In-situ testing of floating thermal piles in soft sensitive clay

Licentiate thesis, 2017

Thermal piles are structural piles with additional function of working as geothermal heat exchangers towards the soil volume. Heating and cooling process of soft soils is known to affect the stress state and soil structure, with potential consolidation settlements and creep as undesired consequences. The aim of this thesis is to investigate the effects of static mechanical and cyclic thermal loading of floating piles in soft clay. A test site with soft sensitive clay is selected and the soil properties are characterised in an extensive field and laboratory test program. A full scale thermal pile and the surrounding soil volume is instrumented with a set of sensors. A thermal loading rig, together with the thermal pile installed, has been successfully designed to absorb and extract energy to the ground. The thermal properties of the clay have been evaluated in the field using a Thermal Response Test (TRT). The field test setup is found to be capable of capturing changes in displacements, pore pressures and temperatures caused by the cyclic thermal loading. The change in pore pressures is strongly linked to the measured change in temperature in the clay. It has been shown that the thermal loading applied neither lead to significant pile head displacements, nor vertical deformations in the soil. The pile bearing capacity, recorded in a maintained loading test, has been found to be similar for both the thermal and the reference test piles. The test is adding novel data on the response from a driven floating thermal pile in soft sensitive clay.