Cyclic Shear Response of Fine-Sand Tailings Using Cyclic Direct Shear Test

Abstract

The extraction and processing of mineral and metal ores in the mining industry comes paired with large amounts of mine waste, also known as tailings. This waste consists of various chemicals, acids, and heavy metals which are used during these extraction processes. The tailings are usually stored off in tailings storage facilities (TSF) in a loose state, gradually consolidating over time. TSFs founded in seismically active areas are susceptible to liquefaction due to earthquake loading. Historic data show that about 35% of dam failures are due to liquefaction of the tailings, thus increasing the need to study the liquefaction responses of tailings under cyclic loading. Extensive studies have already been
conducted using cyclic direct simple shear (DSS) and cyclic triaxial (CTX) tests as these tests under constant-volume conditions can evaluate the change in pore pressure within a soil accurately. Previous study shows what importance the relative density and sloping ground conditions, known as drained shear bias, have on the cyclic resistance to liquefaction of the tailings. However, in practice the pore water is not bounded within the material and excess pore water can flow out through installed drains. A round-robin program, issued by the University of Western Australia (UWA), requested a study on the liquefaction response of a particular fine-sand tailings material. Inspired by this round-robin program, an interest raised in studying the cyclic shear response of tailings by using a direct shear box to investigate the cyclic behaviour under partially drained conditions. With use of the direct-shear apparatus of Wille Geotechnik, a test program has
been set up to study the influences of relative density and drained shear bias under stress-controlled cyclic shearing and under constant normal load conditions.
Results of the experiments met the expectations that denser soils have a 9.6% higher cyclic resistance ratio (CRR), and samples with applied drained shear bias have a 32% lower CRR compared to samples tested with level ground conditions. Furthermore, samples which underwent post-cyclic shearing showed strain-hardening responses and yielded higher shear stresses compared to the monotonic test, indicating that the constant normal load further densified the samples during cyclic shearing. However, during the experiments, it was
quickly found out that the loading frequency was not being applied optimally making it not possible to analyse influence of partially drained conditions . This study showed promise on its capabilities to study cyclic shear loading on a soil. For future work, it is suggested to perform similar tests under a uniform loading frequency with the use of a shear box to evaluate its capabilities to study on partially drained conditions. It is also recommended to conduct tests under constant volume conditions, to evaluate the shear-box apparatus’ capabilities to
study the liquefaction response of a soil due to excess pore pressure generation.