Watercress Beds, Longparish

The ground investigation identified Made Ground and river terrace gravels overlying chalk at approximately two metres below ground level. Groundwater was recorded at depths between 0.7m and 0.75m below ground level, highlighting the challenges of working within a high water table environment.

The site was classified as DS-1/AC-1 in terms of chemical conditions, with potential chalk dissolution features also identified as a consideration due to the underlying geology.

The shallow groundwater levels meant that a conventional foundation approach would have required excavation below the water table, increasing the complexity of the construction process and potentially affecting the programme.

To overcome the challenging ground conditions, we designed a CFA piled raft foundation solution comprising two reinforced concrete rafts serving the main dwelling and detached garage. The total structural foundation area covered approximately 411m², with raft thicknesses ranging from 225mm to 300mm.

The piled raft was supported on 300mm diameter CFA piles, with typical pile lengths ranging between 8.0m and 11.5m and a maximum safe working load of approximately 340kN. The design incorporated a non-voided piled raft solution. This was achieved as the low plasticity clay bands identified during the ground investigation did not require additional heave protection.

A lightweight piling rig was utilised during construction, removing the requirement for a traditional engineered piling mat and helping to minimise site preparation requirements.

We delivered the complete foundation package, including the piling works, reinforced concrete raft slabs and underslab drainage.

The works were completed within a five-week construction programme.

By constructing the piled raft from a prepared concrete blinding layer, rather than undertaking extensive excavation below the groundwater level, we were able to maintain progress while avoiding the additional complications associated with working in saturated ground conditions.

This approach provided a more efficient and controlled construction process while ensuring the foundation design met the required performance and warranty standards.