Tire shreds and tire shred – soil mixtures can be used as alternative backfill material in many geotechnical applications. The reuse of tire shreds may not only address growing environmental and economic concerns, but also help solve geotechnical problems associated with low soil shear strength. In this study, an experimental testing program was undertaken using a large-scale triaxial apparatus with the goal of evaluating the optimum dosage and aspect ratio of tire shreds within granular fills. The effects on shear strength of varying confining pressure and sand matrix relative density were also evaluated. The tire shred content and tire shred aspect ratio were found to influence the stress–strain and volumetric strain behaviour of the mixture. The axial strain at failure was found to increase with increasing tire shred content. Except for specimens of pure tire shreds and with comparatively high tire shred content, the test results showed a dilatant behaviour and a well-defined peak shear strength. The optimum tire shred content (i.e., the one leading to the maximum shear strength) was approximately 35%. For a given tire shred content, increasing the tire shred aspect ratio led to increasing overall shear strength, at least for the range of tire shred aspect ratios considered in this study. The shear strength improvement induced by tire shred inclusions was found to be sensitive to the applied confining pressure, with larger shear strength gains obtained under comparatively low confinement.Key words: tire shreds, shear strength, reinforcement, triaxial testing, stress–strain behaviour.

Gas movement through soils is important for ecosystems and engineering in many ways such as for microbial and plant respiration, passive methane oxidation in landfill covers and oxidation of mine residues. Diffusion is one of the most important gas movement processes and the determination of the diffusion coefficient is a crucial step in any study. Five laboratory methods used for measuring the relative gas diffusion coefficient (D(s)/D(o)) were compared using a loamy sand, a porous media commonly found in agricultural fields and in several engineered structures, such as in landfill final covers. In the absence of macropores, all methods gave rather similar values of D(s)/D(o). Methods allowing the study of microscale variability indicated that the presence of macropores highly influenced gas movement, thus the value of D(s)/D(o), which, near a macropore may be one order of magnitude higher than in regions without macropores. Repacked columns do not allow the study of heterogeneity in D(s)/D(o). Natural spatial variability in D(s)/D(o) due to water distribution and preferential pathways can only be studied in large systems, but these systems are difficult to handle. Advantages and disadvantages of each method are discussed.

A method is proposed to estimate CH(4) oxidation efficiency in landfill covers, biowindows or biofilters from soil gas profile data. The approach assumes that the shift in the ratio of CO(2) to CH(4) in the gas profile, compared to the ratio in the raw landfill gas, is a result of the oxidation process and thus allows the calculation of the cumulative share of CH(4) oxidized up to a particular depth. The approach was validated using mass balance data from two independent laboratory column experiments. Values corresponded well over a wide range of oxidation efficiencies from less than 10% to nearly total oxidation. An incubation experiment on 40 samples from the cover soil of an old landfill showed that the share of CO(2) from respiration falls below 10% of the total CO(2) production when the methane oxidation capacity is 3.8 μg CH(4)g(dw)(-1)h(-1) or higher, a rate that is often exceeded in landfill covers and biofilters. The method is mainly suitable in settings where the CO(2) concentrations are not significantly influenced by processes such as respiration or where CH(4) loadings and oxidation rates are high enough so that CO(2) generated from CH(4) oxidation outweighs other sources of CO(2). The latter can be expected for most biofilters, biowindows and biocovers on landfills. This simple method constitutes an inexpensive complementary tool for studies that require an estimation of the CH(4) oxidation efficiency values in methane oxidation systems, such as landfill biocovers and biowindows.

