Rafted Road Solution, Blackwood
George Wimpey Housing Development, Blackwood, South Lanarkshire
Client: George Wimpey East of Scotland Ltd.
Contract Management: CON-FORM Contracting Ltd.
Engineers: T Lawrie & Partners
CON-FORM was appointed to carry out an enabling works package to the site, including the installation of the estate roads.
Site investigations indicated a 4 m layer of silt underlying a layer of up to 1m of peat.
The original proposal was to remove the peat and excavate a 10m wide strip down to the bottom of the silt, with the excavated silt re-engineered and replaced using CON-FORM's E-Found® methodology. The re-engineered material was to be overlaid with a standard road construction. However, due to the poor strength of the underlying materials it was impossible to traffic the works. Further investigation identified the silt material to be in excess of 6m in depth, making it impractical to treat.
This necessitated a fundamental redesign. A rafted road approach was used to minimise any stress increase on the underlying silt to limit future settlement.
The silt was excavated to a 1m depth below the invert of the drainage, to provide protection from disturbance. A combined geo-textile/geo-grid layer was rolled out on the excavated base as structural reinforcement and clean stone placed on the grid. This was overlaid with a geo-textile to prevent silt/PFA ingress into the stone. 1m of PFA was then pushed out as a lightweight fill. The excavated soil was then modified ex-situ with a cement/PFA blend to make it suitable for re-use. It was then replaced and compacted in 300mm layers to achieve a minimum CBR of 5%. This process was repeated until the road formation level was achieved, with the final layer cement stabilised.
A layer of stone was pushed out to protect the surface and to provide a good working platform for the surface water drainage installation, prior to the final road construction.
Key Benefits of the CON-FORM Solution
- Provided a viable solution to extremely difficult geotechnical site conditions.
- Eliminated more complex/costly and potentially infeasible solutions, e.g. supporting the carriageway on a piled raft or attempting to remove all the silt and replace with crushed rock.
- Allowed re-use of site won materials.
- CON-FORM solution cost and time effective with savings estimated to be in the region of 30%.