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Research papers, University of Canterbury Library

The collapse of Redcliffs’ cliff in the 22 February 2011 and 13 June 2011 earthquakes were the first times ever a major failure incident occurred at Redcliffs in approximately 6000 years. This master’s thesis is a multidisciplinary engineering geological investigation sought to study these particular failure incidents, focusing on collecting the data necessary to explain the cause and effect of the cliff collapsing in the event of two major earthquakes. This study provides quantitative and qualitative data about the geotechnical attributes and engineering geological nature of the sea-cut cliff located at Redcliffs. Results from surveying the geology of Redcliffs show that the exposed lithology of the cliff face is a variably jointed rock body of welded and (relatively intact) unwelded ignimbrite, a predominantly massive unit of brecciated tuff, and a covering of wind-blown loess and soil deposit (commonly found throughout Canterbury) on top of the cliff. Moreover, detailing the external component of the slope profile shows that Redcliffs’ cliff is a 40 – 80 m cliff with two intersecting (NE and SE facing) slope aspects. The (remotely) measured geometry of the cliff face comprises of multiple outstanding gradients, averaging a slope angle of ~67 degrees (post-13 June 2011), where the steepest components are ~80 degrees, whereas the gentle sloping sections are ~44 degrees. The physical structure of Redcliffs’ cliff drastically changed after each collapse, whereby seismically induced alterations to the slope geometry resulted in material deposited on the talus at the base of the cliff. Prior to the first collapse, the variance of the gradient down the slope was minimal, with the SE Face being the most variable with up to three major gradients on one cross section. However, after each major collapse, the variability increased with more parts of the cliff face having more than one major gradient that is steeper or gentler than the remainder of the slope. The estimated volume of material lost as a result of the gradient changes was 28,267 m³ in February and 11,360 m³ in June 2011. In addition, surveys of the cliff top after the failure incidents revealed the development of fissures along the cliff edge. Monitoring 10 fissures over three months indicated that fissured by the cliff edge respond to intense seismicity (generally ≥ Mw 4) by widening. Redcliffs’ cliff collapsed on two separate occasions as a result of an accumulated amount of damage of the rock masses in the cliff (caused by weathering and erosion over time), and two Mw 6.2 trigger earthquakes which shook the Redcliffs and the surrounding area at a Peak Ground Acceleration (PGA) estimated to be around 2 g. The results of the theoretical study suggests that PGA levels felt on-site during both instances of failure are the result of three major factors: source of the quake and the site affected; topographic amplification of the ground movement; the short distance between the source and the cliff for both fault ruptures; the focus of seismic energy in the direction of thrust faulting along a path that intercepts Redcliffs (and the Port Hills). Ultimately, failure on the NE and SE Faces of Redcliffs’ cliff was concluded to be global as every part of the exposed cliff face deposited a significant volume of material on the talus at the base of the cliff, with the exception of one section on the NE Face. The cliff collapses was a concurrent process that is a single (non-monotonic) event that operated as a complex series of (primarily) toppling rock falls, some sliding of blocks, and slumping of the soil mantle on top of the cliff. The first collapse had a mixture of equivalent continua slope movement of the heavily weathered / damaged surface of the cliff face, and discontinuous slope movement of the jointed inner slope (behind the heavily weathered surface); whereas the second collapse resulted in only discontinuous slope movement on account of the freshly exposed cliff face that had damage to the rock masses, in the form of old and (relatively) new discontinuous fractures, induced by earthquakes and aftershocks leading up to the point of failure.

Videos, UC QuakeStudies

A video of an interview with Mark Yetton, Engineering Geologist at the Port Hills Geotech Group, about their work to assess the risk of rock fall and cliff collapse on the Christchurch Port Hills.

Images, UC QuakeStudies

A damaged house perched on the edge of the cliffs above Sumner. The photographer comments, "This house has slowly gone over the edge since the big earthquake in Christchurch in February 2011. Subsequent earthquakes has caused the rock face to crumble more and more".

Images, UC QuakeStudies

Part of a stone wall left sticking out over the edge after the cliff below it collapsed. The photographer comments, "A viewing platform in Clifton has its foundations exposed after the cliff face collapsed".

Videos, UC QuakeStudies

A video of a helicopter using a monsoon bucket to wash loose debris off a cliff in Redcliffs. The debris is from an earthquake-damaged building on the edge of the cliff which has had to be urgently demolished after wet weather and aftershocks made it unstable. The video also includes an interview with Brenden Winder, Red Zone Cordon and Access Manager at CERA, about the removal of the debris and the need to check other properties.

Videos, UC QuakeStudies

A video of an interview with Terry Huggins about a boulder crashing into the side of his neighbour's house in Sumner. The boulder came lose from the cliffs above and smashed through the deck of the house into one of the supporting poles. It is the third large boulder to hit the house, with one also lodged in a bedroom.

Images, UC QuakeStudies

A digitally manipulated image of a sign reading "A bit of dirt never hurt". The photographer comments, "This was a sign put up on a section of land in the Port of Lyttelton where an earthquake damaged building had been removed. The cliff at the back had collapsed down probably during the demolition process".

Videos, UC QuakeStudies

A video of a press conference with Mayor Bob Parker, Roger Sutton (CEO of Orion), Superintendent Dave Cliff (Christchurch Police), and Chief Fire Officer Dan Coward. The speakers talk about the work that is being done to bring power back to Christchurch residents, and to assess the safety of buildings in the Christchurch central city.

Images, eqnz.chch.2010

Cliff Face Above Sumner Beach (10.03.2011) Sumner Christchurch Canterbury New Zealand © 2011 Phil Le Cren Photo Taken With: Canon EOS 1000D + Canon EF/EF-S lenses + 10.1 effective megapixels + 2.5-inch TFT color LCD monitor + Eye-level pentamirror SLR + Live View shooting. + EOS Built-in Sensor cleaning system + Wid...

Images, eqnz.chch.2010

RSA - Rocks From Cliff Face (10.03.2011) Sumner Christchurch Canterbury New Zealand © 2011 Phil Le Cren Photo Taken With: Canon EOS 1000D + Canon EF/EF-S lenses + 10.1 effective megapixels + 2.5-inch TFT color LCD monitor + Eye-level pentamirror SLR + Live View shooting. + EOS Built-in Sensor cleaning system + Wide-...

Videos, UC QuakeStudies

A video of a press conference with Gerry Brownlee announcing a CERA review which will change the zoning of 270 Port Hills properties. Brownlee announces that 247 properties will change from green zoned to red zoned and 33 properties will change from red zoned to green zoned. The properties that have been rezoned red have an unacceptable level of life risk from cliff collapse and the potential of debris inundation.