The Beach Haven house with the massive slip. Photo / Chris Keall
More details are emerging about the land beneath a new Beach Haven home where a cliff collapsed last month but Auckland Council is yet to say what caused the slip.
The site near Charcoal Bay was never built on until a 343sq m two-level $1.12 million dwelling and palisade wall were completed at the $2.25m property last year, records also showed.
The Herald reported this month on the huge slip beneath the home when land collapsed from the cliff down the Waitemata Harbour.
Despite this, the resident-owners said they had decided to stay. They had been reassured by a geotechnical engineer that 300 cubic meters of concrete under the ground, 38 piles and a palisade wall were still doing their job and keeping the rest of the cliff from giving way.
The collapse had caused them to lose about 400 cubic metres of land from their property and access to the reserve below. No other amenities were damaged or lost, they said.
"We lost about 30 nearly 40sq m of territory in front of us, but it's about 17m above sea level, so we are talking about hundreds of cubic metres lost," the owners told the Herald on August 4.
This included hundreds of native plants they said they had spent hours planting down the side of the bank to replace felled pine trees. The house was only completed last Easter after six years of planning and building, they said.
The owners said they didn't receive any warning about building on the land and got multiple geotechnical investigations done before construction.
An engineering report submitted for the resource consent application for the new house last decade noted the presence of extensive vegetation on the cliff beneath the site back then and also how the site had never previously been built on.
Resource consent documents from 2018 recorded the presence of vegetation on the cliff underneath 52 Brigantine Dr.
Albany-based consultants Engineering Geology's cliff regression analysis four years ago said: "The cliff is covered in vegetation with a good cover of residual soil indicating that regression of the cliff is slow."
Bushes and large trees covered the north, west and southern slopes around the property, the engineers noted at that time.
Historical records indicate the North Shore cliffs are regressing on average about 2m to 6m each century, Engineering Geology's report said.
The engineers also clearly warned what could happen to the cliff. It wasn't at the top that their warnings focused on, but at the bottom at the beach and the foot of the cliff, referred to as the toe.
"If erosion occurs at the toe, it is likely that the slope will steepen as the toe is progressively eroded," Engineering Geology cautioned.
Cliff regression "may occur sporadically". To stabilise the building platform, the construction of a deep in-ground palisade wall on the seaward side of the site should be undertaken to protect the land from potential slumping, Engineering Geology's report said.
The site was underlain by sandstone and mudstone of the East Coast Bays Formation.
"There is the potential for some ground loss due to slope failure and cliff erosion within a 100-year time span," the engineers said. "The site is well-vegetated and shows no sign of regression in the last 50 years."
A council spokesperson told the Herald applicant Ben Wilson had uplifted two building consents to allow him to build the new dwelling above the cliff.
"Stage one [of the consent process] dealt with the building platform and palisade wall or inground barrier pile wall. This structure stabilises the building platform as calculated by the Geotechnical engineer.
In the approved report, the geotechnical engineer carried out cliff regression and stability calculations and estimated the 100-year cliff regression line. Because of the location of the regression line and taking into account soil loss and the location of the building structure, the house was designed to be founded on piles, some 8m deep, she said.
"During the course of assessing the application, the council requested for the palisade wall to be designed for seismic loads," she said indicating the authority sought an even stronger wall than was originally proposed.
"Also, the council requested the geotechnical engineer to review the foundation piles designed by the owner's structural engineer (information submitted and accepted," the council spokesperson said.
Stage two of the consent dealt with the pile foundations, floor slab and the rest of the building. The house has a concrete precast panel and floor slap frame.
"This site has been unoccupied. Early photos in 1963 show no nearby buildings adjacent to or on the site. Around 1996 some buildings do appear adjacent to the site and indications are that the dwelling would be the first permanent structure recorded on this site," the council spokesperson said.
Council records show the home has a capital value of $2.25m of which $1.13m is the land value. The section is listed as being 966sq m although how much has been lost with the cliff collapse is unknown.
Plans lodged by SMC Design Studio show a 17m-tall pine between the house and the cliff, an 18m high pine on the other side, a number of pohutukawa and manuka. Erosion and sediment control measures were to be installed before major earthworks at the site, plans showed.