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A design procedure is proposed to minimize water infiltration into landfills by optimizing the water diversion length of inclined covers with capillary barrier effect (CCBE). This design procedure is based on a conceptual, mathematical and numerical approach and aims at selecting materials and optimizing layer thickness. Selection among candidate materials is made based on their hydraulic conductivity functions and on a threshold infiltration rate imposed on the designer. The capillary break layer (CBL; bottom layer) is characterized by a weak capillarity, while the moisture retention layer (MRL; upper layer) is characterized by a compromise between strong capillarity and high hydraulic conductivity. The thickness of the CBL corresponds to the height where suction reaches its maximum value for a given infiltration rate. This height can be calculated using the Kisch [Ge´otechnique 9 (1959)] model. The optimal thickness of the MRL is determined by applying an adaptation of the Ross [Water Resources Research 26 (1990)] model. The results obtained using the proposed design procedure were compared to those obtained from numerical simulations performed using a finite element unsaturated seepage software. The procedure was applied for two cover systems; one where deinking by-products (DBP) were used as MRL and sand as CBL and another where sand was used as MRL and gravel as CBL. Using this procedure, it has been shown that an infiltration control system composed of thin layers of sand over gravel is highly efficient in terms of diversion length and that its efficiency can be enhanced by placing a hydraulic barrier -such as a layer of DBP -above the MRL.

Stable isotope analyses were performed on gas samples collected within two instrumented biocovers, with the goal of evaluating CH 4 oxidation efficiencies (f 0 ). In each of the biocovers, gas probes were installed at four locations and at several depths. One of the biocovers was fed with biogas directly from the waste mass, whereas the other was fed through a gas distribution system that allowed monitoring of biogas fluxes. While the f 0 values obtained at a depth of 0.1 m were low (between 0.0% and 25.2%) for profiles with poor aeration, they were high for profiles with better aeration, reaching 89.7%.Several interrelated factors affecting aeration seem to be influencing f 0 , including the degree of water saturation, the magnitude of the biogas flux and the temperature within the substrate. Low f 0 values do not mean necessarily that little CH 4 was oxidized. In fact, in certain cases where the CH 4 loading was high, the absolute amount of CH 4 oxidized was quite high and comparable to the rate of CH 4 oxidation for cases with low CH 4 loading and high f 0 . For the experimental biocover for which the CH 4 loading was known, the oxidation efficiency obtained using stable isotopes (f 0 =55.67% for samples taken inside flux chambers) was compared to the value obtained by mass balance (f 0 =70.0%). Several factors can explain this discrepancy, including the high sensitivity of f 0 to slight changes in the isotopic fractionation factor for bacterial oxidation, α ox , uncertainties related to mass flow metre readings and to the static chamber method.

An experimental passive methane oxidation biocover (PMOB) was constructed within the existing final cover of the St-Nicéphore landfill. Its substrate consisted of a 0.80-m thick mixture of sand and compost. The goal of this experiment was to evaluate the performance of the PMOB in reducing CH 4 emissions when submitted to an increasing methane load. The CH 4 load applied started with 0.3 g CH 4 m −2 h −1 . When the site had to be closed for the winter, the CH 4 input was 27 g CH 4 m −2 h −1 . Throughout the study, practically all the CH 4 input was oxidized, absolute removal rates were linearly correlated to methane loading, and the oxidation zone was established between 0.6-0.8 m. These results seem to indicate that the upper limit potential of this PMOB to oxidize CH 4 was not reached during the study period. Surface CH 4 concentration scans showed no signs of leaks. The substrate offered excellent conditions for the growth of methanotrophs, whose count averaged 3.91 x 10 8 CFU g dw -1 soil.

Biocovers constitute a promising technology to reduce fugitive and residual emissions from landfills throughout their operational life and after gas collection systems are turned off. The aim of this study was to assess the efficiency of two substrate materials to oxidize CH4 into CO2 under field conditions and in the laboratory (column tests). The two substrates evaluated were: 1) a mixture of sand and compost, and 2) a mixture of the sand-compost with gravel. The oxidation rates obtained in the field attained a maximum of 576 g CH4 m -2 d -1 for one of the substrates and 352 g CH4 m -2 d -1 for the other. These maximum values were much higher than those obtained in the laboratory: 115 and 118 g CH4 m -2 d -1 , respectively, for an oxidation efficiency of 96% in both cases. The exact causes for the discrepancy between field and laboratory results were not identified, but it was hypothesized that vegetation on the surface of the passive methane oxidation biocovers (PMOBs) greatly improved methane oxidation efficiencies in the field. The results obtained in this study show that laboratory column tests constitute a reliable means to evaluate potential candidate materials for biocovers.However, the maximum oxidation efficiencies may not be the same as those obtained in the laboratory.