Dynamic properties of sandy and gravelly soils
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A two-part study was undertaken to evaluate the dynamic properties of nonplastic sandy and gravelly soils. The first part involved the development of a large-scale, multi-mode, free-free resonant column. This device is called the multi-mode device, MMD. Key benefits of this device are: (1) “relatively” large specimens (15 cm in diameter) can be tested, (2) background noise is greatly minimized, and (3) the point of “fixity” in resonance testing is conveniently positioned at the specimen mid-height during the first-mode resonance. With the MMD, three different measurements can be performed. These measurements are: (1) torsional resonance measurements, (2) longitudinal resonance measurements, and (3) direct-arrival, constrained compression wave measurements. These measurements allow linear and nonlinear measurements of shear modulus, G, and material damping ratio in shear, Ds, in addition to measurements of small-strain Young’s modulus, Emax, small-strain material-damping ratio in unconstrained compression, Dc,min, and small-strain constrained modulus, Mmax. These measurements can all be performed sequentially on the same specimen. The second part of the study involved using the MMD to evaluate the dynamic properties of sandy and gravelly soils. A total of 59 reconstituted specimens were tested to systematically investigate the effects of void ratio, e, effective confining pressure, σo', median grain size, D50, and uniformity coefficient, Cu, on modulus and material damping in shear as well as the effects of these parameters on Emax, Dc,min, and Mmax. Some findings are: (1) the large-scale, free-free device works well and is adaptable to construct larger devices, (2) the primary effect of increasing D50 is to increase small-strain shear modulus, Gmax, and decrease small-strain material damping ratio, Ds,min, (3) D50 has a similar effect on Emax and Mmax as on Gmax, (4) Ds,min and Dc,min of dry granular soils are generally lower than 1.0%, (5) σo' has a slightly larger effect on Gmax, Emax, and Mmax of loose, well graded granular materials than dense, uniform materials, and (6) reference strain, γr, (a shearing strain at which G/Gmax = 0.5) is mainly a function of σo' and Cu, while γr increases as σo' increases and Cu decreases.