The normalized shear modulus reduction $(G/G_{max})$ curve has been widely used in the soil dynamics to evaluate the in-situ non-linear deformational behavior combined with field seismic method. In this study, the characteristics of $(G/G_{max})$ curves on cohesionless soils have been investigated using resonant column/torsional shear (RC/TS) tests.
In order to obtain a reliable test result and to facilitate the testing procedures, a computer-aided RC/TS testing system was developed, and the entire testing procedure, signal monitoring, and data analysis can be performed fully-automatically. To characterize the $(G/G_{max})$ curves of natural soils, RC/TS tests were performed on 3 granite weathered residual soils, 7 subgrade soils, and 7 subbase soils. Test results showed that $(G/G_{max})$ curves of natural soils exist between Seed and Idriss sand and gravel curves, and can be grouped depending on the particle gradations, particularly on the nominal particle size, D95. To investigate the effects of particle distribution and size on $(G/G_{max})$ curves, the soils of various sizes were prepared by sieve analysis and the controlled soil specimens constructed by mixing various size particles were tested. The $(G/G_{max})$ curves of controlled specimens are consistent with Seed and Idriss sand curve, and the particle size and distribution does not affect the shape of $(G/G_{max})$ curves, in contrast to the results of natural soils. Further investigations are required to clarify these differences.
