4 Background Alternative disposal techniques for slimes Types sub-aerial depositionThickenedPastefilteredCommon aspectsIntermitent: cycles of waiting and disposal periodsPhysical processes: sedimentation, consolidation and desiccation
5 Background Desiccation Importance Triggering mechanisms The most effective phenomena for rehabilitation work, optimizing storage, and reducing risks regarding containment structure failureTriggering mechanismsSurface dryingLowering the GWTDriving force and PhasesSuction1D shrinkage3D shrinkage (cracking)Comprehensive worksAbu-Hejleh & Znidarcic (1995) and Yao et al. (2002)Konrad & Ayad (1997)Fujiyasu (1997)
6 Background Compressibility Constitutive relations Permeability Main input relationships for analyses with CONDES (Yao et al. 2002)ConstitutiverelationsCompressibilityPermeabilityCracking functiona - function
7 Field experimental studies Goal & strategiesTo gain some understanding of the main mechanisms that play a role in the sub-aerial method applied to Samarco’s slimesTo investigate consolidation separately from desiccation using a field experimentTo focus on slimes desiccation because was lesser known and more challenging
8 Field experimental studies Site & operationsLocation: inside the Germano tailings impoundment (Samarco Mineração S.A.) in Mariana, MG.Impoundment figures: 3 m high ring dyke, confining an area of 4,850 m2.Foundation: 2 m of coarse siliceous tailings, grading from fine sand to medium silt, underlain by a deep layer of iron tailings slimes.Ground water table: at the contact of those layers, 2 m below the surface.Drainage system: stop logs installed at the lower part (bottom at 2% slope)Access: a pier to the centre of the testing area for instrumentation maintenance and sampling operationsFilling: slimes pumped from an adjacent slimes pond at the Germano impoundmentWater cover: 5 to 10 cm deep during the filling process and consolidation period to prevent early desiccation.Surface water removal: for the desiccation part of the test.
9 Field experimental studies Instrumentation, testing & instalationDevices:Geotechnical: Thermistors, settlement devices (staff gauges), tensiometers, piezometers, time domain reflectometry probes (TDRs)Climate: Weather station and a class A pan testPlacement:Thermistors and TDR probes launched at certain pond elevations during the deposit filling (movable position)All other instruments at fixed positionsTestingPeriodic sampling using a stationary samplerGravimetric water content determinationSpecific gravity and bulk density.
10 Field experimental studies Overview of the testing site
11 Analyses of experimental data Input data for analyses with CONDESAnalysesData/ParametersConsolidationCompressibility and Permeability parametersA (kPa-1)2.5438BC (m/dia)9.45 x 10-4D4.2370Z (kPa)0.0495Boundary conditionsBottomhp= variableTopSurcharge nullSpecific gravity - G3.89Filling rate (m/day)0.0603Filling period (day)34DesiccationCracking function parametersA0.38590.0508C1.36650.626 x 1018hp= mEvap. rate =0.002 m/day
12 Analyses of experimental data Progress of the deposit height (at day 84 starts desiccation)
13 Analyses of experimental data Progress of volumetric water content (day 84 starts desiccation)
14 Analyses of experimental data Progress of gravimetric water content (desiccation starts at day 84)
15 Analyses of experimental data Progress of bulk densities (desiccation starts at day 84)
16 Analyses of experimental data Progress in solids content (desiccation starts at day 84)
17 Analyses of experimental data Progress of porepressure at base (desiccation starts at day 84)
18 Analyses of experimental data Meteorological data during field experiments (desiccation starts at day 84)
19 Analyses of experimental data Progress of evaporation with data from the weather station and Class A device
20 Analyses of experimental data Progress of cracking (a) day 89, (b) day 92, (c) day 94
21 ConclusionsCharacterization of the desiccation behaviour of a fine tailings from the iron ore milling operations was successfully reached.A test section was built and monitored, using an extensive sort of instruments and tests.Material behaviour and boundary conditions were assessed such as settlement, water content, bulk densities, solids content, porewater pressures, evaporation rates, etc.Cracking morphology has also been described.Sub-aerial deposition conditions was examined as a part a large study of alternative methods for slimes.A brief comparison of field data with a numerical modelling of the problem was presented and the results have shown consistent agreement.Overall, the research seems to suggest that reasonable efficiency with slimes thickening can be achieved by the sub-aerial disposition method.