30 June 2008

International Symposium of Landslides – Wenchuan (Sichuan) earthquake landslides

Posted by Dave Petley

This week is the 10th International Symposium on Landslides and Engineered Slopes, which is a once every four years get together. This time round it is being held in Xi’an in China, which is particularly appropriate given the recent events a few hundred kilometres to the southwest. The organisers managed to timetable a session of two keynotes and six other presentations on landslides associated withe the earthquake. The session was pretty interesting and highlighted both the amazing job that the Chinese authorities and people did in the response and recovery stage after the earthquake, not least with respect to the valley blocking landslides, and the incredible amount of information that has been collected already. For example, 15,000 landslides have been mapped, hundreds of unstable slopes have been identified and in some cases hazardd assessments have been conducted. One speaker mentioned that 700 geologists have been working on landslides in the earthquake zone – if so then this is a remarkable response that few if any other countries could match.

I don’t intend to cover all aspects of the presentations, not least because there was some inevitable and understandable duplication and contradictions between them. Highlights for me included:
1. The first speaker was the conference convenor, Prof. Chen, who presented a paper on behalf of Ning Liu, who was the engineer in charge of the successful draining of Tangjiashan. The presentation made the achievement of mitigating this slide all the more notable. Interestingly, the first part of the talk was on another valley blocking landslide, Yigong, which was successfully breached by the same team in June 2000. This landslide was much bigger than Tangjiashan – 300 million cunic metres, creating a lake of about 1.5 cubic kilometres. The same team, called the hydro-police, successfully drained that lake too, although the peak discharge was c.140,000
cumecs! Thus, the team that tackled Tangjiashan had experience of undertaking similar tasks. The approach used was exactly the same – i.e. to build a small channel and then to allow the water to slowly excavate it to allow the lake to drain away. They were confident that it would be OK as there were rock fragments and boulders in the landslide mass – and they were quite right of course.

He also stated that all of the other landslide lakes have now been resolved too. If so this is an unprecedented achievement for which the authorities and individuals involved deserve high praise.

2. Yin Yuping of the China Geological Survey gave a more general overview of the landslide problems, suggesting that the key issue from a slope stability perspective was probably the very high peak ground accelerations (although unfortunately these were estimated rather than empirical data being produced). He talked at length about Beichuan. The landslide that struck the old town (near the apex of the bend) killed 1600 people, partly through burial and partly as a result of a ferocious air blast that flattened every building bar one that it struck. The other was the slide that hit the New Town and buried the Middle School. 350 children were killed. Sadly it appears that this was a slope in a known state of instability – and indeed it seems that 16 piles had been installed on the slope on 2006 to improve safety.

He suggested that 15,000 landslides had been mapped to date (this looks a little conservative to me?) and that 9,000 unstable slopes had been identified. All of the landslide dams occurred close to the fault surface trace, but one large landslide occurred >300 km from the fault!

3. Qiang Xiu presented a study of the Donghekou landslide, which had a volume of 12 million cubic metres. It inundated three villages, killing about 300 people. The pictures of the landslide were very impressive indeed, although I think it is fair to say that more work is needed to understand the landslide mechanisms.

Tomorrow includes a series of keynote lectures, a round table discussion (in which I am involved) and a series of parallel sessions, all wrapped up with a cultural evening. I will try post the highlights (excluding the cultural bit) tomorrow.