The project involves the installation of complex piling, 750mm diameter concrete bored piles, which are embedded at a depth of at least three times the pile diameter into the dense sandy gravel layer known as Riccarton gravel, located approximately 20 metres below ground level (bgl). Moore Construction was brought on board by Armitage Williams early in the project for their input on the construction methodology and to offer in-house engineering expertise. Our role was crucial in determining the best options for constructing these cast-in-situ piles to a final depth of 25 metres bgl.
One of the key challenges in this project was dealing with artesian water pressure encountered at such depths. To mitigate this, Moore Construction adopted a specialised drilling method, utilising polymer fluid during the boring process. The polymer fluid served a dual purpose: it stabilized the borehole and reduced the risk of groundwater ingress, which could have otherwise damaged the structural integrity of the pile. To ensure optimal control of the artesian conditions, the polymer fluid head was maintained above the ground level, providing the necessary counterpressure to manage the artesian water forces.
This methodology was essential in minimising groundwater intrusion into the pile borehole and maintaining its stability. Without this, the uncontrolled inflow of groundwater could exert destabilising forces, potentially leading to the collapse of the pile hole. By employing this advanced technique, Moore Construction ensured the safe and stable formation of deep foundations, using pumped support fluid to maintain the integrity of the piles during construction.
Additionally, the piling platform itself was designed with precision to support these operations. It consisted of a 400mm thick layer of well-compacted pit run hardfill, underlain by A19 Bidim geotextile. This platform was carefully designed, checked, and verified by Moore Construction’s in-house piling engineer, ensuring full compliance with the Federation of Piling Specialists’ Working Platform Spreadsheet and Guidelines. The robust design of the platform provided a stable and secure base for the piling machinery, contributing to the overall success of the piling works.