Although it is well established that heat conduction in unsaturated soil depends on liquid saturation, there are several models available to consider the changes in thermal conductivity during drying ...and wetting. The key factors affecting thermal conductivity of unsaturated soil are evaluated through a critical examination of these different models and their development. Depending on the principles and assumptions employed, these models are categorized into three groups: mixing models involving series/parallel elements; empirical models where thermal conductivity values at dry and saturated states are used; and mathematical models based on phase volume fractions. Experimental data for different soils are used to assess the quality of prediction for these models. It is found that all the existing models do not realistically account for pore structure or interface properties, and thus are not capable of properly predicting thermal conductivity as a function of liquid saturation. A conceptual model based on soil–water retention mechanisms, is proposed to overcome the pitfalls of the existing models and can be used to establish quantitative thermal conductivity models for variably saturated soils in the future.
AbstractThe roles of particle breakage and drainage conditions on the quasi-static compression response of sand were evaluated by comparing the results from drained and undrained isotropic ...compression tests on dry and saturated specimens up to a mean total stress of 160 MPa. For dry sand specimens, the compression curves from drained and undrained tests were similar because of the high compressibility of air. The isotropic compression curves of the dry sand specimens at mean stresses greater than 30 MPa reflect a transition toward void closure, reaching a minimum void ratio of 0.04 at 160 MPa. Dry sand specimens with different initial relative densities showed similar behavior during isotropic compression in drained conditions for mean stresses greater than approximately 30 MPa. As expected, saturated sand specimens tested under undrained conditions showed a much stiffer response than in drained conditions, with a bulk modulus greater than that of water. Increasing trends in particle breakage quantified using breakage factors from the literature with increasing mean stress were observed for the dry sand specimens, but negligible particle breakage was observed for the saturated sand specimens tested in undrained conditions. To highlight the linkage between particle breakage and transition to void closure at high mean effective stress, an empirical relationship was developed using a breakage factor from the literature to match the experimentally-derived compression curves of dry sands under drained conditions.
AbstractThe axial and radial thermal responses of a field-scale energy pile installed in dense sand and subjected to monotonic and cyclic temperatures are examined. It is found that the axial thermal ...strains in the pile are more restricted to thermal expansion/contraction compared to radial thermal strains. The radial thermal strains are close to that of a pile expanding/contracting freely, indicating minimal resistance from the surrounding soil in the radial direction. As a result, very low magnitudes of radial thermal stresses developed in the pile compared to axial thermal stresses. The pile–soil radial contact stresses estimated from the cavity expansion analysis are up to 12 kPa for a pile temperature change of 22.5°C and are likely to stay low for the range of commonly encountered operating temperatures for cast-in-place concrete energy piles installed in dense sand. During cyclic heating and cooling, unstable changes in axial and radial thermal strains were observed initially during initial cycles, indicating a ratcheting response. The changes in strains became more stable over further cycles without significant changes in side friction or pile–soil contact stresses.
AbstractThis study presents a centrifuge modeling approach to characterize the transient thermomechanical response of energy foundations during heating-cooling cycles to provide data for calibration ...and validation of soil-structure interaction models. This study focuses on the response of a scale-model energy foundation installed in an unsaturated silt layer with end-bearing boundary conditions. The foundation response was assessed using embedded strain gauges and thermocouples. Other variables monitored include foundation head displacements, soil surface displacements, and changes in temperature and volumetric water content in the unsaturated silt at different depths and radial locations. Measurements during the initial heating process indicate that the thermal axial stress is greater near the toe of the foundation as a result of the restraint associated with mobilization of side shear resistance along the length of the foundation. The thermal axial strains were close to the free-expansion thermal strain near the soil surface and decreased with depth. The thermal axial displacements calculated by integrating the thermal axial strains correspond well with the independently measured head displacements. The mobilized side stresses calculated from the thermal axial stresses increased with height and were consistent with the shear strength of unsaturated silt. During successive heating-cooling cycles, slight decreases in upward thermal head displacement were observed because of changes in the stiffness of the unsaturated soil from thermally induced water flow away from the foundation and potential downdrag effects. However, little change in the thermal axial stress was observed during the heating-cooling cycles.
Thermal Conductivity of Biocemented Graded Sands Xiao, Yang; Tang, Yifan; Ma, Guoliang ...
Journal of geotechnical and geoenvironmental engineering,
10/2021, Letnik:
147, Številka:
10
Journal Article
Recenzirano
Odprti dostop
AbstractThis paper includes an investigation of the thermal conductivity of biocemented soils to better understanding the regimes of heat transmission through soils treated by microbially induced ...calcium carbonate precipitation (MICP). A series of thermal conductivity tests using the transient plane source method (TPS) was performed on biocemented silica sand specimens with different gradations, void ratios, and MICP treatment cycles. The results showed that MICP treatment greatly improved the thermal conductivity of sand specimens. An increase in uniformity coefficient or a decrease in void ratio of the sand resulted in an increase in the thermal conductivity of MICP-treated specimens for a given MICP treatment cycle. The increment of thermal conductivity of MICP-treated specimens with respect to that of untreated specimens was also affected by gradation, void ratio, and content of calcium carbonate. The greatest improvements in thermal conductivity were achieved for sands having an initial degree of saturation between 0.82 and 0.85. An empirical equation was established to predict the thermal conductivity of MICP-treated silica sand with different variables, which may be useful in designing energy piles in biocemented sand layers.
