8 March 2012
Analysing the pre-landslide image from Tumbi Quarry
Posted by Dave Petley
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.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Observing the lateral scarp on the left side in the post-movement photo, one could argue that the cultivated slope above the access road may have moved initially, especially if it was loaded with spoils. The slope adjacent to and above the quarry would have gone for the ride, secondarily. It looks like the slope above the road but below the quarry failed and was not draped by debris, as one might expect if only the highwall of the quarry failed.
The report describes the pre-existing quarry as, “…located on a limestone surface expression in the hill slopes above the Tagari River.” I see no signs of benches or ramps on the weathered but apparently competent quarry face, so I wonder if most of the limestone surface must have been already exposed prior to historic quarry activity.
Since the slide photos indicate a high degree of regolith weathering, which you would expect here, doesn’t a large exposed limestone face with little weathering indicate a slide potential? If so, were there any site investigations that investigated that supposition? Also, if the exposed face was the result of a large slide, there must be an oral history in the area mentioning it. Was that investigated prior to site work?
In any case comparing the pre and post slide photos, the slide appears to have been at the extreme SE of the site, along an apparent drainage path. Since the drawing indicates the quarrying should stay between major drainages, I’m wondering if perhaps the quarry activity extended across the southern drainage afterall, exposing faulting to surface drainage.
The Google Maps link below shows the area with ‘terrain’ showing. Not sure of the DEM resolution, but if you look at the uplands contours the topography looks karstic, so drainages could be unpredictable.
To orient yourself, zoom out two levels and Komo is in the SE corner of the map. The area shown is similar in scale to the inset map on the quarry drawing. You can trace the roadfrom the Tegari River crossing to the NW (shown on the inset near the Juni training facility), and the road goes past the quarry SE down to Komo.
http://goo.gl/Y4QtW
[…] together with the eye-witness description of the failure event in the NZ Herald article and the pre-landslide aerial photos, in a post in a couple of days. I think together they provide a better understanding of what […]
[…] together with the eye-witness description of the failure event in the NZ Herald article and the pre-landslide aerial photos, in a post in a couple of days. I think together they provide a better understanding of what […]