20 August 2017
Dynamic analysis of the Oso landslide – a contribution by Oldrich Hungr and colleagues
One of the last important research contributions by Professor Oldrich Hungr is a paper to be published on 1st September 2017 (but now available online) providing a dynamic analysis of the Oso landslide. This paper (Aaron et al. 2017) plays of course to Oldrich’s greatest strength – his understanding of, and contributions to, the mobility of landslides. As I have noted previously, Oso remains controversial in terms of its pre- and post-failure behaviour. This paper is a follow-up to an earlier article (Stark et al. 2017) that examined the material properties of the landslide. I note that the conclusions of that earlier paper are not accepted by some prominent people who have studied the Oso landslide.
Aaron et al. (2017) model the runout behaviour of the landslide using two well-documented and highly regarded models for the dynamic analysis of landslides – DANW (which is a 2 dimensional model) and DAN3D (the three dimensional equivalent), both developed by Oldrich and his colleagues. The approach that they have used has been to model the landslide in two dimensions first to understand the parameters that control the landslide behaviour, and then to use these to model the landslide in three dimensions. They have calibrated the models on the basis of detailed mapping of the landslide deposit – in other words, they required that the models were able to account for the spatial distribution of different remnants of the landslide as well as the overall morphology of the deposit. In particular, they observe three distinct zones of the landslide mass post-failure (quoted from Aaron et al. (2017):
1. The source zone, which contains a nearly intact although heavily deformed block at the head, bordered by highly sheared but massive blocks that appear to have travelled for a limited distance down a sloping, steplike surface from a higher elevation;
2. The valley floor, which consists of widely spread fluid deposits bearing rafts of intact sand and clay; and
3. Distal splash zone, which consists only of fluidized material and organic debris.
These features can be quite readily identified on the Google Earth imagery of the landslide:-
I noted above the controversy about the failure mechanisms of the landslide. Aaron et al. (2017) note this controversy, but observe that the various mechanisms make little difference to the runout analysis. I suspect that this may be a point of contention. The models proposed by Aaron et al. (2017) are reported to be able to reproduce the key features noted above, and many of the fine-grained elements such as the location of various displaced rafts containing identifiable trees. The most important aspect of this model is that not all of the landslide material is assumed to have undergone liquefaction. In the model presented here, material in the upper part of the landslide, consisting of overconsolidated glaciolacustrine silt and clay, underwent brittle failure but did not undergo liquefaction. Material on the lower part of the slope, consisting of colluvium (effectively debris from earlier landslide events) failed as a result of the impact and loading of the upper portion of the landslide, and underwent “significant undrained strength loss (liquefaction)“, allowing it to travel more than 1.4 km across the valley floor. It was this mobility that caused the landslide to induce such heavy loss of life.
Aaron et al. (2017) note that their analysis does not explain why this colluvial material was so prone to liquefaction; further laboratory testing is going to be needed to explore this behaviour, and thus to provide evidence for what is likely to be a controversial mechanism. But the approach described in this paper, and the use of detailed mapping, evidence from previous high mobility failures and high quality 2d and 3D modelling is a fitting tribute to the skills, knowledge and contributions of Oldrich Hungr and of course his co-authors.
Aaron, J., Hungr, O., Stark, T. and Baghdady, A. 2017. Oso, Washington, Landslide of March 22, 2014: Dynamic Analysis, Journal of Geotechnical and Geoenvironmental Engineering, 10.1061/(ASCE)GT.1943-5606.0001748, 143:9, (05017005).
Stark, T. D., Baghdady, A., Hungr, O., and Aaron, J. 2017. “Case study: Oso landslide on 22 March 2014—Material properties and failure mechanism.” Journal of Geotechnical and Geoenvironmental Engineering, 10.1061/(ASCE)GT.1943-5606.0001615, (05017001).