Geo-Congress 2022: Geoenvironmental Engineering; Unsaturated Soils; and Contemporary Topics in Erosion, Sustainability, and Coal Combustion Residuals

Biochar, a solid porous product derived from waste biomass, has garnered profound attention from the geoenvironmental engineers in the recent years, due to its ability to absorb wide range of gaseous and liquid phase contaminants. One important application of biochar that has been explored recently is the use in landfill cover soil to enhance microbial methane oxidation. The unique properties of biochar such as high-internal porosity, high-moisture retention, and presence of recalcitrant carbon compounds offer conducive environment for the methane oxidizing microbes to proliferate and thrive in the long-term exposure to landfill methane, thereby enhancing aerobic methane oxidation and mitigating landfill methane emissions. Although biochar has shown promising potential to enhance methane oxidation, the addition of inert biochar in the landfill cover soil leads to an initial lag phase due to the time required for microbial acclimation and may result in lower methane oxidation rates in comparison to the soil alone which has already been exposed to the landfill gas. However, once the microbes are acclimated, the effect of biochar kicks in and transcends the capacity of soil alone to oxidize landfill methane. This paper investigates reduction of the initial lag phase caused by addition of inert biochar using an activated biochar. Activated biochar was prepared by soaking biochar in a methanotrophic bacterial consortium isolated from a landfill cover soil. Several series of laboratory batch and column experiments were performed with activated biochar amended soil, non-activated biochar amended soil, and unamended soil to quantify methane oxidizing potential. DNA based 16S rRNA gene amplicon sequencing was performed to characterize and compare the microbial community. The activated biochar amended soil showed higher methane oxidation rates and efficiency from the beginning of the incubation in batch and column experiments in comparison to the non-activated biochar amended and unamended soils. The cumulative methane uptake of 10% non-activated biochar amended soil was lower than the unamended soil until initial 50 days of batch incubation which increased rapidly thereafter. The carbon dioxide generation and significant increase in methylotrophic relative abundance confirmed methane oxidation. Overall, the activated biochar showed promising potential to reduce the initial lag phase and enhance microbial methane oxidation in landfill cover soil.

The usage of polymer-modified clays to construct hydraulic barriers has gained popularity due to their superior hydraulic properties. Altering the clays using polymer enhanced the chemical resistance of composite material in containing aggressive inorganic solutions to which the untreated virgin clays fail. The present study is focused on evaluating the free swell index of clays modified through the in situ polymerization of acrylic acid. Tests were conducted on two mineralogically diverse clays, i.e., Na-bentonite and kaolinite, using interacting liquids such as NaCl, CaCl₂, AlCl₃, and solutions with extreme pH conditions. The swell index of polymerized clays was in the order of monovalent > divalent > trivalent solutions and was higher than the swell index of the untreated clays. Test results showed that the polymer addition had improved the swelling capacity of clays. However, hydraulic conductivity tests are needed to be performed to ascertain their long-term hydraulic performance.

Hydraulic conductivity tests on geosynthetic clay liners (GCLs) to evaluate chemical compatibility can require months to years to reach equilibrium. There is a need for alternative methods to screen GCLs for chemical compatibility that are more expedient. In this study, a neural network machine learning (ML) algorithm was used to predict the hydraulic conductivity of Na-Bentonite (NaB) GCLs to leachate chemistry. Development of the ML predictive model (MLPM) included five steps: data collection, data cleaning and normalization, algorithm selection, parameters optimization, and model validation and evaluation. The MLPM is based on data collected from two decades of tests conducted on NaB GCLs with a broad range of leachates. Bentonite characteristics, permeant chemistry, and stress conditions are incorporated into the MLPM. Validation showed that the MLPM predicts hydraulic conductivity within one order of magnitude of the measured hydraulic conductivity in 85% of the cases.

In Vietnam, geofoam is little known as a construction material for highway embankments. Geofoam is a lightweight, closed-cell, and rigid plastic foam material. Bridge approaching embankments can be constructed using geofoam blocks as a fill material directly on soft ground. This study investigated the field performance of a geofoam as a fill material of a bridge approaching embankment on soft ground in Ho Chi Minh City. A full-scale bridge abutment test was constructed using a domestic geofoam at the experimental site. The full-scale embankment test was experienced by 10 cycles of a 300-kN truck. The results indicate that the domestic geofoam with a density of 21 kg/m³ (EPS-22) worked properly for the bridge embankment as the fill material on soft ground under the truck load. The truck load diminished markedly throughout the concrete slab and reduced sharply in the embankment height. Elastic vertical displacement at the surface of the concrete slab measured at a distance of 0.5 m from the rear wheel was 5–6 mm under 9 cycles of the truck. The concrete slab distributed the truck load on a larger area than an area calculated by the assumption that the contact area of a wheel load distributes across the concrete slab by an angle of 45°.

Column experiments were conducted to select a suitable reactive material for a permeable reactive barrier (PRB) installed in a low-lying acidic floodplain located 130 km south of Sydney, Australia. Limestone (98% CaCO₃) could neutralize the groundwater acidity while effectively removing excessive concentrations of Al and total Fe. A computational approach that captures coupled biochemical clogging of a granular limestone assembly was adapted to simulate the behavior of the PRB installed in the acidic terrain. At the end of bicarbonate buffering, the predicted reduction of hydraulic conductivity due to clogging at the inlet and outlet of this PRB was 45% and 33%, respectively. PRB monitoring framework was also affected by coupled clogging, which was evident from the electronic images and changes in piezometer and data logger readings.

Wildfire-burnt hillslopes are vulnerable to erosion, precipitation-induced landslides, and debris flows. Xanthan gum (XG) and polyacrylamide (PAM) were recently shown as attractive alternatives for controlling surface erosion. However, their mobility on the hillslopes or effects on downstream waters are unknown. This study showed that after three wet-dry cycles, vertical movement of XG and PAM into the soil with rainfall is possible. Controlled water quality experiments showed that XG could have negative impacts on acidity and turbidity if its concentration in water is 0.25% or higher. Both additives showed potential concerns related to total nitrogen at concentrations of 0.25% (XG), 0.1% (PAM), and higher. Further, even at the lowest concentration (0.01%), both increased the total organic carbon concentration, which can be problematic for water treatment and control of toxic disinfection byproduct formation. However, further research is needed as the benefits of controlling post-fire erosion may overcome the water quality degradation from XG and PAM seen in controlled experiments.

