consideration when
preparing a pile design. Piles have little ability to resist the forces that result frommajor
slip failures in the soil around them, since these forces are often very large.
Large diameter piles may have their bases enlarged in stiff stable water-free ground
by underreaming, but if there are severe water inflows or bands of cohesionless material
at the level chosen for the pile bases, then it may be impossible to ensure completely
stable base excavations or a clean bearing surface. The conditions where instability is
likely are often indicated by silt or sand seams in a clay or by significant reductions in
moisture content as compared with those of the general clay mass.
In the re-use of sites that have previously been occupied by chemical works or gas
works, it is frequently found that the ground contains residual chemicals which could
adversely affect the durability of one or several types of pile. Protective coatings may be
required in the case of preformed piles, or in the case of cast-in-place piles a protective
lining or sleeve may be needed.
It is clear from these few but important examples of difficulties which result from
particular soil conditions that the means of avoiding or minimizing undesirable effects
can differ between one pile type and another and that in certain cases particular types
of pile maybe ruled out as unsuitable. In practice there are many ground-condition
influences that bear on the economy and proper choice of piles for any job and one
ignores or underestimates the importance of carrying out a proper and appropri-
ate site investigation at one's peril. In general terms, the larger the job in question,
the more dramatic and expensive the end consequences of insufficient information
become.
10.2.3 Pre-boring for driven piles
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
