'Poorly constructed' landfill major factor in landslip

A house is demolished after it was left teetering on the edge of a massive slip in Kelson, Lower Hutt. Picture / Mark Mitchell

An investigation into the huge landslide in the Lower Hutt suburb of Kelson earlier this month has revealed a major cause was poor construction of a landfill 40 years ago.

Contracters were forced to demolish a house in Kelson after the landslip left the home teetering over the edge of a gully.

The massive slip on the evening of August 7 all but destroyed the house forcing the owners Ann and Nigel Nation and three neighbouring homeowners to be evacuated.

Hutt City Council decided to demolish the house after engineers reported that it was too dangerous to try to save the property.

Council communications manager Don Carson told NZPA today engineers had found the landfill -- about 40 years old -- was poorly constructed.

"One of the basic requirements of doing this sort of thing (construction of a landfill) is that you don't leave the original vegetation there.

"You strip it off otherwise it decays and leaves an organic layer between the original land surface.

"The fill you are putting on top simply lubricates the slip."

Despite pinpointing one of the major contributing causes of the landslip, it was unlikely the Hutt City Council would pursue legal action, Mr Carson said.

"The council are not really trying to find out who or what was at fault 40 years onwards -- from the evidence of case law that is a very hard road to hoe.

"And it's rather pointless, the evidence is really of academic interest."

Mr Carson said there was no point in trying to find out who did it unless there was overwhelming evidence and somebody to take action against.

"Neither of them apply and not only was there poor construction but there had been some amateur drainage work carried out around that area of the slip some time after the original landfill."

Mr Carson said the council's main focus was health and safety -- which it was satisfied with.

The council also had to decide what to do about the sewer pipeline, he said.

"We have three options -- find another way for the pipe to go around on the remaining land, make a more permanent suspended pipe or reinstate the area with fill so the pipeline went under the ground as it originally did but more securely," he said.

Mr Carson said the council did not have a timeframe for the project.

"We will be having discussions very soon with the Earthquake Commission as they are a party to this as well."

Mr Carson said he was confident that where there were dwellings the slip would not continue to undermine them.

One person had been advised to move out but had chosen to remain in their house, he said.

Some residents along Vista Crescent had also been advised not to go out on their decking, he said.

Mr Carson said by summer the council hoped to know what the longterm future of the area was.

"Obviously the people in those surrounding houses would prefer to have flat land out the back of their properties rather than a whopping great big slip."

"But there is a lot of dollars in doing that work and it's 50 metres across and it's a very steep slope."

The other contributing factor was the record amount of rainfall the Hutt had in July, he said.

"I hope we don't have that sort of rain again and hopefully its shaken those loose properties which were on the point of going anyhow."

- NZPA