Soil Cap Research Articles

Bearing capacity of circular footings on multi-layered sand-waste tire shreds reinforced with geogrids

The presence of waste tires poses an environmental challenge as they occupy a significant amount of land and are expensive to dispose in landfills. However, reusing waste tires can address this issue when waste tires are used in geotechnical applications. To determine the viability of this approach, laboratory-scale tests were conducted to investigate load-bearing capacity of circular footings on sand-tire shred (STS) mixtures with shredded waste tire contents of 5%–15% by weight and three different widths of shreds. The investigation focused on analyzing the thickness of layers composed of STS mixtures, the soil cap, and the impact of geogrids on bearing capacity. The results indicate that a specific mixture of sand and tire shreds provides the highest footing-bearing capacity. In addition, the optimal shred content and size were found to be 10% by weight and 2 cm × 10 cm, respectively. Furthermore, for a given tire shred width, a particular length provides the largest bearing capacity. The results agree well with that of previous research conducted by the first author and his colleagues in direct shear and California bearing ratio (CBR) tests. The primary finding of this research is that the use of two-layered STS mixtures reinforced by geogrids significantly enhances the bearing capacity.

Rapid evolution of salt glacier caves on a mountain diapir in a semiarid climate

Salt caves occur mostly in hyperarid and arid climates and are typically a few thousand years old. There is little data on the evolution of salt caves in semiarid climates, and no data on caves developed in salt glaciers, i.e., salt masses spreading under their own weight, similar to ice glaciers. The age of salt caves and the incision rates of cave streams were studied on a salt glacier of the Jahani diapir in a semiarid climate, Zagros Mts., Iran. Wood fragments from 3 caves sampled 0–11.3 m above the present streambed were dated by AMS 14C. The cave passages accessible today were developed during the last ~600 years at the Jahani. The cave level 5–6 m above the present streambed was formed 1520–1660 CE or slightly earlier, then the stream incised upward and reached its highest level between 1690 and 1920 CE or slightly earlier. Then, the downward incision started. This indicates that caves in semiarid climates evolve extremely rapidly. The maximum vertical incision rate of the cave stream on the semiarid Jahani diapir is 20–110 mm/yr, which is faster than 0.4–25 mm/yr in caves in hyperarid climates. The incision rate of the cave stream at the Jahani exceeds the denudation rate of the thick cap soil surface, being comparable to the denudation rate of salt exposures. The persistence of the steep longitudinal profiles of the integrated salt caves at the Jahani is probably caused by the transport of very coarse sediment. Aggradation was probably caused either by blocking of cave passages by earthquakes or by more intense erosion of the cap soil. The salt glacier at the Jahani is moving slowly (<10 mm/yr); claims that this glacier is the fastest on Earth (velocity in the range of m/yr) are based on a study that did not follow geodetic standards and are in strong disagreement with the age of the caves and other lines of evidence.

Sustainable use of End-of-Life-Tires (ELTs) in a vibration isolation system

Huge stockpiles of End-of-Life Tires (ELTs) are of major global concern. This paper investigates and helps to demonstrate the beneficial utilization of ELTs to provide geotechnical seismic isolation (GSI) of machine foundations located on a rubber-soil mixture (RSM). A series of laboratory tests is conducted to extend the understanding of RSM behavior beyond that previously elaborated, particularly by demonstrating the efficiency of the GSI-RSM system and by defining its optimal parametric values. The results indicate that increasing the thickness of the RSM layer and soil cap, along with the addition of rubber content, up to the optimum values lead to enhanced performance. Beyond these optimum values, the benefits start to decrease. For the determined optimum rubber content (8%), RSM layer thickness (0.1B), and soil cap thickness (0.15B) at a vibration moment level of 0.17 N.m, the GSI-RSM system achieves maximum reductions of 36%, 43%, and 77% in resonant frequency, resonant displacement, and resonant acceleration, respectively, compared to untreated beds. Moreover, the GSI-RSM system effectively reduces the developed peak particle velocity (PPV) and kinetic energy, independent of the excitation frequency. The increased damping and significant energy dissipation demonstrates the system’s effectiveness in mitigating vibrations. This environmentally-friendly technique presents a low-cost alternative for improving the vibration behavior of soil beds. It provides the basis for the wider adoption of ELTs for GSI, for machine foundations and for other infrastructure projects with similar soil vibration concerns, including building foundations, transportation infrastructure and industrial facilities. In this way the research contributes to the growing body of sustainable practices in the construction industry.

