Unsaturated behavior of rammed earth: Experimentation towards numerical modelling

•Effect of hygroscopic conditions on the mechanical characteristics.•Double compaction method to obtain samples of uniform density.•Effective stress equation which intrinsically introduce the effect of suction in the failure criteria.•Non-linearity of Mohr-Coulomb failure envelope in unsaturated conditions. This study concerns the coupled hydro-mechanical behavior of rammed earth, which is a real difficulty for the development of this construction technique. Unconfined compressive strength tests on samples conditioned at different relative humidities were performed to determine the variation of mechanical capacity with suction. Both compressive strength and Young’s modulus increase with suction. Further, the effect of shear characteristics with the hydric conditions was studied by direct shear test. As suction induced cohesion contributes a significant part of strength, the apparent cohesion reduced with the reduction of suction. In addition, a considerable variation was observed in the friction angle. Unconsolidated undrained triaxial tests on unsaturated samples were performed to plot the failure envelope for a greater value of normal stress and to complete the failure envelope. These three test makes it possible to put in evidence a non-linearity in the failure envelope over all the suction range studied. Consolidated undrained triaxial tests on saturated samples were performed to determine the intrinsic cohesion and intrinsic friction angle. It was observed that Mohr-Coulomb criterion is not realistic for rammed earth and the failure envelope is non-linear for unsaturated conditions. Thus, a failure criterion was proposed in which both cohesion and friction angle are dependent on normal stress applied. The expression for Bishop’s effective stress for unsaturated soil was evaluated from the results of triaxial and unconfined compressive strength test together. The non-linear failure criterion obtained for a particular suction can be extended to other suction states through Bishop’s effective stress formulation. It can further be used for coupled hydro-mechanical modeling of rammed earth structures.