£22m project improves water quality

Paul Boughton
Scottish Water has completed the Airdrie Environmental Project, resulting in a significant improvement in the water quality of the area and the alleviation of flooding issues dating back to the 1960s.
The completion of the £22m investment in the town of Airdrie brings to a close one Scottish Water’s largest environmental undertakings in the 2010-2015 investment period.
The Scottish Environment Protection Agency (SEPA) determined that the water quality in the South Burn in Airdrie was suffering from storm water discharges at various locations in the town. [Page Break]
Scottish Water designed and costed a project to remove these discharges into the South Burn, which contributes to the River Clyde, and redirect them to a new storm transfer sewer.
The project also aimed to alleviate the likelihood of flooding at the Cairnhill Road railway bridge. One of the lowest points in the town centre, it is also surrounded on all sides by shallow vertical slopes and a lack of permeable land to soak up surface water. There was also a requirement to remove flooding risk from eight properties in the town.
Brian Dalton, Project Manager, Scottish Water, said: “The project consists of several interconnected pieces of infrastructure. Combined Sewer Overflows are tied into the existing and some new sewer tunnels that then feed into a large diameter storm transfer sewer that runs 1.2km in length to a storm tank.
“Thereafter, storm flows discharge to the North Calder water. Attenuated flows from the storm tanks are returned to the existing sewer network as the flows subside."
Byzak Ltd was selected as the principal contractor due to its proven track record of delivering similar schemes such as the River Exe Tunnel for the National Grid.
Byzak proposed a tunnelling route that minimised the disruption to the people of Airdrie, as well as maximising the benefits gained from using the company’s specialist tunnelling plant. [Page Break]
Richard Aitchison, Byzak Ltd, said: “As well as being less disruptive to residents and businesses, the proposal resulted in savings to the client of around £2m. There has also been a major reduction in service diversion saving approximately £1m.”
Work commenced by excavating a shaft at a brownfield site near the Go! Outdoors store at Locks Street. This was an ideal entrance shaft location as there was enough space to establish site offices and ample storage for building materials. It was also secluded and screened from nearby residential properties and a short distance from the storm tank location at the former Airdrie WWTW.
The tunnel was constructed using a Herrenknecht EPB tunnel boring machine (TBM) named ‘Molly the Mole’ by Aimee Stewart, a local school pupil. Byzak had previously used this machine for similar projects in the UK and it was a proven technology.
Launched from a 38m x 8m x 9m deep sheet piled pit, the TBM was driven at an upgrade of 1 in 37 towards a reception shaft in a car park near Airdrie railway station. The ground conditions were mainly mudstones, shales and sandstones of the Coal Measures Series, but also included areas of made ground (demolition materials, abandoned coal workings, industrial waste etc.). At full production ‘Molly the Mole’ covered 60-70m per week.[Page Break]
Storm water storage is provided mainly by a 1,202m long, 2.44m diameter segmental tunnel constructed as single span, with a horizontal and a vertical curve, at depths up to 40m beneath a residential secondary road.
The storm water storage tunnel is connected to the existing sewerage network by two pipe jack tunnels. The first, 206m long and 1,500mm in diameter, and the other, 316m long and 1,600mm in diameter. The 1,600mm diameter pipe jack is comprised of three lengths radiating from a common 9m deep 7m diameter segmental drive shaft located close to Airdrie town centre. The 1,500mm diameter pipe jack extends the main storage tunnel to a new 25m diameter storm tank.
The pipe jacks were constructed using a backactor pipe jack shield, and were completed using ‘Wullie Worm’ (also named by a local pupil), a Herrenknecht AVN 1600D remote controlled full face slurry machine. A major under track crossing of Airdrie Train Station was completed at end of January 2012.
Two lengths of auger boring (70m and 65m) were constructed in the town centre area to allow one of the CSOs to be removed from the existing network.[Page Break]
Constructed within an open excavation, the new 11m deep storm tank downstream of the tunnel comprises 25m diameter smooth face precast concrete segments. The tank incorporates an innovative CSO chamber and flow switching chamber. The CSO chambers constructed for the project include factory made precast concrete components founded on in-situ reinforced concrete bases. Construction of the storm tank was completed by the attachment of precast hollow core roof slabs.  Controls for the storm screens and the four transfer pumps are housed in the brick MCC building constructed at ground level.
At Milton Street, the 3.9m long x 2.7m wide CSO chamber comprised three main components – the main chamber, the roof slab and the weir wall – which were craned into position and assembled on site. After making connections to the newly constructed pipe sewers, the 6mm static peak screen from Hydrok UK Ltd was installed.
To the south of the town, as part of the Cairnhill Road Flood Alleviation Scheme, the existing sewerage network has been upgraded.  A total of 900m of new sewer in sizes 300mm diameter to 1,050mm diameter has been laid in open cut in Cairnhill Road and adjoining streets at depths up to 5m.
The conclusion of this project will make a lasting improvement to water quality in watercourses across Airdrie and in the wider River Clyde catchment.

For more information, visit www.scottishwater.co.uk

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