Study on Utilization of Biopolymer in Liner

Abstract: Biogeotechnology is a branch of Geotechnical Engineering that deals with the application of biotransformation, biogrouting and mineral precipitation (MICP) to geotechnical engineering problems. Soil capping is a technology to curb the interaction between a waste body and the ground surface. A final cover system in landfill capping play a major role in intercepting rainwater infiltration and percolation of surface water into the subsurface waste dumped area. In order to minimize the water infiltration through different barrier layers such as single liner system, single composite liner system and double liner system in cover system are recommended. A single composite liner system comprises of two barrier (made up of different material), placed over one another to provide beneficial effect whereas the single and double liner system consists of same material to attain the effect. The present study deals with the action of sea weed in the form of powder (produced by the cell wall) in the layer of soil capping which intercept the rainwater intrusion in liner. The results obtained from the hydraulic conductivity of different treated soils indicate that the effect of single liner system is equivalent to single composite liner system. It has been observed that the addition of sea weed powder causes dissociation of hydroxyl groups attached to silicon in silica sheets with increase in optimum water content. Thus the addition of sea weed powder decreases the infiltration rate by clogging the soil layers.

Phytostabilization alternatives for an abandoned mine tailing deposit in northwestern Mexico

PurposePast mining activities in northwestern Mexico have left a large number of abandoned mine tailings deposits, such as in San Felipe de Jesus, Sonora, that is known to be transferring metal(oid)s to nearby agricultural fields. Given the risk and the need to implement mitigation measures, in this paper, we describe the results of two experiments evaluating the phytostabilization potential of five plant species collected in the area.MethodsWe evaluated the assisted phytostabilization approach using compost and nutrients and the soil capping approach using combinations of soil, gravel, clay, and tailings layers.ResultsThe assisted experiment revealed that seedlings were unable to establish under unamended treatments and only Ricinus communis showed potential under this approach. Compost and nutrients reduced the accumulation of As, Pb, Mn, and Zn in leaves of R. communis, but some were above the maximum tolerable levels for domestic animals. Under the capping approach, R. communis also showed better performance under some combinations of soil, gravel, and tailings layers than the other species. The accumulation of As, Pb, and Mn in leaves was below the maximum tolerable levels for domestic animals, indicating that soil capping has greater potential in this abandoned tailing deposit.ConclusionThe capping approach has more phytostabilization potential than the use of amendments, reducing the risk of incorporating metal(oid)s in the trophic web.

Phosphogypsum impacts on soil chemical properties and vegetation tissue following reclamation.

Phosphogypsum (PG) is a by-product of phosphorus fertilizer that is typically stacked near production sites. Phosphogypsum contains trace elements and naturally occurring radioactive materials which may be hazardous to the surrounding environment. Phosphogypsum stack reclamation typically involves placing a soil cap and seeding grass to create a barrier for reducing environmental impacts; using woody species is uncommon. This study used three soil treatments with grass and woody species to determine whether mixing PG with soil affects soil chemical properties, and metal and radionuclide concentrations in tissue. None of the elements in soil was above Canadian guidelines for industrial land use. Aluminum, beryllium, chromium, copper, iron, magnesium, manganese, nickel, and vanadium were significantly higher in both study and reference sites than in pure PG; cadmium, calcium, fluoride, and strontium were significantly higher in pure PG. There was a poor correlation between soil and plant concentrations for most elements indicating trace elements were not in a bioavailable form. Trace elemental concentrations in plant tissue generally differed significantly with vegetation type but not within similar species. Trace elements and isotopes in PG were not high enough to affect plant growth. Among the isotopes, 222Ra emissions differed significantly with vegetation covers; activity of 226Ra in pure PG was above Canadian guidelines, but lower in vegetation tissue. This study suggests 15cm soil mixed with PG can be used for PG stack revegetation when fast-growing Salix and Populus species are used in reclamation.

