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Buffer materials that are used to isolate heat-emitting waste canisters must bear a strong thermal load and have good thermal conductivity and stability. This study investigated the strengthening of internal thermal conduction via bentonite sealing. An admixture of quartz and graphite accelerated the heat transfer into the host rock. Samples containing different bentonite-sand-graphite (BSG) mixtures were prepared. The influence of the volumetric water content, degree of saturation, dry density, porosity, sand content, and particle size on the thermal conductivity of the BSG mixtures was analyzed by conducting a series of thermal needle tests. Electrical resistivity tests were conducted to examine the electrical resistivity of the BSG mixtures, and the dependency of soil thermal conductivity on the volumetric water content based on electrical resistivity data. The results indicated that the dependency of thermal conductivity on the volumetric water content was closely related to electrical resistivity. Based on the thermal conductivity inflection point as determined by the volumetric water content corresponding to the electrical resistivity inflection point, an improved series-parallel thermal conductivity prediction model and the method to determine model parameters for the unsaturated BSG mixtures were proposed. The precision of the proposed prediction model was verified based on laboratory data. Key words: BSG mixtures, buffer material, thermal conductivity, electrical resistivity, prediction model, 1. Introduction and background In the conceptual design of high-level radioactive waste (HLRW) repositories, the thermal performance of the buffer material is one of the key factors considered in the [...]

The performance of a flexible pavement depends on the resilient modulus (MR) of subgrade soil. Thus, MR is a key design parameter for mechanistic-empirical pavement design of flexible pavements. Generally, the resilient modulus is determined by conducting repeated load triaxial (RLT) tests in the laboratory and has been used to characterize the subgrade soil behavior under repeated traffic loading conditions. The use of cement to stabilize natural subgrade soils is widely accepted by transportation agencies. Several research studies were conducted on the resilient behavior of cementtreated soils. However, limited research studies have been conducted on the resilient behavior of cement-treated silty soil. Therefore, the current research study assessed the resilient moduli properties of cement-treated silt. Cement-stabilized soil specimens were statically compacted and cured in a humid room for a stipulated curing period before conducting RLT tests. RLT tests were conducted on cement-treated specimens at different cement dosages and curing periods to study the effect of the cement dosage and curing time on the resilient modulus. Test results indicated that a significant improvement in performance was observed after cement treatment. The untreated soil specimens exhibited stresssoftening behavior with an increase in deviator stress, whereas the cement-treated specimens exhibited stress-hardening behavior. The resilient modulus was increased with an increase in cement dosage. Regression analyses were conducted on RLT test results using three-parameter universal model and model parameters were determined. It was observed that the three-parameter universal model exhibited an excellent fit with experimental data.

A new in situ thermal piezocone penetration test (T-CPTU) system is developed to determine the thermal properties of soil. It is expected to overcome most of the shortcomings observed in existing in situ test techniques. Based on Fourier's heat conduction equation and pore pressure dissipation theoretical equation, a method for calculating the thermal conductivity, namely the predicted temperature method, was proposed. The accuracy of the T-CPTU probe testing process and thermal conductivity calculation results were verified by numerical simulation, laboratory large-scale model tank tests, and thermal needle tests. Finally, the field data of T-CPTUs at three sites in Nanjing, China, were collected and compared with the laboratory thermal needle tests. The results indicated that the thermal conductivities obtained using T-CPTU were accurate and closer to those of laboratory thermal needle tests for most soils. The thermal conductivities of the undisturbed soil samples measured in the laboratory were lower than those obtained by T-CPTU. Key words: thermal piezocone penetration test (T-CPTU), in situ tests, temperature distribution, thermal conductivity. Un nouveau systeme d'essai de penetration thermique in situ par piezocone (T-CPTU) est developpe pour determiner les proprietes thermiques du sol. Elle devrait permettre de surmonter la plupart des lacunes observees dans les techniques d'essai in situ existantes. Sur la base de l'equation de conduction thermique de Fourier et de l'equation theorique de dissipation de la pression des pores, une methode de calcul de la conductivite thermique, a savoir la methode de la temperature predite, a ete proposee. La precision du processus d'essai de la sonde T-CPTU et les resultats du calcul de la conductivite thermique ont ete verifies par simulation numerique, par des essais en laboratoire sur des reservoirs modeles a grande echelle et par des essais par aiguille thermique. Enfin, les donnees de terrain du T-CPTU sur trois sites a Nanjing, en Chine, ont ete collectees et comparees aux essais par aiguille thermique realises en laboratoire. Les resultats ont indique que les conductivites thermiques obtenues a l'aide du T-CPTU etaient precises et plus proches de celles des tests d'aiguille thermique en laboratoire pour la plupart des sols. Les conductivites thermiques des echantillons de sol non perturbes mesurees en laboratoire etaient inferieures a celles obtenues par T-CPTU. [Traduit par la Redaction] Mots-cles : essai de penetration thermique par piezocone (T-CPTU), essais in situ, distribution de la temperature, conductivite thermique., 1. Introduction With the rapid increase in demand for the utilization of geothermal energy resources, and the importance of soil thermal properties to the design of energy piles, ground source [...]

