The mechanism of the Highway 3 landslide in Taiwan

That bastion of "quality" journalism, the Daily Mail, has an article about the Highway 3 landslide that includes a set of high quality images of the site:

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The article correctly identifies the failure as being a dip-slope slide:

"The hill had a dip slop [sic - should be slope] on the side nearest the motorway. The other side of dip slopes are steep and irregular, while the slope itself makes it easier for rocks to slide down, experts said.  The Ministry of Transportation said it as [sic] investigating up to 20 similar dip slopes near major roads in Taiwan."




A dipslope means that the beds in the slope were inclined sub-parallel to the slope surface, creating a plane of weakness on which sliding can occur, like this (from here):

The sliding surface can be clearly seen in the second image, though it was less steeply inclined than in the cartoon above.  The inclination of the beds parallel to this surface on the displaced block is very clear.  Excavation of the debris will have to be done very carefully indeed to avoid the upslope material slipping down onto the workers.

The lack of a trigger implies a progressive failure - but I am surprised in that case that signs of distress were not observed prior to failure. 

Finally, it is interesting that the Ministry of Transportation says it is investigating "up to 20 similar dip slopes near major roads in Taiwan".  Having spent a great deal of time in Taiwan I would be surprised if there were just 20 such slopes, but maybe these are just the very large ones by major highways.  The 18th August 1997 Lincoln Mansions landslide in Taipei, which killed 28 people, was also a dipslope failure, and the 2009 Shiaolin disaster was a wedge failure with a dipslope component.

The government really needs to get a grip of the management of slopes in Taiwan.

The geological structure of the Hsiaolin slide

Thanks to reader Chingying Tsou, who has answered my request for information about the geological structure of the Hsiaolin landslide. He has provided a link to the website of the Sino-Geotechnics Research and Development Foundation, which provides the key information. The page is in Chinese, but the diagrams are really helpful, as is the Google translation.



So here is a topographic map of the landslide site, with the major faults and of course the landslide itself marked on:

You will note two cross-section lines on the map, one of which (A-A') runs down the axis of the landslide. This cross-section is reproduced below:


It is clear from this that the landslide is a dipslope failure - i.e. the slide has occurred on beds that are orientated parallel to the slope, and thus facilitate failure. The cross-section indicates that the rocks are a mixture of sandstone and shale. This can often cause problems as the shale is weak, impermeable and prone to weathering, whereas the sandstone is often stronger but allows the accumulation of water (i.e. pore pressure generation). The presence of the fault is an additional factor - it may well be that the movement on the fault has caused the beds to be disrupted and thus weakened. It should also be noted that this cross-section is probably only indicative. It would not surprise me to find that the river has actually eroded out the lower portions of these beds, then filled in the spaces with the terrace deposits upon which the village was built, further weakening the slope.