| Dynamic shear modulus and damping capacity of soils are two important properties required in the analysis of a number of soil dynamics problems. An experimental study that is directed primarily toward improving our knowledge and understanding of these properties as a function of fundamental state and environmental parameters is undertaken. The investigation consists of two parts involving, respectively, sands and clays.; The dynamic response of a large number of uniform-sized, dry sands and a limited number of improved gradations was obtained from resonant column tests, and the data were analyzed to evaluate the effects of grain characteristics (size, gradation, shape, surface texture) and other test variables (e.g. ambient stress conditions, void ratio or relative density, strain amplitude, number of loading cycles) on the modulus and damping capacity. For strain amplitudes less than 2 x 10('-3) radians, the test results indicate that mean principal stress, density, and strain amplitude exert the strongest influence on both the modulus and damping capacity. Shear modulus is further influenced by grain roundness; the effects of the other parameters are either virtually zero or very small. Empirical relationships correlating shear modulus and damping capacity to the important controlling parameters were obtained. The applicability of these relationships on other sands was assessed in a secondary testing program utilizing six different sand samples. The proposed relationships and relationships proposed by other investigators were compared with the experimental data.; Clay samples were prepared by consolidation of kaolinitic clay slurries with different chemistries following isotropic and anisotropic stress paths to various stress levels. The fabric (or microstructure) of these samples is determined qualitatively and quantitatively by the combined use of scanning electron microscopy and x-ray diffraction techniques and a new way of expressing clay particle orientation is developed. Saturated clay specimens were trimmed in different orientations from the consolidated block samples and tested in resonant column apparatus at different times after the application of isotropic and anisotropic ambient stresses. In addition to particle orientation, the test variables included strain amplitude, void ratio, ambient stress conditions, stress history and path, and long-term time effects. The test results, in general, substantiate the previously reported trends of the influence of the latter factors with a few notable exceptions. The influence of microstructural anisotropy is to increase the modulus (up to 70%) as the particle orientation plane rotates 90(DEGREES) with respect to the dynamic shear plane. An elliptical relationship is found to represent the modulus at any direction with respect to the fabric axes satisfactorily. The influence of fabric anisotropy on damping capacity is found similar in magnitude but opposite in direction of that on modulus. In order to study the influence of moisture regime, certain specimens were desaturated by equilibrating to various suction values in a ceramic plate soil-moisture extractor and subsequently were tested for the dynamic properties. Dynamic shear modulus of partially saturated clays increases significantly (7 to 10 folds) as suction increases from the initial saturated state up to a critical suction (about 400 to 700 kPa for the clay used) beyond which it tends to decrease. Damping capacity is not affected significantly up to the critical suction; however, damping increases significantly beyond it. The results establish the general trends regarding the role of fabric and moisture regime in controlling the dynamic behavior of clay soils. |
