Assessment of suitable loading rate for suction-controlled triaxial testing on compacted silty sand via axis-translation technique

摘要

State-of-the-art automated triaxial test systems have allowed for testing of saturated and unsaturated soil specimens along sophisticated stress paths. Selection of an appropriate shearing rate is crucial in the accurate determination of shear induced volume changes in the specimen, and making sure that there is no bifurcation of the target effective stress path from the measured stress path. However, in triaxial testing of unsaturated soils (UNSAT testing) at constant suction values, the axial shearing rate should also be slow enough to maintain equilibrium of matric suction as well as uniformity in water content throughout the shearing stage. In this case shearing rate is mostly controlled by the high-air-entry ceramic disk, regardless of how pervious the soil is. The maximum suction that is to be applied during the test program at highest confining pressure has to be chosen to quantify this rate. In medium to dense silty sands, with tendency to dilate upon shearing, it is essential to have shearing rates slow enough to permit upward flow of water into the specimen through the low-permeability ceramic disk. As part of an ongoing research on statically compacted unsaturated silty sands at UT Arlington, the suitable shearing rate is determined empirically through a series of strain-controlled UNSAT Triaxial CD tests at three axial shearing rates (0.014%/min, 0.0086%/min and 0.0029%/min), with highest suction applied via axis-translation technique. The most suitable shearing rate would be the maximum rate for which the specimen is subjected to constant matric suction throughout the test, with induced axial strains similar to those induced at lower shearing rates.