A methodology for the calibration of the effective stress parameter: From macroscopic considerations to microstructural validation

This study proposes an experimental methodology to calibrate the effective stress parameter χ for unsaturated soils, ensuring a unique shear failure criterion. The study integrates water retention behaviour and shear strength to calibrate the χ parameter through an experimental method. The experimental study was conducted on a reconstituted clayey soil with a plasticity index of 98.3%. Unconfined compression tests were conducted under six suction levels (5 controlled RH, and laboratory conditions) to assess soil strength and stiffness. Triaxial consolidated undrained tests were performed on saturated samples to determine the intrinsic failure criterion. Then, combining uniaxial compression strength (UCS) at various suctions and shear failure criterion under saturated conditions, the χ parameter was calculated for different suction levels. Finally, a power law equation relating χ parameter to the degree of saturation is calibrated. This method ensures that the Mohr-Coulomb failure criterion remains consistent, regardless of suction conditions. In this approach, the strength gain observed at high suction is integrated in the effective stress formulation. At the end of the study, the calibration is validated from microstructural considerations. From pore size distribution obtained by mercury intrusion porosimetry, it is demonstrated that the χ parameter is essentially affected by the water retention properties of macro-porosity while the water retention of micro-porosity has no effect on the macroscopic effective stress.