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Articles, Christchurch uncovered

Mountains cannot be surmounted except by winding paths.     Johann Wolfgang von Goethe The Port Hills may not be mountains as such, but they formed a formidable barrier for the first European Settlers. Of course, Māori had a number of well-established … Continue reading →

Images, eqnz.chch.2010

The magnitude 7.1 Christchurch earthquake broke off an enormous chunk of Castle Rock in the Port Hills which has tumbled down towards the Lyttelton tunnel. View from Morgan's Valley (-43.578037° 172.714828°).

Research papers, University of Canterbury Library

The Canterbury Earthquake Sequence (CES) of 2010-2011 produced large seismic moments up to Mw 7.1. These large, near-to-surface (<15 km) ruptures triggered >6,000 rockfall boulders on the Port Hills of Christchurch, many of which impacted houses and affected the livelihoods of people within the impacted area. From these disastrous and unpredicted natural events a need arose to be able to assess the areas affected by rockfall events in the future, where it is known that a rockfall is possible from a specific source outcrop but the potential boulder runout and dynamics are not understood. The distribution of rockfall deposits is largely constrained by the physical properties and processes of the boulder and its motion such as block density, shape and size, block velocity, bounce height, impact and rebound angle, as well as the properties of the substrate. Numerical rockfall models go some way to accounting for all the complex factors in an algorithm, commonly parameterised in a user interface where site-specific effects can be calibrated. Calibration of these algorithms requires thorough field checks and often experimental practises. The purpose of this project, which began immediately following the most destructive rupture of the CES (February 22, 2011), is to collate data to characterise boulder falls, and to use this information, supplemented by a set of anthropogenic boulder fall data, to perform an in-depth calibration of the three-dimensional numerical rockfall model RAMMS::Rockfall. The thesis covers the following topics: • Use of field data to calibrate RAMMS. Boulder impact trails in the loess-colluvium soils at Rapaki Bay have been used to estimate ranges of boulder velocities and bounce heights. RAMMS results replicate field data closely; it is concluded that the model is appropriate for analysing the earthquake-triggered boulder trails at Rapaki Bay, and that it can be usefully applied to rockfall trajectory and hazard assessment at this and similar sites elsewhere. • Detailed analysis of dynamic rockfall processes, interpreted from recorded boulder rolling experiments, and compared to RAMMS simulated results at the same site. Recorded rotational and translational velocities of a particular boulder show that the boulder behaves logically and dynamically on impact with different substrate types. Simulations show that seasonal changes in soil moisture alter rockfall dynamics and runout predictions within RAMMS, and adjustments are made to the calibration to reflect this; suggesting that in hazard analysis a rockfall model should be calibrated to dry rather than wet soil conditions to anticipate the most serious outcome. • Verifying the model calibration for a separate site on the Port Hills. The results of the RAMMS simulations show the effectiveness of calibration against a real data set, as well as the effectiveness of vegetation as a rockfall barrier/retardant. The results of simulations are compared using hazard maps, where the maximum runouts match well the mapped CES fallen boulder maximum runouts. The results of the simulations in terms of frequency distribution of deposit locations on the slope are also compared with those of the CES data, using the shadow angle tool to apportion slope zones. These results also replicate real field data well. Results show that a maximum runout envelope can be mapped, as well as frequency distribution of deposited boulders for hazard (and thus risk) analysis purposes. The accuracy of the rockfall runout envelope and frequency distribution can be improved by comprehensive vegetation and substrate mapping. The topics above define the scope of the project, limiting the focus to rockfall processes on the Port Hills, and implications for model calibration for the wider scientific community. The results provide a useful rockfall analysis methodology with a defensible and replicable calibration process, that has the potential to be applied to other lithologies and substrates. Its applications include a method of analysis for the selection and positioning of rockfall countermeasure design; site safety assessment for scaling and demolition works; and risk analysis and land planning for future construction in Christchurch.

Audio, Radio New Zealand

As the nation prepares for lockdown, Christchurch's leaders says their city is prepared. Over the past ten years Christchurch has dealt with it's fair share of crisis, from earthquakes, Port Hills fires, the March 15 terror attacks, flooding, and a gas explosion. While Covid-19 has a global impact, some Cantabrians say their past experience will help them get through. Eleisha Foon reports.