Bentonite clay is widely used as a hydraulic barrier in geosynthetic clay liners and cut-off walls. The interaction of the clay with divalent electrolytic solutions such as Ca²⁺ or Mg²⁺ deteriorates its sealing properties. The treatment of bentonite with polymers has improved its hydraulic properties; however, sustainability concerns have led to the discovery of an alternate polymer. Sodium alginate was used to prepare a new, more sustainable polymerized bentonite clay. The alginate was selected due to its recent laboratory extraction from wastewater. This paper presents the performance of the alginate polymer-based bentonite clay in terms of swelling and hydraulic conductivity. Overall, the preliminary results were optimistic towards the use of alginate in polymerized clay. In short, the clay has the potential to be used as an effective hydraulic barrier in the construction industry, which would promote sustainable development.

Rapid increase in the demand for electrical vehicles (EVs) and consumer electronics has raised the global demand for lithium (Li) ion batteries because of their high-energy density, longevity, quick and effective discharge, and lower susceptibility to temperature fluctuations compared to conventional lead batteries. Besides imparting green solutions to the environment by means of reducing CO₂ emissions the exponential rise in Li batteries has created vulnerable effects to humans and other species on the earth. Sourcing the raw materials, production, usage, recycling, and disposal of these batteries lead to contamination of nature at each stage. This effect is more pronounced at the disposal stage, as the maximum percent of spent lithium batteries reach landfills without appropriate treatment. The present paper aims at discussing the possible ways of soil and water contamination by leachate released from spent Li ion batteries within the landfill. A thorough literature survey has been carried out on sorption and desorption characteristics of Li in different plastic soils exhibiting varying mineralogy. Due consideration has been given to effective cation exchange capacity and specific and non-specific adsorption of Li in plastic soils. The influencing factors such as hydration radius, presence of competing monovalent and divalent cations, pH of pore solution, and exchange sites in the soil are considered to explore the possible fixation studies to minimize the movement of Li-ion. The review forms the basis to better understand the mechanism involving the movement of Li in plastic fines and proposes suitable disposal strategies to be adopted.

The shale caprocks of the saline aquifers, used for geological storage of CO₂, are highly heterogeneous and anisotropic due to their diverse mineralogical composition. A thorough understanding of the mechanism of interaction of the clay minerals to CO₂ adsorption is unavoidable. The expansive clay minerals (smectites) are of specific importance as they can accommodate CO₂ in the interlayers and undergo volumetric expansion. The present study focuses on the effect of the interlayer cation in a smectite clay on CO₂ adsorption. Adsorption isotherms studied under moderate temperature and pressure ranges showed that sodium exchanged bentonites adsorb more CO₂ followed by potassium and calcium exchanged ones. The study revealed that the effect of exchangeable cation influences the energy heterogeneity on the clay surface and BET surface area but does not influence the mechanism of CO₂ adsorption. The significance of temperature on the clay surface energy characterization is also reported.

Slurry trench cutoff walls have been widely used for over 70 years to control groundwater flow, seepage through dams and levees, and contaminant transport. In the US, soil-bentonite (SB) slurry walls are frequently the best and most economical vertical barrier to stop the horizontal flow of groundwater and minimize contaminant transport. This paper reviews the development of SB cutoff walls from both a construction and design standpoint. Lessons learned regarding trench stability, the type of bentonite, the makeup of the backfill, specifications, quality control, interface connections, longevity, hydraulic conductivity, state-of-stress, and compatibility with contaminants are presented. Items of particular importance in specifications including viscosity of the fresh slurry, viscosity of the in-trench slurry, unit weight of the slurry, slump, gradation, and hydraulic conductivity of the backfill are discussed. Guidance for quality control testing in both the lab and field are provided including recommendations regarding stresses for testing. The paper provides summary opinions regarding the limitations of SB slurry walls. Issues that require special consideration are identified and include excessive depth, limited available working platform width, excessive underground or overhead obstructions, artesian ground water conditions, layers of extremely weak soil, and rock or boulders in the soil profile.

Sodium bentonite (NaB) is widely used in containment barriers to limit contaminant migration into the environment. The performance of NaB barriers in applications such as landfills, impoundments, and radioactive waste disposal relies on the ability of the NaB to limit liquid and contaminant flux through the barrier for extended durations and under changing environmental conditions. The results of previous studies indicate that NaB may exhibit significant membrane behavior (solute restriction), potentially enhancing the performance of the barrier by reducing both liquid and contaminant flux. However, diffusion and membrane behavior in NaB under variable temperature and chemical conditions is not well understood, largely due to experimental testing challenges. This knowledge gap limits our ability to predict contaminant transport through NaB barriers exposed to elevated temperatures (e.g., high-level radioactive waste disposal). To address this challenge, an innovative testing apparatus was developed for measurement of coupled membrane behavior and diffusion in clays under controlled temperature and chemical conditions. The setup was equipped with heating elements, temperatures sensors, and temperature controllers, along with remote data logging and hydraulic control features. The apparatus design and testing procedures, as well as example results for NaB are presented. Preliminary results indicate that the testing apparatus maintained consistent temperature conditions throughout testing and that membrane behavior decreased with increasing temperature.

While most municipal solid waste (MSW) landfills maintain temperatures below 55°C (131°F), a relatively few MSW landfills have temperatures exceeding 93°C (200°F). In order to understand the key mechanisms that allow heat accumulation and temperature rise, commercial geothermal model TETRAD was used to simulate the heat transfer in an MSW landfill located in the southeastern United States. Field temperatures of the landfill were monitored using thermistor sensor arrays. The measured peak temperatures ranged from 68°C to 77°C (155°F to 170°F). Waste heat generation, infiltration, and leachate flow were the key variables evaluated in the modeling. The model results indicated that the average waste heat generation rates for this landfill ranged from 0.3 to 1.4 W/m^3. The higher heat generation rate corresponds to a relatively young (≤4 years) portion of waste containing a relatively high mass fraction of coal combustion fly ash. The lower heat generation rate corresponds to a 14-year old portion of waste predominantly containing MSW.

In situ properties, including tip resistance, sleeve friction, pore pressure, Hydraulic Profiling Tool (HPT) pressure, and electrical conductivity along to depth, were measured to characterize solid waste using cone penetration test (CPT) with HPT probe at a closed landfill in Nebraska. The normalized results of the properties showed that all the measured values increased with depth. The existing equation and soil behavior charts were used to obtain unit weights and Soil Behavior Type (SBT) of the two sites. The obtained total unit weights were found to fall between 16 and 20 kN/m³ which are similar wet dense soils, while they are higher than unit weights obtained from an active landfill. The SBT chart showed that the waste has similar behavior to fine soils, as opposed to the more granular behavior of active landfills. The results show that this low-cost and rapid testing tool can be used to correlate between hydraulic and physical properties of landfills to make MSW landfill characterization more efficient.

