Comparative Analysis of the Post-Liquefaction Shear Strength of Cyclone Tailings based on SCPTu and Laboratory Testing

There have been many cases of flow failure of sand tailings dams triggered by liquefaction in the last 20
years. Currently, there is a need to assess the stability of tailings dams constructed in the past against flow
failure, since their design and construction quality is unknown, and did not follow safety guidelines. In
engineering practice, it is common practice to ensure the safety of dams against flow failure by carrying
out post-liquefaction stability and deformation analyses. However, this procedure requires an appropriate
estimation of the post-liquefaction shear strength, which is a difficult task due to the high level of
uncertainty involved and the lack of understanding of its mobilization mechanism and controlling factors.

This paper examines the engineering evaluation of the post-liquefaction shear strength for the stability
analysis of an existing non-plastic cyclone tailings dam located in an area of great seismic activity in
Peru. The purpose is to compare methods based on in-situ and laboratory testing, aiming to provide
insight on how to deal with the uncertainty and limitations of the evaluation of post-liquefaction shear
strength from the practitioner viewpoint.

The research took a case study approach, which consisted of the revision and diagnosis of the
activities of geotechnical exploration and design conducted for one project, discussion of issues and
limitations found, and evaluation of implications on similar projects. The diagnosis comprised a literature
review and interviews to establish factors controlling the post-liquefaction shear strength, as well as
limitations in the methodologies for its estimation.

The results indicate that the post-liquefaction shear strength obtained through in-situ testing is more
conservative than values obtained from laboratory testing for this particular project. Furthermore, several
key weaknesses in the evaluation of the post-liquefaction shear strength were found. These key
weaknesses included poor strain-rate application in undrained monotonic tests, sample disturbance effects
during sampling and laboratory preparation, and lack of strain-softening response in post-cyclic monotonic tests for the laboratory-based approach. For the approach based on in-situ testing, the main
issues found were the lack of accurate data in fines correction factors, and great scattering in the result
obtained. Some considerations to address such limitations are provided.

Roman, B., H. Barriga and A. Najarro. 2019. “Comparative Analysis of the Post-Liquefaction Shear Strength of Cyclone Tailings based on SCPTu and Laboratory Testing,” in Proceedings of Tailings and Mine Waste 2019, 17-20 November 2019. Vancouver, BC : University of British Columbia.