Built in 1953, Woodhead Tunnel number three was originally used for rail traffic under the Pennines. It gradually fell out of use, being retired completely in 1981. Today, though the trains have stopped running, it continues to be used by the National Grid to deliver power to Manchester.
Making the old tunnel suitable for modern day electricity supply though required a great deal of work and expertise. It is for this reason that Balfour Beatty Civil Engineering Limited appointed Couldwell Concrete Flooring for installation of the concrete floor
At 5km in length and 7.4m in width, installation of the cable supports required a solid concrete platform to work from. The floor required, had to be poured in two strips on either side of the tunnel, falling off to concrete drainage channels from a 500mm by 300mm pre-existing centreline ducting protection
After much discussion, we decided the best approach would be a simple one, using a permanent formwork comprising modified Beta Strip 200. Critically, this met with approval for close operation with high voltage cabling.
This approach worked as it allowed the installation of the 230mm deep unreinforced C35/40 concrete slab to move forward quickly, without the need for stripping and cleaning. With the presence of the draining channels too, the formwork also ensured no damage was caused to these.
We worked with the suppliers of the Beta Strip 200, Permaban, to modify the material to meet the exacting requirements. Upper and lower welded stiffening strips were required, requiring multiple holes for the AlphaFoot studding to pass through. These in turn needed to be levelled and located with flange nuts above and below the upper stiffener.
The modified strip was secured through shot fired fixings. Following on site and off site discussions with Permaban and Avantafix, this was deemed the best approach as it offered flexibility where the holes would need to be placed. With the concrete understructure already in place not designed for this task, any irregularities in its laying could also be circumnavigated.
Our teams were confronted with a very confined space. Requiring the stringent control of traffic and materials too, health and safety on site was also paramount. In order to deliver the project to time, our teams spent several days setting up the formwork, before the in-situ concrete flooring was laid.
When installation of the floor commenced, it had to be done with close cooperation of CEMEX, who supplied the concrete. Due to the site limitations, the supply had to be carefully controlled, to ensure S3 consistency as the trucks were filled, journeyed, and delivered the superplasticised material.
In order to achieve the brush-texture finish required, Couldwell Concrete Flooring used laser screed flooring. A perfectly level and even side was completed first, allowing for access of vehicles and workers, whilst the other side of the 35,000m square floor was joined using Miothenefoam, creating a longitudinal debonded expansion joint along the centreline.
At the heart of the Woodhead Tunnel project’s success was teamwork between all parties. Working closely together allowed a complex and challenging engineering venture to be completed, ensuring Manchester homes and business can be powered long into the future.
