Fairmile Lane Cobham Residential Development

An initial desk study classified the site as having high volume change potential (HVCP). Rather than designing around that initial classification, further Atterberg testing was undertaken to establish the actual ground conditions. The additional testing refined the classification to low-to-medium volume change potential, allowing the design to be optimised accordingly.

A 160mm voidformer was therefore adopted for the piled raft, providing the required protection against clay heave without the additional depth that would have been required for a high volume change classification. The ground profile comprised Made Ground over the Bagshot Formation, with the underlying strata consisting of gravelly sand, sand and gravel, and sandy to gravelly clay.

Made Ground extended between approximately 0.15m and 0.5m before reaching the Bagshot Formation, which was proven to 20m.

Groundwater encountered at between 0.8m and 1.0m in several boreholes was assessed as perched water rather than the true groundwater table. A representative groundwater level of approximately 14m below ground level was therefore adopted and did not govern the foundation design.

Each piled raft was supported on 300mm diameter piles, typically between 7m and 13m long, with a maximum safe working load of approximately 320kN. The foundation comprised a uniform 225mm RC35 structural slab over the voided system.

CBR values of greater than 2% across the site allowed SPEEDECK to use a lightweight piling rig, removing the need for an engineered granular piling mat.

The four plots were delivered as a single substructure package, combining piling and raft construction under one scope. SPEEDECK also installed the underslab drainage and services as part of the works.

From the start of piling, just six weeks were allowed to complete and hand back all four plots at structural slab level, ready for the superstructure trades to begin.