Pile Foundation: What Can Make A Pile Fail - Engr. Osaz’ ENOBAKHARE

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