Growth and survival of native wetland species in shallow capped centrifuged tailings and co-mixed tailings: a meso-scale greenhouse study

This 3-year meso-scale greenhouse study used 55-gallon columns to evaluate the survival and growth of boreal wetland communities planted on centrifuge (CF) tailings and co-mixed (CM) tailings capped with different reclamation cover soil capping designs. The CF tailings were capped with a shallow layer (10 and 30 cm) of peat reclamation material (PRM) and the CM tailings were capped with a shallow layer (5 cm) of PRM above 15 or 35 cm of reclamation subsoil (till). After 3 years, plant survival and growth on CF tailings showed significant improvement with a 10 cm PRM cap compared to the uncapped tailings, and plants growing on a 30 cm PRM cap outperformed those on the 10 cm PRM cap. Plant growth on CM tailings was significantly improved with a soil cover containing 5 cm PRM and at least 15 cm till. Among the seven native wetland species included in this study, the top performing species in terms of survival and above-ground biomass were Salix bebbiana, Scirpus microcarpus, and Carex aquatilis.

Chemical speciation of phosphorus in farmland soils and soil aggregates around mining areas

Phosphorus (P) speciation determines its bioavailability, mobility, and environmental risk in soils. Farmland soils around mining areas are worldwide distributed, yet the P speciation in such soils remains poorly understood. In this study, we determined the P speciation in the farmland soils and soil aggregates around mining areas from China and evaluated their relationship with soil pH and Fe/Al fractions using chemical extraction, P K-edge X-ray absorption near-edge (XANES) spectroscopy, and correlation analysis. Chemical extraction revealed that soil P was dominated by inorganic P (Pi) (68.0 – 82.5%) largely associated with pedogenic Fe/Al oxides (43.4% – 94.8%). The P was mainly adsorbed and occluded by Fe/Al oxides in acidic soils, while predominantly existed as Ca-P and occluded species in alkaline soils. The presence of heavy metals appeared to have no significant effect on P speciation in these soils. Compared with the macroaggregates, the available P, total P, and their ratio all increased dramatically in the soil colloids (< 0.002 mm) due to the accumulation of Fe/Al oxides. The XANES analysis indicated that the P speciation in the acidic soils and soil aggregates was dominated by Fe-P, followed by Al-P, and little Ca-P; in contrast, the proportions of Ca-P increased but Fe-P decreased in the alkaline soils and soil aggregates. Compared with the macroaggregates, the proportions of Fe-P increased while Ca-P decreased in the soil colloids. Correlation analysis indicated that amorphous Al and isomorphic substitution Al played critical roles in P immobilization by pedogenic Fe/Al oxides. These new insights into the P speciation are essential for environmental risk assessment and efficient utilization of P in the farmland soils.

Controlling internal nitrogen and phosphorus loading using Ca-poor soil capping in shallow eutrophic lakes: Long-term effects and mechanisms

Clean soil is a potential capping material for controlling internal nutrient loading and helping the recovery of macrophytes in eutrophic lakes, but the long-term effects and underlying mechanisms of clean soil capping under in-situ conditions remain poorly understood. In this study, a three-year field capping enclosure experiment combining intact sediment core incubation, in-situ porewater sampling, isotherm adsorption experiments and analysis of sediment nitrogen (N) and phosphorus (P) fractions was conducted to assess the long-term performance of clean soil capping on internal loading in Lake Taihu. Our results indicate that clean soil has excellent P adsorption and retention capacity as an ecologically safe capping material and can effectively mitigate NH4+-N and SRP (soluble reactive P) fluxes at the sediment-water interface (SWI) and porewater SRP concentration for one year after capping. The mean NH4+-N and SRP fluxes of capping sediment were 34.86 mg m−2 h−1 and -1.58 mg m−2 h−1, compared 82.99 mg m−2 h−1 and 6.29 mg m−2 h−1 for control sediment. Clean soil controls internal NH4+-N release through cation (mainly Al3+) exchange mechanisms, while for SRP, clean soil can not only react with SRP due to its high Al and Fe content, but also stimulate the migration of active Ca2+ to the capping layer, thus precipitating as Ca-bound P (Ca-P). Clean soil capping also contributed to the restoration of macrophytes during the growing season. However, the effect of controlling internal nutrient loading only lasted for one year under in-situ conditions, after which the sediment properties returned to pre-capping conditions. Our results highlight that clean Ca-poor soil is a promising capping material and further research is needed to extend the longevity of this geoengineering technology.

