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Sunday, 11 March 2012

Prof Petley's Landslide Analysis Reiterates Need for Investigation

By Prof Dave Petley
Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom. 
(Originally Posted on the Landslide Blog)

Yesterday I posted a photo of the Tumbi Quarry site before the landslide.  Today I thought I’d look at this image in a little more detail.  The photograph was collected on 27th January 2010, i.e. almost two years before the landslide:

For reference, lets compare this with the post-landslide image:
So lets start with the quarry, and home in on the section of the new photo that shows the landslide source area (see image below). It is clear that the entirety of the workings in the pre-landslide photograph was destroyed in the landslide.  The section of the quarry that survived (see the second photo) was upslope and across from the original workings – I have marked this as point a on the image below:
It is quite helpful to take a look at the Exxon-Mobil plans for the development of the project, as highlighted in an earlier post:
The dark grey area is the original quarry (as per the pre-landslide photo), the yellow is the new haul road up to the higher quarry section, and the light grey, light blue and green hatched areas are the new quarry, or lands to be cleared for quarrying.  The plans appear to be more-or-less consistent with the configurations in the two photos.  Note that the landslide appears to have destroyed most of the haul road, and it is somewhat unclear as to how much the new quarrying had expanded into the light grey area.  A key question remains as to where the haul road was located, and where the quarry spoil was being dumped.
Going back to the new photo, in the older (weathered) section there is clearly a stream issuing from the quarry face (marked as b on the image) – this water course is also shown on the map above.
Let’s take a look at the land-use.  The photo shows that most of the area that slipped was forested with mature trees, which agrees with the map above.  There are some cultivated areas near to the main road, but these are for the most part not in the landslide area.  The source area is densely forested, which suggests that it is unlikely that deforestation was the cause.  I have previously noted that in such a deep-seated landslide, land-use change is unlikely to be a primary factor.
There are some features in the landscape that are almost certainly small (but certainly not trivial) landslides.  Point c on the image above is almost certainly a slip, and point d is probably another.  Note though that in both cases these appear to be slips in soil or regolith, not in bedrock as per the main landslide.
The most intriguing features remain these linear structures in the slope above the main quarry (point e).  Superficially these look like either tension cracks or footpaths – from an image like this it is impossible to discriminate.  I am erring slightly in favour of them being footpaths simply because it is hard to imagine a quarry being operated in an area with such tension cracks.
So what does this tell us?  Well, we can in effect rule out land use change as being a major factor in this landslide unless there was catastrophic felling between the image being collected and the landslide (and even then I do not believe that it would be a major factor in such a deep landslide).  The presence of the stream suggests that the limestone was well-drained, but of course blockage of the source might have serious implications for the slope.   The landslide has removed most of the quarry plus the associated infrastructure.  It is impossible to say that the landslide was caused by the quarry, but it is also clear that there is nothing in the image that would definitely indicate that the landslide was not associated with it.
The need for a proper independent inquiry
Of course all of this indicates that there is a need for a proper, independent assessment of this landslide.  I know that there are now moves by some to either try to get a court order to undertake such an investigation, or to commission such a process independently. Clearly either route would be expensive, so those involved are trying to raise the funds to support these efforts.  We must remember that at least 25 people died in this event, and maybe many more.  Personally, I would have thought that it is in the interests of all parties, including the quarry operators, to understabd what has happened here.  It could well be that those responsible for the quarry are completely exonerated by such an investigation.

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