Fresh doubt has been cast on the strength and durability of the huge concrete basement at Auckland's highest skyscraper project.

Photo: RNZ / Patrice Allen

An independent review also questions if the concrete walls around the 18-metre deep hole surrounded by other buildings just off Queen Street, are up to Building Code standards.

But the builder has protested about how the review was done and says the basement has "the expected structural integrity".

Problems with the five-storey-high walls in the below-ground basement have stymied any work on the Seascape tower in Customs Street for more than a year.

It is meant to open next year, rising almost 200 metres and boasting 200 high-end apartments - but it can't even get out of the ground.

The defects in the concrete basement triggered a temporary safety stopwork in late 2018 - newly released emails show there were worries about a tower crane's stability - and have caused delay upon delay, and many investigations since.

Now this review, done by engineering firm Beca for the Auckland Council, has been released under the Official Information Act to RNZ.

Beca has found fundamental fault with the repair plan put forward by the builder China Construction New Zealand, a subsidiary of one of the world's biggest contractors, Shanghai-based global giant China State Construction.

'Reasonable doubt'

The argument is around the severity of defects on the outside, or far face, of the basement walls, what impact these might have and how extensive the repairs should be.

China Construction has repaired the inside faces of the four basement walls. These repairs have not been independently assessed for the council.

Beca said in its 12-page review that China Construction appeared to be suggesting that having done these inner-face repairs, that this was enough and it did not need to repair the far faces of the walls.

Read the peer review of Beca (6.4MB)

But there was a good chance the far faces, hidden under soil, had "significant defects" too.

Crucially, the repair plan's evidence "is not sufficient to remove reasonable doubt that the strength and durability of the basement walls may be lower than the design intended or may be below the minimum standard required by the NZ Building Code", Beca said.

Photo: China Construction NZ

The reports show that there has been a lot of monitoring of the basement and, as of a few months ago, had not detected any sinking or tilting.

The inside-face defects, mainly in the top 5m of the wall, consisted of concrete contaminated with soil or bentonite - a slurry used to help control the pouring of the walls - and places where steel reinforcing was fully exposed or was not covered with the required 70mm of concrete.

These defects have been scoured out with water blasters and filled up again.

The exposed steel at Seascape "may have a high risk of corrosion" particularly because surrounding groundwater appeared to be salty, Beca said. The sea is just 250m away and the basement is well below sea level.

Back in November 2019 the Auckland Council told RNZ: "There is no evidence of seawater in surrounding soils."

Now, however, reports say the "groundwater at the site is tidal".

Beca goes on to suggest the council should ask the builder to prove that project was still code compliant.

If it could not prove this, that would be a major problem.

Read China Construction's response to the Auckland Council (3.2MB)

'Very sound'

However, China Construction's managing director Timothy Yang maintained the basement is up to Code.

"This is a very sound structure," he told RNZ.

"There's no movement, no crack, no nothing."

The technology in use, called diaphragm-wall technology, was unusual in New Zealand and so there were misunderstandings, he said.

His company is promising it will check the defects on the far, hidden faces of the walls adequately; but at the same time, its reports show the company is relying in part on visible checks of what it considers high-risk areas, but not of the walls as a whole, which are mostly hidden behind soil.

"We have never been asked to investigate on the far side of the D-wall in the whole world," Yang said, adding China State was putting up many skyscrapers worldwide.

"Like, how can you dig out a pile and check the surface? You don't do that.

"You can't have an inspection under the ground."

He declined a taped interview.

Independent checking

The Beca review is the key independent check of problems with the basement.

The other firms involved so far - subcontractors, engineers or consultants - are on the payroll of China Construction, or the developer Shundi Customs, also based in Shanghai. The project has hired Aurecon as an outside engineer to do reviews for it.

Yang protested that his company had not been shown the review, and that Beca never approached them.

"Beca know very little about this job," he said, suggesting the better measure of it was from the consultant engineers hired by Shundi.

Photo: China Construction NZ

The council shot a host of new questions raised by the review to China Construction two weeks ago.

Beca's review suggested a large part of the repair plan - at least half a dozen sections - appeared to be irrelevant to the question of whether or not to repair the far-face defects.

The contractor responded to the council in late January, and has sent RNZ its six-page response.

This is essentially a defence of the 400-page repair plan, that had concluded the walls have "the expected structural integrity".

The walls' far faces probably had fewer defects than the inner faces, because the soil surrounding them was more dense than the reclamation material abutting the upper five metres of the inner face, the plan said.

"This is further proven by the [good] condition of the inner face at lower levels," it said.

The design was unchanged, so was compliant, and new documentation was not needed, it said.

Visual checks were continuing of high-risk far face areas "to corroborate the evidence to suggest that the far-face condition is better than that of the internal face", before a risk assessment was made.

Also, the salt levels in surrounding material were "not aggressive" so corrosion was not a threat, the contractor has told the council.

"We will make a final decision on our actions once we have reviewed these responses," the Auckland Council said.

Costs and tensions

The review adds to the risks, costs and tensions at a project where the contractor and regulator - the council - have sometimes been at odds.

In early September 2018, the council told China Construction: "It's disappointing that building control didn't receive any communication regarding the serious issues found ... last week" at the basement.

That's according to emails newly released to RNZ under the OIA, which also show the council expressing annoyance in 2018 it had not been consulted about repairs.

China Construction said in October 2018 the concrete defects would be a quick-fix.

Photo: China Construction NZ

But it took until March 2019 to prepare the repair plan, and has now taken another 10 months for the council to get this reviewed, then ask for still more information.

Yang said that no one involved with the Beca review had sought to contact the project team "to better understand the quality control process that is in place, or the current status of it. One would expect this as normal code of conduct in a professional working environment".

The council is likely to be sensitive to any reputational damage from the delays.

Late last year, the New Zealand Chinese Building Industry Association told RNZ that convoluted sector regulations and quality assurance standards were holding them back.

Causes and investigations

China Construction's repair plan, released in full to RNZ, does not pin down why the basement walls have more defects than expected, though factors included:

some of the concrete being quite old - five to six hours - when it was poured (though still up to standard)

and especially, the very permeable soil, and obstructions the wall builders, March Construction, part of the French multinational Vinci, hit on the way down.

Beca's review latched on to the soft ground conditions - "considering," it said, "... the risk of reduced durability and strength at the sites where defects were not repaired."

Photo: China Construction NZ

China Construction conceded the far-face of the walls was mostly hidden, but still contended the defects affected only 5 percent of the wall area. It said that, even so, it took this seriously and had engaged global experts to do investigations, core drilling and non-destructive testing.

One of these, Zhongkan Metallurgical Institute, used x-ray-type imaging that found some contamination but concluded the concrete was "good".

However, Beca said the test area was probably too small for this to count.

Another scanning company, Concrete Structure Investigations (CSI) of Wellington, was also used but its findings not included in the repair plan. The council asked why not. China Construction replied that it decided CSI's type of scanning wouldn't be suitable on the 600mm-thick, heavily reinforced walls.

CSI is the same company that last October spoke out about finding hundreds of major buildings nationwide had defective or missing concrete or reinforcing steel. It has not named any specific buildings.

Beca said the China Construction appeared to be relying on its consulting engineer which concluded the Seascape basement wall design was up to waterproofing standards, so must be structurally strong enough despite any defects; and an opinion from an investigation company, Sol Expert, owned by the same multinational as the wall-builder March, saying the reinforcing steel exposed to the soil or bentonite would not corrode because the alkaline levels surrounding it were high.

Developer Shundi Customs, of Shanghai, declined to comment. It invited RNZ to visit the site.