Staffordshire: M6 J13-15 Smart Motorway

DAWSON-WAM was contracted by Kier to provide all sheet piling works on the M6 J13-15 Smart Motorway Project. The M6 Motorway between Junction’s 13 to 15 had 3 running lanes in both the north and southbound carriageways. The aim of this project was to convert the existing hard shoulder into a 4th running lane with state-of-the-art technology throughout to control traffic flows. 

Sheet piles were to be installed in areas were the existing highway boundary was limited and tradition earthwork solutions were not feasible. 

A contraflow system was utilised  to control the traffic during construction with the work being carried out in a linear fashion on the Northbound carriageway followed by the Southbound. The majority of walls on this scheme were of KSP design saving the project approximately 25% in material purchase. 

Dawson WAM commenced sheet piling works on site in April 2019. 104 nr. sheet pile walls were installed on this scheme in total with an overall linear length of 4600m. All piles were Z profile with an average length of 12.5m. At the schemes peak there were 4no. Sheet Piling Rigs on site. 

The ground conditions consisted of sandstones, mudstones and clays across the length of the scheme, however the sandstone was most prevalent in the northern section closest to J15. 

Due to the ground conditions Dawson WAM chose to pre-auger the ground utilising our 450mm cased CFA system. Once preaugering was complete our ABI TM22 leader rigs were able to utilise their quick docking system and quickly change over to vibro hammer for sheet pile installation. 

DAWSON-WAM helped deliver value engineering solutions to the project which were able to be translated into considerable savings in both cost and time. This was help considerably by Kier Highways engaging DAWSON-WAM early in the process so that value engineering potential could be fully realised.

Value engineering included the following; 

• use of high grade S430 steel sheet piles which provided a more efficient structure, saving material purchase; 
• use of KSP™ system throughout, delivering 25% saving in material and savings in installation
• Use of advance preaugering systems to allow sheet pile installation into ground strata which would normally be difficult or require significant levels of impact driving
• Detailed analysis of sheet pile requirements every 10m and selection of most efficient sheet pile solution to meet the particular requirement.  

Sheet piles were delivered to site in a just in time basis throughout this project. Piles were ordered from supplier as per design and delivered to DAWSON-WAM’s 13 acre Bedfordshire yard where they were stored until required. Having the benefit of a storage facility allowed for a greater control over sheet piles and benefited the collaborative design approach which was taken throughout the scheme in order to save costs and mitigate material wastage. 

If piles were removed from the scheme following purchase and delivery, they were able to be stored until they could be utilised at a later date. On this scheme the client had left over sheet piles from another scheme and these were able to be stored in our yard and utilised throughout the whole project, thus preventing the expensive purchase of more piles. 

Throughout this scheme Dawson WAM encountered a number of challenging issues which we were able to overcome with our broad range of capabilities. 

Cresswell Defect

Whilst work was being carried out on the Northern Carriageway in October 2019 it became apparent to the client that a geotechnical slip was occurring on the southbound carriageway adjacent to a Rail Bridge. Dawson WAM were able to react to this slippage and under night time closures were able to mobilise, install 10m long sheet piles to depth and demobilise again. This work was carried out over a number of nights with a 4-hour working window. Due to the nature of the ground and our powerful 30VV Vibro Hammer we were able to install without a pre-augering requirement. All Plant was removed from the carriageway each night before reopening. 

Once the temporary pile installation was complete Dawson WAM provided technical assistance to the designer in how a permanent solution would work here. At this point the carriageway was also due to be widened, however the highway boundary did not allow for the large piling platform which would be required. Through our in-house manufacturing department Dawson WAM were able to design and manufacture steel plates which fixed to the temporary sheet piles. The rig would then sit upon these plates and reach further off the carriageway to install the permanent wall. 

Due to the ground conditions, retained heights and loads which would be present here a HZM Combi wall system was designed to be installed here. Utilising a gate system and our powerful 36VV Vibro Hammer we were able to install 15m long HZM piles to depth without the need for any impact hammers. All HZM Piles were installed to level first and the shorter intermediate AZ piles installed after. Finishing ahead of programme on this wall allowed us to pass a time saving onto the Client.  

Installation into Sandstone 

A number of sheet pile walls on this scheme required installation of sheet piles into sandstone. Sandstone was present predominately close to Junction 15 and at varying levels. One wall in particularly had sandstone from the surface to depth. In order to create a shelf for sheet pile installation on this wall the client had to utilise a rock breaker to break the sandstone as it was so hard. 

Dawson WAM mobilised to this location with one of our powerful ABI TM22 leader rigs. Utilising our cased CFA system, we were able to accurately predrill each clutch location on the AZ piles, ensuring verticality was highly accurate at all times. Once adequate predrilling had occurred, we were then able to install the sheet piles utilising our powerful 36VV vibro hammer. Piles were installed to depth with no requirement for impact driving at this location. A wall of 60m in length was installed in 7 days, ahead of programme, and in very tough ground conditions. 

Sheet Piling Adjacent to Existing Services 

At a number of locations on this scheme sheet pile retaining walls were to be installed adjacent to some large exiting services. These included medium pressure gas mains and cast-iron trunk mains in excess of 600mm diameter. At an early-stage dialogue was started between ourselves, the client and the service providers to find a clear and safe path forward.

Through this dialogue a safe system of work was employed that satisfied the design and also the service provider in each occasion. In one occasion when working adjacent to a trunk main, which was laid at sandstone level, Dawson WAM utilised a cased 630mm CFA system to pre-auger. This disturbed more ground along the pile line, thus reduced the force required to install the pile. With this additional preaugering Dawson WAM were able to install the sheet piles using the crowd force of the rig and only very small levels of vibration. A benefit of installing loose AZ Pairs is that our leader rig can detach from the pair and drive each pile in singles. This reduces the amount of resistance against the hammer, so smaller levels can be used. 

Overall, this scheme was a great success for Dawson WAM and the client. Through collaborative working, from design to site activities, the works were completed ahead of programme with no complaints from the public. This helped to contribute to the overall civils work on M6 J13-15 SMP finishing over 6 months ahead of programme.