This paper presents the physical, standard Proctor compaction and shear strength properties of alum water treatment residues derived from the production of potable water at municipal works. In particular, the effects of catchment geology, chemical treatments at the municipal works, hardening phenomena and shearing rate on the constitutive and shear strength responses of these high-plasticity organic clays were studied. Slurry residues have low bulk and dry densities (0 . 96-1 . 13 and 0 . 21-0 . 36 t/m 3 respectively) and low specific gravity of solids (1 . 83-1 . 99), and are highly compressible, although the consolidation rate is low. The dewatered residues have high values of effective angle of shearing resistance of 39-448. Low concentrations of polyelectrolyte added to the residues altered the constitutive response (more elastic perfectly plastic, with shear failure occurring between 2% and 10% compressive strain): the undrained shear strength was enhanced by 10-20% and the effective angle of shearing resistance increased by 28. Alum residues derived from peaty catchments were found to have marginally higher undrained shear strengths than those from a limestone-bedrock catchment. Recommendations are made regarding adequate dewatering of the slurry residue and the efficient landfill disposal of the pressed residue cake.
The index, compression and consolidation properties are presented for alum water treatment residues (WTRs) derived from three different catchments and dewatered using different methods. Oedometer, consolidometer and triaxial consolidation tests were carried out on specimens of different size and consistency over a wide range of applied stresses (3-800 kPa) and under different drainage conditions. The solids fraction mainly comprised organic matter and micro-organisms (loss-in-dry-mass on ignition of between 40 and 60%) and hence the residues had a low specific gravity of solids of 1 . 83-1 . 99 and a low dry density of 0 . 14-0 . 26 t/m 3 measured over the mass water content range of 300-700%. The wet residue was highly compressible (primary compression index of 2 . 1-3 . 1) and the lagooned or landfilled material will settle significantly, but over a long period of time, due to its low consolidation and high creep rates (coefficient of consolidation of 0 . 1-0 . 8 m 2 /year and secondary compression index of 0 . 005-0 . 010). The WTRs had a very low hydraulic conductivity (coefficient of permeability of 10 ÿ9 -10 ÿ11 m/s) due to the microstructure of the constituent flocs, the high organic content and alum's exceptionally high affinity for water. Although the non-chemically coagulated residue was found to be slightly more compressible, the geoengineering properties were generally independent of seasonal variations in the source waters and the catchment geology.
This paper presents the geotechnical properties of the zinc/lead mine tailings from Tara Mines in County Meath, Ireland. The coarser and finer materials from the mechanical crushing and grinding processes were classified as slightly sandy silt and clayey silt, with high specific gravity of solids values of 2.78 and 2.82, and bulk density values of 2.01 and 2.08 tonne/m 3 , respectively. Chemical analysis indicated that the tailings comprised high proportions of calcium and magnesium (derived from the limestone ore body), with high residual concentrations of zinc and lead. The materials in the tailings pond were in a loose to medium dense state, with low to very low hydraulic conductivity of the order of 10 -6 -10 -8 m/s, and values of effective angle of shearing resistance of 37° (coarse) and 32° (fine). An indicative inert capping layer is presented that will form part of the rehabilitation works towards reintegrating the tailings pond into the surrounding landscape when its storage capacity is reached.
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