Thermal strain in a water-saturated limestone under hydrostatic and deviatoric stress states

- Irreversible thermal strain of limestone is measured under three different stress states.
- A new method is proposed to quantify matrix thermal expansion by strain separation.
- Inelastic processes induced by thermal and mechanical stresses are discussed.

The present study is aimed at investigating the evolution of thermally induced bulk strain in a water-saturated limestone (Blaubeuren) at three different stress states. Three cylindrical rock samples are respectively loaded to a constant stress state (σ1 = σ3 = 15 MPa; σ1 = 45 MPa , σ3 = 15 MPa; σ1 = 75 MPa , σ3 = 15 MPa) at drained conditions in a conventional triaxial rock deformation apparatus before the sample temperature is cycled between 30 °C and defined levels up to 120 °C. Strain measurements are performed by one circumferential and two axial extensometers. Irreversible strain in both the lateral and axial sample directions are measured after each temperature cycle indicating permanent increases in diameter and in length. The measured bulk strain is separated into different strain components related to (1) initial stress loading, (2) reversible thermal expansion and contraction of rock matrix, and (3) some residual strain. The magnitudes of the residual strain increase with increasing deviatoric stress (σ1 − σ3) in the lateral direction but decrease with increasing deviatoric stress in the axial direction. The derived matrix thermal expansion coefficients range from 6 × 10− 6 °C− 1 to 1.8 × 10− 5 °C− 1 and from 9 × 10− 6 °C− 1 to 1.5 × 10− 5 °C− 1, respectively in the axial and lateral directions and present lower values at higher deviatoric stresses. Microstructural analyses evidence tensile cracks which are interpreted to have been induced during the temperature cycles. These cracks have the potential to offset matrix thermal expansion yielding lower matrix thermal expansion coefficients at higher deviatoric stresses.