Piling completed ahead of Glasgow TBM launch 02 Dec 2015

The Costain/Vinci Joint Venture team reaches a key milestone in its £85 million construction of the Shieldhall wastewater tunnel in Glasgow, Scotland, for Scottish Water.

Geotechnical specialist Bachy Soletanche has completed the task of installing the project’s 675 piles in just three-and-a-half months. The piles play a vital part in the preparation of the ground ahead of the next stage of the project: the start of the TBM drive.

Advance piling works for Shieldhall tunnel

Work completed out by Bachy Soletanche includes the construction of cased secant piles (CSP) for the first shaft of the tunnel, the service chamber and TBM launch chamber, along with 400m of continuous flight auger (CFA) contiguous piled walls for the cut-and-cover section of the tunnel. The reinforcement cages placed in the piles each weighed approximately a tonne each, were more than 15m long, and were filled with 9m3 of concrete.

As well as being technically challenging, the work had to be carried out within a residential area and in a restricted workspace. The two 25m-tall piling rigs – the 100 tonne CSP and 70 tonne CFA – two huge service crawler cranes and concrete pumps had to work in an area equivalent to the size of just three full sized soccer pitches.

The Shieldhall Tunnel project marks Bachy Soletanche’s first CSP piling project in Scotland since strengthening its team in the region earlier this year with the appointment of Paul Doyle as Contracts Manager for Scotland.

Fig 1. 5.4km Shieldhall tunnel alignment (red)

Brian Walker, Costain Senior Project Manager for the Shieldhall tunnel, said: “The location and size of our site is very challenging but with some careful planning and collaboration with Bachy Soletanche, a major part of this development is now complete and, very importantly, without any issues or incidents.

“It was great to see that final pile being completed and now we are set to begin the next part of the project, when we connect the 150m-long TBM and begin tunnelling. We’d like to thank the Bachy team for all their assistance and look forward to working with them again on future projects.” One such collaboration project will be construction of the 5.53km long East Tunnel section of the Thames Tideway Tunnel, which was awarded in August to the Costain/Vinci/Bachy Soletanche JV for a contract price of £605 million.

The Shieldhall tunnel will be Scotland’s largest wastewater tunnel, measuring 4.7m in diameter and 5.4km in length, and will form part of the largest upgrade of Glasgow’s wastewater network in more than a century. In addition to the TBM launch structure and reception shaft, the project features three further permanent shafts of 15m diameter. The alignment runs between Craigton Industrial Estate and Queen’s Park, and will resolve water quality and reduce flooding issues at key locations in the area served by the Shieldhall Waste Water Treatment Works.

Work on the project began in October 2014 and is scheduled for completion in early 2018, with the procured Herrenknecht slurry TBM expected to arrive on site in January next year (2016). The TBM will launch from a shaft in Queens Park and work northwards under Pollock County Park and the M77 motorway, and under three railway tracks, with the alignment finishing near the M8 (Fig 1).

Ground conditions are expected to comprise soft and highly compressible alluviums with layers of running sand, a variable bedrock depth, a complex interconnected hydrogeological system, all of which is further complicated by historic mining operations in the area. Such conditions have led to early development of a geological model, taking information from a considerable wealth of archive information, to better identify potentially difficult sections.