Adapting a Driven Pile Design to meet the Challenges encountered during Construction

A recent multi-storey commercial development undertaken in Christchurch, New Zealand following the 2010/11 Canterbury Earthquake Sequence presented a range of geotechnical foundation constraints. These included loose, liquefiable silts and sands layered with dense, non-liquefiable gravels; predicted large post-seismic settlements; and significant demand loads from the multi-storey superstructure; all of which had to be addressed via an economic capital cost design solution. A relatively “straightforward” driven concrete pile foundation design was designed for the development, which required adaption during construction due to latent conditions and difficulties encountered. When construction commenced, several piles refused above their target depth. Where this was due to factors such as localised variations in the soil profile, adverse pile performance during driving, or the striking of buried obstructions, automatic acceptance of these piles could not be guaranteed. Careful investigation as to the cause of these issues, and reanalysis of the piled foundation design was necessary to resolve the problems, meet the design requirements and account for the constraints of construction on the site. Various tools were used throughout this process, including high-strain dynamic PDA testing, CPT investigations and engineering judgement. The solutions ranged from acceptance of the refused piles ’as-is‘, to predrilling through surficial fill for the installation of remedial piles. Ongoing critique and assessment of the PDA results, including variation of the testing schedule as construction progressed resulted in value engineering opportunities, including the reduction of the overall number of piles compared to the original design. This ultimately offset the pile remediation costs such that both a successfully constructed foundation and a net cost saving on the original consented design were achieved.