Chris Massey

Determining Rockfall Risk in Christchurch Using Rockfalls Triggered by the 2010–2011 Canterbury Earthquake Sequence

The Canterbury earthquake sequence triggered thousands of rockfalls in the Port Hills of Christchurch, New Zealand, with over 6,000 falling on 22 February 2011. Several hundred families were evacuated after about 200 homes were hit. We characterized the rockfalls by boulder-size distribution, runout distance, source-area dimensions, and boulder-production rates over a range of triggering peak ground accelerations. Using these characteristics, a time-varying seismic hazard model for Canterbury, and estimates of residential occupancy rates and resident vulnerability, we estimated annual individual fatality risk from rockfall in the Port Hills. The results demonstrate the Port Hills rockfall risk is time-variable, decreasing as the seismic hazard decreases following the main earthquakes in February and June 2011. This presents a real challenge for formulating robust land-use and reconstruction policy in the Port Hills.

The integration of terrestrial laser scanning and numerical modelling in landslide investigations

Abstract Terrestrial laser scanning (TLS) can be used to either complement or replace traditional methods of characterizing both the geometry and structural geology of unstable slopes. TLS data collected from a failed bedrock slope threatening the main east–west highway in the Bhutan Himalaya are presented and interrogated for structural information. The structural data, along with TLS-derived slope geometry and cross-sectional profiles, are suitable for use within commercially available slope stability packages to derive solutions for the causes of instability, likely geometry of failure, and future activity under varied scenarios. The methods also allow the possibility of future model verification and calibration though TLS monitoring. The results of TLS-based numerical modelling utilizing a commercially available code are presented and the implications for slope surveying, numerical modelling, monitoring and management are discussed.

GIS modelling in support of earthquake-induced rockfall and cliff collapse risk assessment in the Port Hills, Christchurch

Ground shaking associated with the 22 February 2011 Mw 6.2 Christchurch earthquake exceeded MM10 and dislodged boulders from cliffs on the upper slopes of the Port Hills, southeast of Christchurch City. Boulders rolled into the urban areas below. Cliffs on the lower slopes collapsed and debris avalanches inundated homes built at their bases and undercut homes built at their crests. Boulders and debris avalanches impacted over 200 buildings and killed five people, resulting in widespread evacuations. Large aftershocks caused further boulder rolls and cliff collapses. Before buildings could be reoccupied a life-safety risk assessment was required. This study included pilot investigations of the 19 worst affected areas and these were used to develop hazard and risk models. Once ground verified, the models were extended to the entire Port Hills area. GIS was the main tool used in the development of the models, but other tools and techniques were also utilised.

Evolution of an Overflow Channel Across the Young River Landslide Dam, New Zealand

New Zealand’s Young River was dammed on 29 August 2007 by 11 million m3 of debris at 44° 08′ 44.6″S, 169° 06′ 46.0″E. The fractured biotite-schist debris travelled up to 1.8 km to form a 74-m high dam. Water flowed from the new 2.5 km-long, 23 million m3 lake after 5 weeks. The slope on the dam face is about 10°. The mean surface particle size is about 1 m (uniformity coefficient, 8). Lake level and rainfall at the dam have been monitored since 10th October 2007. There have been many high flows since then; the largest, 3.4 m deep at the outlet. The outlet had lowered 0.7 ± 0.2 m by January 2011; the channel mostly widened but locally aggraded and degraded as boulders 2.1 m. The rainfall which put 3.4 m water depth through the outlet had an annual exceedence probability of 0.1 a−1. We estimate the expected (most probable) longevity of the lake as twice the current age, but recognise that this probabilistic estimate will eventually be 2x too large.

Numerical Evaluation of 2D Versus 3D Simulations for Seismic Slope Stability

Open image in new window Seismic slope stability hazards assessments based on two-dimensional (2D) analyses limits the interpretation to in-plane motion. This means that the effects on ground motions due to lateral heterogeneity contributed by internal wave reflection are neglected. In this study we assess to which extent a 2D model of a simple topographic crest (spur) is valid, using equivalent 3D simulations. The study shows that seismic analysis based on the analysis of 2D cross sections of slopes with irregular topography such as spur crest lines can misrepresent the response of the slope at the surface. Regular slope geometries representing a laterally infinite slope can be well represented by 2D analyses, however as the slope becomes laterally irregular, the topographic contribution needs to be given careful consideration by way of 3D simulations.

