Testing Rockfill Under Relative Humidity Control

Rockfill stress-strain behavior is strongly controlled by the breakage of rock particles. Particle breakage is shown to depend on stress level and the ambient relative humidity. A technique to perform tests on rockfill samples under relative humidity control has been developed. It has been applied to testing large diameter samples of a crushed quartzitic shale under oedometric conditions. Relative humidity is imposed by means of a double stage approach: moist air in thermodynamic equilibrium with a given saturated salt solution is first circulated through the sample. Circulation of moist air is interrupted and the specimen is allowed to reach equilibrium. A technique to extend the feasible suction range of the technique down to 2.5 MPa is described. Details of the evolution of collapse and swelling/shrinkage strains, as changes in Relative Humidity are imposed, are reported in the paper. Experimental findings are consistent with an underlying mechanism of particle breakage explained by the phenomenon of subcritical crack propagation.