Port Hills slope stability report released

Rockslides seen on the Port Hills after the 6.3 earthquake in Christchurch. Photo / File

A long-awaited report looking at the slope stability of the Port Hills above Christchurch has identified 36 areas of "mass land movement" since the earthquakes.

Christchurch City Council commissioned GNS Science to complete the report in response to changes in the stability of slopes in the area following the 2010/2011 quakes.

The Canterbury Earthquakes 2010/11 Port Hills Slope Stability: mass movement Stage One Report released today details investigations of 36 areas in the Port Hills where mass land movement has occurred.

The report assesses the nature of hazards and if these hazards pose a risk to life, homes and critical infrastructure.

'Mass movement' describes slope instability from the movement of significant volumes of soil and/or rock, says Mike Theelen, general manager of the council's strategy and planning group.

The Stage One Report, he says, will give affected Port Hill land owners with a preliminary level of knowledge on the slope stability of the area in which their property is sited.

The council this week sent letters to landowners in the 36 affected areas to let them know their property is located either completely, or partly, within the areas of slope instability identified in the report.

A total of 134 homes are located in the worst zone, with 53 of them being deemed green zoned and therefore could be occupied.

"The information in this GNS Science report, and further investigations in subsequent stages of the project, will provide the council with more information about these areas when making decisions on consenting, land use and infrastructure planning and development," Mr Theelen said.

"These areas will need to be managed in a different way to reduce the likelihood of causing additional instability.

"This could include stricter control of earthworks, surface and subsurface water control, vegetation clearance and retaining walls in the Port Hills. These will be addressed by changes to the District Plan.

"For property and landowners looking to repair and rebuild, we want to assure them the council will continue to process consents in these areas, but for future development we are likely to need them to provide more site-specific geotechnical information than we do at present."

The council has been working with the Engineering Advisory Group and the Ministry of Business, Innovation and Employment (MBIE) to produce guidance for engineers working on foundation solutions for some of these areas.

"We understand that Port Hills property owners face ongoing uncertainty and delay, but decisions involving people's lives and homes need to be well informed, and based on good quality and sufficient scientific information and advice," said Mr Theelen.

Christchurch mayor Lianne Dalziel hoped the report will give residents further information which will help them understand what has happened to land stability on the Port Hills as a result of the quakes.

"Many residents will want further information," she said.

"We are planning public meetings for people most affected, and council staff will be available to meet with people one on one to discuss what the report findings mean for them."

A full copy of the Stage One Report and more information is available on the council's website at www.ccc.govt.nz/porthillsgeotech.