22 February 2012

The Tumbi Quarry landslide – an initial forensic examination of the images

Posted by Dave Petley

The LNG Watch blog has posted online some new images of the aftermath of the Tumbi Quarry landslide in Papua New Guinea.  This is a good time to look in a little more detail at the landslide itself.  The most interesting images are these two:

The second of these, assisted with some information from the first, suggests that this was a multi-phase landslide event.  Let’s start by looking at the central portion of the landslide:-


I have annotated on the image above three key areas.  In A there is an area of disrupted but intact vegetation, which must have come ass an almost intact block from the slope above.  It is not back tilted, so the slide is not rotational, which is consistent with the very planar form of the (presumably joint-controlled) back scarp.  At B there is an arcuate secondary scarp cut into the block that forms A, and below this is a flow deposit, of which the upper component clearly derives from B.

If we now look at the upper part of the landslide we see another secondary failure:

This landslide, marked D above, appears to be a late stage earthflow (it is mostly soil) over the back scarp.

Let’s now take a look at the mid-part of the landslide.  There is a marked difference between the left and right (as viewed from the image) sides of the landslide.  The left side (marked E below) has a steep scarp that appears to be at least 10 m high:

The right side on the other hand still shows the extension of the ridge that runs across the slope, albeit with the vegetation stripped off, suggesting that the landslide was shallow in this region.  So the landslide was deep on one side and quite shallow on the other.  At G the landslide debris appears to have in part ridden over a section of rock, but presumably most of the debris was diverted around this into the centre of the landslide.

Done at the toe the landslide debris clearly spilt into two and followed the drainage lines.  On the left side there is some evidence that it “cut the corner” on the inside of the first bend (marked H below) and super-elevated on the outside (marked as I):

On the other (right) side the debris appears to have travelled straight down the drainage line:

Of course what is not shown by all of this is the first event – i.e. the major failure that started the sequence of failures.  We really need some idea of the form of the topography before the slides to get an idea of this, and we need to get on the ground to look in detail at the deposits that are not covered by the secondary failures.  The  most intriguing aspect is that the distribution of volume change in the head scarp source area does not seem to match the distribution along the track very well.  In the image below, the zone of largest volume change is at J (ignore the superficial earthflow that has partially filled this area), but most of the debris appears to have passed through the area marked K (look at the flow lines, and bear in mind the almost vertical scarp on the left side).  This implies that the initial part of the landslide may have followed the general trajectory shown by the arrow:

However, for the material highlighted in A in the third image above to move into the landslide, material from this zone must have moved out first.  It seems likely therefore that the initial failure occupied the deeper portion of the area from J to K.  To understand how and why this section failed we really do need to have an idea of the pre-failure topography and, critically, the location of the quarry and its spoil.  I am particularly interested in the latter as none of the images show any  waste tips that I can see, which means that this debris was either removed from the site or it has been incorporated into the slope failure.

So the burning questions remain:

1. Where was the quarry excavation?

2. Where was the spoil?

3. What was the rainfall in the period leading up to the slip?

It is deeply frustrating that we seem to be no closer to an answer to any of these questions.

Comments and thoughts, and alternative suggestions, are very welcome. It would be good to crowd-source an interpretation of this landslide head of the official investigation (?).  My interpretation above should be considered to be no more than a set of hypotheses that need testing properly by the investigation team.