15 February 2012
A question that has arisen a few times about the Tumbi Quarry landslide in Papua New Guinea is why it is that a quarry can increase landslide occurrence. The first point to be made is that many quarries, when properly managed, do not cause slope instability. However, some of the most spectacular landslides are caused by mines or quarries. Two come immediately to mind – this is the Yallourn open cast mine landslide in Australia (I have never seen one like this anywhere else!):
And this (courtesy of Caner Zanbak), is the Collolar lignite mine landslide in Turkey two years ago:
So why do quarry walls fail? There are many reasons, but the most important include:
- Almost always the quarry walls are cut more steeply than the existing slope. If this slope is too steep for the rocks in question a slip can develop;
- Cutting the slope sometimes “daylights” (exposes the toe of) an existing discontinuity or plane of weakness, along which sliding can occur. In Collonar above it is likely that sliding occurred on a pre-existing weak layer;
- Cutting the slope can change the groundwater conditions, which can lead to pore pressure build up in weaker areas;
- Blasting the slope can cause fractures to develop that allow the release of a block;
- Cutting the slope can remove layers of strong materials, exposing weaker strata that are prone to failure.
The design of quarry walls, and the maintenance of them, is a highly specialised task into which responsible operators invest a great deal of resource. As a result monitoring of quarry wall movement is a key part of quarry operations, and many quarries operate safety systems to allow them to evacuate is instability develops. As a result, catastrophic quarry wall failures remain rare. Of course it is not clear that any of these are the case at Tumbi Quarry, but these are some of the processes that the investigation should be considering.
It would be good to have some examples of quarry or open cast mine landslides highlighted in the comments. Would anyone like to suggest an example or two?