TAILINGS MATERIAL PROPERTIES

Transcription

1 TAILINGS MATERIAL PROPERTIES Tailings and HLP Workshop 28 April to 1 May 2010

2 Content Introduction Important properties Particle size distribution Slurry density Insitu dry density Permeability Coefficient of consolidation Shear strength Shrinkage limit Other properties

3 Introduction Geotechnical characterisation of material important Central to understanding tailings behaviour Will define design approach and basis of design Determine capacity, wall building method and deposition strategy

4 Important properties Particle size distribution Slurry density Insitu dry density Permeability Coefficient of consolidation Shear strength Shrinkage limit

5 Particle size distribution Indicates sand, silt and clay fractions Determines settling characteristics Will dictate potential for segregation or lack thereof Could dictate selection of deposition method Will dictate beach profile and freeboard

6 Example

7 Slurry density Defined as M/V Solids concentration Controls flow behaviour of tailings once deposited Determines how much water is available for losses and return Could impact on selection of deposition method

8 Insitu dry density Defined as Ms/V Controls impoundment volume for given production rate Dry density and void ratio goes hand in hand, p d = G s p w /(1+e) Typically, dry density increases with depth Dry density depends on three factors: Specific gravity Type of tailings Clay content Grain size and clay content control insitu void ratio

9 Insitu dry density (cont) Hard rock tailings, e = 0.6 to 0.9 Slimes from these tailings, e = 0.7 to 1.3 Slimes from tar sands, kimberlite, red muds, e = 5 to 10 Dictates interstitial losses

10 Permeability Permeability is difficult to generalise Spans from 10-4 m/s for coarse sand tailings to 10-8 m/s for well consolidated slimes Varies as a function of grain size, plasticity, method of deposition and depth in deposit Governs amount of expected seepage to foundation

11 Permeability (cont) High permeability sands Intermediate permeability Low permeability slimes

12 Coefficient of consolidation Primary consolidation governs the rate of pwp dissipation Time dependent process Rate of consolidation governed by: Nature of finer fraction Amount of fine fraction Nature of drainage conditions Rate of consolidation governs: Rate of rise

13 Coefficient of consolidation (cont) Rate of consolidation governs: Rate of rise (excess pwp of deposited layer must dissipate before next layer is placed) Deposition and drying cycle Time to access top of dam Water losses due to interstitial storage Approach to closure design

14 Shear strength Shear strength parameters measured by means of Consolidated Undrained (CU) triaxial tests with pore pressure measurements Careful interpretation of test results required Tailings normally have high drained (effective) shear strength With rare exceptions, tailings are cohesionless Results used in stability assessment for certain geometries and phreatic surface conditions Un-drained shear strength used to measure total stress parameters not all the excess pwp are dissipated

15 Shrinkage limit The gravimetric water content below which no further volume change takes place when the soil is drained When tailings is wetter than SL, loss of moisture results in proportional decrease in volume of the material Once SL is reached, inter particle contact resists further decrease in volume as moisture is withdrawn This may result in shrinkage cracks forming SL provides a target moisture content as the end point of the drying cycle At moisture contents below SL, tailings will tend to be partially saturated favourable Corresponding density could be such that dilative behaviour occurs upon shearing (volume increases and creates suction)

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17 Other properties Water retention ability ability to retain moisture when subjected to suction (negative pore pressure) Particle specific gravity Moisture content Settled density Liquefaction potential