The TBM, named Caroline, has bored 3.5km of sewage tunnel.
The project is being undertaken for United Utilities by KMI+, a joint venture between Kier, J. Murphy & Sons and Interserve.
The scheme began two years ago and involves a mix of interceptor sewers, stormwater storage tunnels, 6km of tunnelling, rising mains and 13 access shafts.
Shafts were initially dug at three locations just outside Preston city centre to sink the TBM 26m underground to create a network of tunnels between the sites.
United Utilities senior project manager Carl Sanders said: "The tunnel is very much the lynch-pin of the whole Preston project. It is the biggest tunnel and rainwater from all the other tunnels will flow into, before the water is pumped to Clifton Marsh wastewater treatment works.”
The scheme is part of the overall United Utilities overall UID (unsatisfactory intermittent discharge) scheme in the Preston area to improve the quality of the Fylde Coast bathing waters and designated shellfish beds in the Ribble Estuary.
KMI+ contracts manager Andy Parker added: “The whole scheme has been very challenging but teamwork and innovation have helped us overcome this. Even before work started we knew the sewer upgrade would be complex. The existing century-old combined sewerage system regularly exceeded capacity, with frequent flood-induced overspills direct into the river.
“Our solutions have included one of the most complex grouting and dewatering systems ever installed, and the use of a German pipe thrusting technique, never before used in the UK, which proved to be fast and cost effective.”
Once fully operational the new network will intercept the flows and divert them 10km westward, via a new pumping station, to the town’s main treatment works at Clifton Marsh. A 2.5km stormwater storage tunnel and two linked shafts will hold back up to 40,500m³ of stormwater, regulating its flow to the treatment works.
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