The CSL guidance has been published following a study of the overall viability of sonic logging in foundation piles, to address some of the safety issues inherent in the placement and jointing of the logging tubes in piles. There have been several injuries in recent years, and anomalous results.

Crosshole sonic logging is a non-destructive testing method for deep foundations, involving access tubes installed inside the rebar cage before placing concrete. It is a good test for pile integrity but has “a highly undesirable downside in terms of safety”, says the FPS.

In a 2017 article it warned: “Ultimately a ‘flexible’ reinforcement cage is incompatible with rigid tubes attached within it that require operatives to use their hands to join them, the FPS warns. Tubes attached to dummy cages inserted after the main cage have been tried but the need to join tubes still remains. The requirement to align multiple cage bars, any of which can catch on each other requires concentration from operatives. They cannot watch say 20 plus bars and 4 tubes at the same time. There is plenty of opportunity for spacer bars and temporary brackets to come loose and fall. The whole process is unforgiving of any momentary lack of attention at any stage.”

The guidance advises: “In accordance with their duty under Construction Design & Management regulations, specifiers must eliminate, substitute or reduce safety risks and must do this when specifying CSL. This can be achieved in the following ways: CSL should be avoided in dry stable rotary piles and small diameter piles where the sonic logging tubes have <300mm clear separation.”

It continues: “Consider alternative integrity testing methods; where CSL must be used, limit its use to the top of the pile within a single cage length to eliminate the need to splice tubes, reduce testing frequency, limit the number of tubes (tube spacing as great as 1.5 metres is acceptable) and where applicable reuse inclinometer or base grouting tubes.”

The full guidance is available only to FPS members.

The industry review was led by Toby Hayward of Expanded Piling with support from Arup, Bachy Soletanche, Franki Foundations, Cementation Skanska and Socotec. The joint industry working group investigated how many genuine defects were identified, compared to innocent anomalies and their causes, the sustainability cost of the methodology, and any correlation possible between sonic logging results and results from alternative methods such as thermal integrity profiling (TIP) testing.

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