Highway Rockfall Cause A Mystery


No-one seems to be able to get to the bottom of a rockfall in the Mangorewa Gorge on State Highway 36 between Tauranga and Rotorua, that damaged two cars, trapping and injuring three people.

The rockfall happened last August near where contractors were working to replace two bridges and widen the highway.

NZTA and the Rotorua District Council have received the last of three independent reports.

It  found the rockfall couldn’t have been anticipated and NZTA and Rotorua District Council took all practicable steps to protect the safety of employees, contractors and sub contractors, and other people in the area.

While the report could not identify a single cause for the rockfall, it concluded that one or more of the following factors could have contributed:

  • blasting vibrations
  • seismic activity,
  • vegetation and tree roots,
  • vertical cracks in the rock strata
  • water expansion (due to freezing)
  • frost and thermal expansion.

Separate reports by the Institute of Geological and Nuclear Sciences and Opus International Consultants Ltd also found that a single cause for the incident could not be identified.

The labour department says it will take no further action over the incident but has recommended that the lessons learned from the incident be shared with other roading contractors.

But the NZTA will remind contractors to take the highest form of control practicable when facing similar hazards. SH36 was closed to traffic for three months following the rockfall to allow debris to be cleared and remedial works to be carried out.

The work included installing rock anchors up to 6m deep into the rock faces and draping steel mesh over the cliff areas to secure the gorge site.

The overall gorge improvement project is expected to be finished in early April.




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