Small Strain Deformation Characteristics of Granular Materials in Torsional Shear and Triaxial Tests with Local Deformation Measurements

Abstract

The soil behaviour under normal working loads usually mobilizes strain increments that are less than 0.1 %. Therefore it is very important to measure the small deformation of soil by laboratory experiments accurately. Among others, the local deformation transducer (LDT) has been used to measure locally small deformation of cylindrical or rectangular specimens in triaxial tests. In this study, a hollow cylindrical specimen was used in torsional shear and triaxial tests. By using a modified version of LDT, called PLDT (Pin-typed LDT), to comply with shear deformation of hollow cylindrical specimen under torsional loading, it was confirmed that the modified version could be successfully used to evaluate quasi-elastic deformation properties. The small deformation characteristics of Toyoura sand, Hime gravel and glass beads were investigated using both PLDTs for the local static measurement and gap sensors and a potentiometer for the external static measurement. In addition, a dynamic measurement system by using a set of small triggers and accelerometers was used to evaluate S wave velocity of glass beads. The specimens at different densities were subjected to small vertical and torsional cyclic loadings during otherwise isotropic consolidation from 50 kPa to 400 kPa and isotropic unloading from 400 kPa to 50 or 100 kPa.

It was confirmed that small strain Young’s and shear moduli values of the tested granular materials which were measured by PLDTs, external transducers and accelerometers are consistent with each other in terms of their stress state dependencies. The externally measured Young’s and shear moduli were by about 5 to 20 % larger than locally measured ones except for Young’s moduli of Toyoura sand. The measured Young’s and shear moduli showed an increasing trend with the density at the same stress level. Based on these test results, applicability of several void ratio functions was also studied.