Electrochemical techniques for remediating metal-contaminated fine-grained soils of low-hydraulic conductivity (in the order 10^–5–10^–9 m/s) have received increased attention over the last decade. However, the influence of electrochemical methods on the mechanical and hydraulic properties of treated soils is not fully understood. This laboratory study adopted the modified Fenton’s reagent using hydrogen peroxide (H₂O₂) and a citrate buffer (Na₃C₆H₅O₇) solution to remediate a low-plasticity clay from Rock Falls, IL. The soil was initially contaminated with 726 mg/kg manganese (Mn) and 7.9 mg/kg arsenic (As). Following a 28-day treatment period using a custom-made high-density polyethylene (HDPE) reactor, approximately 23% and 9% extraction efficiencies for Mn and As, respectively, were achieved. The post-treatment geotechnical tests including plasticity (1% reduction on average), unconfined compressive strength (6% reduction on average), and water retention analysis indicated a moderate correlation between the metal concentrations in the soil and the corresponding hydraulic and mechanical behavior of the soil.

Hydraulic conductivity field testing of compacted clay liners (CCLs) is an important control for landfill construction. The Two Stage Borehole (TSB) test, as described by ASTM D6391-11, is one of the field tests commonly used to evaluate CCLs. To evaluate the methods proposed in ASTM D6391 (Methods A, B, and C), a laboratory experiment to evaluate the results obtained from the different methods was performed. The experiment was to perform a TSB test on three field-scale test pads constructed in the laboratory and in accordance with the three methods. The results obtained from TSB testing led to the conclusion that Methods B and C, as written in the ASTMs D6391-11 standard contained errors that impacted the obtained results from hydraulic conductivity testing. The value of hydraulic conductivity determined from Method C (1.43E-07) is off by a factor of four; when the data were reevaluated utilizing a corrected Method C equation the values compared favorably with the Method A results (3.58E-08 and 4.88E-8, respectively). The values obtained utilizing Method B provided results as much as two orders of magnitude greater than the hydraulic conductivity determined from other methods, this is attributed to Method B being unconstrained.

The need to understand the fate and transport of Per- and Polyfluoroalkyl substances (PFAS) has grown due to the widespread contamination of the environment by them. PFAS are persistent, mobile, toxic manmade chemicals of great concern that contribute to the contamination of soil and groundwater. The presence of PFAS in unsaturated soil complicates their transport due to the impact of the air–water interface and solid-phase adsorption. The air–water interface can significantly increase the retention of PFAS during its transport. In this paper, a numerical model has been developed to study the transport of PFAS by coupling transient seepage and advection-dispersion, also accounting for the air–water interface and solid-phase adsorption. The numerical model was then used to study various scenarios.

The class of emerging per- and polyfluoroalkyl substances (PFAS) is a challenge for environmental remediation. In lined ponds with PFAS contaminated water there are several options for treatment, including in situ treatment within the pond. In the case of a lined pond, in situ treatment of the PFAS was proposed via flocculation and subsequent stabilization of dissolved and particle associated PFAS with chemical flocculants. This pond has a high-density polyethylene liner system that could be vulnerable to damage or distress from treatment activities. To evaluate the integrity of the liner system during and after treatment, an integrity testing program was undertaken. After aging in solution, coupons were visually inspected and then mechanically tested for tensile capacity. Both visual inspection and mechanical tensile testing showed that none of the flocculants or other chemical treatment alternatives resulted in significant deterioration of the liner, although high concentrations of one polymer flocculant showed low levels of attack on the liner material.

The main objective of this study is to investigate the effects of sample thickness and shape on desiccation cracking patterns and crack depth of a compacted clay. For this purpose, six specimens of a high-plasticity clay were tested in the laboratory to determine the effects of cross-sectional geometry (circular and square), and varying thicknesses (approximately 5, 10, and 15 cm) on crack surface patterns and crack depth. The soil was compacted at the optimum moisture content and the compacted samples were placed in a temperature-controlled chamber at 50°C for 48 h to undergo cracking. The crack patterns and progressions were photographed every 12 h and the samples mass was monitored throughout the experiment. An image processing technique was employed to quantitatively examine the evolution of cracking patterns. The results indicate that the sample shape and thickness play an important role in the cracking behavior of compacted clays. Surface crack ratio, average crack width, and total crack length generally increased for deeper specimens and were larger for the rectangular versus the circular cross-sections. Additionally, the rectangular cross-section attained deeper cracks than the circular cross-section.

This study was performed to show the effects of incorporating different percentages of recycled crushed roof shingles into different soils. Fiberglass-based asphalt shingles are the most common roofing material used for residential roofing in the United States. Over time, roof shingles begin to deteriorate due to weathering and are typically disposed of in landfills. Since shingles take over 400 years to completely break down in a landfill, recycled shingles have been used in a variety of roadway and parking lot paving projects in an effort to remediate this significant material waste. Recently, the geotechnical engineering community has been investigating the potential of using recycled crushed shingles in soil remediation projects. Integrating recycled roof shingles into the soil poses a potential hazard, as permeating water may tend to transport contaminants from the shingle material through the soil. This could potentially lead to plant and wildlife hazards as well as groundwater pollution. In this study, pH, conductivity, turbidity, chloride, hardness, Iron, and Phosphate tests are performed on soil, with different percentages of crushed shingles mixed into the soil, without any contaminants. Results are presented that show the effect of different amounts of recycled crushed shingles on the alteration of groundwater properties.

Expansive soil poses major problem when they come in contact with water. Few traditional stabilizers like lime, cement, and fly ash are being used for reducing swelling characteristics of expansive soils. However, recently, several studies indicate that the usage of traditional stabilizers are non-sustainable solutions may causes harmful CO₂ emissions to the environment and pH spike in groundwater. In this paper, the by-product of phosphorous industry (Phosphogypsum) was investigated as an alternative to traditional stabilizers in reducing swelling characteristics of expansive soils. Two soils of different plasticity were chosen to investigate the effectiveness of Phosphogypsum (PG) as stabilizer. A detailed geotechnical investigation on untreated and treated soils with different percentages PG was carried out. The results indicate that the liquid limit gradually decreases with the increase in the PG up to 10% for Soil A, and 30% for Soil B, beyond which, no significant change in the liquid limit was noticed. At an optimum dosage of 3% PG, unconfined compressive strength of Soil B at 28 days curing increased by 300 kPa compared to an identical untreated sample. For the same chemical dosage, swell pressure reduced from 238 kPa (untreated) to 19 kPa. Similarly, an increase in the strength of 100 kPa with a reduction in the swell pressure of 30 kPa was noticed for Soil A treated with an optimum dosage of 10% PG. It is found that the PG reduces swelling by exchange of cations and formation of cementation bond.