AbstractThe effects of daily cyclic temperature variations on the thermal response of an energy pile built under a six-level residential building are examined. The axial and radial thermal strains ...along the length of the pile followed stable, linear reversible paths during daily active heating and cooling cycles corresponding to a pile temperature range of 10°C to 23°C (ΔT of −8°C to 5°C) around a baseline temperature of 18°C. The stable responses of the thermal strains indicate that plastic deformations did not occur in the pile during the daily cyclic temperature changes coupled with the mechanical load in the pile corresponding to 52% of its estimated ultimate capacity. A complex distribution of axial thermal stresses with depth was observed in the pile, with higher stress magnitudes near the pile ends particularly at the end of cooling because of larger temperature changes in the cooling cycle. The magnitudes of radial thermal stresses were considerably smaller than the axial thermal stresses along the length of the pile and are not anticipated to play a significant role in the development of thermomechanical loads in the pile. The temperatures over the cross section of the pile were uniformly distributed at the end of cooling and heating at all depths, while the axial thermal stresses had a nonuniform distribution but with magnitudes less than the calculated ultimate capacity of the pile.
AbstractThis study presents the results from physical modeling experiments on centrifuge-scale energy foundations in dry sand and unsaturated silt layers. These experiments were performed to ...characterize end restraint effects on soil-structure interaction for energy foundations in different soils and include tests on foundations with semifloating and end-bearing toe boundary conditions and free-expansion and restrained-expansion head boundary conditions. Two scale-model energy foundations having different lengths were constructed from reinforced concrete to simulate end-bearing and semifloating conditions in soil layers having the same thickness. The foundations include embedded thermocouples and strain gauges, which were calibrated under applied mechanical loads and nonisothermal conditions before testing. The variables measured during the experiments include axial strain and temperature distributions in the foundation, temperature, and volumetric water content measurements in the soil, vertical displacements of the foundation head and soil surface, and axial stress at the foundation head. These variables were used to calculate the distributions in thermal axial stress and thermal axial displacement, which are useful in evaluating soil-structure interaction mechanisms. The results confirm observations from full-scale energy foundations in the field for end-bearing foundations and provide new insight into the behavior of semifloating foundations. Heating of the semifloating foundations in compacted silt led to a clear increase in ultimate capacity, potentially due to changes in radial normal stress and thermally induced water flow, while heating of the semifloating foundations in dry sand led to a negligible change in ultimate capacity.
Eight full-scale energy foundations were constructed for a new building at the US Air Force Academy. The foundations are being used to demonstrate this technology to the United States Department of ...Defense and have several experimental features in order to study their thermal–mechanical behavior. Three of the foundations are instrumented with strain gages and thermistors, and their thermo-mechanical response during a heating and cooling test was evaluated. For a temperature increase of 18 °C, the maximum thermal axial stress ranged from 4.0 to 5.1 MPa, which is approximately 25 % of the compressive strength of concrete (estimated at 21 MPa), and the maximum upward displacement ranged from 1.4 to 1.7 mm, which should not cause angular distortions sufficient enough to cause structural or aesthetic damage of the building. The end restraint provided by the building was observed to change depending on the location of the foundation. The heat flux per meter was measured by evaluating the temperatures and flow rates of a heat exchanger fluid entering and exiting the foundations. The heat flux values were consistent with those in the literature, and the foundation with the three continuous heat exchanger loops was found to have the greatest heat flux per meter. The transient thermal conductivity of the subsurface measured using the temperatures of the subsurface surrounding the foundation ranged from 2.0 to 2.3 W/mK, which is consistent with results from thermal response tests on energy foundations reported in the literature.
AbstractThe small-strain shear modulus G0 is a property fundamental to the deformation response of soils to dynamic loading. It depends on soil mineralogy, particle characteristics, void ratio, ...effective stress, and the degree of saturation. Some theories have been established to predict G0 of soils under variably saturated conditions. Accordingly, a new unified model was developed for G0 of different soils under variably saturated conditions. The model is based on two conceptual mechanisms: material hardening/softening, which is considered using the degree of saturation as a variable, and interparticle contact forces carried by the soil skeleton, which is considered using the effective stress as a variable. By utilizing suction stress theory, it is demonstrated that G0 can be correlated to the parameters of the soil-water retention curve. Results from the literature for 22 soils along with new results for 7 soils were used to validate the model under variably saturated conditions and a range of total stresses.
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