A full-scale mock-up geothermal bridge was built in Texas, USA, to evaluate the field performance of a novel external geothermal bridge de-icing system. The geothermal bridge system comprises a single U-loop ground heat exchanger (GHE) 131 m deep, a geothermal heat pump, four circulation pumps, and hydronic heating loops attached to the bridge deck. A comprehensive monitoring system was implemented to monitor the spatial ground temperature response. The geothermal bridge system was operated and monitored for solar collector and bridge de-icing operations totaling 99 days between August 2019 and April 2020. An average heat injection of 5.7 W/m was observed during the solar collector operation. Based on the defined control volume, approximately 48.9–380.8 kWh of energy, corresponding to 3.7%–28.5% of the total injected energy, was available at the end of the solar–collector operation. Moreover, test results showed that 31.7%–39.4% of stored thermal energy was preserved for utilization in the first winter de-icing test. The radius of the influence zone of the GHE operation was found to be 3.45 m. The outcome of the field experimental studies and further data collection performed over a year of operations showed that the heat injection into the soil by bridge solar collector was 54% more effective than heat extraction from the ground during the several de-icing tests performed from 2019 to 2020. [ABSTRACT FROM AUTHOR]

The addition of calcium (Ca)-based stabilizers to sulfate-rich expansive soils is associated with the formation of ettringite, a deleterious reactant that can cause moderate-to-severe swell-related damage to overlying lightweight infrastructures. This research study was conducted to understand the effects of combining nanosilica admixtures with a traditional Ca-based stabilizer to effectively treat high-sulfate soils with an intent to suppress the ettringite formation. Engineering and microstructural studies were thus performed to gain a comprehensive understanding of the behavior of sulfate-bearing soils treated with lime in the presence of amorphous nanosilica. The engineering studies on treated and untreated soils included strength tests before and after capillary soaking, free swell strain tests, and resilient moduli studies that were performed to study and understand the macrostructural behavior of these soils at different curing periods. Supplemental studies using scanning electron microscope imaging and energy dispersive X-ray spectroscopy, thermal analyses using differential scanning calorimetry, and X-ray diffraction studies were also conducted to determine the microstructural changes that occur within these sulfate-rich soils. The results showed that additional silica phases furnished from nanosilica suppressed the precipitation of ettringite and correspondingly increased the formation of cementitious phases. This study also provided ample evidence that the application of amorphous siliceous nanomaterials positively impacts chemical treatments and reduces the precipitation of ettringite in sulfate-rich soils, thus enhancing their engineering performance. [ABSTRACT FROM AUTHOR]

The present contribution to the Regional Editors themed issue offers a concise yet focused overview of some of the key technical and scientific issues, as well as of current trends and future challenges, related to the broad discipline of environmental geotechnics in the US region. Particular attention is devoted to current policy and societal drivers as well as future professional and research capacity requirements in critical areas such as innovative recycling and improvement of compost, construction and geologic materials; solid waste management, landfilling and geoenvironmental remediation techniques; and crucial geotechnical engineering aspects of renewable energy production, storage and distribution.

The mechanistic-empirical pavement design guide (M-EPDG) recommends the use of resilient modulus (MR) for characterization of subgrade soils. Subgrade soils may not always have enough strength and stiffness to support the pavement structure. Therefore, a certain type of soil improvement method using cement, lime, or other stabilization techniques is often needed to enhance the strength and stiffness properties of weak subgrade soils. The cement-stabilized soils show brittle behavior under compression loading, which can induce cracking in overlying pavement layers. In general, polymer-treated soils show a semiductile or ductile behavior. It is important to look for combined cement and polymer treatments to address brittle behavior issues as well as moisture susceptibility while maintaining strength and moduli properties. A research study was conducted to understand the strength, resilient, and ductile behaviors of sandy soils treated with cement and a combination of cement and vinyl acetate ethylene (VAE) copolymer. Engineering tests such as unconfined compressive strength (UCS) and resilient modulus tests were conducted on both control and treated soil specimens cured for 7 days. Tests were conducted on specimens before and after immersing in water bath for 4 h to investigate the moisture susceptibility. In these tests, an increase in UCS was observed after cement and cement-VAE treatments as compared to control soil specimens. Results showed that cement-VAE-treated soils exhibited an increase in the axial strain at failure, indicating the semiductile behavior compared to cement-treated specimens. An improvement in the resilient moduli was observed after treatments. Subsequently, two of three-parameter models were used to analyze resilient modulus formulations with stress conditions and determined the regression constants. In conclusion, the study revealed that the use of VAE copolymer improved the stress-strain responses of cement-treated soils and imparted closer to the semiductile behavior, which will reduce cracking in overlying pavement structures. [ABSTRACT FROM AUTHOR]