The granular bentonite (GB) in the conventional geosynthetic clay liners (GCLs) seals the macro-voids due to wetting induced swelling by the diluted salt leachates. The sealing ability of GB is lost in the presence of high-ionic strength salt leachate. Non-conventional GCLs, containing the polymer-amended GB, are currently explored for sealing the macro-voids. In this study, the hydraulic and volume change behaviour of polymer-amended GB high-strength salt solutions and loading conditions was studied to evaluate the long-term ability of non-conventional liners to contain the leachate in the landfills. The super-absorbent polymer, a commercial sodium polyacrylate, was used in the present study. It was shown that the polymer-amended GB with super-absorbing polymer was not able to seal the macro-voids under the studied conditions. The observations were in disagreement with the earlier work to use super-absorbing polymer for containing the high-ionic strength salt leachates.

Landfill gas, generated as a byproduct of municipal solid waste landfilling, poses adverse impacts to the environment and human health. Fugitive emissions of methane, a potent greenhouse gas, from landfills are a significant climate-change concern. This investigation was conducted to evaluate methane emissions from landfill final covers by comparing simulated emissions from a conventional and an alternative cover. The Subsurface Transport Over Multiple Phases (STOMP) numerical simulator was used to predict multifluid flow and reactive transport of methane through the covers, where representative soil properties, cover thickness, and placement conditions were applied. Simulated cumulative methane emissions were two orders of magnitude greater from the alternative cover than the conventional cover. Mechanistic explanations were provided for the observed behavior. Flow of gases must be considered in the design of landfill final covers in addition to considerations for hydraulic performance.

Microbially induced carbonate precipitation (MICP) is one of the techniques to improve ground. One critical part of this technique is the crystallization of calcium carbonate, where the calcium ions are commonly provided by calcium chloride (CaCl₂). While, the threat of chloride ions raises serious concerns; consequently, calcium acetate [Ca(CH₃COOH)₂] acting as a promising alternative to offer calcium ions has received growing attention. However, the relevant observations reported by previous studies are not comprehensive. To fill this gap, this study selects Ca(CH₃COOH)₂ and CaCl₂ as the calcium sources, then examines and compares their effects in two types of cementation environments including aqueous and silica sand environment, respectively. The micro characteristics via XRD analyses and SEM images indicate that the calcium source and cementation environment greatly affect the bio-cementation results, changing the crystal polymorph and morphology. Notably, the aragonite precipitated in the silica sand environment under the involvement of carboxyl groups can potentially contribute to the mechanical behaviors.

Collapsible unsaturated soils experience large volumetric compressive deformations upon wetting. Compaction is a simple and effective soil improvement technique that could significantly reduce soil collapsibility. This paper deals with the numerical modeling of effect of soil compaction on collapsible-soils settlements upon wetting under field conditions. The finite elements program CODE_BRIGHT was adopted for the numerical simulation of field tests using the model parameters that were previously obtained from independent laboratory experiments. The numerical simulations show that the proposed approach is able to model the behavior observed in the in situ tests involving footing prototypes on collapsible-soils subjected to soaking.

Soil water characteristic curve (SWCC) is a fundamental property of geomaterials to determine its unsaturated behavior. The SWCC is essentially important to design the geo-infrastructures such as earthen dams and levees, pavement subgrade, clay liners, soil covers, etc., and determine the ground response under varied environmental conditions. Although there are numerous methods to determine the SWCC in the laboratory, field measurements and evaluation of SWCC are very limited, especially in the geo-environmental application. The objective of this study was to determine and evaluate the change in the SWCC flow parameters of an earthen final cover of landfill under varied climatic conditions. To fulfill the objective, field test section (lysimeter) was instrumented with pairs of moisture content sensors and tensiometers at varied depths to capture the hydraulic response of the cover to a series of variable environmental cycles. The field SWCCs were monitored and evaluated for two years. The SWCCs determined from field investigation were also described mathematically using the van Genuchten’s equation, and two extreme conditions: maximum drying curve (MDC) and maximum wetting curve (MWC) were identified. Based on the field investigation, considerable changes in the SWCCs were observed at different depths. The upper portion of the earthen lysimeter was found to be significantly responsive under the influence of the environmental variation than the lower portion. Non-equilibrium occurrences were detected while portraying the field moisture-suction characteristics, especially in the lower region of the cover soil. Considerable discrepancies were observed in the van Genuchten’s curve fitting parameters in the MDC and MWC.

This study aims to calculate the effective stress parameter from the findings of both shear and volume change behavior of an expansive silt-bentonite mixture. A proposal will be made for a novel methodology to estimate the Effective Stress parameter (χ) from isotropic drying along with the oedometric loading. The suggested methodology comprises an elaborate stress path via which the χ is calculated through an iteration procedure. It is assumed that the isotropic and oedometric consolidation lines are parallel. Several oedometric consolidation and direct shear tests under a wide range of desired matric suctions are carried out and the findings are plotted in the plane of the effective stress. The capability of the effective stress to simulate the shearing and volume change responses of the soil under a wide range of matric suctions applied via osmotic and vapor equilibrium techniques is verified. Incremental form of the effective stress parameter is also determined to model the non-linear hydro-mechanical behavior of the studied expansive soil and the experimental results are compared with the empirical relationships suggested in the literature.

The safe and permanent disposal of high-level nuclear waste (HLW) is an unresolved problem in the nuclear industry. It is therefore imperative to find a solution for this relevant and timely problem. Previous studies are focused on the determination of the clay-barrier gas breakthrough pressure in the field and laboratory testing to determine the safety of the Engineered Barrier System. Most of these experiments are conducted in one-dimensional oedometric cell in which the saturated bentonite sample is enclosed in a stainless-steel container. Hence, a need was felt to conduct experiments which are more representative of the field conditions to which the barrier material (MX-80 clay in this paper) is exposed to in deep geological repository. Therefore, attention is given to gas flow through the interface as this flow through the dissimilar interface is not fully understood yet. These interfaces may offer a preferential pathway for the release of gas from the HLW to the host rock and hence the environment. This paper studies gas breakthrough mechanisms through the rock-clay contact under controlled thermo-hydro-mechanical (THM) conditions in the laboratory using a new and advanced THM multipurpose cell specifically developed to study this phenomenon in collaboration with UPC Barcelona. The paper discusses the time-dependent breakthrough pressure test through a rock barrier prototype and concludes that the rock-barrier interface plays a critical role in determining the overall efficiency of the Engineered Barrier System (EBS) and can allow the gas outflow at a lower breakthrough pressure. Hence, it is important to understand the mechanism involved in the breakthrough of gas through the interface of dissimilar material in EBS.

