Scottish Water’s flagship Shieldhall Tunnel project, in the south of Glasgow, passed the milestone yesterday when the state-of-the-art tunnel boring machine (TBM) building it reached a point under Pollok Park.

The £100 million tunnel, which will enable Scottish Water to improve water quality in the River Clyde and tackle flooding, will at 3.1 miles long be five times longer than the Clyde Tunnel.

Engineers working on the 1000 tonne, 180-metre-long TBM, started construction when it was launched last July and are expected to complete it later this year.

This week they installed a full circle of giant concrete rings that form the tunnel at a point about 10 metres (about 32 feet) under the east of Pollok Park that is 1.55 miles along the route - the exact midway point of the tunnel.

Having reached the half-way stage, the work so far has produced some interesting facts and figures including:

Paul Kerr, Scottish Water’s capital investment general manager, said: “We are delighted to have reached this milestone half-way stage in the tunnel construction.

“The Shieldhall Tunnel team includes some of the best and most experienced tunnellers in the world and they are making great progress with what is the biggest project of its type Scottish Water has ever undertaken.”

The TBM is tunnelling at a speed of about two millimetres per minute, with a rotating cutting wheel with 25 cutters at the front, through challenging ground which includes boulders, clay, hard sandstone, glacial tills and old coal workings.

The first half of the tunnel construction has taken it under Bellahouston Park, the Glasgow Paisley Canal railway line and the M77.

As it inches its way along the route, preparatory work is progressing at Queens Park where, in the past few days, work has been continuing on the construction of an exit shaft for the TBM and a smaller tunnel from that shaft to another shaft which will use a newly-installed flume to connect the tunnel to the existing network.

The route of the tunnel was chosen to maximise the use of parkland and minimise disruption.

The tunnel, which at 4.7 metres in diameter will be big enough to fit a double decker bus inside, is a key part of Scottish Water’s £250m, five-year programme of work, launched in 2013, to improve river water quality and the natural environment.

The programme is the biggest investment in the Greater Glasgow area’s waste water infrastructure in more than a century.

The tunnel will enable Scottish Water to improve water quality in the River Clyde and its tributaries and tackle flooding in various parts of the south of Glasgow area.

The team building the Shieldhall Tunnel for Scottish Water, known as the Glasgow Tunnel Partnership, is a commercial joint venture between Costain and VINCI Construction Grands Projets (corr.) called CVJV. Costain and VINCI have been involved in some of the world’s major engineering projects, including the Channel Tunnel.