State highway bureaucrats were repeatedly told that rockfalls posed "a significant hazard to motorists" on a King Country road where a massive boulder struck and a killed a passenger this year.
Major engineering works were never carried out to remedy the problem. Instead, standard maintenance, emergency responses and minor risk-reduction operations were done, including a $45,000 contract that was scheduled to begin three days after the lethal incident on March 28.
The revelations were exposed in Transport Agency documents released to the Waikato Times under the Official Information Act.
They stretch back to 2000 and show engineering contractors were "certain" rocks would continue to fall onto State Highway 3 and potentially strike vehicles long before Joy Thompson, 74, was killed beneath the southern Mangaotaki Bluffs.
The rock that struck her and her sister hit with such force that it stoved in the passenger side and spun the car 180 degrees on to the opposite side of the road.
Thompson's daughter Karen Ngatai was "disgusted" and in tears when she heard the details.
"I want someone to put their hand up and I want them to suffer in the way that we've all had to suffer," she said.
"I know I can't hurt anyone the same way our family's been hurt by this, but it was preventable - it didn't have to happen. I didn't have to lose my mother like that."
But the Transport Agency's acting highway manager for Waikato, Karen Boyt, was confident their contractors did all they could to prevent the deadly rockfall.
The 3-kilometre Mangaotaki Bluff highway corridor is one of the highest-risk sections in the region for rockfalls and she said their contractors monitor it closely.
Rockface hazard inspections are carried out monthly during stormy and high-rainfall months of the year and bimonthly the other six months. High-risk areas are also re-inspected after heavy rains.
But rockfalls were impossible to predict, she said.
"At the time, we didn't believe there was a high risk to vehicles and we wanted to get in prior to the risk becoming higher . . . This site wasn't seen as a high priority. If it had been seen as an imminent risk to vehicles, then it would have been prioritised."
Yet documents show a history of warnings and repeated rockfalls in and around where Thompson was killed. The southern Mangaotaki Bluffs consist of relatively hard limestone and sandstone overlying weaker mudstone.
The soft lower layer fritters away, undermining the rocks above.
Following a cliff failure on June 26, 2000, Transit's contractor, Transfield Services, reported in a preventive maintenance funding application that: "The large rockfalls are a significant hazard to motorists."
Blasting overhanging rock would have cost $41,000; removing all overhanging sandstone and limestone, $463,000; and additional work to lay siltstone back to a stable angle had a $2 million price tag. The proposal was rejected.
In 2002, Transfield made another application. It said the probability of a cliff failure within 12 months was judged to be certain. If nothing was done, the contractor said the bluffs will continue to drop material onto the highway with the possibility of rocks hitting vehicles.
"A truck has already been hit this year by a large block failure . . . The bluffs will fail continuously as the mudstone continues to undermine the sandstone/limestone. There is the distinct possibility of another vehicle being hit by falling debris."
Its recommendation was to install rock bolts, hang rock netting and smother the siltstone with shotcrete and vegetation. A large hanging rock was also to be removed. The application, at a cost of $200,000, was rejected. Another application in 2003 said large falling blocks caused "a major hazard for road users".
"These bluffs have failed many times historically and last year a particularly large failure blocked the highway. A passing truck pushed the rock off the road."
It recommended a geotechnical investigation and suggested the final solution would include rock bolting/rock netting on the sandstone/limestone, particularly on exposed blocks, and
vegetation on the mudstone.
The cost would have been in the region of $300,000 to $400,000. The application was rejected.
A 2004-05 request was approved for $15,000. The works included knocking down a precarious block, vegetating the mudstone and reassessing the following year to see if an engineered option was required. "Likely that rock bolting in the upper sandstone will be required. Netting and highly visible options are not preferred due to the aesthetic character of the area."
All applications above the general maintenance contract were assessed and prioristised on a national basis from head office in Wellington, Boyt said.
"If we go and spend a lot of funding in here, yes I know this is the site where it happened, but it could have happened anywhere. We could have put the bolts and netting on this slope and it could have happened on the northern slope."
All preventive work applications are assessed, perhaps by several staff, and they are prioritised based on a scoring system known as the "rockfall hazard rating score". It takes into account slope height, ditch effectiveness and average vehicle risk, among other factors.
Each category is given a rating from 3 to 81 - low risk to high.
Despite these earlier reports, Mangaotaki South was repeatedly given a low rating for vehicle risk, including the latest report. Yet the ditch buffer was always listed as "totally ineffective".
The 2012 rockfall hazard rating report said emergency work - blasting and vegetation removal - was carried out in 2011-12. It was expected to reduce the frequency of small-medium rockfall events, but "any further remedial works to prevent large events is outside the scope of rockfall hazard funding".
A 2013-14 hazard report was funded and the $45,000 rockfall mitigation was due to start on March 31. The report notes "ongoing falls per year, small to large rocks reaching carriageway common".
"A recent crash involving a truck hitting a rock on the carriageway has highlighted the ongoing risks along this section."
Thompson was cremated at the Taranaki Crematorium on April 3, the day after her granddaughter's birthday.
About 300 mourners paid their respects to the mother of two and foster mother to many.
Among them were two NZ Transport Agency staff - the regional director and acting highway manager at the time.
They attended "to show our sympathy to the family", Boyt said.
LONG TERM FIX SOUGHT
Design of a long-term engineering solution to the rockfall hazard at Mangaotaki South has now started.
The NZ Transport Agency's acting highway manager for Waikato, Karen Boyt, said their regular inspections, including the most recent on February 26, did not indicate there was a heightened risk of a rockfall in the area.
Mitigation works that began on March 31 have been completed.
"We have also carried out additional geological mapping of the rockface, blasting of large, at-risk boulders and removing vegetation and small rocks," Boyt said.
"Design of a long-term engineering solution to the rockfall site has now commenced, and we are mapping areas within this corridor which are at high risk for rockfalls, providing us with a more detailed picture of the rockfaces so we can see any changes."
She said this work is done by specialist contractors who abseil across these rockfaces to gather detailed geological information on their condition.
"The rockfall which claimed the life of Mrs Thompson is a terrible tragedy, and our deepest sympathies remain with her family and friends," Boyt said.
- Waikato Times
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