An Experimental Investigation on the Effects of Temperature on Soil-Concrete Interface Systems

Geothermal energy pile incorporates the structural pile and ground source heat exchanger to generate environmental and economic benefits. While the popularity of geothermal energy pile has been increasing in the recent decades, there is limited understanding of the pile-soil interaction, especially the effect of temperature on the interface shear strength. This is important for accurately predicting the load-carrying capacity of the pile shaft and other thermo-mechanical coupled behaviour. This study investigates the effect of temperature on the pile-soil interface shear strength using a modified shear box, and the effect of confining stress on the thermal conductivity of soils using a modified triaxial cell apparatus. With the modified shear box, tests were conducted at different temperatures (20⁰C, 35⁰C and 50⁰C) including thermal cycles to examine the pile-soil interface shear strength under the effect of heating. Also thermal conductivity tests were performed on four different soils including two from Melbourne, Australia, at different confining stresses (up to 200 kPa) with different initial moisture contents and densities.

The shear strength of smooth concrete-soil interface was found to increase when the temperature rose from 20 to 35⁰C, but independent of temperature between 35 and 50⁰C. Also, thermal cycles appeared to have no effect on the interface shear strength. Only minor drop in interface shear strength was observed when the number of thermal cycles increased. However it seems that the increase of the number of thermal cycles influenced the soils moisture state leading to dryer soil involved in the soil-concrete interface shearing. In terms of the modified triaxial thermal conductivity test, results showed that the confining stress had slight influence on highly compacted soils. For loosely compacted soils, a considerable increase in thermal conductivity was observed when the confining stress was increased, this was caused by density changes. It was concluded that the thermal conductivity of a soil was predominantly determined by its level of compactness, moisture content and confining stress.