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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]