6 November 2013

Our strange desire to find a landslide trigger

Posted by Dave Petley

When I was studying Physics at school, we were shown the famous regelation experiment in which a wire is placed over a block of ice and a weight is attached to each end.  Though time the pressure on the wire causes melting of the ice, and the wire slowly cuts its way through the block, and eventually the weights and wire fall to the ground.  As the wire passes through, the water refreezes, such that the wire appears to pass magically through the ice.

There are various versions of this experiment on Youtube, of which this is about the best:


The interesting part of this experiment starts at about 1:22 and ends at 1:40 (in the speeded up sequence).  Of course when the wire finally cuts through the ice the weights collapse to the floor with a great crash – the very last moments before this are shown below:

landslide trigger


This experiment, though fun, illustrates an important aspect of first time failures in rock slopes.  If you weren’t watching the experiment closely then the collapse looks spontaneous – I remember when we were shown the experiment one of my class rates immediately asked “What caused that?”.  However, the collapse had no trigger – it was a spontaneous event caused by a progressive process.  Now, if towards the end of the experiment that same classmate had leant on one of the weights then he would have triggered the collapse.  However, he didn’t cause the failure, which was generated by the wire cutting its way through the block.

The Tumbi Quarry landslide brings this into sharp focus.  Very often in the aftermath of large slope failures we spend a great deal of time looking for a landslide trigger.  We analyse rainfall records, look at seismicity, examine temperature data and investigate the actions of people.  Sometimes we even look at a combination of all these things.  And, lo and behold, eventually we find the event, or combination of events, that caused the collapse to occur at that specific moment in time.  Occasionally, we can’t even find a landslide trigger, as was the case in the Mount Cook landslide in 1991.

The point is not that triggers are unimportant – of course they are, especially in slopes in weak materials, and it is sensible to try to understand the relationship between the number of landslides and the magnitude of large triggering events in any landslide-prone environment.  However, for individual first time failures in rock slopes the search for a landslide trigger is often a red herring.  The reality is that the slope has weakened through time until it is on a hair-trigger; when a large enough event comes along the collapse occurs.  If no event occurs then the slope collapses spontaneously; this is a true progressive failure.

So, spending time looking for a trigger for the Tumbi Quarry landslide is interesting and worthwhile, and I have no problem with this sort of work.  However, we must first recognise that the basic question should be whether there is a landslide trigger at all, and if so what?  It may well be the case that no landslide trigger can be identified.  Second, we should not allow the search for a landslide trigger to distract us from the real issue – i.e. what were the processes that led to the rock mass becoming unstable, such that the trigger became effective?

To me the key issues at Tumbi Quarry remain unanswered, and the multiple victims of the landslide mean that they deserve more attention.