Yielding of Rockfill under Relative Humidity Control

Abstract

This paper presents a fundamental study on the evolution of particle breakage during triaxial compression tests, focusing on the effect of total suction. This analysis was based on triaxial compression tests, performed with RH control. The tested material comes from the sta- bilizing shoulders of Montesinho CFRD and three RHs were stud- ied: 50%, 75%, and 100%. Experimental procedures are presented, with details of the prep- aration of specimens, and stress–suction paths are given. Special at- tention is given to the compaction and the application of different suctions. First, the influence of confining stress is analyzed. Results show that the confining stress led to a contractive behavior, de- creasing dilatancy. For a confining stress of 0.75 MPa, the dilatancy almost disappears. Considering the analyzed values of total suction, the maximum difference of deviatoric stress between specimens with different values of total suction was obtained when the mate- rial reached the peak condition, and was attenuated for larger values of axial strain. By determining the volumetric and axial strains, as well as the shear strains, it is possible to plot the stress paths and the plastic strain increment vectors. It is also possible to separate the elastic and plastic components. This allows for confirmation that speci- mens compress volumetrically during the initial part of the loading curve, up to a certain level of stress. Near the peak deviatoric stress, the material exhibits dilatancy, probably because of particles fitting together intricately, as a result of particle breakage and rearrangement. Another aspect analyzed was particle breakage; this tends to in- crease with the confining stress and the decrease in total suction. To assess the amount of breakage during triaxial loading, the Einav breakage factor was determined. The results corroborate the con- clusion that the values of the breakage factor increase with confin- ing stress and with the decrease of total suction. A 10% increase was observed in the breakage factor when the total suction was de- creased from 93.86 to 0 MPa. Dilatancy is described in terms of stress ratio, particle breakage, and average confining stress, and the experimental results show thatthe major effect of the decrease of total suction is contraction of the peak failure envelopes in the p–q plane by about 5.5% (for mean stresses up to 2.5 MPa). At the end of the tests, dilatancy was prac- tically independent of suction, decreasing only with confining stress to 1/3 (for the studied confining stresses: 0.20, 0.40, and 0.75 MPa). The research findings indicate that suction has a limited effect on the measurement of dilatancy. In any case, its effects, al- though noticeable and broadly consistent with observations in other tests with controlled suction on weak soils and gravels, are substan- tially smaller than the rest of the variables mentioned.