A Rockfill Triaxial Cell with Suction Control

This paper describes the development of a new triaxial cell specifically built to test coarse granular material under partially saturated conditions. The partial saturation is achieved by imposing a total suction that is correlated with the relative humidity. Relative humidity control was achieved by water vapor transfer using a forced convection system, which is driven by an air pump transporting the air humidity to the sample, and it is controlled by saturated saline solutions placed in a vessel. A double-wall cell was designed to monitor global volume changes of the material on compression and shearing. In addition, the cell uses a novel technique to measure local axial and radial deformations. The use of both global and local measurements allows detecting experimental problems such as membrane penetration on isotropic compression and shearing, as well as membrane sliding on shearing at low confining stresses. Selected test results are presented to show the capability of the cell during isotropic compression and shearing made with strain and stress control. The results of the test with strain control show a nonlinear increase in strength and dilatancy of the material in relation to the decrease of the relative humidity. Also they have a tendency to reach the critical state, and it can be seen that such a critical state line is a function of the relative humidity. Under a constant deviator stress, a collapse test of the rockfill was carried out and the results showed that the deformation depends strongly on the time. Furthermore, it is larger than that produced by making a test under saturated conditions, commonly taken as final reference.