Background to specification
Valued at £900 million, the Manchester Metrolink Phase 3 contract sees the construction of the largest light rail scheme in the UK, with 60km of new track to be laid and 55 new stops installed to effectively treble the size of the network.
Technically it has called for a high level of efficiency and problem-solving to keep it on track for completion in 2016. But problem-solving is one of Tubosider’s great strengths, notably when Laing O’Rourke’s Expanded Structures division came to installing the new track and stops at Manchester Airport.
Tubosider’s first planned stormwater solution ultimately needed considerable reworking – but all in response to various unforeseen changes in available space and discharge criteria.
At the start of building the Airport line, a conventional helibore steel tank of suitable capacity adjacent to the incoming flight path looked to provide a complete answer to requirements – and at the most competitive price.
But at this critical stage in the project, the culvert due to serve it was transferred in authority from Manchester City Council to the Environment Agency. The allowable discharge level was halved at a stroke, and having to reduce the footprint of the first tank really put pressure on Laing O’Rourke.
Solution & benefits
With Tubosider’s area manager quickly on the spot to liaise with the contractors, the engineers and both authorities to propose and agree the best solution, the decision was taken to reduce the footprint of the first tank to four legs just 27.3m long by 13m wide, all constructed in 2800mm diameter pipe to give a capacity of 1000m³.
The remaining storage requirements could then be accommodated by fitting a smaller Twinstore tank and flow control system into the tight space between the new Ringway Road and the Metrolink carriageway.
The advantage of Twinstore is that its pipe ends are unsealed to allow water storage to balance out into the surrounding backfill, which is lined with an impermeable membrane.
This second 3 leg tank of just 1400mm diameter, with a footprint of 41.6m by 5.4m, also provided much more than its dimensional storage capacity of 280m³. With the tanks able to take more weight to help positioning, they offered an even more cost effective solution to both carriageway and Metrolink track drainage alongside the Airport.