All hill areas face ongoing slope stability risks, but earthquake damage has increased this risk to some land in the Port Hills.
As part of the Port Hills land damage assessment programme, in November 2013 Christchurch City Council released the GNS Science Stage One Report which identified potential areas in the Port Hills that were at risk from mass movement (sometimes called landsides).
The Report gave mass movement areas in the Port Hills a preliminary Class I, II or III category. Class I areas are where any further mass movement could see lives lost, and homes and/or critical infrastructure severely damaged. In a Class II area, any further mass movement could damage homes and aﬀect critical infrastructure, and in Class III could cause damage to homes. Lives are not thought to be at risk from the mass movement hazard in Class II and Class III areas.
In August 2014, the Council released eight further GNS Science Reports looking further at the level of risk from mass movement in eight areas between Mount Pleasant and Sumner. The reports showed 37-green zoned homes were in areas where the risk to life from mass movement is considered ‘intolerable’ by the Council. This means the risk to life for a resident from mass movement in any one year is equal to or greater than one in 10,000.
The Council and Crown has purchased properties, where there was no cost-effective engineering solution to reduce the risk to them to an acceptable level, or their properties are needed to carry out engineering works to reduce or remove the hazard.
Engineering concepts have been developed for each of these areas to reduce the risk to the other homes to an acceptable level. The approach has been to remove the hazard wherever possible.
Quarry Road - Main Road
Work here has removed the hazard through benching the slope. This work was completed in early 2015.
Quarry Road - The Brae
Remediation work will comprise of earthworks to remove unstable soil. The work, undertaken by SCIRT for the Council, started in January 2016 and is expected to take about six months.
A bund is being constructed here to contain debris from a slope failure, and protect green zoned houses in this area. Works began in September 2015, and should take around 10 months. The work is being done by Fulton Hogan, working for SCIRT.
Defender Lane - Egnot Heights/Taupata Street
Works here, completed in mid 2015, are a combination of risk mitigation measures, including slope excavation at Egnot Heights, and the construction of a debris barrier (or bund) at Taupata Street.
This area is affected by mass movement as well as cliff collapse and rockfall affecting the road corridor. Remediation works will be carried out as part of the Sumner Lyttelton Corridor programme.
If you have any questions about the works in any of these areas, please contact SCIRT through the Council's customer call centre on (03) 941 8999 or 0800 800 169.
If you have any concerns over possible land movement and you believe there is a risk to life, self-evacuate, and call 111.
If you notice land changes, but don't believe there is an imminent risk, please contact the Council on 941 8999 or 0800 800 169.
Canterbury Earthquakes 2010/11 Port Hills Slope Stability Reports - released August 2014
Stage One Mass Movement GNS Science Report [PDF 18MB] released in November 2013