The use of a capillary barrier system (CBS) to control the water infiltration into the native slopes is widely studied at present as a counter-measure against rainfall-induced landslides. The CBS consists of finer-graded material over the coarser graded material to suspend water at the interface. Experimental determination of suitable material combinations for a given rainfall region is tedious and time-consuming. Theoretical evaluation of CBS is thus receiving significant interest. The determination of breakthrough head and pore pressure variations in the CBS slopes requires considerable study. In the present study, a model based on Richard’s flow equation and proposed interface boundary conditions was presented to study the influence of the CBS effect for a given material combination. The influence of different hydraulic conductivity models (HCF) on the breakthrough head was studied. The proposed model is helpful to find the optimal combination of layered systems to design efficient CBS.

This paper presents the results of cyclic triaxial tests conducted on unsaturated soil samples recovered from the railway embankment of a South African coal line to investigate the influence of cyclic loading frequency on a formation layer material, in this case a clayey sand. The testing involved suction-monitored repeated loading tests accounting for the conditions likely to be encountered in practice under varying train speed. The results of single-stage loading tests with a frequency applied individually in a range of 0.5–4 Hz were compared to the results of multistage loading tests with a frequency increasing from 1 to 4 Hz. The results of the single-stage loading tests indicated that the accumulated axial strain increased with an increase in the cyclic frequency, implying that a higher train speed would result in a higher level of the permanent deformation. The staged increase in the frequency of the multistage loading tests showed that the permanent deformation mainly took place during the first packet of cyclic loads whereas the deformations measured during the following repeated loads were negligible. Suction reduction and an increase in degree of saturation during cyclic loading was dependent on the water-retention state of the sample. The results were then interpreted in terms of resilient modulus.

Soil-structure interaction has recently been increasingly considered in the design of structures located in areas of high seismicity. Design codes typically assume the soil to be in either the dry or fully saturated states; however, dynamic soil properties such as stiffness and damping are influenced by the degree of saturation in the underlying soil. This paper presents some findings from two series of centrifuge tests to evaluate the performance of a mid-rise structure, shallowly embedded in sandy silt with varying water table depths. In one test series, the physical model was designed to remain within its elastic range, in another, the physical model was designed to behave inelastically during seismic loading. Results indicate that as the water table elevation decreased, the soil behaved stiffer, leading to reduced period lengthening and system damping. Reductions in period lengthening and system damping that were correlated with the depth of the water table, were also related to the behavior of the superstructure, with greater reductions observed during the experiments performed on the elastic structure.

Recently, a consistent three-dimensional elasto-plastic model was proposed to study unsaturated soil behavior with consideration of coupled hydro-mechanical hysteresis (hereafter referred to as C3DHM). This model uses a unique yield curve to represent both mechanical and hydraulic yielding (i.e., the occurrence of plastic water content changes and mechanical strains). Moreover, it provides consistent predictions for different soil phases that is considered a significant limitation in many existing models. In this paper, the C3DHM model is used to simulate the behavior of unsaturated soils subjected to cyclic shear loading. Experimental results for suction-controlled (i.e., drained) triaxial tests for soils subjected to cyclic shearing were collected from the literature. The model capabilities were investigated by comparing its predictions against the measured results. The model showed reasonable predictions for both volumetric and shear behaviors. Results showed that the model neglects the shakedown phenomenon that is obvious from experimental results. This phenomenon is critical for many transportation applications. Accordingly, possible modifications for the C3DHM are proposed to better capture the soils’ shakedown.

We present a computational nonlocal poromechanics framework for modeling unguided cracking in unsaturated soils. In this mathematical model, the initiation and propagation of cracks in unsaturated soils is autonomous and modeled through a novel fracture criterion considering solid-fluid coupling effects. The nonlocal coupled poromechanics model is implemented numerically using an implicit fractional step algorithm in which the coupled problem is transformed into two sub-problems (i.e., mechanical and fluid flow). First, the mechanical sub-problem is solved using Newton’s method with an explicit predictor of water pressure. Second, the sub-problem of fluid flow is solved using Newton’s method in the updated solid configuration. Two numerical examples are presented to demonstrate the robustness of this proposed computational nonlocal poromechanics framework for modeling arbitrary cracking in unsaturated soils.

This paper focuses on the impact of elevated temperatures on the water retention of MX80 bentonite under constrained or constant volume conditions. This topic is necessary to simulate the combined heating and hydration of bentonite in engineered barrier systems for deep geological repositories for nuclear waste, where the restraint from the surrounding host rock prevents swelling. In addition to summarizing the coupled thermo-hydraulic properties of MX80 bentonite, a water retention curve considering capillary and adsorption effects was extended to elevated temperatures and compared with data from the literature for compacted bentonite, with insights into the effects of different mechanisms of water retention over the full suction range.

It has been recognized the capillary interface area has a profound impact on the capillary pressure and water retention capacity of unsaturated clay. This paper deals with the soil–water retention characteristics of unsaturated clay through full-scale molecular dynamics (MD) modeling. We run a series of MD simulations of unsaturated clay under drying and wetting conditions. The capillary interfacial area is measured through a numerical algorithm based on a point cloud technique. Based on the numerical results, we propose a relationship among pore water pressure, water–air interfacial area, and mass water content. The contribution of adsorption to pore water pressure is also quantified at the microscale based on density distribution.

The depth of the active zone (DAZ) plays a critical role in the estimation of the ground surface movements in problems involving shrink-swell soils (SSS). The DAZ can be defined as the maximum depth of the SSS that contributes to the ground movements due to the moisture content variations. Several methods have been proposed to estimate the surface movement in SSS. However, in most of these methods, the procedure to calculate the DAZ has not been clearly defined. This paper proposes a methodology to estimate the DAZ based on numerical analyses performed with a fully coupled hydro-mechanical FEM code. The proposed approach allows to properly account for the effect of the initial and boundary conditions, soil properties, and weather history, amongst others. The paper shows that the proposed method can provide a very satisfactory estimation of DAZ, and also discusses how some critical parameters and factors affect the movements predicted by the model.

The cracking behavior of clay material due to desiccation has been a subject of intense research in the last few years. However, the enhance in gas permeability induced by the presence of drying cracks in clays has not been well investigated yet, particularly in applications related to clay barrier materials to isolate dangerous pollutants like high level nuclear waste (HLW). The aim of this research is to better understand how clay barrier materials behave upon heating and drying processes, and also to investigate how gas permeability will change as the moisture decrease and drying cracks evolve. This information will be instrumental to achieve a safe and optimal design of repositories for HLW. The MX-80 bentonite from Wyoming (US) is the material adopted in this study. Two compacted samples were prepared at two initial dry densities (i.e., ρ_d = 1.3g/cm³ and ρ_d = 1.6g/cm³) and at the same initial liquid degree of saturation Sr = 46%. The samples were then heated and dried in the oven at 90°C up to different target Sr values. Gas permeability test under controlled conditions in the lab were conducted afterwards. A significant impact of drying cracks on gas flow properties was observed.

