Work to mitigate the geotechnical risk around Sumner Road is progressing well, with rock scaling and detailed design for the permanent rockfall mitigation works in the upper half of the road nearly complete.

Read more below about other developments in the work to mitigate risk and repair the road so it can be re-opened.

A section of the work above Sumner Road on the Lyttelton side has been redesigned from a significant excavation involving the removal of up to one million cubic metres of material over eight benches across the bluffs, to one main catch bench below the bluffs.

A catch bench is a rock bench, designed to ‘catch’ falling rocks before they land on the road. Original consented plans were for eight benches across the bluffs in the area known as area C in the project.

After further design consideration, a single catch bench will now be constructed instead. This latest design is not only more cost-effective but also has considerable environmental benefits. The area of the new catch bench design is shown in the top image, and the bottom image shows where the eight catch benches would have gone.

A single catch bench will cost less than the original multiple bench design and leave the bluffs intact. It is a highly resilient solution, which would not be significantly damaged by future rockfall. It would contain debris coming down, and reduce rockfall risk to road users in the area by intercepting debris.

Under the new design, there is ready access for the debris clearance needed prior to construction of the catch bench. There will also be the flexibility to adapt design to ground conditions during construction.

A small temporary upper safety bench will be built to make the area safer for construction of the catch bench, and the design minimises long-term ongoing maintenance.

Excavation of the catch bench, using explosives, will start from 15 June. No explosives will be stored on site. The explosive charges will be placed into holes that have been drilled into the ground to break the ground apart. This will also limit the noise and vibration.

It’s a very different method than the one used in March which was designed to split surface rock and released more noise and vibrations.
There will be about three to four blasts a week for the next seven months to remove around 100,000 cubic metres of rock.

During that time if you would like updates on blasting please email your name and phone number to sumner@mcdgroup.com. If you have any questions or concerns please contact the team for more information on 0508 623266.

The design is complete for the rock interception bund in area F (area shown in the blasting map above). The design is based on 3D rockfall modelling, that shows this design offers high levels of resilience and can sustain multiple rock strikes.

Built of rock wrapped in Geotextile steel mesh, the bund will be low-maintenance. There will be quick and easy access for people clearing any rockfall that lands behind the bund, increasing safety for those who will maintain it. 

Considerable work has been undertaken by Council landscape architects to ensure it blends in with the environment.

The repair and rebuild of key retaining walls along Sumner Road is being fast tracked. Retaining walls below and to the west of the bund in area F are being designed now. Construction may start in around a month.

You haven’t been able to see it until now, but the Sumner Road Risk Mitigation Project team has been very busy scaling or removing approximately 10,000 cubic metres of unstable rock from the bluffs above Sumner Road.

The photos below show how much rock has been scaled over the last seven months and how dangerous the road was before and during the mitigation work. The mountain of rock that was perched precariously on the bluffs will now be recycled onsite and moved safely down past Windy Point or up Sumner Road to the Gollans Bay quarry.