A specialised team of experts, some from as far away as Germany and Australia, have been called in to help reinforce the historic brick barrel beneath Bridge Street.

Work is now underway to preserve and reuse the structure, which was designed in the 1860s for use as a drainage system.

“It’s in incredible condition considering it was laid in the 1870s,” says Mike Allen, Managing Director of NuFlow Tasman.

Working with the international team, a glass-reinforced plastic liner made from corrosion-resistant glass fibres will be installed inside the brick barrel. Relining the brick barrel will allow it to be repurposed as a stormwater pipe, while also preserving the historic structure and extending its life for decades to come.

The team moved onto Bridge Street East in early March to carry out final investigations and preparation work ahead of installing the liner. There are close to 100 old laterals – smaller pipes that connect to the main line – made from a range of materials that needed to be carefully trimmed so they don’t puncture the liner.

The team has also removed nearly 20 tonnes of silt from inside the barrel, some of which has likely been there since the late 1800s.

“We even found some one-penny coins from the 1860s in the silt, so this has been building up for some time,” says Mike.

“Twenty tonnes sounds like a lot, but in the grand scheme of things, 20 tonnes in 150-odd years is pretty minimal. The barrel has really stood the test of time.”

Final preparation includes CCTV inspection of the barrel, followed by a thorough flush and hydrovac to make sure the pipe is completely clean and clear of debris.

Then the real work begins.

“A glide foil will first be installed, which acts as a buffer between the pipe and the liner,” Mike says. “Using the manholes along the street and a Hiab with a spreader bar, the liner will be carefully fed into the pipe while a winch at the other end pulls it into place. Once it’s in position, compressed air is used to inflate the liner.”

Unlike a standard round pipe, the brick barrel has an egg-shaped profile - a design commonly used in older wastewater systems - which presents a challenge.

“Getting the shape right is crucial. In a circular pipe, the liner inflates evenly and settles quickly into shape. With an egg-shaped pipe, the tighter curves at the bottom mean the liner has to stretch and conform differently in different places.”

After the liner is installed, a camera will be sent through the length of the pipe so the team can find and smooth out any creases or wrinkles before the UV light train is introduced.

The light train is a row of high-intensity UV lamps with 1000-watt bulbs. As it moves through the pipe, it cures and hardens up to 1.5 metres of liner every minute.

“It looks like something from a sci-fi movie,” Mike says.

For the next week there will be some additional noise on site, mainly from generators. At times, Bridge Street may look busy with crews working above ground, while at other times, work will be happening out of sight, under the street.

“When you see people gathered around a hole, it usually means someone is underground doing the hard work. Others are monitoring the cameras and controlling what’s happening below ground, Mike says.