Since its emergence by Cundall in 1971, the Discrete Element Method (DEM) proved its efficiency in simulation and explanation of particulate media micro behaviors. The erosion process is the result of micro-interaction between fluid and soil particles. By parallelizing the Computational Fluid Dynamics (CFD) method with the DEM to simulate fluid action on the soil particles, erosion can be simulated at the particle scale. This paper presents a study of CFD-DEM applications in the simulation of erosion. The research focuses on the CFD-DEM technique to explore the fundamental erosion mechanisms in the Erosion Function Apparatus (EFA) test. One of the problems in soil erosion that needs to be investigated is the particles’ incipient motion. A three-dimensional CFD-DEM model is utilized to simulate the EFA simulation to derive velocity-based and shear stress-based erosion functions. The results of the above-mentioned numerical simulations are in good agreement with the laboratory test measurements, which indicates the potential accuracy and reliability of the CFD-DEM technique in simulating erosion at the particle scale and applying the result to full-scale problems.

Underground buried pipes are extensively used in water and sewer networks to meet different domestic and agricultural needs. Underground water networks may suffer from leakage problems through cracks or holes. Pipe leakage can lead to economic, environmental, health, and safety consequences. Leakage from pipelines can induce soil fluidization due to the increase of pore water pressure in the region of the leak. The soil fluidization reduces the bearing capacity of the supporting ground, ultimately leading to the collapse of the ground and the buried utilities. In this study, the soil fluidization mechanism due to pressurized pipe leakage is numerically modeled with the Material Point Method (MPM). MPM represents the continuum with a set of integration points, so-called material points (MPs) that move attached to the media, carrying all the material information. Two MPM two-phase approaches are used to understand the soil-fluid interaction mechanisms associated with a leaking buried pressurized water pipe embedded in fully saturated sand. The single-point approach represents the saturated medium with one set of MPs; the double-point approach separately models the solid and the liquid phases using two sets of MPs. The capabilities of the two approaches to simulate the onset and evolution of soil fluidization are presented and discussed. Finally, the displacement field and failure mechanism around the leaking pipe are analyzed in terms of the soil porosity.

The interaction between streamflow and bed-load sediment material plays a vital role in sediment transportation, and the modeling of hydrodynamic systems of coasts, oceans, and rivers should be considered in hydraulic and geotechnical studies. In this model, smoothed particle hydrodynamics (SPH) is used to simulate the dam-break wave propagation on two erodible beds with different materials of sand and PVC granular grains; however, sediment particles are represented by two models of spring-dashpot which is used mostly in Discrete Element Method (DEM) and Bingham models. In the first method, when the hydrodynamic force is exceeded and satisfied the fracture criterion of the bond, the particles are separated in the form of a crack. In the Bingham model, sediment area is treated as a non-Newtonian fluid which can be expressed on two different modes of solid and Newtonian fluid, and they are related to yield stress point of material.

Mangroves stabilize coastlines, partially through a porous ‘forest’ of trunks and aerial roots, which reportedly dissipate wave energy and reduce flow velocity. It is hypothesized that not only the vertical but also the horizontal components of the ‘forest’ contribute to the altering of flow field which mitigates scour. To test the hypothesis, the effect of a modified sacrificial pile group consisting of vertical mini-piles and (semi-) horizontal crossbars on the flow field around a downstream monopile is studied using computational fluid dynamics (CFD). The simulation results confirmed that the group of mini-piles significantly reduces both the horizontal and vertical velocity in front of the monopile as well as the flow-induced shear force at the bed. The presence of the crossbars reduces the horizontal velocities further, but more importantly, it helps significantly reduce or even reverse the downward flow at the front near the base of monopile. Such changes to the flow field are expected to reduce the strength of the horseshoe vortex and reduce local scour.

A dam or levee breach caused by overflow erosion is difficult to evaluate during an overtopping event due to difficulty in accessibility and quickly changing conditions. However, for assessment of risk associated with breach time and ensuing downstream consequences, the erosion rate of embankment soils during this process needs to be evaluated. Soil erosion and water depth measurements were taken during flume tests using a Shallow Water Lidar (SWL) system scans. The tests were conducted in a 1-m-wide tilting flume on three clean sand and gravel soil mixes with a median grain size D₅₀ of 2, 5, and 20 mm. The box measured 0.45-m-wide × 1.2-m-long × 0.25-m-deep. Due to the confined environment of the flow in the flume, the acting bed shear changes with hydrodynamics of the flow differently from under a uniform flow. The scour hole generated in the test box reaches equilibrium when the acting bed shear is equal to the critical shear. Standard machine learning techniques were used to image soil and water profiles from noisy Lidar data. First, the data are filtered using zonal-averaging and then based on the filtered data; the methodology selects the best profiles from a competing set based on the minimum error each profile produces on the data. Once the profiles are obtained, erosion rates and bed shear are computed, and a qualitative assessment is carried out to understand the relationship between temporal and spatial dependence of erosion rate on bed shear and soil particle size. Erosion rate and shear stress reached their maximum value within the first 60–70 seconds of the test and spatially within 0.3 m from the upstream end of the test box. The erosion rate decreased by about 4 times from 0.13 cm/s to 0.03 cm/s as D₅₀ increased from 2 mm to 20 mm at the same acting bed shear. The erosion rate for both mixes is reduced over time; however, the rate of reduction for D₅₀ of 20 mm is much higher over the same test duration. The erosion rate was shown to be strongly correlated to the acting bed shear nonlinearly. The results indicate that the calculated spatial variation of shear stress over the duration of the tests is consistent with the formation of maximum depth of scour hole.

Erosion of coastal dunes during storm events is an increasingly common problem in the face of global climate change and sea-level rise. To investigate the efficacy of bio-mediated ground improvement for reducing the impact of extreme events such as hurricanes, a near-prototype-scale experiment was performed. In the experiment, a model sand dune was constructed in a large wave flume and divided into treated and untreated zones which were instrumented with pressure and moisture sensors. One of the treated sections was subjected to a surface-spray technique to apply bio-cementation. Afterward, the dune was subjected to a discretized severe storm event (a scaled Hurricane Sandy) consisting of 25 trials. Surge runup and drawdown cause surface erosion and also internal instability due to liquefaction. Pore pressure sensors were embedded in different depths of the dune to study the pressure fluctuations during the wave action and the consequent momentary liquefaction phenomenon. Momentary liquefaction leads to detachment of fine sand particles and the initiation of internal erosion and sediment transport. In this project, remote assessment technology (lidar) was used between each trial to evaluate the performance of the dune under the surge flow by detecting the eroded volume of the sand. To better quantify material properties in-situ, a series of triaxial experiments were conducted on bio-cemented cores taken from the formed crust. The strength and stiffness of the cemented sand were measured under different drainage conditions. Element test results indicate a significant increase in critical bed shear stress (τ_c) due to cementation.

