Static and dynamic behaviour of compacted sand and clay: Comparison between measurements in Triaxial and Oedometric test systems

In rock mechanics and rock physics, like in many other branches of research, it is important to compare results obtained in different kinds of apparatus that are meant to measure the same properties. Differences may in general be due to differences in samples, or in test procedures. Here we compare uniaxial compaction experiments in oedometric and triaxial tests systems, using brine-saturated samples made from pure kaolinite or from Ottawa sand. Small differences in sample manufacturing or in initial loading of the specimens were found to cause significant differences in static behaviour and in ultrasonic velocities during the tests. The influence of differences in sample geometry (wide and thin samples in the oedometer versus long and slim samples in the triaxial set-up) and the influence of different boundary conditions caused by the confining medium (steel in the oedometer, thin soft sleeve in the triaxial system) were studied, amongst others with the use of discrete particle modelling. Although the boundary conditions may have an influence, the most significant sources of discrepancy in our experiments were associated with the manufacturing and preparation of the samples to be tested. The test data show that the drained static compaction modulus for sand is close to its dynamic counterpart, while for kaolinite, the dynamic modulus is significantly larger than the static one.