Liquefaction assessment of a thickened tailings deposit in an arid environment using empirical approaches and critical state laboratory testing to calibrate cone penetration tests

A case study of a liquefaction assessment undertaken on a thickened tailings deposit at a copper mine in an arid environment is presented. A program was undertaken that consisted of cone penetration tests with pore pressure measurement (CPTu), with variable rate penetration testing, and specialist critical state laboratory tests, with the objective of estimating undrained shear strengths and potential for cyclic liquefaction to occur under the design earthquake. During the CPTu program, the results were analysed to assess zones where the tailings provided low penetration resistance, generally indicative of looser tailings than adjacent depth intervals. At these locations, multiple piston-tube tailings samples were collected for laboratory tests allowing calibration of the field results. It was found that current available methods applied to the CPTu to infer the in-situ state parameter did not provide reasonable results, in particular for estimation of in situ strength of the tailings, and hence they could not be applied to the tailings studied. Nevertheless, the testing gave insight to the behaviour of the tailings and the estimates of the normalised dimensionless penetration rate of the cone indicated that undrained conditions controlled the shear behaviour of the tailings. Therefore, the method proposed by Robertson (2009) using Nkt method was found to provide a reasonable estimate of the tailings undrained shear strength in the TSF, with comparisons to laboratory testing showing good agreement.