Effect of the volume of the drainage system on the measurement of undrained thermo-poro-elastic parameters

For evaluation of the undrained thermo-poro-elastic properties of saturated porous materials in conventional triaxial cells, it is important to take into account the effect of the dead volume of the drainage system. The compressibility and the thermal expansion of the drainage system, along with the dead volume of the fluid filling this system, influence the measured pore pressure and volumetric strain during an undrained thermal or mechanical loading in a triaxial cell. The correction methods previously presented by Wissa [Pore pressure measurement in saturated stiff soils. ASCE J Soil Mech Found Div 1969;95(SM 4):1063-73], Bishop [Influence of system compressibility on observed pore pressure response to an undrained change in stress in saturated rock. Geotechnique 1976;26(2):371-5] and Ghabezloo and Sulem [Stress dependent thermal pressurization of a fluid-saturated rock. Rock Mech Rock Eng 2009;42(1):1-24], only permit one to correct the measured pore pressures during an undrained isotropic compression test or an undrained heating test. An extension of these methods is presented in this paper to correct the measured volumetric strain, and consequently the measured undrained bulk compressibility and undrained thermal expansion coefficients, during these tests. Two examples of application of the proposed correction method are presented for an undrained isotropic compression test and an undrained heating test performed on a fluid-saturated granular rock. A parametric study has demonstrated that the porosity and the drained compressibility of the tested material, and the ratio of the volume of the drainage system to the one of the tested sample, are the key parameters that most influence the error induced by the drainage system.