The Federation of Piling Specialists will carry out a review into the effectiveness and safety of cross-hole sonic logging as a testing method for foundation piles.
Cross-hole sonic logging is a method used for pile integrity testing. It involves the placement and jointing of logging tubes in piles, which has resulted in several injuries in recent year, the Federation of Piling Specialists (FPS) explained.
The FPS technical committee has reviewed sonic logging before; however, it is now focussing on assessing its continued efficacy as a test method now that there are “safer” alternative methods in common use across the industry.
The technical committee’s review will be led by Roger Bullivant chief geotechnical engineer Jon Ball. The FPS health and safety and operations committees will also assist in the investigations.
FPS chair John Chick said: “It is incumbent upon the FPS to investigate any methodology that presents a safety issue, and sonic logging has directly caused a number of injuries.
“Sonic logging must be scrutinised to understand if the benefits it offers are worth the risk given the number of alternative safer and more sustainable testing methods now commonly available.”
He added: “Over the next few months, the FPS’s technical committee will fully investigate the technique and evaluate whether it is still a viable integrity testing method to be specified, with a view to publishing the results and any subsequent guidance by the end of 2022.”
What are the risks associated with sonic logging?
Sonic logging requires the installation of four or more full length steel tubes in the pile and these tubes are usually fixed to the reinforcement cage. Many bearing pile schemes involve long, bored piles dictating cages of two or three sections or more. The assembly of these cages requires the bottom section to be suspended in the pile or in a “rat hole”. The next section is then lifted into place and spliced. The new whole is then lowered into the bore, suspended and the process repeated for the next section of cage.
Piling contractors and reinforcement suppliers have made great strides in developing innovative, safe and simple to use splicing systems. These have in many situations eliminated the need for hands and arms to be inserted into the cage to attach bolted clips and fixing brackets.
Sonic logging tubes, however, still present a major hazard.
The tubes sit within the reinforcement cage, rigidly attached to the inside of the bars or the cage helical or bands. This is essential as logging is sensitive to accuracy of positioning and divergence or convergence of adjacent tubes. The telescopic tubes are held up above the splice with ties until released, when they are free to slide down a given distance in order to enable jointing of the tubes. The tubes are usually a screw fit to be watertight and to bar the entry of grout and concrete. Some tubes are still sealed by wrapping with adhesive tape which further increases the risk prolonging the time operatives have to have their hands inside the cage.
At any time after release, a momentarily jammed tube can free itself and fall. A 60kg tube falling say 1m under gravity can cause serious injuries. A team concentrating on one jammed tube amongst 20 plus spliced cage bars, can take their eyes off the alignment of another tube. Fixing brackets for the tube can spring off the more flexible cage and fall from height. There are far too many cases of tubes subjected to unexpected loads during handling, or transportation, catching on other parts of the cage or on the ground. These then can fall out of the cage during the lift. Some records describe tubes falling from the upper section all the way down the pile bore.
Piling contractors and reinforcement fabricators have worked hard to mitigate the risks; stirrup bars and impact blocks are welded into cages to act as stops for sliding tubes, different ways of screwing the tubes together have been tested. However, the fundamental risk has not been able to be removed.
Over the years operatives have lost fingers and thumbs in the course of this operation. People have suffered hand injuries when using stilsons to tighten tubes whilst keeping their hands out of the cage. In one case the top cage section slipped unexpectedly and knocked the stilsons downwards at speed, pulling the users hands into the cage.
Read John Chick’s full article on sonic logging.
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