Experimental artefacts in undrained triaxial testing

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 undrained thermal or mechanical loading in a triaxial cell. A correction method is presented in this paper to correct these effects during an undrained isotropic compression test or an undrained heating test. A parametric study has demonstrated that the porosity and the drained compressibility of the tested material and the ratio of the vol-ume of the drainage system to the one of the tested sample are the key parameters which influence the most the error induced on the measurements by the drainage system.

💡 Research Summary

The paper addresses a subtle but significant source of error in conventional triaxial testing of saturated porous materials under undrained conditions: the dead volume of the drainage system. Although undrained tests are intended to isolate the specimen from any fluid exchange, the drainage lines, pressure transducers, and associated fittings inevitably contain a finite volume of fluid. This fluid, together with the compressibility and thermal expansion of the drainage hardware itself, reacts to mechanical loading or temperature changes, thereby contaminating the measured pore pressure and volumetric strain.

A theoretical framework is developed in which the specimen and the drainage system are treated as two coupled compressible bodies. The specimen is characterized by its drained compressibility (β_s) and thermal expansion coefficient (α_s), while the drainage system is described by β_d, α_d, and its dead volume V_d. By imposing the undrained condition (total volume change ΔV = 0), the authors derive explicit relationships linking the recorded pore‑pressure increment Δp_m and volumetric strain ε_v,m to the true values Δp_t and ε_v,t. The key dimensionless parameter emerging from the analysis is the volume ratio r = V_d/V_s, where V_s is the specimen volume. The correction formulas are:

Δp_t = Δp_m /