Soak holes not to blame for landslides, say geologists

Soak holes cannot be blamed for the devastating landslides that destroyed homes and caused millions of dollars of damage in Tauranga last month, experts have concluded.

Geologists Bernard Hegan and Dr Laurie Wesley brought in by Tauranga City Council to probe the cause of the slips along the Otumoetai ridge.

The geologists told 120 affected residents at Baycourt last night there was no direct relationship between the presence of soak holes, which are created for drainge, and the slope failures.

Tauranga had endured a once in a century storm -- a very severe event -- and there was probably so much water entering the ground from many sources that the influence from soak holes was not great, the geologists said.

They also pointed out there were similar sites in Tauranga that would be subject to slips in future severe rainstorms and these sites could not be made totally secure.

The council has begun a far-reaching investigation into the effects of the flooding and intends to appoint a hydrologist to give advice on the best way to make the stormwater systems more efficient.

Stephen Town, the council's chief executive, said "we want to make sure the (stormwater) systems are working... and we want to provide better solutions to keep people safe in their homes".

The investigation is expected to take up to six months, but the council plans to report initial findings to another public meeting in three months.

The council, however, is already planning to add $500,000 to the 2005/06 budget to investigate ways to improve stormwater systems in three major trouble spots.

The geologists said in their preliminary report that very few of the slips they inspected could be considered surprising or unusual given the steepness of the slope.

The landslides generally occurred in places likely to be vulnerable to intense rainfall -- there had been ancient or relic slips there in the past.

A large number of houses in Tauranga were located on sites similar to where the slips occurred and would be vulnerable in a future severe rainstorm.

It was virtually impossible, for technical and economic reasons, to implement measures that would make these houses totally secure but limited measures could be taken.

They suggested that soak holes located close to the edge of steep slopes could be eliminated, as could surface run-off flowing toward the edge of vulnerable slopes -- particularly water coming down concrete driveways and cul-de-sacs.

They said the construction of pole walls retaining significant quantities of fill was clearly undesirable as it placed added load at the top of the slope, thus increasing the chance of failure. Some slips had taken walls and fill with them.

Mr Town said the council would assess whether the planned stormwater projects were sufficient and whether their implementation was rapid enough.

"We suspect you want us to spend more on stormwater and we will have to look at how we pay for these improvements," he said.

A total of $2.6 million had already been included in the long-term community plan for works in many affected streets during the next five years; and in Links Ave, Mount Maunganui, in 2006/07.

Real estate agent Paul Tozer said property values shouldn't fall in Otumoetai because of the flooding and landslides.

If the properties were fixed up and they were certified then "I don't see a problem".

- BAY OF PLENTY TIMES