Civil Engineering Reference
In-Depth Information
the construction stage or subsequently, perhaps entailing a change of method during
the work, a requirement for a different type of plant, and in certain conditions even
remedial work. Some examples of items that can give rise to difficulties are outlined
in the following paragraphs.
The presence of groundwater and its possible fluctuations can be significant in certain
construction procedures in relation to most bored piles and care should be taken to
obtain as much relevant information as possible. It is particularly relevant to pile head
casting levels, lengths of temporary casing necessary to maintain bore stability, the use
of stabilizing fluids such as bentonite or polymer suspensions, concrete quality and the
method of placing concrete.
Hard strata, such as dense granular soils and rocks, must be identified and their
properties quantified. If for example a driven pile is chosen in circumstances where
considerable penetration of dense upper soils is required, then time and energy will
be consumed in achieving this and piles may be damaged during the driving process.
It is not in general sensible to seek to achieve significant penetrations into hard rock-
bearing strata by any straightforward driving process and the specification of very
small sets and prolonged hard driving is often counterproductive and very expensive.
In so far as bored piles are concerned, the costs of hard rock penetration are also high,
and lack of adequate information may add considerably to the expense of the work
because larger penetrations are called for than are necessary.
Boulder clays pose a particularly intractable problem because conventional site
investigation cannot easily lead to identification of either true boulder dimensions or
frequency, nor are Standard Penetration Tests capable of defining the true nature of the
soil. It has sometimes been recommended that, where practicable in such ground, large-
diameter bore holes and direct inspection should be used rather than the small-diameter
standard investigation holes. Boulders can severely hinder many pile installation proce-
dures and can exert a very important influence on the cost of piling. Piles, particularly
preformed types, can be damaged, and for all types of pile, delays in construction can
occur. In the presence of large quantities of boulders it is worth considering how the
boring length in the difficult strata may be minimized.
It is not without reason that it has sometimes been said that most claims for extra
payment in piling result either from battles with adverse groundwater conditions or
in seeking to penetrate rock or other hard strata.
10.2.2 Other ground-related difficulties
Other ground-related difficulties that can present a problem include down drag on piles
which may be due to consolidation of upper soil layers after pile installation, or heave
which may result from swelling of clays around piles when trees have been removed
from a site prior to construction. A similar condition may arise because of a deep
excavation following pile installation and both these effects are often underestimated
in terms of the forces that are caused in the piles. It is frequently also not realized that
if attempts are made to improve pile performance by base grouting, and subsequently
there is substantial removal of overburden around the pile heads, the effects of the
base grouting can be lost.
The forces resulting on piles in either conditions of down drag or heave may be
reduced by providing a slip layer around the pile over the affected length, or the pile