Research papers, University of Canterbury Library

Following the 22 February 2011, MW 6.2 earthquake located on a fault beneath the Port Hills of Christchurch, fissuring of up to several hundred metres in length was observed in the loess and loess-colluvium of foot-slope positions in north-facing valleys of the Port Hills. The fissuring was observed in all major valleys, occurred at similar low altitudes, showing a contour-parallel orientation and often accompanied by both lateral compression/extension features and spring formation in the valley floor below. Fissuring locations studied in depth included Bowenvale Valley, Hillsborough Valley, Huntlywood Terrace–Lucas Lane, Bridle Path Road, and Maffeys Road–La Costa Lane. Investigations into loess soil, its properties and mannerisms, as well as international examples of its failure were undertaken, including study of the Loess Plateau of China, the Teton Dam, and palaeo-fissuring on Banks Peninsula. These investigations lead to the conclusion that loess has the propensity to fail, often due to the infiltration of water, the presence of which can lead to its instantaneous disaggregation. Literature study and laboratory analysis of Port Hills loess concluded that is has the ability to be stable in steep, sub-vertical escarpments, and often has a sub-vertically jointed internal structure and has a peak shear strength when dry. Values for cohesion, c (kPa) and the internal friction angle, ϕ (degrees) of Port Hills loess were established. The c values for the 40 Rapaki Road, 3 Glenview Terrace loess samples were 13.4 kPa and 19.7 kPa, respectively. The corresponding ϕ values were thought unusually high, at 42.0° and 43.4°.The analysed loess behaved very plastically, with little or no peak strength visible in the plots as the test went almost directly to residual strength. A geophysics resistivity survey showed an area of low resistivity which likely corresponds to a zone of saturated clayey loess/loess colluvium, indicating a high water table in the area. This is consistent with the appearances of local springs which are located towards the northern end of each distinct section of fissure trace and chemical analysis shows that they are sourced from the Port Hills volcanics. Port Hills fissuring may be sub-divided into three categories, Category A, Category B, and Category C, each characterised by distinctive features of the fissures. Category A includes fissures which display evidence of, spring formation, tunnel-gullying, and lateral spreading-like behaviour or quasi-toppling. These fissures are several metres down-slope of the loess-bedrock interface, and are in valleys containing a loess-colluvium fill. Category B fissures are in wider valleys than those in Category A, and the valleys contain estuarine silty sediments which liquefied during the earthquake. Category C fissures occurred at higher elevations than the fissures in the preceding categories, being almost coincident with bedrock outcropping. It is believed that the mechanism responsible for causing the fissuring is a complex combination of three mechanisms: the trampoline effect, bedrock fracturing, and lateral spreading. These three mechanisms can be applied in varying degrees to each of the fissuring sites in categories A, B, and C, in order to provide explanation for the observations made at each. Toppling failure can describe the soil movement as a consequence of the a three causative mechanisms, and provides insight into the movement of the loess. Intra-loess water coursing and tunnel gullying is thought to have encouraged and exacerbated the fissuring, while not being the driving force per se. Incipient landsliding is considered to be the least likely of the possible fissuring interpretations.

Audio, Radio New Zealand

At least five companies are busy working in and around Christchurch blasting rock on unstable slopes in the hope of reducing danger since the earthquakes. Spectrum's Deborah Nation joins backcountry construction company Solutions 2 Access, as the team blasts rock on the Port Hills above Lyttelton.

Audio, Radio New Zealand

For over a decade, an irreplaceable ring laser has been trapped in a cavern 30m below Christchurch port hills. The Carl Zeiss laser was installed in an old World War Two bunker in 1997 until a rockfall after the 2011 Christchurch earthquake sealed the cavern. But now it's been rescued and is fully operational. Professor Jon-Paul Wells is the principal investigator on the ring laser project.

Research Papers, Lincoln University

Numerous rockfalls released during the 2010–2011 Canterbury earthquake sequence affected vital road sections for local commuters. We quantified rockfall fatality risk on two main routes by adapting a risk approach for roads originally developed for snow avalanche risk. We present results of the collective and individual fatality risks for traffic flow and waiting traffic. Waiting traffic scenarios particularly address the critical spatial-temporal dynamics of risk, which should be acknowledged in operational risk management. Comparing our results with other risks commonly experienced in New Zealand indicates that local rockfall risk is close to tolerability thresholds and likely exceeds acceptable risk.

Images, UC QuakeStudies

Two workers inspect fuses placed in an embankment during reinforcement work. The photographer comments, "This is the reinforcing of an embankment in the port of Lyttelton, which partly collapsed in the Christchurch earthquakes. They are using the same equipment as used for blowing up rock faces to mend them".

Audio, Radio New Zealand

Topics - the latest twist in the Kim Dotcom saga is that the police have announced they're investigating the Government Communications Security Bureau's illegal surveillance of the internet mogul. New research has shown that membership of all mainstream Christian denominations has fallen to historic lows - except for Catholicism. New research has uncovered the role underground rock structures played in stopping the September 2010 Christchurch earthquake from linking up with the Port Hills faultline and causing greater damage to Christchurch.

Articles, UC QuakeStudies

A post on the NZ Raw blog written by Mark Lincoln on 24 February 2011. Mark says, "I think this is the first post I wrote after the Feb 2011 earthquake. That first photo was my first view after coming out of the office. There's a popular wide panoramic photo that someone took from the Port Hills of all of the dust rising up from the city - the photo in the blog post shows what it looked like from within the dust cloud! There are people gathering further down the street where a building has collapsed".

Audio, Radio New Zealand

The Aromaunga Baxters Flowers nursery in Heathcote, Christchurch sits right above the point where the earthquake struck on 22 February 2011. The greenhouses on the steep slopes of the Port Hills, as well as a big old villa and other brick buildings were badly damaged. Ten years on co-owner John Baxter says the earthquake damage is still being repaired, but sales have been boosted by a lack of imported flowers due to Covid-19 restrictions.