Horwich, Rivington Chase Link Road

The proposed scheme for Bolton Council involved construction of a new, 1km long link road intended to open up to new access to further housing and commercial developments and ease traffic congestion in the area. Menard were contacted in 2021 by the client’s consultant engineer to discuss the scheme and collaborate to devise options to overcome the challenging geotechnical constraints. The project consisted of a new roadway to be built on shallow embankments up to maximum of 3.5m in height with a live loading of up to 15kPa.

The proposed road runs over variable depths of made ground with substantial thickness of peat below extending to maximum depths of around 9m below existing ground level. Due to the challenging soils, construction of a new road with these loadings would cause unacceptable long-term total and differential settlements. Initial solutions from others included either piling options or proposals to dig out and replace the peat which all came with substantial cost and time implications along with a high level of embodied carbon on the project which made the scheme unviable to the client.

Menard proposed a solution to utilise Dynamic Replacement (DR) beneath the road embankments in order to limit total and differential settlements to an acceptable range (100mm total and 1/250 differential). DR pillars (around 1.8m in diameter) were installed through the made ground and peat to depths of up to 6m to create a composite material [soil+Pillar] and therefore increase the bearing capacity of the soil and reduce long term settlements. The presence of the granular pillars also helps to accelerate consolidation during the construction period allowing increased drainage through the cohesive layers. The DR pillars were associated with a 1m thick Load Distribution Mattress, designed by Menard and installed by the principal contractor which would also provide a safe working platform for the heavy equipment to work over the soft soils. In the deeper areas of peat (over 6m from EGL) Pre-Fabricated Vertical Drains were also proposed prior to installation of the DR pillars to ensure sufficient consolidation of the underlaying material at depth, this was to be combined with a 750mm thick surcharge for a period of 4 weeks, to replicate the proposed traffic loading. To prevent additional cost, time and carbon from material being required for surcharging, Menard proposed that the granular backfill for the DR solution could be used as this surcharge thickness before being utilised in the pillars.
Menard were also informed of 2no. large, United Utilities assets which included a high-pressure water main and large diameter sewer which would cross the proposed road. To protect the assets from the loading above and site related vibrations, Menard proposed a ground improvement solution of Controlled Modulus Columns (CMCs) to support strip footings beneath protection slabs. The non-vibration technique would provide a bearing capacity of 200kPa for the footings to be designed as regular strip foundations either side of the sensitive assets.