Journal of Geotechnical and Geoenvironmental Engineering

Biochar amendment to landfill cover soil is proposed as an economical solution to reduce methane (${\text{CH}}_4$) emissions from landfills without gas-recovery systems or in conjunction with gas recovery for near-complete ${\text{CH}}_4$ removal. In this study, column experiments were conducted to simulate the effects of biochar amendment to landfill cover soil and investigate whether biochar amendment can promote the growth of methanotrophic bacteria able to oxidize ${\text{CH}}_4$ into carbon dioxide (${\text{CO}}_2$). Acrylic columns were packed with coarse gravel (gas-distribution layer) and then filled with either soil or 20% biochar/80% soil. The columns were fed humidified synthetic landfill gas ($25\%\hspace{0.17em}{\text{CH}}_4:25\%\hspace{0.17em}{\text{CO}}_2:50\%\hspace{0.17em}{\text{N}}_2$) continuously for 4 months. Sampling ports along the length of the column were used to collect gas samples for measurement of the ${\text{CH}}_4$ and ${\text{CO}}_2$ concentrations. Additional isotopic analysis (${\delta }^{13}\text{C}$) and temperature profiles were also used to evaluate the extent of ${\text{CH}}_4$ oxidation as a function of depth. The deoxyribonucleic acid (DNA) extracted from the soil and biochar-amended soil samples collected at different depths of the columns were subjected to quantitative polymerase chain reaction (QPCR) analysis to determine the abundance of the particulate ${\text{CH}}_4$ monoxygenase (pmoA) genes and infer methanotrophic activity. The pmoA results indicated that a higher number of ${\text{CH}}_4$-oxidizing bacteria (methanotrophs) existed in the biochar-amended soil column, which also supported the observed higher rates of ${\text{CH}}_4$ oxidation compared with soil alone. Batch incubation experiments were conducted using column soils retrieved from different depths to determine Michaelis-Menten kinetic parameters for ${\text{CH}}_4$ oxidation. These results indicate that biochar amendment is effective in increasing methanotroph populations and promoting ${\text{CH}}_4$ oxidation.

Various forms of thickened or high-density tailings, characterized by exhibition of a yield stress that facilitates deposition in gently sloped stacks, are increasingly used for a variety of mining operations. Although this technology reduces reliance on containment by dams and the associated risk of catastrophic failure, regulators are concerned with remobilization of such unbounded deposits by earthquakes or extreme rainfalls. Therefore, the monotonic and cyclic shear strength of thickened tailings layers are of significant interest. Thickened tailings experience a particular stress history during deposition, which may include desiccation, rewetting, and subsequent consolidation by burial under new tailings. The effect of this stress history on their shear response is investigated using a Norwegian Geotechnical Institute (NGI)-type simple shear device. Samples are alternately mechanically overconsolidated or subjected to desiccation and subsequent rewetting because of burial by fresh tailings before consolidation. The two sets of data, one overconsolidated by load, the other by matric suction, were compared using the suction stress concept to quantify an equivalent overconsolidation ratio (OCR) caused by desiccation. However, the effects of mechanical overconsolidation and desiccation were found to be qualitatively different for both the monotonic and cyclic responses; therefore, it is not possible to quantify the effects of desiccation using mechanical overconsolidation and one stress-state variable. Desiccated samples exhibited much weaker strength at phase transformation but more strain hardening compared with mechanically overconsolidated samples at the same OCR. For the desiccated samples, both monotonic strength at phase transformation and cyclic resistance continuously increased with increasing desiccation to air entry value (AEV). Past the AEV, a much smaller increase in strength was observed. Even a small amount of desiccation changed the monotonic response of the tailings from contractive to dilative behavior.

A pseudostatic analysis, based on the beam on nonlinear Winkler-spring method considering the seismic earth pressure and side friction acting on embedded pile caps, was developed to estimate pile bending moments and shear forces. To verify the proposed method’s effectiveness, the authors conducted six dynamic centrifuge tests on a superstructure-footing model supported by $2\times 2$ piles in sand deposits comprising a dry sand layer over liquefied soil. The seismic earth pressure, vertical wall friction at the active and passive sides, and friction at the sidewalls of the pile caps were separately measured by two-dimensional load cells during shaking. The seismic total earth thrust and side friction estimated by the proposed method showed good agreement with the experimental results for both the amplitude and phase difference to the superstructure and the pile cap inertia forces. The estimated bending moment and shear force in the piles also showed reasonable agreement with the experimental results; they were able to account for differences in the pile rigidity and soil relative density.

This paper extends Culmann’s vertical-cut analysis to unsaturated soils. To test the extended theory, unsaturated sand was compacted to a uniform porosity and moisture content in a laboratory apparatus. A sliding door that extended the height of the free face of the slope was lowered until the vertical cut failed. Digital images of the slope cross section and upper surface were acquired concurrently. A recently developed particle image velocimetry (PIV) tool was used to quantify soil displacement. The PIV analysis showed strain localization at varying distances from the sliding door prior to failure. The areas of localized strain were coincident with the location of the slope crest after failure. Shear-strength and soil-water-characteristic parameters of the sand were independently tested for use in extended analyses of the vertical-cut stability and of the failure plane angle. Experimental failure heights were within 22.3% of the heights predicted using the extended theory.

