Engineer denies fault

Slips in Whitby are most likely due to builders overloading land after engineers have signed off sections, according to the engineering firm involved.

Whitby Coastal Estates employed Johnsonville- based engineer Cardno when it built the south-east Whitby development over the past decade. The area has suffered numerous slips.

Many section owners later employed Cardno to assess plans for their houses.

But engineers could not guarantee builders and homeowners would comply with engineers’ recommendations, said Cardno manager Ray O’Callaghan.

Too much soil might be loaded on to a site or walls, and struc- tures might be built larger than plans allowed. Builders might fail to install enough sub-soil drains into new fill, he said.

In the case of the Helm Pl properties pictured in last week’s Kapi-Mana News, earthworks to build up a flat lawn happened after Cardno signed off the section, Mr O’Callaghan said.

The non-engineered fill might have overloaded the underlying natural soil.

‘‘The cause of any slip is that the weight of the soil is greater than the internal stresses capable of keeping soil in place.’’

Wet soil was heavy, and poorly compacted soil held lots of water, which could also cause slipping, Mr O’Callaghan said.

Another possible cause of slips was builders dumping topsoil into gullies below Whitby properties. Organic matter under the soil then rotted and became slippery, he said.

If failures could be explained it was possible building codes were failing to meet conditions on the ground, Mr O’Callaghan said.

Then of course there were acts of god, which was why engineers took out insurance, he said.

‘‘Sometimes it’s an unexpected, underlying failure that noone could have seen, and that’s when EQC comes in.’’

Cardno had assessed the majority of developments in the Wellington region for 30 years, and had never had a slip on its sections before further work was done, Mr O’Callaghan said.

‘‘We’ve not had slips or structural failures anywhere in Whitby in the 30 years that we have been working there.’’