A triaxial testing system to evaluate stress-strain behavior of soils for wide range of strain and strain rate

Pre-peak stress-strain behavior of geomaterials, such as soils and rocks, at small and intermediate strains is an important geotechnical engineering topic. Among different laboratory stress-strain testing methods, the triaxial test is the most popular one performed. This paper describes some modifications and improvements of an existing triaxial testing system that were mate to more accurately evaluate the stress-strain behavior of geomaterials for wide ranges of strain and strain rate. An electro-mechanical loading device was specifically designed to control strain states and stress paths and to study the quasi-elastic properties of geomaterials at any given stress state. The device is now driven by an alternating current servomotor, allowing for changing the strain rate about three orders of magnitude without any intermission in each test. A 16-bit A/D card is also used to increase the resolution in data acquisition. The long-term stability of a local axial gage (LDT), has been ensured to evaluate creep deformations as well as post-creep behavior of geomaterials, factors that are important for many practical engineering applications. High performance of the system is demonstrated by presenting some typical results from undrained cyclic tests at very small strain levels for a wide range of strain rate and an undrained monotonic loading test for a wide strain range, both on a compacted silty sand. It is shown that the equivalent Young's modulus and damping ratio at very small strains have a clear sensitivity to both strain rate and aging period. For monotonic shearing tests in a wide range of stain, the isotach property and the effects of changes in the strain rate are also presented.