Laboratory News: Cyclic Triaxial Testing of Soils

In recent years, the world has observed the catastrophic impacts of tailings dam failures due to liquefaction. Liquefaction results in a significant reduction of soil strength and stiffness, which affects the ability to support any loading. The resulting tragic effects of dam collapse are death and injury along with widespread environmental destruction. Correct liquefaction assessment is critical to tailings dam management.

Liquefaction may occur in saturated soils under static or seismic loading conditions. In seismically induced liquefaction, vibrations generated during an earthquake can cause the pore water pressure to surge, while under a static case the liquefaction may be triggered by a number of mechanisms including but not limited to rapid loading, reduction in horizontal stresses due to deformation and variations in ground water levels.

Laboratory assessment of liquefaction

The ATC Williams NATA accredited soil and tailings laboratory is equipped to carry out complex monotonic and cyclic triaxial testing required for assessment of liquefaction.

Test results are analysed to determine critical state parameters and consequently the potential of a material to liquefy under seismic or static cases, together with assessment of strength characteristics. The application of the critical state framework in assessing the liquefaction potential of mine tailings has been used by ATC Williams for over 15 years.

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What is the Critical State Line?

The stress paths for cyclic triaxial testing of a tailings sample prepared to three different densities with identical initial stress conditions and applied cyclic loading are presented below.

As shown in the diagrams, the tailings prepared at a lower density (i.e. higher void ratios) has liquefied under cyclic loading while the denser samples have not. The critical state line (CSL), in simple terms, defines a state at which the soil response changes from a contractive (potentially liquefiable) to the dilative response. The CSL is established by carrying out a series of triaxial shearing tests in the laboratory.

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Predicting liquefaction and deformations

The results of triaxial testing can be analysed to determine and calibrate input parameters for numerical models to predict liquefaction and deformations under various loading conditions. The most used constitutive models for liquefaction and deformation analyses are UBCSAND (Byrne et al 2004) and NORSAND (Jefferies and Been 2016).

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ATC Williams is a leader in this field and has published extensive refereed technical papers on numerical modelling of liquefaction and seismic deformation analyses of tailings and water retaining structures since 2005.