STRUCTURAL CONTROL ON ROCKFALL: A GEOLITHOLOGICAL AND GEOMECHANICAL CHARACTERIZATION OF MIOCENE VOLCANICS

Rockfall releases, or detachment zones, are controlled by planes of weakness in a rock mass such as bedding planes, faults, foliation, joints, and cleavage. Little work has examined the relationship between primary structures (cooling joints and bedding) and detachment zones in volcanic rock. The February 22 and June 13, 2011 events of the Canterbury Earthquake Sequence triggered widespread rockfall within Lyttelton Volcanic Group deposits. This study attempts to isolate primary structural controls on rockfall in volcanic rock by characterizing the geomechanical and geolithological properties of a single rockfall source above Rapaki, New Zealand. The source rock is inaccessible due to the continuing seismic hazard, so a scanline was conducted on an outcrop proximal to the source cliff face and an Unmanned Aerial Vehicle (UAV) collected images of the exposure. The source cliff contains both coherent and brecciated lava flows, which are not equally distributed on the face and show unique joint patterns and rock strengths. Scanline data and rock mass characterization investigated the orientation and persistence of discontinuities within a coherent lava unit and bounding breccias. Images of the cliff face taken by the UAV were collated into a single orthomosaic image, and used to map and classify the cliff exposure and detachment zones. Detachment zones were classified based on the lithologies, the presence of visible joint release, and lithologic contacts at the boundaries of the detachment zone.

Image analysis estimates that roughly 9% of volcanic cliff above Rapaki detached as rockfall boulders in the recent seismic sequence. Detachment zones are semi-randomly distributed across the cliff face, with notable absences where the slopes are shallow or the cliff face is concave into the slope. Detachments are often defined by lithologic boundaries, which is in part due to a decrease in Geological Strength Index values because of the increase of tightly spaced subhorizontal joints. Changes in primary cooling joints in both coherent and brecciated lava flows near lithologic contacts are the strongest control on rockfall detachment zones. This study highlights the importance of understanding the primary structures, which directly influence mechanisms of release both during seismic events and under static conditions.