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Shear strength and stiffness of a Sahara sand from Libya




Badanagki, Mahir, author
Carraro, J. Antonio H., advisor
Shackelford, Charles D., committee member
Shuler, Scott, committee member

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Modern geotechnical analyses rely upon a rigorous characterization of the dilatancy, critical state and stiffness parameters of geomaterials. In order to generate a fundamental database for these parameters for future geotechnical projects in Libya, the shear strength and stiffness of Libyan Sahara sand were systematically studied in drained and undrained axisymmetric compression. The dry funnel deposition method was employed in this study to create homogenous specimens and simulate the natural fabric of aeolian sand deposits from Libya. The fabric of Sahara sand was examined using a Scanning Electron Microscope (SEM). Static, monotonic, isotropically-compressed drained and undrained triaxial tests were performed on specimens with nominal height and diameter equal to 140 and 70 mm, respectively, to characterize the stress-strain-volumetric (or stress-strain-excess pore pressure) response and determine the intrinsic parameters of Libyan Sahara sand. Bender element tests were also performed to measure the shear wave velocity (Vs) and estimate the small strain shear modulus (Gmax) of Sahara sand at mean effective stress levels of 50, 100, 200 and 400 kPa. The intrinsic parameters that characterize isotropic compression, critical-state, dilatancy and small-strain stiffness of Libyan Sahara sand were determined to allow future analyses of mechanical behavior for this soil to be carried out using a rigorous theoretical framework for granular soils. The critical state soil parameters Γ, λ and k of the Libyan Sahara sand were determined to be equal to 1.92, 0.031 and 0.0002, respectively. The critical state friction angle (φc) of the soil was found to be equal to 31.9° based on results from both drained and undrained tests. The value of φc determined from drained tests was found to be in perfect agreement with the value of φc determined from undrained tests, as expected. The intrinsic parameters Q and R of the peak friction angle (φp) correlation (Bolton 1986) were determined to be equal to 8.5 and 0.98, respectively. This allowed prediction of φp values that differed by no more than about 1° from the actual values measured during the triaxial tests performed under a wide range of relative densities and mean effective stresses. The very small strain stiffness parameters Cg, ng and eg of the Libyan Sahara sand were equal to 548, 0.51 and 2.17, respectively. While the Libyan Sahara sand tested has about 25% of nonplastic fines, results from this fundamental study suggest that the isotropic compression, critical-state, dilatancy and small-strain stiffness characteristics of this soil can be reasonably interpreted according to (or predicted by) a rigorous framework that has already been validated for clean sands and/or sands containing fines.


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