The Field Investigation of Tension Cracks on a Cement Grouted Slender Column during the Thermal Response Test-Paper

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The paper reports a field thermal response test on a slender, cement-grouted column installed in London Clay. Built by enlarging an existing borehole, the column housed geothermal pipework and instrumentation. It was subjected to repeated heating and cooling cycles, with vibrating wire strain gauges (VWSGs) and distributed optical fibre sensing (OFS) measuring strain and temperature.<br /><br />OFS detected multiple tension cracks along the column during the first cooling cycle, while the VWSGs did not capture them. The findings demonstrate the value of distributed sensing for monitoring localized changes in geotechnical structures. The cracks altered the column’s thermo-mechanical behaviour: during cooling, the column contracted with weakened interaction between the column and surrounding soil. During heating, the cracks diminished and soil–column interaction was re-established. The middle section was restrained during heating, whereas the ends expanded more freely.<br /><br />Estimated mobilized shaft friction at the column ends was about 30 kPa during the first heating phase, falling to approximately 19 kPa near the top after subsequent thermal cycles. Temperature measurements in the geothermal pipes also indicated stronger heat exchange with the soil during heating than cooling, consistent with the observed changes in mechanical interaction.<br /><br />Overall, the study shows that thermal cycling can generate tension cracks in a cement-grouted column and that these cracks can modify both its load transfer and heat exchange with the surrounding ground.

Keywords

thermal response test

cement-grouted column

London Clay

geothermal pipework

vibrating wire strain gauges

distributed optical fibre sensing

tension cracks

thermo-mechanical behaviour

shaft friction

heat exchange