Every year wildfires burn acres of land around the world and in the United States. One of the consequences of wildfires on soil is hydrophobicity, enhancing the surface runoff and erosion that lead to economic disaster, injury, and life loss. A hydrophobic layer builds upon grain surfaces from burned soil organic matter gases during wildfires and depends on the amount of organic matter, soil texture, fire severity, and vegetation type. However, the effect of geomechanical soil properties, such as grain sizes, on mudflows remains unclear. This study relates the grain size distribution with the amount and different patterns of erosion during rain events. Rainfall experiments are conducted on regular and hydrophobic sands with fine, medium, and coarse grains packed in 30° flumes. The results indicate that a higher rate of debris flow occurs in hydrophobic soil, and this amount is affected by the grain size distribution. In the slopes with finer grain, a significant amount of soil mass is collected at the end of the test. In addition, different patterns of flow are identified in slopes with varying sizes of grain. Wider channels form in the coarse-grain slopes, while the primary pattern includes sequin shapes in the fine-grain slope.

Mudflows and debris flow following rainfalls and storms pose a significant threat to people living in the vicinity of hills and mountainous areas. The risk of the extreme event increases near wildfire burned scars due to sand hydrophobicity. Sand particle surface turns hydrophobic due to the high-fire temperatures and organic matter burning, which change sand hydrological characteristics. Water-ponding, decreased infiltration, and increased overflow are some of the consequences of sand hydrophobicity. In this study, a set of single drop tests were conducted on fine, medium, and coarse hydrophobic and regular sand beds on horizontal and tilted surfaces. The results show that hydrophobicity enhances the splashing on the sand surface, regardless of the grain size. In addition, the drop impact on hydrophobic sand triggers particle displacement, and the drop has more mobility on hydrophobic sand. Furthermore, the eroded sand mass fluctuation on hydrophobic sand proves the impact of hydrophobicity on splash erosion and sand erosion. These observations can enhance the understanding of the initiation of debris flow in burned slopes and better understand the behavior of rain droplets following their impact on the sand surface.

This paper presents a rapid soil classification method using computer vision to improve the productivity of on-site classification of excavated soil. In Singapore, thousands of truckloads of excavated soil are transported from construction sites to transfer hubs (known as “staging grounds”) daily. These soils, broadly classified into “Good Earth” and “Soft Clay,” are reused for various applications in land reclamation works, with or without further treatment. An accurate classification is needed for better re-utilization of these material for a sustainable built environment. However, the current classification methods available are expensive, time-consuming and/or labor-intensive, while visual inspection is subjective and prone to human error. A proof-of-concept study of the rapid classification method using computer vision was conducted at a staging ground. Results showed that the optimized artificial neural network model, using three image textural features (Contrast, Correlation, and Entropy) can classify soils into “Good Earth” or “Soft Clay” in less than three minutes, with an accuracy of at least 85%.

This study aims to evaluate the impact of a full thermal cycle, freezing–thawing–heating–cooling, on the microstructure of kaolinite clay. The macro-scale behavior of clays under various temperature changes has been investigated extensively, but currently, there is a lack of understanding of the micro-scale response of soils to these thermal loads. In this study, a thermo–hydro–mechanical triaxial cell manufactured by GDS capable of changing the temperature of soil specimens between –20°C and 70°C is utilized. Five different triaxial specimens are mounted on the triaxial setup. Once the saturation is achieved, one specimen is subjected to mechanical consolidation only at 20°C. Each of the four other samples is subjected to temperature changes in a sequence to create a full freezing–thawing–heating–cooling cycle. Once each specimen completes its corresponding thermo-mechanical path, the specimens are swiftly extracted out of the triaxial setup, and samples are taken from the bottom of the triaxial specimen for further microstructural analysis by mercury intrusion porosimetry. The analysis of pore size distribution shows an increase in the modal pore diameter due to freezing, while thawing and heating decrease the dominant pore size. Although there is a slight expansion during the cooling stage, the modal pore size diameter of the specimen decreases at the end of a full cycle.

To promote sustainable practices in earthworks, there is a growing impetus to leverage the utilization of mining and industrial wastes. Recent research has established coal gangue (CG) utilization (a mining waste generated during the mineral processing stage of coal mining) as a fill material in earthworks. Further, the earlier researchers have also identified the environmental benefits of reduced carbon emissions attainable through its utilization in earthworks. However, the attempts to comprehensively evaluate the environmental, economic, and societal benefits of CG utilization are sparse. In this context, an attempt has been made to comprehensively evaluate the social, economic, and environmental benefits attainable by CG application in earthworks. A carbon footprint analysis (CFA) was done to construct a CG-based embankment to evaluate the environmental benefits. The cost benefit analysis (CBA) was extended to the above scenario to estimate the economic benefits. The amount of land and fuel savings achievable through CG utilization was estimated to assess the social impacts. The CFA and CA estimated that the utilization of CG in embankment construction could reduce emissions and costs by save 3,210 kgCO_2e/m³ and 48%, respectively compared to conventional fill materials (local soil). Further, it was observed that 24 acres of land and 169k (L) of fuel consumption associated with CG disposal could be eliminated through its utilization (in embankment scenario). Considering both the resources’ value and scarcity, the net savings in land and fuel are substantial. Thus, the findings of the study confirmed that use of CG in earthworks has the potential to provide economic, environmental, and societal benefits.

Coal Combustions Residuals (CCRs) are generally non-plastic sand- and silt-sized materials that are often found in a loose, saturated state and can be susceptible to significant loss of strength or stiffness. Measuring the in situ state is a way to quantify this susceptibility. Recreating in situ conditions in the laboratory is challenging due to the state changes that occur between field sampling and sample setup in the laboratory. Geotechnical practice has thus evolved to use Cone Penetration Testing (CPT) to estimate in situ state. Detailed closure design of CCR impoundments requires more than simple screening-level approaches for estimating in situ state. This paper describes a critical state-based approach to estimating the in situ state from CPT data. CCR properties are first estimated from advanced laboratory testing. A summary and discussion of these properties from various CCR impoundments is presented. Next, a CPT calibration chamber test was simulated numerically. These simulations used a cavity expansion model and NorSand, a critical state-based constitutive model typically used for sand- and silt-sized materials. Results from these simulations provided a correlation between the CPT response and in situ state. The in situ state parameter was then estimated using actual CPT data. The results from the CPT were compared with results from relatively undisturbed samples within the pond, and with results from screening-level approaches.

