Foundations are vital elements of earth-laid
structures because they support and transmit the entire load of that structure
to the earth. Of all foundation types, deep foundations are often considered
the safest and most appropriate especially where cost factor cannot be allowed
to take prominence. Pile foundations are the commonest of all deep foundation
types and they are very critical because of their depth and size; they are
relatively slender compared to the overall size of the structure and they go as
far below the natural ground level as where there is suitable bearing capacity
–reaching beyond 50-metres for some high-rise buildings. Piles with circular
cross-section are very popular in the industry and the usual width (or
diameter) lies between 300 – 600mm. In
Nigeria, reinforced concrete cast in-situ (bored) piles are common compared to
driven piles because it is considered less expensive. The cost involved in
driving a pre-cast reinforced concrete pile into earth is about a quarter times
higher than for cast in-situ piles and then the technicalities involved.
Because piles are a sort of stand-alone
foundation and are mostly designed to carry very heavy loads, it is often
advisable to test their load bearing capacities before building over them. The
common tests for piles are the Pile Load (PLT) and Pile Integrity Tests (PIT).
These tests are not only relevant for the purpose of structural analysis; they
also provide useful information about the composition of the pile throughout
its length. The construction of pile foundations and testing of same are best
carried out by professional structural engineers. However, due to ignorance or
negligence, some project owners either outsource their piling works to random
borehole drillers who are mostly skilled in water works with little or no idea
on the dynamics of piles or do not carry out appropriate tests on their piles
even after taking such unreasonable risk.
Shit happens and piles fail too. There are
many reasons why this occurs but of all, poor construction and faulty
arrangement of piles are the commonest. If a pile is not constructed properly,
for instance the mix ratio or provision for reinforcement in a reinforced
concrete pile is not in tandem with what was stated in the structural design;
the end result might be a weaker pile. Also, if the mix ratio is adequate but
the concrete was not properly loaded into the borehole, the pile may have sizable
gaps and pores and lines of weaknesses may result from it. Likewise if a pile
is designed as a friction pile but constructed as end-bearing, it might become surplus
to requirement. There are reported cases where deliberate attempts were made by
unscrupulous constructors not to reach the specified depth in a bid to cut corners.
This is not a healthy practice and might not help the pile reach its desired
strength. When constructing bored piles without casement and sufficient care is
not taken to avoid excessive backfilling before loading concrete, the result
might be a shorter or a soiled pile and that is not a good one. There are other
factors like faulty designs, poor supervision, poor mixing of concrete and the
use of sub-standard materials especially with regards to the tensile strength
of steel rebars, dirty aggregates and foul water etc.
For driven piles, if the blow (hit) from the
pile hammer on the pile head generates excessive vibrations and stresses down
the pile, the pile may lose strength. Pile
failures can be devastating to a structure as failure of a single pile can lead
to a partial collapse of a building, hence the need to build it right all the
time
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