Arsenic and its management in soil

Arsenic (As) is a toxic metalloid and carcinogenic element that occurs widely in environment around the world. Arsenic is introduced into soil and water through geological (weathering of rocks and minerals) and anthropogenic pathways (mining, pesticides, fertilizers, timber treatment etc). Irrigation of agricultural land with As-contaminated groundwater or wastewater, particularly in India, Bangladesh and South-East Asia resulted in the accumulation of arsenic in both soils and plants posing risks to soil ecosystems and human health. Arsenic in soil and water exists in a different valence state, but predominantly as toxic As(III) and less toxic As(V) form. Arsenic toxicity is one of the most challenging problems in drinking water, as well as food crops (Samal et al., 2021). With greater public awareness of arsenic poisoning in animals and human nutrition, there has been growing interest in remediation technologies for mitigating As-contaminated ecosystems. Remediation of As-contaminated soil improves physical, chemical and biological approaches that may achieve either partial/complete removal of arsenic from soil and water. Many technologies like bioremediation, phytoremediation, soil mixing, soil washing, soil capping, filtration, adsorption and immobilization are used for arsenic removal from soil and water. Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilize, and/or destroy contaminants in the soil and groundwater. But most of the arsenic still remains in the environment and there is always a risk of leaching caused by changes in the environmental conditions. Integrated approach involving a combination of physical, chemical and biological technologies is needed for the successful and effective management of As-contaminated environment.

Keu Contamination in Tuscany: The Life Cycle Assessment of Remediation Project as a Decision Support Tool for Local Administration

In this study, a Life Cycle Assessment (LCA) was conducted on a project to clean up a heavy metals-contaminated site located in central Italy (Tuscany) in order to define the less impactful solution. The study evaluated the contamination in the soil, derived from the leaching of backfill materials composed of quarry aggregates and sintered granules referred to as “Keu”, a waste derived from the pyrolization of sewage sludge from the Tuscan tannery district, rich in chromium. Three action scenarios were compared, namely the no-action scenario, an excavation and landfill disposal scenario, and a permanent material capping and sealing scenario. The LCA results show the lowest impact for soil capping operations due to the high impacts of heavy metal emissions and landfilling of materials for the first and third scenarios. The third scenario has the lowest impact for ten of the eleven categories analyzed by the CML-IA baseline method. The only exception is the ozone layer depletion category due to the binder synthesis process used for the pavement to protect the membrane. Future studies could be devoted to the study of Keu, through extensive characterization, as well as a study of the fate of this material in landfills to define an appropriate process for future LCA analysis.

Effects of Oyster Shell Powder and Lime on Availability and Forms of Phosphorus and Enzyme Activity in Acidic Paddy Soil

