Shear strength of flue gas desulfurization by-product
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Shear strength parameters of flue gas desulfurization (FGD) sludge were investigated by conducting a series of constant volume direct simple shear tests on FGD specimens compacted using standard Proctor energy. Compacted FGD specimens were consolidated to three different confining stresses (500 pounds per square foot (psf), 3000 psf, 6000 psf) then sheared at a deformation rate of 0.0002 inches per minute while the specimens' volume was maintained constant. Compaction test results indicated that for specimens with the moisture content around 50 percent, the dry unit weight of the specimens compacted using standard Proctor energy lies around 70 pcf. Results from the consolidation tests indicated that the FGD specimens used in performing the direct simple shear tests completed primary consolidation in a very short time. Durations of primary consolidation were expected to be shorter than three minutes. During the constant volume shearing stage, increases in effective normal stresses were observed indicating that the compacted and consolidated FGD specimens were trying to dilate which would generate negative pore water pressure when sheared under truly undrained conditions. The effective strength (drained strength) envelope for the compacted FGD resulted in an effective stress friction angle of 34 degrees and effective stress cohesion intercept of zero. The undrained shear strength was found to have a normalized shear strength ([tau]peak/[sigma]c) of about 1.0 for specimens consolidated to 3000 psf or greater. Specimens consolidated to 500 psf resulted in a normalized shear strength ([tau]peak/[sigma]c) of greater than 2.0. It is hypothesized that the higher effective confining stress may destroy some bonding in the compacted FGD thereby lowering the undrained shear strength.
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