1 September 2017
Stunning, unbelievable, high resolution footage of the Zhangjiawan rockslide
Posted by Dave Petley
Stunning, unbelievable, high resolution footage of the Zhangjiawan rockslide
Reader Fabien has kindly pointed out to me that Liveleak has new footage of the Zhangjiawan rockslide in Guizhou Province in China, which killed 26 people and left a further nine missing and presumed killed. This is the recording, which sadly has no further metadata. Frustratingly WordPress no longer allows me to embed Liveleak videos, so I have embedded the Youtube copy of the video:-
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This is undoubtedly one of the best ever recordings of the development of a major rockslide, capturing the precursory rockfall activity as well as the final failure event. The impression is clearly of a progressively disintegrating rockmass, rapidly fragmenting as movement develops. This may well be associated with the development of rotation in the mass. I have captured three elements of the initiation of the final collapse below:-

The initiation of the Zhangjiawan rockslide. Stilll from a video posted pseudo-anonymously to Liveleak
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This sequence shows that at the point of the development of rapid motion the rockslide was undergoing both toppling (i.e. the mass was rotating such that the upper section of the rockmass was moving more rapidly than the lower portion, and the whole mass is in effect tilting forwards) and basal sliding into the valley. This is a large-scale version of the motion that can be imagined from this simple block diagram, although obviously it was far more complex when on the scale of the Zhangjiawan rockslide:-

Simple block diagram to illustrate toppling and slide, via the BC Government
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Meanwhile, in a comment on my previous post my good friend Tim Davies asked about the runout of the landslide, and in particular spectulated that the runout length might have been quite short. This ChinaNews image shows the full landslide track:-

ChinaNews image of the Zhangjiawan rockslide
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This suggests that whilst not being hyper-mobile, the track was probably not very short. There are some signs that the portion on the right side of the image has transitioned into a flow I think. Xinhua reports that the landslide volume was about 600,000 m³, with a fall height (I assume of the initial failure) of about 200 m.
I have not yet been able to locate this landslide reliably. Has anyone else had any luck?
This is interesting on a number of fronts. Firstly, the mechanical degradation and progressive failure points towards the importance of relatively small failures unloading the larger rock mass. It has serious implications for how rock slope stability is assessed. Secondly, and this might only be an issue of perspective, it appears that the very first failures are not downslope but lateral from the main failure so in fact the initial unloading may in face be closer to the sigma-2 direction not sigma 3. In most stability analysis this out of plane stress is never considered. Do you think this is an out-of-plane motion or just the viewing angle of the photographer?
Hi David It seems to me this location close to Nayong
https://www.google.de/maps/@26.616102,105.3966603,1320a,35y,180h/data=!3m1!1e3
From google map the mountain range looks like fault with many relict (dormant) landslides.
http://news.xinhuanet.com/english/2017-08/28/c_136562555.htm
Hi Dave, looks like this is the location of the landslide : 26°38’02.0″N 105°26’46.7″E, already rockfalls at the toe of the slope.
This shows the late precursory stage very well indeed! But presumably this doesn’t happen for an earthquake-triggered landslide…?
The video also shows that during initial large-scale motion the rock mass is disaggregating (collapsing) along existing joints, and that intact rock is not fragmenting to any great extent; the fragmentation process generates voluminous dust clouds, which become prominent only once the mass has started to impact the base of the fall. The famous Mt St Helens collapse in 1980 showed the same – but that was overtaken by release of high-pressure volcanic gasses.