Beach Haven cliff collapse: New house weight 'very unlikely' to have caused huge slip

Cliff collapse beneath the Brigantine Dr home, finished last Easter. Photo / supplied

Construction of a new $2.25m house at Beach Haven is "very unlikely" to have had any effect on a cliff below it, according to an Auckland Council engineering boss who says authorities are continuing to monitor the situation.

Ross Roberts, Auckland Council's engineering resilience head and a geotechnical engineer, was commenting on the fall on a council reserve beneath homes at Brigantine Drive where the cliff collapsed in various falls, one on July 15.

A new home at 52 Brigantine Drive was only completed in Easter last year, the owners told the Herald on August 4 when the first report was published about the collapse.

Roberts said today council staff were continuing to monitor the situation but he held no fears for people's safety.

Asked about the weight of the new multi-level house on the site the council said had never previously been built on, Roberts said it was "very unlikely" that weight was a contributing cause. "This house is surrounded by a piled wall and those piles were buried 8m down," he stressed.

"So that house is not having any effect on the cliff. The only way the house could be affected is if it was literally on top of the landslide - and it's not. The house shouldn't make any difference to the cliff stability because it's far enough back," Roberts said.

"There's been comments raised about how much concrete went into the piles and palisade wall. But the concrete went into the foundations and actually replaced rock so there's not much difference in weight," he stressed.

The cliff beneath Ben Wilson's home at 52 Brigantine Drive, Beach Haven. Photo / Chris Keall

Roberts visited the site this month: "From the walkover I did, it looks like vegetation was removed," but again he questioned what effect that would have had.

He said it appeared the cliff collapsed due to multiple triggers: "Weak rock, you've got the sea eroding the toe so gravity is working against you, then there's an ongoing process of weathering which weakens the site over time and the chemistry of the rock also changes.

"Of course, we had a really wet July so the issue of moisture to the soil and rock increases physical pressure in the gaps between the breaks and as that increases, that adds a compounding effect. Changes in vegetation can have an effect but quite how much is almost impossible to quantify," Roberts said.

"However, in this case, it's not clear how big the effect of vegetation has been. Large landslides and rocks are less likely to be stabilised by vegetation. It tends to be soil that was bound together that moves. So changes in vegetation would not be the only thing that triggered this."

A call was made to the council asking questions about the site following the collapse and asking for it to be checked "so I went to have a look", he said.

Roberts said he was no more alarmed than he had been about any other landslide.

"It's something you see quite regularly. There's no immediate threat to people or property from this. It seems unlikely to me. This landslide is well away from houses and the material is not falling onto an area [where] people are walking because the reserve is fairly inaccessible. "In its current state, I am not concerned about it. But we are going to monitor it and ensure it doesn't get worse," Roberts said.

He did not have enough information to say what factors had caused the collapse: "There's not one single cause."

The massive cliff fall beneath 52 Brigantine Drive. Photo / Supplied

Asked about new wood stairs built between the house above the cliff and the beach, Roberts said: "There did seem to be something of a set stairs left behind. Possibly construction of those stairs could have had an effect on the cliff stability. But I haven't assessed whether construction of those stairs would have had an effect and I wouldn't be able to now anyway."

Recent landslips had occurred elsewhere, including Half Moon Bay, he said.

The council regulatory team had involvement but Roberts said he could only talk about the geotechnical aspects, having walked over the slip "or at least the bits that are safe to access. It's a fairly significant landslide. It's larger than the average but not completely out of the ordinary in Auckland.

"It has taken away a chunk of the rock mass and this happens all around the cliff lines in Auckland. They are constantly being eroded and moving backwards, between 1m and 6m every century, not at a steady rate but in chunks at a time."

Paul Carter, a senior geotechnical engineer of Engineering Geology, co-wrote a report lodged for 52 Brigantine Drive's resource consent. He also disagreed with some comments that the new house could have contributed to the cliff failure.

"The house is piled with loads from the house structure taken down deep into the ground to reduce loading on the slope. There is also an inground wall between the house and the slope. The landslip is away from the house and almost entirely outside the property boundary," Carter said.

The owners acknowledged in the August 4 Herald article they had lost some of their land.

Ben Wilson, whose new home is above the collapsed cliff, said: "I'm all consumed with a family matter this week. The council have all information relating to your questions."

Chris Darby, an Auckland councillor and Planning Committee chairman, expressed alarm about the cliff failure and said he was focused on what had caused that.

The council compliance team issued two abatement notices to Wilson: one on vegetation removal in 2019 and the second on stair construction on the council reserve this year.