Did Radiative Cooling Trigger New Zealand’s 2007 Young River Landslide?

The 11 million m3 Young River landslide (44° 08′ 44.6 ″S, 169° 06′ 46.0″ E) fell at 4:40 h on 29 August 2007 in New Zealand’s Mt Aspiring National Park. It fell from an extensive mass of dilated closely jointed schist with pre-existing evidence of rock-mass creep and prior rockfalls. The remaining slope has only marginal stability, as had the prior slope for a long time. It required very little change to trigger rapid failure. The fall left a seismic record indicating a lack of an earthquake trigger. The landslide fell in winter, and the deposit shows evidence of much groundwater in the source, hence we dismiss a permafrost-thaw trigger. The failure occurred just before the usual coldest time of night. Rainfall and daily maximum and minimum temperatures at climate stations in the region suggest that the landslide fell on a night with extreme radiative cooling to a dry, clear, night sky (the largest diurnal temperature drop that winter), after several weeks of clear, cold nights. We suggest that the immediate trigger was an abnormal rise in groundwater pressure when springs froze on the face of the landslide source area. Since freezing of springs is likely to be frequent in winter, we suggest that the rise in pore pressure on the night of 28–29 August 2007 affected a progressively lowering threshold of rock-mass strength from mass creep. Hence failure was inevitable, and the particular trigger was irrelevant.

TXT-tool 1.064-1.1 Field Guide for the Identification and Assessment of Landslide and Erosion Features and Related Hazards Affecting Pipelines

This Field Guide was originally prepared by GNS Science in 2007 as part of the Institute of Geological and Nuclear Science (GNS Science) Short Course on the Identification and Assessment of Landslide and Erosion Hazards aimed at pipeline overseers and technicians. The guide was originally designed as a reference for technicians to use in the field during pipeline inspections, but can also be applied to other engineered structures. It contains background information on landslides and other erosion processes, including: some definitions; a short glossary of relevant landslide terms; discussion on observation and recording of geological and geomorphic information in the field; a landslide and erosion classification scheme developed for pipeline technicians; and an example “Relative Hazard Exposure Matrix” that could be used as an initial tool to assess and rank the potential consequences to a pipeline should landslide and other erosion hazards occur.

Displacement mechanisms of slow-moving landslides in response to changes in pore water pressure and dynamic stress

Although slow-moving landslides represent a substantial hazard, their detailed mechanisms are still comparatively poorly understood. We have conducted a suite of innovative laboratory experiments using novel equipment to simulate a range of porewater pressure and dynamic stress scenarios on samples collected from a slow-moving landslide complex in New Zealand. We have sought to understand how changes in porewater pressure and ground acceleration during earthquakes influence the movement patterns of slow-moving landslides. Our experiments show that during periods of elevated porewater pressure, displacement rates are influenced by two components: first an absolute stress state component (normal effective stress state) and second a transient stress state component (the rate of change of normal effective stress). During dynamic shear cycles, displacement rates are controlled by the extent to which the forces operating at the shear surface exceed the stress state at the yield acceleration point. The results indicate that during strong earthquake accelerations, strain will increase rapidly with relatively minor increases in the out-of-balance forces. Similar behaviour is seen for the generation of movement through increased porewater pressures. Our results show how the mechanisms of shear zone deformation control the movement patterns of large slow-moving translational landslides, and how they may be mobilised by strong earthquakes and significant rain events.

CrEAM Modelling of Groundwater-Triggered Landslide Acceleration at the Utiku Landslide (New Zealand)

Recurrent acceleration and deceleration of the Utiku landslide (New Zealand), to velocities up to 15 mm/day, is attributed to seasonal changes in pore pressure in the slope. High-resolution monitoring shows that the landslide velocity increases disproportionately as pore pressure increases. This non-linear relationship between pore pressure and displacement rate is investigated with CrEAM (Creep Equilibrium Analysis Method, Engl 2013). CrEAM is a novel analytical tool for displacement analysis of creeping landslides. The computational effort is relatively low, making the approach well suited for time-efficient back-analysis.