Evaluation of the Procedure Used to Determine Nonlinear Soil Properties In Situ
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Soil properties (shear modulus and damping) are normally determined from laboratory tests. These tests provide both values of the shear modulus in the linear elastic range for very small levels of strain, and its variation with the level of strain. It has become more common to measure the maximum shear modulus at low levels of strain directly in the field, using geophysical techniques. The values obtained in situ can differ significantly in some cases from those determined in the laboratory, and a number of reasons and correction factors have been proposed in the literature to account for this variation. As a result, when in situ properties are available, it is normal to use these values for very low levels of strain, but still assume that the variation of the ratio G/Gmax (normalized shear modulus) with shear strain is the same as determined in the laboratory. Recently, tests have been performed using large vibrators (the Thumper and Tyrannosaurus Rex of the University of Texas at Austin) to determine soil properties in situ for larger strains, and the variation of G/Gmax obtained from these tests has been compared to that reported in the literature from lab tests. Observation indicates some generally good agreement, but also some minor variations. One must take into account, however, that in the determination of the shear modulus versus strain in the field from vibration records, a number of approximations are introduced. The objective of this work is to evaluate the accuracy of some the procedures used and to assess the validity of the simplifying assumptions which are made. For this purpose, a shear cone that would reproduce correctly the horizontal stiffness of a circular mat foundation on the surface of an elastic, homogeneous half space, was considered. The cone was discretized using both a system of lumped masses and springs and a finite difference, using second-order central difference formulation, verifying that in the linear elastic range the results were accurate. A number of studies were conducted next, increasing the level of the applied force and using nonlinear springs that would reproduce a specified G/Gmax vs. γ curve. Using a similar procedure to that used in the field tests, the shear wave velocity between hypothetical receivers and the levels of strain were determined. The resulting values of G/Gmax vs. γ were then compared with the assumed curve to assess the accuracy of the estimated values.
Torres, Daniel E. (2010). Evaluation of the Procedure Used to Determine Nonlinear Soil Properties In Situ. Master's thesis, Texas A&M University. Available electronically from