Located on a floodplain, comprised of soft alluvial deposit overlying mudstone, with high sulfate levels, this site came with multiple challenges.
The original scheme proposed driven steel tubes supported by ground beams. This was specified due to the interpreted sulfate levels, which was believed to be high enough to erode concrete piles over time. However, this scheme was not cost-effective and reduced project viability.
Our first step was to review the SI report. Our engineers reviewed the data and deemed the sulfate levels to be lower than originally thought. Based on this insight, we proposed our non-voided system, using CFA piles. By opting for this type of piling we limited the amount of oxidation as CFA piles are formed in minutes. Rather than open bore piling or trench foundations, which greatly disturb the ground and allow oxidation to occur over days. This is an important design feature as the more the ground is oxidised the greater the activation of the sulfates, which increases the degradation of the concrete. We also used sulfate resistant concrete to counter the sulfate’s erosive effects.
Switching from steel tubes to CFA piles resulted in large savings for our client.
To mitigate the flood risk the site levels need to be raised. Due to the soft alluvial deposits, overlying mudstone ground conditions, an engineered solution was required. To raise the site levels, while facilitating the requirements of the piling rigs, a significant amount of crush material would have needed to be imported. However, due to the design of our Housedeck system, we could use our low track pressure midi piling rigs. This allowed the fill material to be changed to an engineered sub-soil. Although using engineered sub-soil would usually require a top layer of crushed fill to prevent surface degradation caused by the piling rigs, we were able to use our Concrete Working Surface (CWS) instead. As the compliant bid opted for using a 500mm piling mat, our CWS added to the material cost savings.
The final challenge of this site was the presence of trees. This was tackled on the original scheme by allowing for heave precautions in these areas. As part of our value engineering process, our engineers noted that the soft alluvial chalk mitigated any risk of heave posed by the tree roots. We were able to liaise with the NHBC on behalf of our client and gain approval to mitigate any heave precautions and use our non-voided Housedeck system.
“Abbey Pynford’s simple but technically insightful scheme helped us avoid an over-engineered design, saved us money and increased the project’s viability”
David Beech, Senior Infrastructure Manager, Taylor Wimpey
“Abbey Pynford`s innovative approach helped us deliver this project for the client.”
Sam James, Technical Director, Agetur
“Abbey Pyford’s technical assistance on this project was very valuable and helped keep the project running smoothly.”
Holly Spense, Technical Coordinator, Taylor Wimpey