Soil acidification improvement in the main grain production regions of southern China is an important issue to enhance the quality of cultivated land and promote grain yield. In order to explore the effects of oyster shell powder and lime on acidity and availability and inorganic forms of phosphorus in acidic paddy soil, a pot experiment was performed using oyster shell powder and lime amendments with dosages of 0.05%, 0.10%, and 0.15%. The results showed that both oyster shell powder and lime significantly (P<0.05) increased the pH and decreased exchangeable acid content of paddy soil. The improvement effects increased with the dosage of soil amendments. Under the same amendment dosage, the effects of lime on soil pH and acidity were higher than those of oyster shell powder. Both lime and oyster shell powder significantly increased the content of available P extracted using H2SO4-P, Bray-1 P, and Olsen-P techniques. The contents of inorganic P in soils decreased in the order of Fe-P>Al-P>Ca-P. The application of lime and oyster shell powder significantly increased the contents of Al-P and Fe-P in soil. Compared with the control treatment, lime and oyster shell power increased Al-P and Fe-P by 26.3%-37.4% and 7.9%-23.7%, respectively. However, there was no significant difference in Al-P content among treatments of the three amendment dosages. The contents of Fe-P and Ca-P in soils increased with an increased dosage of amendments. The activities of DHA, ALP, and IPP and the copy number of the phoD gene in soil increased with the application of lime and oyster shell powder, whereas the activities of ACP and the copy numbers of phoC and pqqC decreased. The application of lime and oyster shell powder at a rate of 0.10% and 0.15% significantly (P<0.05) increased the yield of rice. The lime and oyster shell powder treatments at the dosage of 0.15% increased rice yield by 34.2% and 46.8%, respectively, whereas the amendment had no significant effect on straw biomass of the rice crop. Correlation analysis showed that soil pH and the ALP activity were significantly positively correlated with inorganic P and available P contents, respectively. These results suggested that lime and oyster shell power could effectively increase the content of available phosphorus by eliminating soil acidity and improving the phosphatase activity, which played a positive role in increasing crop yield.

Study on Reinforcement Mechanism and Reinforcement Effect of Saturated Soil with a Weak Layer by DC

This paper presents a numerical investigation on the improvement mechanism of dynamic compaction (DC) in saturated soil of a weak layer with high levels of groundwater, using an improved fluid–solid coupling method with a Drucker–Prager–Cap soil model. The numerical model is verified by comparing with published test data at first, which show that the dual-phase coupling method can approximately reflect the development law of excess pore water pressure and the improvement effect of layered saturated soil foundation under DC. Then, based on the numerical model, the influences of the thickness and depth of the weak layer as well as tamping energy on the development and dissipation of excess pore water pressure, effective stress, and the relative degree of reinforcement during DC were investigated. The results showed that the embedment depth and thickness of the weak interlayer may greatly affect the effective reinforcement depth of DC. Meanwhile, the tamping energy and the groundwater table also play a great role in the improvement of the layered saturated soil under DC. The groundwater table should be lowered by dewatering or adding a drainage layer to achieve a better compaction effect during DC.

Dynamic Responses of a Pile with a Cap under the Freezing and Thawing Processes of a Saturated Porous Media Considering Slippage between Pile and Soil

The freezing/thawing stratification effect of seasonal factors or artificial disturbances in frozen soil regions has an important influence on the vertical vibration of the pile–soil–cap system. Taking into account the slippage between the pile and soil, a simplified layered analytical model of the vertical vibration of the pile–soil–cap system in a double-layered stratum under the freezing and thawing processes of a saturated porous medium was established, and the analytical solution of the dynamic response on the top of the pile cap was obtained. In this model, frozen saturated porous media and Biot’s porous media theory were used to simulate frozen soil and unfrozen soil, respectively. The validation of the slippage model was first verified by comparison with the results of the existing model tests. This was followed by a dynamic model test of the pile–soil–cap system in a self-made, ground-freezing system. In comparison with the analytical results and the experimental results of model tests under the freezing/thawing processes, the validation of the present model is further verified. A comprehensive parametric study reveals that the parameters of the frozen or thawed soil layer have significant effects on the amplitude–frequency curve of the vertical vibration of the pile foundation.

Effect of soil capping depth on phosphogypsum stack revegetation.

