ABSTRACT
ABSTRACT: The stability of an upstream constructed tailings sand dam is highly dependent on the strength of the existing tailings to form the foundation of the subsequent raise. Dams that are raised above the original design height or are constructed with a narrow zone of coarse sand on the upstream face may have portions of the dam constructed over potentially liquefiable tailings. Whereas silts (e.g. fine tailings) have in the past been viewed as non-liquefiable, that view is now changing such that sufficient ‘fines content’ is no longer regarded as sufficient protection from liquefaction and alternative means of assessment are required. The paper will present an example where the liquefaction potential of silt-sized tailings was assessed as part of the stability assessment for a raise of an existing upstream constructed tailings sand dam. The liquefaction assessment was based on the critical state concept, and used two approaches. First, the liquefaction potential was assessed using disturbed and undisturbed samples to define the void ratio profile of the tailings with depth, which was then compared to the critical state locus (CSL) determined from a suite of laboratory tests on the tailings. Second, the liquefaction potential was assessed in-situ using piezocone (CPTu) measurements and soil-specific calibrations. The estimated in-situ state parameter from the two methods was remarkably similar, giving confidence to the overall approach. The calibrated CPTu data was then adopted for assessment of the dam as a whole. With the in-situ state parameter profile defined, estimation of liquefaction potential is straightforward, and independent of ‘fines content’, as described in the paper.
