The mystery of basement flooding in new Christchurch CBD buildings

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Basement flooding in several new Christchurch buildings may be due to rainfall – so why did it disappear during lockdown? Building owners want answers. MARTIN VAN BEYNEN reports.

It’s a mystery.

Basements in several commercial buildings in Christchurch’s central business district are flooding. There’s no doubt a higher water table is behind the problem, but what is causing the water table to rise?

The flooding could be due to a host of factors including weather events, building infrastructure and river flows. But another culprit has emerged. It’s a new aquifer-sourced heating and cooling system installed in new buildings around the city.

The buildings using the technology include the justice precinct, the Town Hall, the Bus Interchange, the Botanic Gardens and the central library Tūranga. The flooding problem – which Stuff understands affects at least two major central city buildings including the Ballantynes building in Colombo St – disappeared during last year’s Covid-19 lockdown when buildings using the technology shut.

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The ground-sourced heat exchange process involves pumping water from a low-lying aquifer and putting it through machinery where heat is extracted or discharged. The water is then reinjected into a higher aquifer structure. Christchurch is ideal for the new systems. It’s located on a series of confined aquifers at depths ranging between 5 metres and 200m. The aquifer water is consistently between 12-13 degrees Celsius.

Building owners Richard Owen and his son Ashton are convinced the ground-source system caused serious flooding problems in their then new Wynn Williams building in Hereford/Montreal St in 2017. The flooding came about a month after a ground-source heating system began operating at Ngāi Tahu Property’s new office building close by.

Ashton Owen said the two events were too coincidental not to be related.

Wynn Williams building owner Ashton Owen says testing of the impact of heat exchange systems were inadequate.

KIRK HARGREAVES/Stuff

Wynn Williams building owner Ashton Owen says testing of the impact of heat exchange systems were inadequate.

Environment Canterbury (ECan) conducted tests in 2017 during which the Ngāi Tahu Property building turned off its heat exchange system over a weekend. The close-down did not appear to have any effect on the flooding problem.

But Owen said the tests were inadequate as his expert had recommended testing over three weeks. His company had installed pumps to deal with the flooding problem, and it was now under control, he said.

“It’s been really, really frustrating. There doesn’t seem to be any real commitment to sorting this out and finding out whether these systems are affecting the water table.”

A hole, the size of several cars, discovered under Christchurch’s new Tūranga library in Gloucester St in May, is also part of the mystery.

STUFF

Christchurch’s Tūranga central library took out the supreme award in the 2019 Property Council Awards. (Video first published in June 2019)

Located under the loading dock outside the library in Gloucester St, it was initially put down to a leaking underground pipe in its heat exchange system, but the cause is still unknown.

Some believe the reinjection of water at 35m under the ground surface has undermined material sealing the aquifer at that level.

The theory is that this has created pathways for the aquifer water to rise thereby undermining the ground under the building.

A big hole appeared under the foundations of the central library building in May.

CHRIS SKELTON/Stuff

A big hole appeared under the foundations of the central library building in May.

Compounding the puzzle is the fact that when the hole was discovered at Tūranga, the Isaac Theatre Royal across the road in Gloucester St was organising the installation of more pumps to cope with flooding problems in its basement.

“We noticed a rise in water table about 12 months prior to noticing the flooding. We already had pumps below the auditorium, but that wasn’t enough. It got so bad there were times where it could have severely affected shows,” theatre chief executive Bob Mangan said.

He wouldn’t be surprised if the ground-sourced heating systems were behind the theatre’s problems, but the theatre did not have the funds to investigate.

Christchurch's historic Isaac Theatre Royal started having flooding problems in late 2019.

JOSEPH JOHNSON/STUFF

Christchurch’s historic Isaac Theatre Royal started having flooding problems in late 2019.

Alarms and cameras had been installed to warn of new flooding.

Mike Thorley, a Christchurch-based hydro-geologist at engineering firm Beca Ltd, which has championed the technology and won an international award for it in 2015, declined to comment.

In a paper he presented to a Water New Zealand conference in 2017, he referred to concerns water reinjection might increase seepage into or around buildings or flood deep excavations such as basements and underground car parks.

“Careful management of reinjection flows and increased monitoring would be required to ensure ongoing access to the resource,” he said in the paper.

The justice and emergency services building in Christchurch uses ground-source heat exchange technology.

Stacy Squires/Stuff

The justice and emergency services building in Christchurch uses ground-source heat exchange technology.

He noted “the cumulative effect of the reinjection” was starting to appear in long-term monitoring sites such as one at the Canterbury Museum, which was showing “record high” flowing artesian groundwater levels.

ECan groundwater science manager Carl Hanson remains sceptical.

He said the question had been investigated in the past, “but we have been unable to make clear links between ground source heat pump systems and the level of the water table in the CBD”.

“We’ve done no further investigation into this question since 2017, and we’ve got no monitoring programmes in place related to foundations and substructures in the CBD … Our 2017 investigation did not show a clear link between ground source heat pumps and the water table, and we have seen no further evidence of broad-scale changes resulting from these systems, so no further investigation has been undertaken.

The central Christchurch Bus Interchange is heated and cooled by ground-source heat exchange.

Joseph Johnson/Stuff

The central Christchurch Bus Interchange is heated and cooled by ground-source heat exchange.

“We note that the groundwater levels in the Canterbury Museum well rose by roughly 0.8 metres in about 2017, but we can’t confirm this increase is related to ground source heat pumps.

ECan did not monitor foundations and substructures in the CBD because its focus was on a regional scale, “with monitoring wells distributed across the city to give us a broad picture of how the groundwater system behaves as a whole”.

The main driver of variations in the depth of the water table over time was rainfall, he said. The 0.8m rise in 2017 referred to the water pressure (upward flow) 30m below the ground surface.

Building owners with flooding problems were unwilling to go public about the problem when approached this week. A Ballantynes spokeswoman said the company did not want to comment at this stage. Another owner also declined.

A Christchurch City Council spokeswoman said the council had not experienced any significant flooding at any of its facilities with basements in the CBD.

“We are not aware of any recent complaints from the public. While we have had some subsurface subsidence caused by water at Tūranga, there is no evidence that this has caused any impacts off-site – the subsidence is in a very defined area.

“When we have had minor issues they are generally caused by high winter water tables or internal leaks.”

The city council’s facilities head, Bruce Rendall, said the hole at Tūranga had been filled, but the surface was yet to be reinstated.

Ballantynes department store in Christchurch has had basement flooding problems.

Alden Williams/Stuff

Ballantynes department store in Christchurch has had basement flooding problems.

There were several theories on the cause of the subsidence and its experts had not formed a definitive view, he said.

“Council is one of several property owners who operate ground source heat pumps due to their thermal…



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