The made ground was quite variable for this site so we requested further trial pitting and a tree plan in order to fully understand it’s extent and identify the plots which were at risk of heave . A heave report was then prepared and submitted to the warranty provider for approval.
Concept Design
Heave, which is the upward movement of the ground, was expected because the soil had a medium to high volume change potential. This is based on Atterberg limit testing and the influence of nearby trees. To handle this, 225mm void formers were used for plots 1 to 16 and 23 to 24 due to the high potential for soil expansion. It was determined that plots 17-22 did not require heave precaution as the non-cohesive made ground accounted for the majority of the heave zone.
Due to the extent of made ground present, the site was designated as Characteristic Situation 2 for Ground Gas, this was overcome by the provision of our cast in situ monolithic slab and an overslab gas membrane, which satisfies the criteria set out within BS8485:2019 for residential properties and eliminates the need for underslab ventilation which would be required for Traditonal Beam & Block Flooring Systems.
Design Engineering
The ground conditions at the site were highly variable, with the Made Ground differing significantly in both depth and composition.
Beneath this layer lies the Whitby Mudstone Formation, which consists of stiff to firm clay. The results indicate that these clays have a medium to high volume change potential. This means that the clay may expand or contract significantly with changes in moisture, making it susceptible to heaving, or upward ground movement. A Voided Piled raft was proposed as a solution for the plots underlain by significant clay and within tree influence.
A comprehensive tree survey was provided, and the influence zones of the trees were thoroughly analysed. The analysis revealed that plots 1 to 16 and 23 to 24 fall within these tree-influence zones. Following these results, it was determined that these plots will need heave protection to mitigate the risk of ground movement.
Construction
We showcased exceptional project management and technical expertise by undertaking a challenging residential project and achieving remarkable results.
Demonstrating efficiency and precision, we successfully installed and completed all 24 housing units within an impressive 8-week time frame.
This achievement included not only the construction of the slabs but also the installation of underslab drainage systems and the completion of test piles, ensuring a comprehensive and high-quality build.
