Some geotechnical work and terrain evaluation had gone on near the spot, but the road crews “hit a very localised section of soil which was completely unique in itself to the geology”, Waka Kotahi said.
“It was unlike any other soil composition that was found within the closure site.”
The report confirmed engineers had not found any other areas of such slippery soils, “but this should be reconfirmed as part of the risk review and works completion procedures”.
The report suggested the project was not yet out of the woods and recommended investigating for any further “uncemented” soils which could present a wider risk.
It also noted a number of “major geological features” that should be further assessed.
Waka Kotahi said the observational approach taken in February was one that let it balance the need to get a thorough understanding of the geology with urgency to get work done in the drier months.
The report laid out how the area was now being stabilised, with diggers removing dirt at the crest of the collapsed slope, then working their way down excavating some or all the residual soil as they went.
“We consider this would be a safe approach,” the engineers said.
The highly weathered rock underneath meant that once the soil was gone “we expect the slope should achieve adequate global stability”.
Without the recovery work going on now, the road would likely have to be closed for six to eight times briefly, and once for a long time, each year, it said.
“These recovery works will significantly enhance the level of service of the road and prevent these long duration closures.”
RNZ