A critical state testing round robin programme was carried out on sandy silt gold tailings. This involved 15 laboratories around the world testing a sandy silt tailings to infer its critical state line (CSL). Methods to be used were intentionally not supplied to participants, to enable the current methods being employed in industry and academia to be obtained in an unbiased manner. All but one of the laboratories involved in the study used the moist tamping sample preparation technique, generally to produce loose, contractive specimens. Void ratio was measured using a variety of means, including cell calibration, end-of-test water content and end-of-test soil freezing (EOTSF) to assist in measuring the final water content. Of the 15 entries, four were excluded from the primary comparison owing to various issues that appear to have led to their divergence from most of the entries received. Of the remaining entries, the best reproducibility was produced by laboratories that used EOTSF to measure void ratio. Most other test procedure variations appeared to have a negligible effect, with the exception of fixing of the top platen and possibly sample size. A CSL elevation range of 0·04 void ratio for laboratories using EOTSF was observed.

A geotechnical investigation was carried out to characterise a subaqueously deposited, primarily silt, iron ore tailings. Piezocone penetration tests (CPTu) were carried out followed by piston tube sampling at a selected target depth. Piston samples provided measures of in situ density (by means of gravimetric water content), and supplied material for reconstituted and intact laboratory testing. Reconstituted samples prepared using moist tamping (MT) for determination of the critical state locus (CSL), along with intact specimens, were both tested. The potential existence of layering within the recovered specimens was also assessed, indicating near-homogenous samples. The laboratory testing of intact specimens suggested that they appeared to tend towards the same CSL as that obtained from reconstituted loose MT specimens. This tentative result differs from some previous comparisons – with the agreement seen in this case being suggested to primarily result from a lack of layering. In situ state as inferred from both CPTu data and comparison of tube densities to the CSL suggested a loose state.

No abstract

The cross-anisotropic nature of soil strength has been studied and documented for decades, including the increased propensity for cross-anisotropy in layered materials. However, current engineering practice for tailings storage facilities (TSFs) does not appear to generally include cross-anisotropy considerations in the development of shear strengths. This being despite the very common layering profile seen in subaerially-deposited tailings. To provide additional data to highlight the strength cross-anisotropy of tailings, high quality block samples from three TSFs were obtained and trimmed to enable Hollow Cylinder Torsional Shear tests to be sheared at principal stress angles of 0 and 45 degrees during undrained shearing. Consolidation procedures were carried out such that the drained rotation of principal stress angle that would precede potential undrained shear events for below-slope tailings was reasonably simulated. The results indicated the significant effects of cross-anisotropy on the undrained strength, instability stress ratio, contractive tendency and brittleness of each of the three tailings types. The magnitude of cross-anisotropy effects seen was generally consistent with previous published data on sands.

No abstract