Beneath one of Nelson’s most important city-centre corridors sits a piece of underground infrastructure that tells its own story about how cities evolve.
Bridge Street, now at the centre of Nelson’s wider Bridge to Better revitalisation program, is being reshaped as more than a traffic route.
The project combines transport upgrades, three-waters works and public-space renewal in a way that reflects Whakatū Nelson’s identity while preparing the area for future growth.
But below the paving, landscaping and streetscape works lies an older layer of the city again: a historic brick barrel dating back to the early days of Nelson’s urban development.
Preserving that asset while keeping the broader renewal moving was always going to demand something smarter than open-cut replacement.
Here trenchless technology comes into its own. Nuflow Tasman Managing Director Mike Allen was approached by Nelson City Council to propose a solution for rehabilitating the historic brick barrel stormwater line running between Collingwood Street and Trafalgar Street.
The stormwater line is around 250m long and a feasibility study considered four different renewal or relay options before Impreg’s GL16 UV cured liner with a 100-year design life was selected. Nelson City Council described the work as a way to preserve and reuse the structure, while extending its service life for decades.
In other words, the chosen method was not simply about repairing a pipe, it was about retaining a historic asset, avoiding unnecessary excavation and making the old structure fit for a modern drainage role.
That decision matters because this was not a standard trenchless rehabilitation job. Bridge Street is part of a major city-centre transformation. Nelson City Council has described the broader program as a significant investment in infrastructure, including increased stormwater capacity, water supply works, wastewater upgrades and a redesigned public corridor.
On a site like that, the normal pressures are multiplied: construction sequencing, stakeholder coordination, traffic, business disruption, heritage sensitivity and public scrutiny.
Digging up and replacing an 18th century structure under a busy urban corridor would have come with obvious cost, program and community impacts.
By contrast, the UV liner solution allowed the project team to work through existing manholes, avoid major excavation and keep the footprint tight.
The investigation and preparation work also reinforces a point that experienced trenchless contractors already know well: success on complex renewals is won before curing starts. CCTV inspections showed the brick barrel was still structurally intact overall, despite signs of age including missing bricks, deteriorated mortar and general wear.
The team then carried out full cleaning and flushing, followed by hydrovac removal of accumulated debris. Around 20 tonnes of silt were removed over two visits.
It was noted some of that material may have been sitting in the barrel since the late 1800s, which says plenty about the longevity of the original structure.
Nuflow also undertook final CCTV and dimensional checks at nine different locations to confirm consistency through the line before installation. That level of pre-install verification is exactly what heritage and non-standard-profile assets demand. Geometry was another major factor.
The brick barrel was an ovoid (egg) shaped profile, a form commonly used in older wastewater systems.
One section was mortared and had a slightly different internal diameter, which led the team to split the installation into three separate shots of approximately 130m, 60m and 55m.
There were close to 50 old laterals, made from a range of materials, that had to be carefully trimmed so they would not puncture the liner during installation.
“This is the kind of detail that separates straightforward trenchless work from the jobs that really test planning, crew discipline and product selection,” said James Haua, Nuflow Australasia Business Development & Technical Manager.
From there, the installation sequence followed a tightly controlled Impreg UV CIPP installation process. A glide foil was first installed to reduce friction and protect the liner as it was pulled into place.
The Impreg GL16 UV liner was pulled in and positioned using a winch.
End cans were then installed and the liner was gradually inflated. The liner was then calibrated by inflating the liner 30 millibar every five minutes until reaching a working pressure of 250 millibar.
“That staged approach was critical to allowing the liners’ fibreglass layers to compress properly to the host pipe and avoid defects such as poor conformance or delamination,” said Mike Allen.
“On irregular shapes and heritage assets, that level of patience is part of quality assurance, not lost time.”
ProKASRO dual light core with a total of eight 1,000-watt bulbs, which emits a particular UV wavelength, was used to cure the liner. This was inserted into the inflated liner and then pulled to the other end.
During this, the technicians visually monitor the liner interior for wrinkles or other defects.
The curing process then commenced, igniting the UV bulbs sequentially to full intensity, then being pulled back at a calibrated speed of 0.6m to 0.8m per minute.
Once the light train reached the other end, the liner was allowed to cool before the end cans and light cores were removed.
The ends were then cut back and sealed with mortar. Nuflow’s NUVLINE solution is used according to Haua, because of its “controlled structural rehabilitation process, with each step tied to achieving final quality in a difficult host asset.”
There is also a bigger industry lesson in this project. Trenchless solutions are often justified on the standard metrics of cost, disruption and program.
Those metrics absolutely matter here, and Nuflow Australasia highlighted reduced trenching, shorter construction duration, lower stakeholder impact and cost savings compared with replacement.
But the Bridge Street job shows something more valuable again: trenchless can be the mechanism that allows infrastructure renewal and heritage preservation to happen together.
That is a stronger story, Haua said, than ‘no dig’ on its own.
“It shows how rehabilitation can support broader city-making objectives while still solving a hard engineering problem underground,” he said.
For the trenchless sector across Australia and New Zealand, that is the real takeaway. As city centres age and expectations around liveability, sustainability and heritage become sharper, the jobs that matter most will increasingly sit at the intersection of engineering performance and place-sensitive delivery.
Bridge Street is a good example of how UV CIPP can answer that brief. It preserved a historic brick barrel, avoided excavation, supported a major public project and delivered a practical stormwater outcome from a structure that might otherwise have been written off.
For the many partners involved, the project’s success is a demonstrates where trenchless technology works at its best, by enabling renewal without erasing what is already there.
“It was great to work with the team from design all the way to the final installation,” said Colin Murphy Impreg Country Manager.
“Our technician from Australia was involved at all stages of the project and accompanied by a German Impreg tech for the final instal.”
For more information, visit
www.impreg.com and nuflow.net/tasman/
.jpg)