Phosphogypsum is a by-product of the phosphorus fertilizer production process and is typically stacked at the production sites. These stacks can potentially pose environmental hazards, which can be substantially reduced through reclamation by capping with soil and revegetation upon decommissioning. We conducted a study on a phosphogypsum stack using five soil capping depths (8, 15, 30, 46, 91cm), an uncapped treatment, and five vegetation treatments (monocultures of four grass species Agrostis stolonifera L., Festuca ovina L., Deschampsia caespitosa (L.) Beauv., Agropyron trachycaulum (Link) Malte ex H.F. Lewis and one mix of the four species with Trifolium hybridum L.) to assess plant growth, health, rooting characteristics, and trace element uptake. Cobalt and nickel concentrations in plant tissue from plots with ≥ 15cm soil capping were within ranges found at reference sites, whereas fluorine was three times elevated. Vegetation cover was significantly greater on capped than uncapped plots, being greatest for Agropyron trachycaulum (26%) and Festuca ovina (26%). Capping depths ≥ 15cm had greater cover, biomass, and healthy plants than the 8cm cover. Soil water content was similar in the 15-46cm capping depth, with the lowest in the 91-cm caps. Fluorine, cobalt, and nickel were elevated in select plant tissue samples on the research plots relative to references, and cap depth affected tissue fluorine and cobalt concentrations but not nickel. Concentrations of these trace elements were lower than maximum tolerable levels for animal consumption. From this 5-year study, Agropyron trachycaulum and Festuca ovina and a soil cover depth of ≥ 15cm are recommended for reclamation of phosphogypsum stacks.

Biochar, Mineral P and Their Co-Application Affects Chemical Fractions of P and Mung Bean Growth in Two Tropical Soils

ABSTRACT Phosphorus fixation reduces P availability and affects crop yields in tropical soils. The ameliorative effect of biochar and its co-application with mineral P fertilizer on the soil labile inorganic P fractions and crop growth are unknown and hence studied in two soils with contrasting properties. Soils were applied with kunai grass (Imperata cylindrica) biochar at 15 t ha−1, mineral P fertilizer at 64 mg P kg−1 either together or solely to the potted soils under greenhouse conditions and mung bean (Vigna radiata) seeds were planted. Treatments were replicated 3 times and pots were arranged in complete randomized design. Soil samples were collected from the rhizosphere on days 30 and 90 and analyzed for pH, soluble P, inorganic Fe, Al and Ca-bound P fractions. Dry matter content and P uptake by the mung bean was also assessed. Soil pH was unaffected by fertilization regimes in both soils. Sole biochar application as well as co-application with mineral P increased the amount of P in Ca-P, Al-P and Fe-P fractions in both soils. However, co-application of kunai grass biochar with 64 mg P kg−1 of fertilizer significantly (p < .05) increased mung bean crop dry matter in acidic (78.8%) as well as alkaline soil (23.1%). The usage of kunai grass biochar should be combined with mineral P for reaping the synergistic agronomic benefits in the tropical acidic and slightly alkaline soils.

Analytical modeling for the load-transfer behavior of stiffened deep cement mixing (SDCM) pile with rigid cap in layer soils

A new composite structure named stiffened deep cement mixing (SDCM) piles has recently been considered and applied in many geotechnical engineering projects. The SDCM pile was formed by inserting a stiffened core into the deep cement mixing (DCM) pile that has just finished construction, thereby combining the advantages of DCM piles and concrete piles. This study developed a load-transfer analytical approach for SDCM pile with rigid cap in layer soils from the theoretical solution view. Herein, the nonlinear mechanical behaviors of the DCM pile–soil and DCM pile–stiffened core interfaces were characterized by a tri-linear bond-slip model, and the action of soil on the pile end was simulated by an elastic-perfectly plastic model. Three-dimensional finite element simulations were conducted for SDCM piles embedded in homogeneous soil and double-layer soils. The reasonable matches between the predictions and the results from FE simulations and physical model tests confirmed the applicability of the theoretical approach. The full-range load-transfer behavior and the bearing performance of SDCM piles were further discussed. Parametric studies were finally conducted, providing insights into the design practice of SDCM piles.

