I.R. Fleming

Shear strength testing of intact and recompacted samples of municipal solid waste.

This paper presents results of shear strength testing of intact and recompacted samples of municipal solid waste (MSW). A method for in situ sampling of MSW from landfills using a push-in sampler was developed and used to obtain intact samples of MSW from a large municipal landfill. Shear strength testing of MSW was carried out using a large triaxial compression apparatus as well as a large direct shear apparatus. The results are presented in terms of cohesion intercept (c′) and angle of shearing resistance (\phi^\prime) and are compared with those available in published literature. Based on these results and their favourable comparison with this literature, it can be concluded that meaningful shear strength parameters for MSW can be obtained using consolidated undrained triaxial tests on large-diameter intact and recompacted samples. A fairly consistent picture of the shear behaviour of MSW obtained from effective stress paths in triaxial tests appears to suggest that shear behaviour of MSW can be explai...

ESTIMATION OF THE MECHANICAL PROPERTIES OF MSW DURING DEGRADATION IN A LABORATORY COMPRESSION CELL

Significant advances have been made during the past two decades in the understanding of the mechanical behavior of municipal solid waste (MSW) landfills. Large post-closure deformations have been observed in landfills, often interfering with the buried infrastructure used to collect landfill gas. Slope failures have also been observed in landfills, in some cases with significant loss of life. Since a major component of MSW is degradable material, it is expected that the mechanical properties of waste likely change over time. It is therefore necessary to understand and quantify the evolution of mechanical properties of waste with time as it degrades. The present study is an effort in this direction intended to explore the long-term changes in the mechanical behavior of waste. Some preliminary results are presented in this paper.

Indirect measurements of field-scale hydraulic conductivity of waste from two landfill sites

Management and prediction of the movement and distribution of fluids in large landfills is important for various reasons. Bioreactor landfill technology shows promise, but in arid or semi-arid regions, the natural content of landfilled waste may be low, thus requiring addition of significant volumes of water. In more humid locations, landfills can become saturated, flooding gas collection systems and causing sideslope leachate seeps or other undesirable occurrences. This paper compares results from two different approaches to monitoring water in waste. At the Brock West Landfill in eastern Canada, positive pore pressures were measured at various depths in saturated waste. The downward seepage flux through the waste is known, thus the vertical saturated hydraulic conductivity of the waste at this landfill was determined to be 3 × 10(-7)cm/s. By comparison, the Spadina Landfill in western Canada is predominantly unsaturated. The infiltration of moisture into the waste was measured using moisture sensors installed in boreholes which determined arrival time for moisture fronts resulting from major precipitation events as well as longer-term change in moisture content resulting from unsaturated drainage during winter when frozen ground prevented infiltration. The unsaturated hydraulic conductivity calculated from these data ranged from approximately 10(-6)cm/s for the slow winter drainage in the absence of significant recharge to 10(-2)cm/s or higher for shallow waste subject to high infiltration through apparent preferential pathways. These two very different approaches to field-scale measurements of vertical hydraulic conductivity provide insight into the nature of fluid movement in saturated and unsaturated waste masses. It is suggested that the principles of unsaturated seepage apply reasonably well for landfilled waste and that the hydraulic behavior of waste is profoundly influenced by the nature and size of voids and by the degree of saturation prevailing in the landfill.

A design chart for estimation of horizontal displacement in municipal landfills

This paper describes the development of a design chart for the estimation of maximum horizontal displacement within a municipal landfill using the height and the side slope of the landfill. The design chart is based on the results of a finite element parametric study in which the behaviour of the municipal solid waste (MSW) was modeled using a non-linear elastic hyperbolic model. The model input parameters, i.e. non-linear stiffness, shear strength and unit weight of MSW, were obtained from laboratory testing data and an extensive stochastic numerical modelling exercise. Non-linear variations of unit weight as well as Youngs modulus of MSW with depth were incorporated in the finite element analyses. The validity of the design chart was assessed using field monitoring results from a large landfill located in Ontario, Canada.

Surficial bitumen in the Athabasca oil sands region, Alberta, Canada

The Athabasca Oil Sands of Northeastern Alberta, Canada, are primarily recovered by surface mining operations. These areas are required by regulation to be reclaimed at mine closure. Recent cuts in the proposed borrow areas for cover material have revealed unusual deposits of hydrocarbons at quantities exceeding clean soil guidelines in the surficial materials. This article outlines recent work in characterising the hydrocarbon composition and groundwater leaching potential of this surficial bitumen. Characterisation consisted of field sampling with analysis by standard Canadian chromatographic methods. An initial investigation of contaminant mobility and biodegradability was conducted using a custom unsaturated soil column leaching experiment. Hydrocarbons were found to be primarily above C30. Concentrations greater than C60 were approximately 10 times those below C60. As expected, minimal leaching occurred, suggesting a minimal risk to groundwater. This result provides meaningful evidence that the material can be directly placed for reclamation. This option in turn represents a meaningful saving in terms of cost, effort and energy for the mine operators.

Estimating degradation-related settlement in two landfill-reclaimed soils by sand-salt analogues

Landfill reclaimed soil here refers to largely degraded materials excavated from old landfill sites, which after processing can be reinstated as more competent fill, thereby restoring the former landfill space. The success of the process depends on the presence of remaining degradable particles and their influence on settlement. Tests on salt-sand mixtures, from which the salt is removed, have been used to quantify the impact of particle loss on settlement. Where the amount of particle loss is small, say 10% by mass or less, settlements are small and apparently independent of lost particle size. A conceptual model is presented to explain this behaviour in terms of nestling particles and strong force chains. At higher percentages of lost particles, greater rates of settlement together with some sensitivity to particle size were observed. The conceptual model was then applied to two landfill reclaimed soils, the long-term settlements of which were found to be consistent with the conceptual model suggesting that knowledge of particle content and relative size are sufficient to estimate the influence of degradable particles in landfill reclaimed soils.