P. Simms, S. Sivathayalan, F. Daliri Carleton University Desiccation in dewatering and strength development of high density hard rock tailings

Stress history of high density tailings stack layers : Effect of degree of desiccation Settling o r s/w consolidation

Stress history of thickened tailings stack layers: Geotehcnical Stability? Cost? Acid Rock Drainage ? Geotechnical Stability ? Settling ICOLD and UNEP (2001) Drying to Shrinkage Limit?

Effect of salts? Cracks? Consolidation Thickness of fresh-layer Rheology Site topography Conceptual model of factors influencing “drying time”

Within a range, initial solids concentration does not affect initial degree of settling (column test data) 5

How we predict the rate of drying? Hypothesis: We can use standard unsaturated flow codes, that are used in soil cover design, to predict rate of drying Models such as SoilCover, SVFlux, Hydrus… Complicating factors with tailings: salts, cracking, significant volume change

Matric suction data from a two- layer test Initial period of hindered settling Placement of Second layer

So we have found that this approach has a certain skill, what happens when we extrapolate to deep layers of tailings? Hypothetical analyses performed to analyze what happens when a fresh layer is deposited over several (1m to 10 m) of previously desiccated tailings We used a range of parameters (Water retention curves, saturated hydraulic conductivity) typical to many hard rock tailings (air-entry value from kPa, saturated hydraulic conductivity between and m/s) Potential evaporation rate varied between 2 and 10 mm/day. Drainage and no-flow bottom boundary conditions applied

Influence of fresh layer thickness on drying rate PE = 5 mm /day These and similar predictions published in 2010 Paste and thickened tailings conference

Multilayer Deposition in a Drying Box Volumetric Water Content Sensors Tensiometers Senix Distance Sensors Scale

Desiccation Process in the Drying Box Day 1 Day 2 Day 3Day 4 Day 5 Day 6 Evolution of Cracks

Drying Time

Modelling of drying box

Generic predictions compared to field and drying box results 16 Predictions from Paste 2010 paper Drying box – only 1 m of tailings

Monotonic Results of Desiccated/Rewetted Samples

Vane Shear Results

The influence of stress history on shear strength Settling

If only desiccation to 25 % instead of 20% (SL) required, significant reduction in needed drying time 20

Important Outcomes: Time to reach end of SW consolidation, for lifts up to 1 m and as low as 65% solids concentration, seems to be within 48 hours. Only small influence of initial solids concentration on final void ratio post-consolidation and post-drying Samples experiencing settling without desiccation exhibited strain softening response. A relatively small degree of desiccation changes the response to strain hardening. Though desiccation to SL and beyond increases the shear strength, it requires substantially longer drying time. The degree of desiccation should be determined based on required site specific shear strength.

Acknowledgements Former and current graduate students who have contributed to this work: Bereket Fisseha, Julio Henriquez, Rachel Bryan, Adedeji Dunmola, Shabnam Mizani, Adrian Manlagnit, Parviz Heidarian, Mary-Ellen Gleeson, and Farzad Daliri Collaborator Siva Sivathayalan (co-supervisor of Farzad Daliri) Thank you Golder Associates, Musselwhite Mine, and Barrick Gold

Actual evaporation in drying box

Preliminary Results 5 Days 8 Days11 Days