SGER: The Elastic Moduli of Soils With Dispersed Oversize Particles

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

To calculate the elastic deformation experienced by soils subjected to static or dynamic loads, knowledge of the elastic moduli is required. Although the response of soils to these kinds of loads may not be strictly represented as linear elastic, solutions based on linear elastic behavior are commonly used for the estimation of stresses or strains in the field. The elastic moduli are normally evaluated in the laboratory using conventional triaxial compressive tests on cylindrical samples. For meaningful test results, it is necessary to maintain a ratio of sample diameter to the maximum particle size of approximately 6:1 or greater. For the conventional triaxial specimen diameter of 2.8 in., the maximum grain size that can be tested is 0.5 in. Thus, when soils are tested in the conventional triaxial apparatus, it is common practice to remove "oversize" particles with an average diameter greater than 0.5 in. Samples larger than 2.8 in. can be used in triaxial testing; however this requires the use of large cells and large loading systems, making the tests too expensive and time consuming to be used routinely. The removal of material larger than 0.5 in. from the samples tested in the conventional triaxial apparatus will provide unreliable elastic moduli values for the calculation of stresses and deformations associated with civil engineering projects built in soils with large rock particles (i.e. glacial tills, residual and colluvial soils). This proposal presents a theoretical method developed by Hashin that calculates the elastic moduli of a composite made of an elastic material containing dispersed large rigid particles. The Hashin method requires only knowledge of the elastic moduli of the matrix that surrounds the particles coupled with the concentration by volume of the rigid large particles in the composite in order to calculate the elastic moduli of the mixture. The objective of this one year exploratory investigation is to assess the validity of the Hashin method by conducting ultrasonic velocity tests on dry mixtures of soils (clay, silt, and sand) and dispersed oversize particles (gravel, glass beads, sands). Also, stress-strain curves reported in the geotechnical literature obtained from conventional static triaxial compression tests on laboratory prepared soils-large particles mixtures will be used to obtain the static elastic moduli. Using the ultrasonic velocity measurements, the laboratory dynamic elastic moduli will be obtained. Using the results from triaxial compression tests, the static elastic moduli will be obtained. The laboratory elastic moduli will be then compared with those predicted by Hashin's theoretical method in order to assess its reliability. The results of the exploratory research will be used as a basis to further explore the validity of Hashin's method as it applies to soil-oversize particle mixtures having a soil matrix with varying degrees of saturation and heterogeneity.