On Friday 3 June FOGG and PV welcomed geologist Ross Cayley from the Geological Survey of Victoria to talk about the recent landslides in the Grampians. We all learned a lot and greatly enjoyed the day .We were most appreciative for the effort that Ross had put in to the preparation for the talk. He even updated it with reference to the minor earthquake just two days previous.
About 29 members, visitors and PV staff were present for the activity.
Ross started his presentation by placing the Grampians in a national and international context with regard to landslide frequency. He made the point that landslides occur with much greater frequency and intensity in many other regions of the world and indeed in Australia. New Zealand and the Otways are good examples. In fact he said that the Grampians landslides would be hardly noticed in NZ!
He showed us a series of slides explaining the various kinds of landslides and their features. He pointed out that the majority of the January events in the Grampians happened in areas where mudstone was the underlying rock. Mudstone is a major component of the Silverband Formation which outcrops along the eastern margin of the Mt Difficult and Serra Ranges. Water can permeate the pores of this rock causing an increase in internal hydraulic pressure as well as a substantial increase in weight. These factors combine to reduce the stability of the area and eventually to its failure.
Many of the slides occurred under joints and gullies in the overhanging sandstone cliffs. As the sandstone itself is impervious, water would have been directed into these joints and gullies thus accelerating the movement of the soil below. Once the soil started to move downhill, obeying the laws of gravity, adjacent parcels would tend to move in sympathy as they were further destabilised.
Ross showed us a compilation of videos from around the world which dwarfed the events of last January in the Grampians. We saw some astounding footage of hundreds of metres of mountains, several million tonnes, sliding downhill. Perhaps one of the most amazing was a slide in Afghanistan which looked like a river of rock pouring down a dry river bed.
An excursion up the Silverband Road allowed Ross to show us some of the features which he had talked about earlier. He made the point that many of the slips were likely to be more stable now than before they took place and that the Grampians landscape has been largely formed as a result of past landslides.
In short the landslides in the Grampians, though surprising and alarming to us, were completely natural events following several years of drought which caused soil to contract and plants to die thus allowing more water to permeate into the soil. This combined with the unusually heavy rain in the aftermath of Cyclone Yasi caused these completely unforeseeable events. If we have a repeat of this rainfall we can expect more landslides.