Development of a new temperature-controlled oedometer
Creators: name: Kirkham, Andrew
Publisher: name: Imperial College London
Year: 2020

description: A new temperature-controlled oedometer has been designed at Imperial College London and commissioned to investigate the thermo-hydro-mechanical (THM) behaviour of soils. The proposed application of the research is the design of underground structures which heat or cool the surrounding soil, such as ground source heating/cooling systems, and geological disposal facilities for nuclear waste. High-quality laboratory test data, which are currently lacking, are required for understanding thermal effects on soil, and for calibrating THM constitutive models, used for modelling soil-structure interactions. In the new equipment, the temperature of the tested specimen is controlled between 5°C and 70°C with a temperature-controlled water bath. For heating above ambient temperature, the water bath is heated directly using three 150W heaters. For cooling below ambient temperature, an external chiller unit is used, in combination with a heat exchanger. Water is continuously circulated through a hollow plate, directly below the specimen, which minimises the temperature gradient across the specimen. A full, thermal and mechanical calibration of the equipment has been carried out. Vertical thermal and mechanical compliance is determined by repeating the full heating/loading path for the proposed test, with no soil specimen in place. The thermal and mechanical corrections are applied separately, not in combination, because of the much larger magnitude of mechanical strains (and associated error). The instrumentation (load cell and displacement transducers) is raised above the lid of the water bath to isolate it from temperature changes. The differential, thermal, lateral expansion of the confining ring and soil has been considered in the calculation of the soil volume, as too has the thermal expansion of the solid phase of the soil in the calculation of the void ratio. The effect of friction, tilting, and compressible components of the equipment (filter papers and porous stones), has been investigated. Based on these tests, the use of filter papers is discouraged for testing in general, and for thermal testing in particular. New methodologies for performing thermal tests using the new equipment have been developed and are presented here. These cover calibration, testing, and data-processing, and make explicit exactly how all measured and calculated values are obtained, including how the calibration is determined and applied. Soil tests have been performed to develop and validate the calibration, testing, and data-processing methodologies, using bentonite clay, Leighton Buzzard sand, and KSS clay (a mixture of kaolin, silt and sand). The results from the final series of thermal tests, on KSS, constitute a high-quality data-set, which could be used to calibrate a constitutive model, such as the IC Thermal model used in the Imperial College Finite Element Program (ICFEP). The effect of repeated thermal cycling on normally-consolidated and over-consolidated soil, and the effect of temperature and temperature history on soil behaviour has been tested. The new temperature-controlled oedometer, together with the new calibration, testing, and data-processing methodologies, can now be used to obtain high-quality data on the one-dimensional thermo-hydro-mechanical properties of other soils.