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Warrington: M6 J21a to 26 Smart Motorway



National Highways under Smart Motorway Alliance


DAWSON-WAM (DW) were contracted by Costain PLC to provide all the king post wall and associated works package on the M6 J21a-26 Digital Road under National Highway’s Smart Motorway Alliance. The M6 forms part of the London to north-west and Scotland strategic corridor, and the wider trans-European route, E05. The M6 between junctions 21a and 26 is a key strategic route carrying around 120,000 vehicles daily between Lancashire, Merseyside, Cheshire and Greater Manchester. The M6 junction 21a to 26 smart motorway scheme will relieve congestion and smooth the flow of traffic, improving safety and journey times.

DAWSON-WAM were asked to get involved by Costain during an Early Contractor Involvement (ECI) phase of the works some 3 years before planned commencement. ECI centred around workshops which allowed specialist contractors to advise on the design and delivery aspects of the works. One of the key aspects of the ECI was the validation of the king post wall solution and that sheet piled walls were not viable due to presence of strong sandstone from a high level. ECI was also used to inform the design, make value savings like the use of sacrificial steel over coating systems, and to make a critical contribution to the specification such as tolerances, minimum bearing areas of panels to post and quick strength gain concrete to allow posts to be loaded a short time after posts were installed.

One of the key challenges of the job, and the reason sheet piling wasn’t the chosen solution, was the presence of medium strong sandstone from a high level in almost every king post wall location. To overcome the sandstone DAWSON-WAM used two of their mighty ABI TM22 piling rigs with Cased CFA techniques to efficiently bore the sandstone which was up to 48Mpa in strength. An added benefit to the project was that these rigs come with rapid docking system which allows quick rigging up and derigging between locations. The rigs also run on 100% HVO and use efficiency drives to cut fuel use by 40% over equivalent systems.

On this project the king posts walls were installed to extremely tight tolerances, to achieve minimum bearing areas for the precast panels, and to provide straight and true finished walls. To achieve this DAWSON-WAM devised an innovate quick guide which allowed the accurate suspension of the 3.5t 305 x 305 x 283 UCs in the wet 50N concrete, controlling both position and level with simple and efficient locking frame. Concrete was then pumped into the bores with the concrete accurately set to the bottom of panel height so that minimal breakout was required to permit the installation of concrete panels.

The package of works included the installation of precast panels, seated on rubber strips against the steel king posts. Panels were made bespoke for the project and included tongue and groove connections and weepholes in the bottom panels. Backfilling of panels then took place with a 300m thick drainage layer placed behind the panels and finally topsoiling and wrap arounds completed where the walls started and finished. All materials we brought to site in a just in time basis to avoid congestion on a very busy narrow working corridor.

One of the key challenges of the job was the construction of ‘Wall C’ located on the Junction 23 Southbound off-slip. Due to its position and the limited working area works had to be completed at night with the rig mobilising and demobilising from the work area each night in a 5hr working window. As an added complication, the piles had to be installed within a sloping embankment, with the top of the posts terminating not far above existing ground level. To provide temporary support and an accurate method for locating posts within the bore, our engineers devised a methodology where temporary steel casings were vibro driven into the embankment to the panel toe level. This allowed us to maintain embankment stability, accurate position of the post and accurate concrete termination level to be achieved. Works were completed on 2 weeks of night work.

Due to the nature of the works, space was always at a premium and works were made more difficult due to the presence of all manner of existing structure such as gantry bases or bridges. In one case at Junction 22 the works to be installed very close to a live off slip road. This required the main contractor to excavate a very steep rock cutting in advance of our works and meant that piles were to be installed into an uneven rock step. This would have the result of pushing the Cased CFA off position due to the uneven surface of hard material. This only came to light as the works were being excavated and only 2 weeks before piling start. To resolve this DAWSON-WAM installed a temporary line of Legato blocks, filling the gap between the uneven rock face and block with a 25N concrete. This allowed a flat surface and relatively homogenous layer to allow piling to commence from, also providing solid support for the temporary work to support UCs.

This project was the first piled retaining wall package precured under the Smart Motorway Alliance (SMA). The DAWSON-WAM team were praised by the SMA team due to our collaborative approach from ECI to hand over, and to our alignment of our own focus to that of the SMA’s three pillars of Safety. Delivery and Customer. The package was a critical path requirement which added to the pressure of the work however our team rose the challenge, delivering to programme (and ahead in some elements), to the agreed budget and crucially, delivered the works to a very high standard, gaining praise from our client for all aspects of our approach to delivering the works.

Key Numbers

•Over 10nr king post retaining wall solutions covering 300 linear metres

•All associated backfilling works

•All temporary working platform construction and removal

•Over 140nr king posts installed

•Drilling through sandstone of strength up to 48MPa

•Rock sockets of up to 10m in length

•400t of steel in the posts

•350t of precast panels

•2nr ABI TM22 piling rigs