Douglas D. Cortes

Thermal conductivity of hydrate‐bearing sediments

[1] A thorough understanding of the thermal conductivity of hydrate-bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate-bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate-bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water- and tetrohydrofuran (THF)-saturated, and THF hydrate-saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid-versus-hydrate filled pore spaces. The thermal conductivity of THF hydrate-bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain-liquid interfaces are transformed into grain-hydrate interfaces.

Numerical Simulation of Inverted Pavement Systems

Conventional pavements rely on stiff upper layers to spread traffic loads onto less rigid lower layers. In contrast, an inverted pavement system consists of an unbound aggregate base compacted on top of a stiff cement-treated base and covered by a relatively thin asphalt concrete layer. The unbound aggregate interlayer in an inverted pavement experiences high cyclic stresses that incite its inherently nonlinear granular media behavior. A physically sound, nonlinear elastoplastic material model is selected to capture the unbound granular base in a finite-element simulator developed to analyze the performance of inverted pavement structures. The simulation results show that an inverted pavement can deliver superior rutting resistance, as compared with a conventional flexible pavement structure with similar fatigue life.

The LaGrange case history: inverted pavement system characterisation and preliminary numerical analyses

Inverted pavement systems consist of an unbound aggregate base placed between a stiff cement-treated foundation and a thin asphalt cover. Unlike conventional pavements, which rely on stiff bound aggregate layers to bear and spread traffic loads, in an inverted pavement structure, the unbound aggregate inter-layer plays a major role in the mechanical response. A comprehensive characterisation study was conducted during the construction of a full-scale inverted pavement section in LaGrange, GA, USA, with particular emphasis on the unbound aggregate base. New laboratory and in situ tests were developed and used to characterise the unbound aggregate non-linear stiffness–stress relationship. Forensic digital image analysis confirmed particle alignment, i.e. inherent anisotropy. Comparison of pre- and post-compaction gradation test results failed to quantify the extent and significance of particle crushing. The information compiled during the field test was used in a complementary numerical study; the results confirm acceptable stress levels within the various layers under standard loads.

In-situ assessment of the stress-dependent stiffness of unbound aggregate bases: application in inverted base pavements

Unbound aggregate bases are the primary structural components in many flexible pavements. The response of the unbound aggregate base is critical to the overall performance of the pavement, particularly in inverted base pavements given the proximity of the base to the traffic loads. The behaviour of granular materials such as unbound aggregate bases is inherently nonlinear and anisotropic. An experimental methodology is developed to assess the in-situ stress-dependent small-strain stiffness of unbound aggregate bases under controlled load using wave propagation techniques. CODA wave analysis is used to detect small changes in travel time. The methodology is applied in two distinct case histories of inverted base pavements. Results show that field-compacted granular bases exhibit higher stiffness, lower stress sensitivity and more pronounced anisotropy than laboratory-compacted specimens. The discrepancy in stiffness observed among the two field case histories is primarily attributed to traffic preconditioning sustained by the older pavement. Additional results show that the effect of suction on the stiffness of coarse-grained granular bases is insignificant.

Flow Test: Particle-Level and Macroscale Analyses

This paper investigates the physical interpretation of the flow test. The research involves conducting image-monitored flow tests on mortars that are prepared with mixtures of natural round sand and crushed angular sand. The goal of the research is to evaluate both how the flow progresses and how the aggregate shape characteristics in cement affect mortar rheology. The research finds that within an energy-based framework, both the base shear at the mortar-plate interface and the loss of internal energy in the shear deformation are functions of the shear resistance within the mortar. As for particle shape, it is found that both packing density and the mobilized friction between aggregates affect it. The authors conclude that the amount of paste required to reduce grain interaction in order to attain adequate flow is defined by particle shape.

Smart Ground-Source Borehole Heat Exchanger Backfills: A Numerical Study

Geothermal heat pump borehole heat exchangers rely on sensible heat for energy storage and low thermal conductivity materials for heat transfer. This paper examines numerically the potential benefits of an engineered backfill on the performance of a borehole heat exchanger. The results show that improving the thermal conductivity of the backfill and introducing a phase change material for energy storage can alter the thermal radius of influence of the borehole, improve the system efficiency, and reduce long-term changes in ground temperature.

A Thermal Direct Shear Device for Testing Polymer-Bonded Sands

This paper documents the customization of a direct shear apparatus to accommodate heat injection into a polymer-bonded sand (PBS) specimen while in the shear box (i.e., before, during, or after shearing). The system is controlled with an electronic microcontroller that is coded to enable tailored heat treatment protocols and data logging in the temperature range between 110°C and 160°C. The use of the system is exemplified with experiments aimed to characterize the strength of PBS specimens subjected to multiple heating—and thus multiple healing—cycles.