The objective of this study is to determine if particle orientation and particle size have an influence on the angle of internal friction for unbound slate slag with an emphasis on low normal stresses. Slate slag samples were retrieved from the Paine Mountain slate tract, located in Northfield VT, USA, and sieved. Using parallel gradations and particle scalping techniques, four different gradation samples were produced for testing. In order to compare data between gradations, a relative density of 50% was used for all tested samples. In order to assess angle of repose of the slag under no confinement a "pour test" was performed. Direct shear testing was also performed on all gradations to determine the angle of internal friction under normal stresses of 6.9, 14, 21, 69, 138, and 207 kPa (1, 3, 5, 10, 20, and 30-psi) normal loads. An additional direct shear test was performed using samples with particles carefully oriented perpendicular to the shear plane. The four random oriented granular gradation samples angle of internal friction ranged between 28-38 degrees while the vertically oriented sample found an apparent angle of internal friction of 75-degrees. The size of particles in the pour test was directly proportionate to the angle of repose found using this test. The smaller the grain sizes present in the sample, the steeper the angle of repose. The direct shear testing showed that as the particle size within the sample increased, the angle of internal friction also increased. The critical void ratio for all compiled slate samples was found to be between 14-21 kPa normal stresses. Most pronounced was the finding of the apparent angle of internal friction differing considerably with careful particle orientation.
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