Upgrading a resonant column apparatus to reliably measure specimen void ratio

The small strain shear modulus of soils is an important design geotechnical parameter for a wide range of civil infrastructure applications. The small strain shear modulus can be determined by using elastic theory as a relationship between the soil density, which is expressed as a function of void ratio, and the measured shear wave velocity. Thus, the void ratio has a fundamental impact on the accuracy of the result. Laboratory testing involving resonant column apparatus allows for determining the changes in stiffness of soils with varying void ratios. Standard of-the-shelf resonant column apparatus works well for clayey soils but presents a number of limitations for coarse soils that makes accurate and reliable void ratio measurement difficult due to both pore fluid and (often) sample loss during removal from the equipment. This paper presents the development of a modular set-up which allows for complete specimen sealing at the end of shear wave velocity testing. The modular set-up along with the specimen can be removed from the resonant column apparatus and the void ratio can be determined by means of end-of-test-freezing similar to triaxial testing. From this, reliable small strain stiffness at well-determined void ratio can be computed. The void ratio measurements using the new modular set-up were compared to those from triaxial tests performed on identically prepared specimens. The difference in void ratio at any given isotropic confining stress was between 0.001 and 0.011.