Chris Darby, a North Shore councillor, found older Google Streetview photos showing pine trees between the land and the sea some years ago.
In 2012 Google photos, tall pines appear on the skyline between the existing homes and the sea, in the vicinity of the site Wilson later bought.
By August 2019, when Wilson's new home is shown under construction, no pine trees appear in the photo in front of his new house lot. Some pines do still appear to the right in front of other homes along the clifftop.
Darby said discovering what caused the cliff failure was impossible without expert reports but his first question was whether vegetation had been removed between the house and the sea and if so, who had removed that vegetation and why.
The Herald reported this month that heavy rain had battered Auckland in July and that had caused the land made up of clay, dirt, sandstone and vegetation to slide away in front of the Brigantine Dr property on July 15.
But Darby said geotechnical engineers, arborists and council officials would need to examine the site, cliff and fall to ascertain the reasons for the failure.
"From the photo, it's a very significant loss of land and it's cost the council reserve," he said referring to the area between the house and the sea.
"It's in the interests of the council to examine and ascertain why there's been a failure on its land and has that been caused by activities it was not aware of?" Darby asked.
He is discussing that with officials now.
He looks across to the Bayswater peninsula and says he has seen many landslips in that area over the years.
"I've seen lots of cliff failures in the last 25 years and they appear to be related to significant vegetation loss such as the topping of pohutukawa. I've asked council officers how we keep people up to speed with caring for the coastal environment. What sort of vegetation should people be planting and not planting? Why shouldn't you put a stormwater outfall at the top of a cliff? Look, people do. They don't know," he said stressing he was speaking generally and not about the Beach Haven property.
"We do have a responsibility as a council to properly inform people," Darby said.
"When you remove significant trees, the root matter dies off and stops bind soils anymore. There's nothing above the roots that is demanding food. When those roots die, the soils become unstable," he said.
The geotechnical reports covered relevant issues, he said.
Darby has also examined the Beach Haven home's resource consent documents and said the new palisade wall had been built to a high standard, with a tremendous amount of steel and concrete above the cliff top to attempt to protect the residential site.
"I notice the palisade wall is designed as per specifications. There's an incredible depth of piles in that wall. They are 8m deep. It says the steel for each pile had to be inspected by a structural engineer before it was dropped into the hole. And there's a whole lot of holes. It's a large arc that goes around that development," he said of the palisade wall.
He asked if the construction of such a wall in itself could cause disturbance to the fragile cliffs beneath but stressed he had no geotechnical expertise.
Such matters should be left to the experts to examine, discuss and comment on, Darby said.
Paul Carter, an engineer of Engineering Geology who co-wrote a report on the property for the resource consent, said construction of the house and palisade wall would have had no effect on the cliff's stability.
"Rain would, of course, be a contributing factor," Carter said.
Such cliff movements were not uncommon in Auckland.
"It's important to know it was considered in the design and the owners took advice," Carter said.
The 2019 plans for the new residence by SMC Design Studio labelled the area between the house and the sea a "public reserve". Around 29 per cent of the site was to be covered by the house, the architects estimated.
Auckland Council's geomap calls the area between the house and the sea Jacaranda Avenue Esplanade Reserve, fronting 48-60 Brigantine Dr residential properties.
The Herald asked homeowner Ben Wilson if vegetation had been on the cliff when he bought the property, what the vegetation was, if that had been removed, who had removed it and what approvals had been gained if that was the case.
Wilson said he was dealing with a family matter and the council had all the information relating to those questions.
On a Beach Haven and Birkdale community social media page, one person responded to the Herald article in early August: "Felled pine trees were holding cliff together. Your new planting doesn't have enough root system to hold the soil so of course when it rained, heaps slipped. The drilling and pounding in of piles probably didn't help either."
Another member referred to "land scraping" and the load the new house placed on the site.
An Auckland Council spokesperson said earlier this month the council was aware of the cliff collapse but has not been alerted to any harm to property or people that have arisen from it.
The council was aware a geotechnical engineer had assessed the slip and advised in early August there were no immediate concerns about further subsidence or risk of damage or harm.
'Apartments on the site and more than likely offices' – Andrew Moore, CMP Construction.