Temperature effects on dredged slurry performance under vacuum preloading

Under appropriate temperature conditions, vacuum preloading can effectively accelerate the rate of soil consolidation. If the temperature is low (30 °C), vacuum preloading is less effective at consolidating the soil. If temperature is extremely high, vacuum preloading is less efficient at consolidating the soil due to the consummation of excess energy consumed. In this study, a series of laboratory tests was conducted to analyse the effects that temperature has on dredged slurry consolidation via vacuum pressure using constant and variable heating modes. During these tests, heat transfer, water discharge, surface settlement, and pore-water pressure dissipation were observed in the soil samples. Based on the laboratory test measurements, each soil sample’s horizontal coefficient of consolidation, water content, and shear strength were determined. To quantify the energy consumption of the different heating modes, the ratios of energy consumption as a function of the soil’s total water discharge and mean shear strength were determined. Using these parameters, an optimal soil consolidation temperature was obtained. The results indicated that vacuum preloading was most effective in consolidating the soil under a constant temperature of 75 °C rather than variable temperatures.

Physical and mechanical properties of a compacted silty sand with low bentonite fraction

Compacted granular soils with small additions of bentonite have been used to build geotechnical structures such as impervious liners and cores of zoned earth dams. This paper presents a laboratory study showing how physical and mechanical characteristics of a silty sand are modified by a low percentage of bentonite. The effects of the addition of bentonite on the silty sand are reflected by an increase in the plasticity index, a reduction in maximum modified Proctor density, and a decrease in hydraulic conductivity. The most significant consequences on the mechanical properties are an increase of compressibility and secondary consolidation coefficients, and a reduction in shear strength. Different mixtures were either dynamically compacted at the optimum water content (compacted samples) or prepared after slurry consolidation from the minimum density (remoulded samples). Although the compacted and remoulded specimens show different isotropic compression lines, their critical-state lines in the v:p':q space are identical, where v is specific volume, p' is average effective stress, and q is deviator stress. Comparisons of the mechanical parameters with the existing literature database show that the compression coefficients of the remoulded mixtures are comparable to those of normally consolidated clayey soils of similar plasticity; nevertheless, those of the compacted mixtures are considerably lower. Also, the slopes of their critical-state lines in the q:p' plane are in good agreement with those predicted by empirical correlations for fine-grained soils.Key words: bentonite, silty sand, compaction, physical properties, compressibility, critical state.

Geotechnics of nonsegregating oil sand tailings

The present method of oil sands tailings disposal results in a tailings pond with a fine tailings zone that will take many decades to consolidate fully. The fine tailings accumulate as a result of the segregating characteristics of the tailings stream. Nonsegregating mixes of total tailings are desirable to prevent or greatly reduce the formation of a fine tailings zone. This study investigated the use of lime and sulphuric acid to prevent segregation of the tailings stream. Two batches of Syncrude tailings were tested. These averaged 48 and 55% solids and 17% fines (< 44 μm). The hindered settling and consolidation properties of nonsegregating mixes were determined using large-diameter standpipe and slurry consolidation cells. Nonsegregating mixes were achieved by adding 600–800 ppm CaO, based on total weight, or 7.5–10 mL/L of 10% H2SO4, based on the total volume of tailings. Predictions of field performance using a finite strain consolidation program indicate that a deposition rate of up to 20 m/year of treated total tailings is possible, depending upon the simultaneous degree of consolidation desired. The discovery of nonsegregating mixes with sufficient self-draining capability opens many options to address this important waste-management problem. Key words : oil sand tailings, nonsegregating, hindered settling, consolidation, permeability, laboratory.