Hereward Hall, March Piled Raft Foundations for 19 Homes

Made Ground and topsoil extend to around 0.6m. Below that is the March Gravel Member (sandy clay, clayey sandy gravel and sand) down to between 1.0m and 2.2m. Under that is Oadby Member Till, a sandy, silty, gravelly clay proven to more than 20m. Groundwater was shallow, at 1.0–2.3m below ground level.

Our volume change assessment found that no plot needed heave precautions. The March Gravel is granular and non-plastic, with less than 35% fines, so it isn’t shrinkable. The Oadby Till lies mostly below the water table and showed no signs of desiccation.

With no heave risk to design for, a voidformer would have added cost and about a week to the programme without any engineering benefit. A non-voided piled raft was the right fit for this ground.

Each raft is a 225mm RC35 reinforced concrete slab on 300mm diameter piles founded in the Oadby Till. The maximum safe working load is around 250kN. Fourteen of the homes are paired on seven shared rafts, and each of those rafts uses the same 14-pile layout, so the design repeated cleanly from plot to plot.

A working platform assessment confirmed that our standard 50mm concrete working surface met BRE 470 Case 2 loading, as long as the formation achieved the required CBR value. That meant the working surface could also act as the piling platform, so no separate granular piling mat was needed.

We delivered piling, underslab drainage and services, and the raft slabs as one firm-priced package. A lightweight piling rig installed all 144 piles, and every pile was integrity tested. Apart from piling concrete, all deliveries were kept within the council building’s delivery hours.

All 19 plots went back to Snowdon Homes at structural slab level within the eight-week programme.