Most of the previous research has been focused on developing and validating constitutive models to predict response of unsaturated soils in low-medium suction range. However, there is a scarcity of efforts in developing soil models to simulate its mechanical response in high suction range, particularly above the residual suction. This article presents a new constitutive model introducing net stress and suction as two independent variables. Furthermore, non-associative flow rule incorporating modified stress-dilatancy relationship to take unsaturated state into account is introduced to improve the model results in low-medium to high suction range. The essential soil model parameters are calibrated using suction-controlled triaxial test results for predictions of compacted silty sand response at high values of total suction above residual suction. Preliminary simulations show that proposed model can reasonably simulate the post-peak strain softening response obtained from suction-controlled CTC tests above residual suction value with reasonable accuracy. Although, the proposed model captures initial compression followed by dilation volumetric response with reasonable accuracy, it needs some improvements to be able to capture volumetric response accurately over entire suction range.

The stress-strain response of compacted silty sand with over-consolidated stress history often exhibit distinct peak stress before reaching the critical stress type of response when subjected to suction-controlled triaxial shearing. Such heavily consolidated soil also tends to simultaneously manifest initial compression which transitions into dilational type volumetric response. Modelling such strain-softening type response, especially emulating the smooth transition from peak to critical state is a challenge. In this paper a previously developed generalized plasticity constitutive model, called MPZ (Modified Pastor-Zienkiewicz) is fine-tuned and calibrated using a set of suction-controlled consolidated drained triaxial tests conducted on compacted silty sand specimens. Firstly, the saturated and unsaturated silty sand characteristics and the experimental test program are briefly introduced. Secondly, the calibration of each component of the constitutive model, namely critical state, dilatancy, peak state, loading direction, water retention curve and bounding function are briefly explained. Furthermore, the material parameters are estimated, model performance is displayed, and finally discussed. Preliminary simulations show that the MPZ model is able to mimic overall suction controlled triaxial test response of compacted silty sand decently well by taking into account the changes in density, pressure and suction. However, the peak states are not accurately modelled for low-high suction levels which needs further modifications in proposed model.

An experimental program has been undertaken to assess both peak and residual shear strength parameters of statically compacted, moderate plasticity clayey soil under suction-controlled conditions, resulting in a defined set of suction-dependent peak and residual failure envelopes over a relatively wide range of suction states, from 0 to 300 kPa. The experimental program was accomplished in a servo/suction-controlled ring shear apparatus, which is suitable for testing unsaturated soils under large deformations via the axis-translation technique. Test results substantiate the crucial role that has been observed to be played by the imposed matric suction on the residual shear strength of compacted clayey soils. For the range of net normal stress (0-200 kPa) and matric suction (0-300 kPa) states investigated, the increase in either peak or residual shear strength, with increasing matric suction, was found to be manifestly nonlinear. Furthermore, a distinct correspondence was observed between the nonlinearity of the peak shear strength envelope, with respect to increasing matric suction, and the soil-water retention properties of the clayey soil. Results, in general, suggest that a conceptual residual shear strength framework for unsaturated soils, similar to that postulated for peak shear strength, can eventually be formulated as more experimental evidence of this kind is made available.

In recent years, the geothermal heat pump de-icing system (GHDS) is introduced as a sustainable solution for bridge deck de-icing, which utilizes renewable geothermal energy. Existing GHDS designs mostly rely upon hydronic loops embedded in concrete decks. To extend the GHDS for existing bridges, a new external hydronic deck has been developed recently, which employs a hydronic pipe being attached to the bottom surface of the bridge deck. In this study, the performance of the externally heated geothermal bridge deck is investigated through winter deicing and summer recharging tests with the focus on the ground loop heat exchanger (GLHE), a key component of the GHDS. The test results show that the de-icing system was successful in maintaining the deck surface temperature above freezing in all winter tests. The soil temperature measurements indicate, the 132.5 m vertical U-tube ground heat exchanger is benefited from the undisturbed soil temperature of around 21 °C. The overall average hourly heat extraction of 0.67 kW during winter operation and average hourly heat injection of 0.69 kW during the summer operation were observed. Also, the ground thermal recharge test showed the increase of undisturbed soil temperature at 1.5 m from the geothermal borehole by 0.36 °C after 50 days of system operation.

Current soil stabilization methods are often limited by durability and leaching issues and do not always offer sustainable treatments. This research explores the use of geopolymers to stabilize clays in the North Texas area. In recent years, geopolymer has received much attention as an eco-friendly and sustainable alternative to conventional chemical additives, since it can be processed at room temperature from aqueous solutions by utilizing waste materials and/or abounded natural sources. Two subgrade soils from North Texas were treated with GP mix at a ratio of 8 wt% dry GP to dry soil. GP is shown to reduce swelling and shrinkage potential of soil considerably while an increase in unconfined compressive strength is observed as well. Therefore, further studies are recommended to understand the mechanism of GP and soil bonding resulting in said changes.