Soak holes not to blame

By GRAHAM SKELLERN
Geologists have revealed their initial findings on the cause of last month's landslides.
SOAK holes can not be blamed for the devastating landslides that destroyed homes and caused millions of dollars of damage in Tauranga's worst flooding last month.
This is the preliminary finding of two expert geologists, Bernard Hegan and Dr Laurie Wesley, who were brought in by Tauranga City Council to probe the cause of the slips along the Otumoetai ridge.
There was no direct relationship between the presence of soak holes and the slope failures, the geologists told 120 affected residents at Baycourt last night.
Tauranga had endured a one-in-100-year 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 Bureta catchment, Te Maire Stream which flooded the Mount Maunganui industrial area, and State Highway 2 from Domain Rd to Te Maunga where water crossed the road and flooded parts of Papamoa.
The council will also stage suburban meetings to enable residents to have their say.
Tauranga Mayor Stuart Crosby said "we will focus on street by street and work with all the affected people. We will not take our foot off the accelerator as we work through this process and put your lives back on track."
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.
The Otumoetai ridge was the remains of sea cliffs formed by erosion and consisted of layers of volcanic ashes susceptible to the weakening influence of rainfall. There were also ancient buried valleys where groundwater seepage was concentrated.
At some sites there were vertical cracks in the upper soil layers - water entering these cracks not only softened the soil but also created a large horizontal force from water pressure within the crack.
"The discharge of large volumes of water at the top of the slopes could be a significant factory to the instability," the geologists said.
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 both 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 Balmoral Rd/Landscape Rd during 2005/06; Pillans Rd, Karaka St, Milton Rd, Goods Rd, Shelley St and upper Bureta Rd during the next five years; and 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."
The district plan outlined hazard zones and 50-year flood-prone areas and properties there had sold at good market values.