Rock blasting reviewed

The Conservation Department (DOC) is reviewing the way it decides when to destroy nature to protect the public.

DOC came under fire from conservation groups in July after it blew up a sandstone overhang on the popular Truman Track in the Paparoa National Park, following a rockfall near a viewing platform.

It was accused of taking a ‘‘blast now, think later’’ approach to a spectacular natural feature and registered archaeological site.

This month’s West Coast Conservation Board meeting heard DOC had chosen to extend the track into a dangerous area without any expert assessment of the risk.

A report by board member and caving authority Neil Silverwood said DOC had recently invested in new stairs and a viewing platform, making the site more accessible to the 35,000 tourists who visited the Punakaiki track each year.

‘‘This section passed beneath a sandstone cavern. Prior to building, no geotechnical assessment of the cavern, or risk to new structures was undertaken.’’

In July, a 4-tonne block fell off the cliff near the cave, and the track was closed for geotech assessment. The findings showed that while there was no immediate danger, the sandstone cliff had natural weaknesses and might collapse over time, Silverwood said.

The overhang was then brought down using explosives, releasing about 70 tonnes of rock.

‘‘The cavern was a unique geological feature, containing a remnant blowhole and it was a registered geopreservation site,’’ Silverwood said.

The blasting had been done largely to protect access, but tourism and recreation appeared inconsistent with

The popular Truman Track in the Paparoa National Park. the conservation of a landform that DOC was required by law to protect.

‘‘The section of the track contains several other similar sandstone cliffs, which are a large part of the attraction . . . all are vulnerable to collapse and there is a significant risk to visitors. That risk cannot be mitigated without removing the very thing visitors come to see,’’ Silverstone said.

The Opus consultants’ report supplied to the Greymouth Star recommended the removal of the overhang.

However, it recommended against the use of explosives.

‘‘We recommend engaging a contractor to scale back the overhang in small blocks ... the use of high explosives is discouraged to prevent blast damage to surrounding rock mass.’’

A second Opus report, done after DOC blew up the overhang, found the blast had caused further damage.

‘‘Unfortunately the blasting has fractured the rock mass in the residual section of the overhang . . . this needs to be monitored ... especially after heavy rain,’’ it says.

Robert Dickson, DOC’s Buller district operations manager, said the department had taken further advice.

‘‘We engaged an experienced geotechnical ground-engineering geologist independent of Opus to provide a more detailed site assessment of the overhang.’’

That had confirmed the necessity to remove the overhang as it posed a significant public safety risk, Dickson said.

DOC has been accused of ‘‘blast now, think later’’ over a spectacular natural feature.