Effects of boundary conditions on cyclic thermal strains of clay and sand

The thermal oedometer has been widely adopted to determine soil deformation under heating and cooling. Very often an oedometer ring made of steel is used to confine a soil specimen laterally. To investigate and quantify the effects of boundary conditions (i.e. thermal expansion and contraction) of an oedometer ring on soil thermal strains, steel and invar rings were adopted for comparisons. The former has a linear thermal expansion coefficient (TEC) of 1·5 × 10⁻⁵ m/(m °C), which is within the typical range of 5 × 10⁻⁶ to 3·5 × 10⁻⁵ m/(m °C) for soil skeletons. The linear TEC of the latter is 5·0 × 10⁻⁷ m/(m °C), which is substantially smaller than those of soils. Axial strains of saturated normally consolidated clay and loose sand subjected to thermal cycles were measured in these two types of oedometer rings. The accumulated plastic strains of clay and sand measured in the steel ring were 32 and 11% larger than those measured in the invar ring, respectively. As expected, the expansion and contraction of the steel ring itself resulted in additional soil deformation during thermal cycles. It was also found that a popular equation, which assumes that the soil volume remains constant during the expansion and contraction of the odometer ring, overestimates the influence of boundary conditions on the incremental axial strain of soil by more than threefold.