Jack Caldwell - Mining Engineer - Robertson GeoConsultants

Here is a link to a Licentiate Thesis by Muhammad Auchar Zardari of the Lulea University of Technology in Sweden. Its title is Stability Analysis of Tailings Dams – Focus on Numerical Modelling. It is a big document and the connection was slow, but well worth the patience. I recommend that you download a copy and read it.

The thesis consist of three technical papers submitted for publication:

o Paper I Numerical analysis of strengthening by rockfill embankments on an upstream tailings dam (To be submitted to Canadian Geotechnical Journal)

o Paper II Numerical analysis of curved embankment of an upstream tailings dam (To be submitted to Electronic Journal of Geotechnical Engineering)

· Paper III Static liquefaction of tailings: A review of constitutive models (Manuscript)

Here is a summary of the papers:

Upstream tailings dams may experience slope stability problems when the rate of raising is too fast. Tailings consolidate slowly due to low hydraulic conductivity. The excess pore pressures can build up due to accelerated rate of raising. The cumulative increase in excess pore pressures due to successive raisings can endanger slope stability of a tailings dam. The stability of a tailings dam is closely related to the consolidation process.
The consolidation process and associated stability of an upstream tailings dam during staged construction was modelled with the finite element program PLAXIS. The analysis indicated that the stability of the dam reduced during raising due to increase of excess pore pressures. The safety of the dam was improved by adding rockfill banks on the downstream side. The volume of the rockfill banks was minimized with an optimization technique. This technique involves (i) construction of a rockfill bank on the downstream side when the factor of safety is less than a permissible limit, (ii) utilization of a minimum volume of the rockfill that is necessary to stabilize the slope. This technique can be practicable when the rate of raising is moderate, and partial consolidation occurs between consecutive raisings.
Numerical analysis was also performed on a curved embankment of an upstream tailings dam in order to investigate the possible risk of hydraulic fracturing and internal erosion in a corner of the dam. The analysis showed that low compressive stresses occurred above the phreatic level, near the zones of filter and rockfill banks. These zones contain coarse material, and are, therefore, not susceptible to hydraulic fracturing and internal erosion. An increase in the radius of the corner is suggested in order to prevent large reductions in compressive stresses that may occur due to future raisings. Presently the curved dam section is stable. However, an additional rockfill bank on the downstream side will be required for future raisings.
Static liquefaction is considered as a common cause of disastrous flow failures of tailings dams. These flow failures can be predicted with numerical modelling using suitable constitutive models. In this context, some constitutive models capable of simulating static liquefaction behaviour of loose saturated sands are reviewed. It is generally concluded that the finite element method can be a helpful tool for modelling stability of tailings dams.

Here is a link to another fine document that I have not hitherto come across. It is called Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. It is from the European Commission published in 2009.