Coal ash is one of the largest industrial waste products generated in the United States. Since 1967, several failures in ash impoundments have been reported, releasing millions of gallons of impounded fly ash slurry resulting in loss of life and mass environmental destruction. Several studies have been carried out to understand the flow behaviors of fly ash slurries. This study evaluated the flow behavior of fly ash slurries with two additives: polyacrylamide and synthetic fibers. Two different types of tests, rheological and flow tests, were performed to assess the flow behavior of two hydrated fly ash slurries. An anionic polyacrylamide and G-Nano fibers were used to modify the flow behavior of fly ash slurries. The results showed that the addition of additives increased yield stress and decreased the flow of fly ash slurries.

Federal regulations concerning disposal of coal combustion products (CCPs) have prompted action across the United States to cap and close older, unlined coal ash storage ponds that do not meet the new federal, state, and local regulations. During operations, dewatering is performed to increase the strength and stability of the ponded ash, so that heavy construction equipment can work on the ash surface. However, some deposits of fly ash are not easily dewaterable and exhibit significant water retention capacities. This work seeks to identify why coal fly ash has some unusual water retention characteristics. First, particulate mixes of fine sand and highly porous additives were tested for saturated hydraulic conductivity. Results indicated that including activated carbon or diatomaceous earth in the sand matrix decreased the saturated hydraulic conductivity by one to two orders of magnitude. Second, the saturated and unsaturated characteristics of samples of highly water-retentive ponded fly ash (FA) were investigated. The FA samples (treated and untreated) had a soil water characteristic curve of a silt. The high-residual water content of the ash was found to be due to the combination of water-retentive, highly porous diatoms and kaolinite. The main conclusion from this work is that the presence of porous unburned carbons, porous diatoms, and kaolinite contributed to the increased water-retentive capacities of ponded fly ash and ash/granular mixtures.

Understanding and characterizing the mechanical behavior of fly ash underpins the continued maintenance, risk assessment, and planning of closure procedures for fly ash ponds. Saturated fly ash can undergo flow failure or static liquefaction that exacerbates the consequences of an impoundment failure, such as the run-out of large volumes of ash. Dewatering of fly ash deposits is a potential mitigation technique that may reduce impacts of material run-out. This paper presents results from miniature CPT soundings from centrifuge models of fly ash deposits performed prior to the simulated failure of the containing structure. The tests feature variations to the initial deposit density and were fully saturated or dewatered in flight. Normalization of the measured cone resistances is shown to help address experimental uncertainty of the initial deposit density. Further, combining the results from CPTs performed at different penetration rates reveals whether the volume-change tendencies of the ash is contractive or dilative. These findings can be related to the observed run-out behavior, in terms of run-out distance and failure mechanism, and thus risk posed by failure of the containing structure. Ultimately, the dependence of the impoundment stability on the deposit density and potential improvement arising from dewatering, in addition to the use of CPTs to quantify these changes, is highlighted.

Hydraulic properties were measured on twelve undisturbed specimens of sluiced coal combustion products (CCPs) collected in thin-wall sampling tubes from two CCP impoundments undergoing closure. The saturated hydraulic conductivity (K_s) and soil-water characteristic curve (SWCC) were measured for each specimen. Saturated hydraulic conductivity of the CCPs varied from 10^–8 to 10^–5 m/s, with lower hydraulic conductivity (K_s < 10^–6 m/s) obtained for CCPs with higher fines content. The van Genuchten α parameter for the SWCC varied directly with saturated hydraulic conductivity and was inversely related to van Genuchten’s n. Consistent sets of hydraulic properties corresponding to more permeable (K_s = 10^–6 m/s), typical (K_s = 10^–7 m/s), and less permeable (K_s = 10^–8 m/s) CCPs are suggested for use in variably saturated flow modeling of CCP impoundments for which site-specific data are not available.

This study presents the preliminary results of coal fly ash (CFA)—flue gas desulfurization (FGD) brine co-disposal by compaction as a method to achieve the zero liquid discharge (ZLD) strategy in coal-fired steam electric power plant operation. While the method utilizes the FGD brine to reduce water consumption for CFA landfilling, the compacted mixture serves as a reservoir to store brine salinity. To test the method under the field condition in Charlotte, NC, area, a testbed instrumented with volumetric water content (VWC) and temperature sensors was used to investigate the leachate/runoff generation and variation of moisture and salinity storage of the co-disposal mixture. The results from 11/2018 to 12/2019 showed the total collected runoff and the leachate was 11% and 37% of total precipitation, respectively. The analysis of leachates suggested the majority of salinity from added brine was removed by 754 mm precipitation. The preliminary results from this ongoing research indicate that the co-disposal method fails to retain the stored brine salinity under the local weather conditions due to a lack of stabilization ability, which can lead to an increase of the landfill operational cost and will thus necessitate additional leachate treatment.

The coal-fired electric power generation industry historically managed the long-term storage of coal combustion residuals (CCRs) by sluicing these generally sand- and silt-sized materials into impoundments. These materials present significant challenges when assessing their engineering parameters as the materials are not typical of traditional geo-materials and standard methods for interpretation are generally inadequate. Sample disturbance is inevitable between sample recovery and laboratory testing and so characterization based on in situ data is generally preferred. This paper presents a summary of in situ data from four CCR impoundments in the Eastern United States. Estimates of unit weight, shear strength, and stiffness are presented alongside comparisons to laboratory test data where available. Approaches for classifying different types of CCRs are presented.

This paper describes the design and construction of embedded retaining structures for temporary support of excavation at a site for a new multi-story building in Atlanta, Georgia. The ground conditions comprise fill overlying Piedmont residual soil, partially weathered rock, and gneiss. A soldier pile and lagging wall was designed for a maximum depth of excavation from street level of about 11.4 m (37.5 ft), with a maximum retained height at the wall face of about 9.9 m (32.5 ft). Lateral support was provided by up to three rows of drilled and grouted post-tensioned ground anchors (tiebacks) on three sides of the site. The soldier pile members were designed as pairs of back-to-back MC channels installed in drilled holes. Tiebacks were installed between the webs of the channels to eliminate walers. Threaded bar was used as the stressing tendon in each tieback. Where site constraints did not permit use of tiebacks, the soldier piles were wide flange sections with walers and steel pipe corner braces or steel rakers for internal support. This paper presents the results of two verification load tests that were performed on non-production gravity grouted tiebacks in the Piedmont residual soil. These sacrificial tiebacks verified an ultimate unit grout-to-ground bond stress of 140 kN/m² (20.5 psi). This value is compared to bond stresses from other projects in Piedmont residual soil for different ground engineering elements and installation techniques, and values published in the literature for similar soils.