Thermo-hydro-mechanical characterization of Ottawa sand and Kaolin clay through experimental models.
Authors:   ,   University of Louisville, Joshaghani Mohammad 
Published: February 2023

Recent advances in energy geotechnics and increased utilization of ground source heat pump systems (e.g., geothermal boreholes and energy y piles) and energy geo-structures require improving the understanding of the thermo-hydro-mechanical behavior of soil. Energy geo-structures consist of above-ground facilities, buried structures, and the soil surrounding the buried structure. A buried structure transfers the energy between the ground and above-ground facilities. Energy transfer between above-ground facilities and the soil will cause temperature changes in soil and this will affect soil hydro-mechanical properties. The changes in soil temperature can also significantly alter the soil-structure interaction and its mechanical strength. Soil temperature change is not limited to the energy geo-structures. There are other sources of soil temperature changes in geotechnical engineering. Transferring samples from the field to the lab, daily and seasonal variations of the shallow subsurface soil, chemical reactions in landfills, buried high voltage cables and buried waste disposals change soil temperatures. This study aims at gaining a better understanding of changes in soil properties under variable temperatures, including soil hydraulic conductivity and intrinsic permeability (as hydraulic parameters that control seepage and fluid flow), thermal volume change (as a mechanical parameter to estimate soil deformation and settlement), and thermal pressurization (that is the reason of pore pressure generation and thermal fluid flow). The mentioned parameters are used in numerical modeling, and the outcome of this research can help to elaborate the accuracy of the numerical models by considering variations with temperature and also verifying those numerical models. In addition, the results of the volumetric changes with thermal load will affect foundation desing and required standards of the soil in the vicinity of foundations in buildings with geothermal energy. To study the mentioned parameters, a novel method was engaged to modify the present triaxial cell in the geomechanics lab at the Civil and Environmental Engineering Department at the University of Louisville. Compared to the similar modification by other researchers, this cell with minimal expense was adjusted and modified to a temperature-controlled triaxial cell which can provide a uniform temperature around the soil sample using a novel approach, and has the flexibility to host different thermal tests by a simple change in the internal setup. Tests were conducted on Ottawa sand and Kaolin clay which are examples of coarse and fine-grained soils and are common soils in practice. A series of hydraulic conductivity tests were conducted to study the effect of temperature on intrinsic permeability. Results showed a reduction in both hydraulic conductivity and intrinsic permeability of Ottawa sand (35% reduction in H.C of and 50% reduction in I.P) . In Kaolin clay, hydraulic conductivity showed an average of 150% increase and intrinsic permeability showed an average of 5% reduction most cases and a slight increase in confining pressure of 690 kPa. This increase could be because of the degeneration of the immobile water into the free water. For both sand and clay, void ratio showed a reduction is the average range of 5 %. The effects of temperature on volume change of Kaolin clay was studied. For this purpose, normally consolidated (NC) kaolin clay was tested under different cycles of heating/cooling and different confinement pressure. Besides, overconcolidated (OC) samples with OCR=6.5 and OCR=1.6 were also tested. The experimental results showed about 1 % irreversible thermal volume contraction which is known as thermal consolidation for normally consolidated Kaolin clay while 1 % thermal expansion was observed for highly overconsolidated Kaolin clay (OCR=6.5) and almost no change for slightly OC sample (OCR=1.6). Based on the finding of thermal volume change, further investigation of permeability variations on NC Kaolin clay with temperature demonstrated that the changes in intrinsic permeability by temperature change are time-dependent. This confirms that thermal consolidation which results in void ratio reduction with time reduces the Kaolin clay absolute permeability. Results showed a reduction of 8% for both H.C and I.P af two different confinfing pressure of 345 kPa and 690 kPa at different temperature when time of measurement was imcreased from 1 hour to 48 hours.