Scottish Water’s £100m ($131 million USD) Shieldhall Tunnel project reached a major milestone when a state-of-the-art tunnel boring machine broke through into the reception shaft, completing the initial construction phase. The Shieldhall Tunnel will be the biggest waste water tunnel in Scotland and will enable Scottish Water to improve water quality in the River Clyde and reduce the risk of flooding.
The tunnel boring machine, nicknamed Daisy the Driller, spent 15 months creating the 3.1-mile-long tunnel beneath southern Glasgow.
The Shieldhall Tunnel is one of the most significant infrastructure projects in Glasgow since the Victorian era and will provide 90,000 cubic meters (23 million gal) of extra stormwater storage, the equivalent of 36 Olympic-size swimming pools. The increased capacity of the wastewater network will improve screening of overflows into rivers and reduce the risk of flooding in parts of the Mount Florida/Toryglen and Giffnock areas.
Appointed by Scottish Water in 2012 to advise on various tunnel options, CH2M prepared the reference design, carried out site investigations and assisted with tender preparation and evaluations. In 2014 the tender was issued, with contact award in 2015. Throughout the process, CH2M provided technical assistance and a site team to supervise the tunneling works construction. The tunnel is due to be completed in May 2018.
“The Shieldhall Tunnel is an extraordinary feat of modern engineering, which builds on the endeavors of those pioneers who sought to improve Glasgow’s wastewater network more than a century ago,” said Roseanna Cunningham, the environment secretary. “Providing an excellent, effective wastewater network which serves our growing communities is vital to the city’s infrastructure and environment, now and for future generations.”
“With Daisy the Driller having reached her destination, we have completed the most challenging part of the project,” said Douglas Millican, Scottish Water’s chief executive. “We now move to linking the tunnel to the existing Glasgow wastewater network and bringing the whole new system into operation next year.”
Measuring 140 meters long and weighing 1,000 tonnes, Daisy the Driller tunneled through some of the most difficult and challenging ground conditions in the United Kingdom, including mine workings. While tunneling, Daisy installed more than 3,200 concrete rings (6 segments each weighing over 2 tonnes) to form the tunnel, advancing through the ground at 2 cm per minute. More than 500,000 tonnes of earth, stone, clay and other aggregates were excavated, 90 percent of which are being recycled.
The challenging geology ranged from soft clay, glacial till to hard sandstone and mudstone. The cover to the crown of Daisy the Driller was less than 1 diameter in places, making control of face pressure critical. The tunneling team successfully overcame obstacles, particularly where the alignment crossed beneath Paisley Canal railway line and cover was reduced to less than 1 diameter with soft clay.
The ground beneath Glasgow is riddled with old coal mine workings, some of which are unrecorded. Before any tunneling could start, the team had to grout infill in four areas along the alignment, a challenging undertaking. Grouting not only allowed Daisy’s safe passage through old mining areas, but also ensured the tunnel’s long-term stability. The team used approximately 15,000 tonnes of grout to fill the voids left behind by these workings.