Scottish Water completes 1.2km tunnel
One of Scotland's largest project in AMP5 has been completed in Airdrie. Scottish Water's William Ancell explains
An earth pressure balance machine (EPBM) for tunnelling has completed a 1.2km stormwater sewer under Airdrie, in Scotland's central belt. The machine - named Molly the Mole by a local schoolgirl - has been removed from the ground, which means that the new storm transfer sewer can be connected to the local sewer network and made operational later in the year.
The external diameter of the tunnel is over 3m wide and runs as deep as 40m in places. It will create a relief in times of heavy rain, diverting the water to a new storm tank at the site of the old Airdrie sewage works.
The project will lead to improved water quality in the South Burn and will reduce the risk of flooding in Cairnhill Road at the railway station. In addition eight nearby properties will be removed from the flood register.
Brian Dalton, project manager, Scottish Water, said: "During 2011 our delivery partner Byzak successfully completed the 1.2km tunnel below Victoria Place using Molly the Mole, who ended her journey in late December 2011. They needed this tunnel as part of the overall solution for the project.
"There are currently discharges to the local watercourses which breach our consent with SEPA, our environmental regulator. These flows will now be transferred via interconnecting pipework from their source, transferred along the tunnel to a storm tank, thereafter storm flows discharge to the North Calder water.
Dalton continued, "Attenuated flows from the storm tanks are returned to the existing sewer network as the flows subside. The tunnel option was selected to minimise disruption to the area.
"Our original route was actually going through the centre of Airdrie and when we were in talks with Byzak at the planning stage, they highlighted an alternative route that caused much less disruption, required less surveying work and would be quicker to complete."
Dalton explained that the tunnel runs from Airdrie Railway Station car park to a brownfield site behind the bingo hall in Coatdyke - a single drive of approximately 1,250m. The tunnel structure is unlined, one pass segmental pre-cast concrete rings.
The EPBM tunnel machine was chosen due to ground conditions and availability. Byzak, had previously used this machine for similar projects in the UK.
Dalton explained, "The ground conditions along the track of the tunnel varied - boulder clay, mudstone, sandstone and mine workings. We came across a small void and some heavy water ingress, but no major issues emerged during the tunnelling. At full production Molly was covering 60-70m per week.
"Meeting the programme dates has been the biggest challenge so far. We set Byzak a tight programme and they have done very well to stick to the foreseen timescales thus far."
In the coming months another tunnelling machine named Wullie Worm will be creating smaller tunnels under the area around the railway station and adjoining streets.
"These will connect the new storm transfer sewer to the local network," Dalton explains. "To enable this, work will be carried out at the car park to the rear of Stirling Street in phases to maximise available car parking space for local residents.
"By autumn 2012 the work will be finished and improve water quality in watercourses across Airdrie and beyond.
Geoff Aitkenhead, Scottish Water's asset management director, said: "This is one of the largest projects that Scottish Water is doing in Scotland in the 2010-2015 investment period. We have communicated regularly with the local community to ensure the work has as little impact as possible. Our team has also volunteered for a community event cleaning up Centenary Park in Airdrie by weeding plants, painting and cleaning up play equipment."