This paper discusses both design methodology and construction procedures for stabilizing expansive subsoils of moderate active depths, using deep soil mixing (DSM) technology/construction. These procedures were derived as a part of the research focusing on the evaluation of effectiveness of DSM technology in mitigating shrink-and-swell behaviors of expansive subsoils under actual field conditions. The design methodology formulated was based on an analytical model proposed for a DSM-treated composite section by modifying the existing heave prediction model for untreated and unsaturated expansive soils. The required area treatment ratio was determined based on the target heave magnitude for the composite section. Design charts were developed depicting estimated heave for increasing treatment area ratios and for various initial swell pressures. Based on this design methodology, DSM construction was implemented under actual field conditions at two test sites. Upon construction, both test sections were instrumented and monitored. In addition, nondestructive tests were also conducted at select time periods. Results from both instrumentation and nondestructive tests revealed that the DSM technology was effective in mitigating shrink-and-swell behaviors of expansive soils. Also, the analytical model used in the present research study has provided reasonable predictions of in situ swelling behavior of composite and untreated sections.

Recent stability issues in ash ponds have highlighted the importance of estimation of in situ void ratio or water content of disposed fly ash to assess the field performance and stability of these disposal units. One standard prohibits the use of fly ash with carbon content greater than 6% in concrete, making low-cost methods of quantifying the carbon content of fly ash attractive, especially for applications that might consider mining and reuse of previously disposed fly ash. In this study, electrical conductivity/resistivity measurements were used to measure the in situ carbon content and void ratio of fly ash, with the ultimate goal of quantifying the level of unburned carbon in the material. Because unburned carbon particles are electrically conductive, the resulting media conductivity is a function of carbon content, allowing determination of the carbon content of fly ash as well as the void ratio, or water content, of the soil matrix. The authors found that the measured electrical conductivity generally increased with an increase in carbon content, and the dependency of carbon particle conduction on applied stress increased with an increase in carbon content owing to the increased number of direct contacts between electrically conductive unburned carbon particles. In addition, this study evaluated the effect of pore-water conductivity on media conductivity and carbon particle conduction.

The objective of this work is to analyze, based on the results of uniaxial creep tests, the effect of organic matter content and binder quantity on the creep behavior of a Portuguese soft soil chemically stabilized with binders. In general, the results show that the application of a creep load induces a slight degradation of the mechanical properties of the composite material of the same age, as a consequence of the breakage of some cementation bonds owing to the creep deformation. This degradation grows with the increases in binder quantity and the decreases in organic matter content. The uniaxial creep tests indicate an increase in the creep strain rate with log time, allowing the definition of two creep compression indices, $C_{\alpha 1}^{\ast }$ and $C_{\alpha 2}^{\ast }$. Whereas the initial creep compression index ($C_{\alpha 1}^{\ast }$) is independent of the strength and shows a low scatter of values (0.00013–0.0003), the final creep compression index ($C_{\alpha 2}^{\ast }$) decreases with the increment of the strength, i.e., for higher binder quantities and lower organic matter contents. The results also reveal that the ratio $C_{\alpha 2}^{\ast }/C_{\alpha 1}^{\ast }$ is almost unchangeable, 2.0–3.0.

This paper presents the results of an investigation into the role of peat fibers in the development of anisotropic stiffness and strength within peat samples taken from below three Canadian National Railway embankments across Canada. These sites include two from northern Alberta on the Edson and Lac-La-Biche subdivisions and one from the Lévis subdivision in southern Quebec. The peat samples were collected in Shelby tubes and were subjected to a program of consolidated undrained triaxial testing. These samples represented a wide variation in fiber content and degree of humification. The measured pore-pressure response during undrained loading indicated that the peat samples were undergoing an anisotropic elastic response to loading. This pore-pressure response was similar in the peat from all three sites, with an anisotropic pore-pressure parameter, $a$, of approximately 0.3. The stiffness and strength of the different peats were also similar, with an elastic modulus of approximately 1.7–2.2 MPa and an effective shear strength of 38°. The yielding behavior of the laboratory specimens suggested that plastic yielding occurs without the development of distinct failure planes and is accompanied by strain hardening. The combined effect of the effective strength parameters and the anisotropic pore-pressure response on the reinforcement provided by the peat fibers is also presented. This analysis showed that the net effect is dependent on the initial stress state of the peat. Under elevated mean effective stresses and low deviatoric stresses, the fibers may have a detrimental effect on strength.

Shallow slides in levee and other embankment slopes are usually controlled by effective normal stresses less than 12 kPa (250 psf). For first-time slides in fine-grained soils, the fully softened shear strength is frequently used to model the strength of embankment soils because it represents the shear strength remaining after the effects of overconsolidation, compaction, desiccation, or other strengthening processes have been removed because of wetting, infiltration, stress relief, swelling, and weathering. However, there is limited fully softened strength data at effective normal stresses less than 50 kPa (1,000 psf), so existing correlations in this stress range must be extrapolated. This paper presents new fully softened shear strength data at an effective normal stress of 12 kPa (250 psf) and recommendations for estimating and modeling the stress-dependent fully softened shear strength envelope directly or as a power function in stability analyses.