Competitive adsorption of multicomponent volatile organic compounds on biochar

Crude oil contaminated lands are recognised to have significant contributions to airborne volatile organic compounds (VOCs) with adverse effects on human health and tropospheric ozone. Soil capping systems for controlling harmful emissions are critical engineering solutions where advanced soil remediation techniques are neither available nor feasible. Studies on the adsorption of single VOC species in biochar have shown promising results as a potential capping material; however, current understanding of mixed gas system and multi-component adsorption of VOCs on biochar which would represent more realistic in situ conditions is very limited. We present, for the first time, the results of a study on competitive adsorption of mixed VOCs, including aromatic and non-aromatic VOCs commonly emitted from crude oil contaminated sites on two types of biochar pyrolysed at 500°C from wheat straw and bagasse as feedstock. The kinetics of sorption of multicomponent VOCs including acetone, hexane, toluene and p-xylene in biochar are studied based on the results of an extensive experimental investigation using a bespoke laboratory setup. Both biochar types used in this study presented a high sorption capacity for VOC compounds when tested individually (51–110 mg/g). For the multicomponent mixture, the competition for occupying sorption sites on biochar surface resulted in a lower absolute sorption capacity for each species, however, the overall sorption capacity of biochar remained more or less similar to that observed in the single gas experiments (50–109 mg/g). The chemical interactions via hydrogen bonds, electrostatic attraction, and pore-filling were found to be the main mechanisms of adsorption of VOC in the biochar studied. The efficiency of biochar regeneration was assessed through five cycles of adsorption-desorption tests and was found to be between 88% and 96%. The incomplete desorption observed confirm the formation of likely permanent bonds and heel build-ups during the sorption process.

Effects of seasonal weathering on dewatering and strength of an oil sands tailings deposit

Considerable research has been conducted over the past decade by oil sands mining companies to improve the dewatering and strength properties of fluid fine tailings deposits in an effort to meet the regulatory and closure requirements. Commercially employed dewatering treatment technologies (inline flocculation, thickening, and centrifugation) may not be sufficient to develop the strength for the creation of trafficable landscape without the use of soft soil capping technologies. These treated tailings are continuously deposited creating soft and saturated deep deposits. Seasonal weathering may be an additional promising technology to further dewater the treated tailings and promote the development of shear strength at the surface. This paper presents the effects of seasonal weathering on dewatering and strength that were investigated by performing multiple cycles of freeze–thaw and alternate drying–wetting cycles on two types of treated tailings deposit. The results indicate that multiple cycles of seasonal weathering significantly increased the dewatering and strength properties. However, different parameters such as freezing gradient, number of seasonal cycles, and pore water chemistry play an influential role in changing the magnitude of the strength. The results also suggest that a minimum threshold strength value is required where the effects of rainfall rewetting had a minimal impact on strength reduction (the strength corresponding to the moisture content approaching the plastic limit).

Effect of soil cap and nitrate inflow on nitrous oxide emissions from woodchip bioreactors

Woodchip bioreactors have the capability to promote the reduction of reactive nitrogen in the nitrate (NO3−) form to dinitrogen (N2), a harmless gas in the atmosphere. Nevertheless, during the reaction the potent greenhouse gas nitrous oxide (N2O) is produced and can be released if denitrification is not complete. The aim of this experiment was to quantify the effect of a soil cap, the concentration of NO3− inflow and drying-rewetting cycles (DRW) on N2O emissions from bench top bioreactors (BTBs, 36.2 × 24.2 × 16.8 cm). The soil cap effect was quantified by comparing the performance of two treatments (n = 3): soil cap (CAP) and soil cap free (UNCAP). The NO3− inflow was simulated by feeding the BTBs with two NO3− concentrations (10 and 5 mg N L−1), and DRW were simulated by saturating and draining the BTBs. Nitrous oxide was quantified in the water samples as well as measured from the surface of the BTBs.The soil cap proved effective at decreasing surface N2O emissions with a reduction of total N2O emissions (calculated as the sum of dissolved N2O and surface N2O emissions) ranging from 30.4 to 42.9%. The NO3− inflow affected dissolved N2O and surface N2O emissions with higher values (average of 3.41 and 0.36 mg m−2 d−1, respectively for CAP, and average of 2.92 and 2.52 mg m−2 d−1, respectively for UNCAP) measured at high NO3− inflow. Drying-rewetting cycles influenced dissolved N2O and surface N2O emissions, with values following rewetting that accounted for more than 56% of the total N2O emissions for both treatments. This study confirmed that soil caps are effective at mitigating N2O emissions and contributed to a better understanding of N2O dynamics induced by two different NO3− inflow concentrations and DRW.