Rock slope kinematic instability controlled by large-scale variation of basalt column orientation

Field observations of columnar-jointed basalt lava flows at Organ Pipes National Park near Melbourne, Australia, show varying columnar cooling joint orientations. The valley-confined basalt lava flows have columnar joints forming a downward spreading fan as the joints adopt orientations perpendicular to the margins of the flow. The variation in the pattern of columnar joint orientations controls local slope failure mechanisms, including toppling and planar sliding. Manually collected field data and terrestrial laser scanning data for joint faces have been used to assess the kinematic stability of the rock slopes. The terrestrial laser scanning data obtained includes a greater number of joint face orientations than manually collected data, including data for otherwise inaccessible locations. However, the bulk data collected by TLS requires careful interpretation at sub-sites within the field of observation to enable variation in column orientation and the associated slope stability mechanisms, to be recognised. The collection of larger datasets by remote methods has the risk of smoothing out local variations. The data in this case study demonstrates that the commonly assumed simple parallel pattern of columnar joints cannot be assumed, even in large, thick lava flows such as the Organ Pipes National Park.