Clay and moon blamed for Omokoroa slips

THREAT: A slip in Omokoroa on Friday. PHOTO / ANDREW WARNER

Scientists suspect the moon's gravity and a unique type of clay are causing landslides in Omokoroa as forecasters warn Western Bay residents to brace themselves for more heavy rain.

Researchers from the University of Waikato and University of Bremen have spent five years investigating the relationship between landslides and sensitive soils in the Bay of Plenty town.

Yesterday they said they had found an unusually-shaped type of clay mineral called halloysite that makes the soil more "sensitive".

The news comes during a month in which landslides have damaged buildings and sections belonging to at least five Omokoroa families, with warnings that more slips could follow heavy rain this week.

A severe weather warning has been issued for the Western Bay, with heavy rain expected to hit from midday today to midday Friday, with up to 250mm expected.

The ground is already saturated from last week's deluge, and the Western Bay of Plenty District Council is urging residents to make sure they're prepared.

Council spokesman Frank Begley urged residents to clear gutters and stormwater grates, prepare evacuation plans and stock up on fresh water and food.

Motorists should minimise their travel, and people should avoid areas where there might be slips.

In Omokoroa, the university researchers found upper and lower aquifers separated by a zone that limited the flow of water between them. This area had previously been identified as the failure surface for large landslides at Bramley Drive.

Waikato senior lecturer in earth sciences Willem de Lange said they had examined ways that sensitive soils formed and the processes that could trigger "long runout" landslides.

"Sensitive soils are capable of rapidly liquefying when disturbed, and produce landslides that travel further away from their source than normal landslides," he said.

The soils, which are found in volcanic deposits around the Bay of Plenty and Waikato, are a hazard to a large number of people.

"The halloysite clays at Bramley Drive have an unusual shape, like the cap of a mushroom, which had not been previously recognised," Dr de Lange said.

The shape allowed the clay to release "a significant volume of water" and cause "very fluid landslides."

Researchers were investigating how the halloysite formed and were looking at ways to increase the stability of sensitive soils.

"Initial laboratory tests are promising, and we are looking at obtaining funding to progress to large scale field trials," Dr de Lange said.

Another interesting finding was that ground at Omokoroa moved by varying amounts in response to the moon orbiting the Earth.

"So far these changes are very small. However, there is growing evidence that we can use the Earth tide movement to predict at what time of day a landslide will occur if all the factors contributing to instability are present.

"Unfortunately we cannot use this to predict what day it will happen."

Halloysite

Is a clay mineral
Contains aluminium, silicon and hydrogen
Was first described in 1826 by a Belgian geologist
Forms during a "hydrothermal" process
Can occur in an unusually pure form in New Zealand

Source: Wikipedia