Auckland Transport rejected advice by engineers to install an "early warning system" to monitor unstable land long before two massive slips in Birkenhead that could cost $24 million to put right.

The revelation has shocked local residents, businesses and Northcote MP Jonathan Coleman, who said AT had the information about unstable land at Rawene Rd carpark, did not act on it and have let down the community.

Auckland Council's chief operating officer, Dean Kimpton, has defended the actions of AT staff, saying although they did not follow engineers' recommendations, they carried out regular visual inspections of the carpark and acted when large cracks appeared.

I'm shocked they didn't do anything. It's starting to make me very angry

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The warning bells were sounded in 2015, two years before the first slip swept away 25 carpark spaces in October last year, followed by a second, larger landslide in November that sent a drilling machine tumbling into the Rawene Reserve and workers scattering.

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Papers obtained by the Weekend Herald under the Official Information Act show a geotechnical assessment by GHD engineers in 2015 found signs of pavement cracking and kerb stones separating.

Photo from GHD report showing pavement cracking in the car park. Source / GHD report.
Photo from GHD report showing pavement cracking in the car park. Source / GHD report.

Boreholes revealed poorly compacted fill to build up the car park was capable of moving under its own mass, particularly if saturated.

GHD recommended two options to fix the unstable land at the carpark for $500,000 or $550,000, and a "do minimum" option of geotechnical monitoring. The latest council estimates put the cost of fixing the slips between $14m and $24m.

GHD said the option of monitoring the site monthly was the most feasible and appropriate and would act as a "early warning system" to respond to any undue movement efficiently and cost-effectively.

In a statement from AT chief engineer Andrew Scoggins attached to the OIA response, he said the car park was monitored for cracks on a "nominal monthly basis ... however the site visits were not documented, which is normal process for an area that is not showing change".

The monitoring did not include ground monitoring pins and sealing existing cracks as recommended by GHD because it was considered unlikely to give any enhanced level of warning over and above visual monitoring, said a joint statement from AT and council.

It was not until two years later on September 14, 2017, when large cracks appeared in the car park, that AT brought back a geotechnical specialist to check the ground and its stability.

"Auckland Transport needs to explain why they didn't act on this independent advice ... something major has happened which could have been averted," said Coleman.

Birkenhead Brewing Company co-founder Brad Boult said "somewhere along the line their systems have failed dramatically". Highbury optometrist Tony Chadwick said: "You do have to wonder why in a body funded by the people for the people there is so much closed door secrecy going on."

Local resident Graham Tucker on his balcony overlooking the site of the slip. Picture / Doug Sherring.
Local resident Graham Tucker on his balcony overlooking the site of the slip. Picture / Doug Sherring.

Claire Balfour, the body corporate chairwoman at the nearby Mokoia Ridge Apartments, said the latest revelations showed a bureaucratic aversion to do anything. Another resident, Graham Tucker, said AT had put his family and others at risk by not acting on the GHD report.

"I'm shocked they didn't do anything. It's starting to make me very angry," Tucker said.

Kaipatiki Local Board chairwoman Danielle Grant said the slips had impacted on the community and caused huge distress, both financially and emotionally. The GHD report should have been shared with the Local Board to have the opportunity to consider the options, she said.

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No one from AT would be interviewed about the GHD report, on the basis Auckland Council is now managing the issue, overseen by Kimpton.

Kimpton said AT staff decided not to do geotechnical testing on the carpark but to visually monitor the site, which was successful because when they noticed additional movement they got engineers back and protected public safety.

He said there is cracking and movement at hundreds of sites across Auckland, saying AT made the right call based on experience and the evidence at hand.

"They have no interest in losing an asset down a hill and when they did spot the cracks they got Opus (engineers) in to have a look," he said.

Kimpton said even if an inclinometer - a geotechnical instrument for monitoring movement below ground - had been set up at the carpark as recommended by GHS, it would not have bought enough time to do anything because the slip happened quickly.

The AT-council statement said the options to fix the unstable land may have provided some extra stability to the slip but given the scale of the slip it is quite possible they would not have prevented it occurring.

It said most of Auckland has soils that swell in winter and shrink in summer. Building controls take this into account and council created a small team of geotechnical specialists last year to respond to issues and recommend solutions where public land may be affected, including coastal erosion.

Asked by the Weekend Herald several times how much it would cost to fix the slips, Kimpton said he had "no idea" or "nothing that can be relied upon" beyond a minimum of $1.8m for temporary repairs so far.

A report to this week's finance and performance committee said urgent repair works have cost $2m, design and other work is expected to cost $1.85m and preliminary costs for a permanent fix are between $10m and $20m.

It is also costing $80,000 a month to pump stormwater and wastewater out of the landslide, which continues to move slowly and remains a significant risk, the report said.

The GHD costs of $500,000 and $550,000 for a permanent fix did not include detailed design, site supervision, land purchases, GST and a recommended contingency of 30 per cent.

Meanwhile, a $9m stormwater project to strengthen unstable land at Chelsea Heritage Estate below the slip means the project cannot be built as planned for safety reasons and "may have to be cancelled or subject to significant cost and programme overruns", said the report.

Timeline

2010:

Serious stability issues given as a reason for not granting consent to lease Rawene Reserve below the carpark.

October 2015: Routine inspection of carpark prompts Auckland Transport to hire GHD for a geotechnical investigation of subsoil conditions relating to potential slope instability.

Late 2015-September 2017: Casual monitoring of the carpark, which is not documented for an area "not showing change".

September 14, 2017: Geotechnical specialist hired by AT when large cracks appear.

September 19, 2017: Carpark closed off.

October 9, 2017: First slip swallows about 50m of carpark.

November 28: Second slip sends a drilling machine and more debris tumbling into Rawene Reserve.

December 18: Herald reveals Auckland Transport and Auckland Council officers knew from February 2017 that a leaking stormwater pipe was causing cracks to the carpark but it was not fixed before the first slip. Council says it is unlikely stormwater pipes played a significant role.

March 3, 2018: Herald reveals GHD report.