Laboratory characterization of #2Q-ROK sand
A series of laboratory tests were performed in a sand commercially known as #2Q-ROK to determine its response to loading. #2Q-ROK is a poorly-graded, clean silica sand composed of 99% of quartz (SiO2). Its specific gravity is 2.65 and its maximum and minimum void ratios are 1.03 and 0.7, respectively. In the absence of crushing, the #2Q-ROK sand direct shear critical-state friction angle is 32.8° and the ring shear critical-state friction angle is 29.6°. One-dimensional compression tests were performed on samples compressed to a normal stress of 24,851.36kPa for time periods ranging from 4 hours to 11 days. The maximum increment in relative breakage was 0.02. Time effects did not play a significant role in particle crushing of #2Q-ROK sand. The evolution of strength, particle gradation and morphology parameters with crushing were investigated by performing ring shear tests. The results of the ring shear tests showed that particle crushing increases significantly with shear displacement. Samples compressed to a normal stress of 300kPa and sheared for displacements ranging from 0.5m to 11.3m showed a maximum increase in relative breakage of 0.41. As crushing increased, the particle gradation of #2Q-ROK evolved from a poorly-graded sand to a more well-graded sand. Samples tested in the ring shear apparatus presented cycles of stress and vertical deformation, which indicated the occurrence of particle crushing. Each of these cycles started with a stage of contraction followed by a stage of dilation and a final stage of vertical displacement stabilization. As a result of the shear stress cycles observed with crushing, the mobilized friction angle of #2Q-ROK sand oscillated between 31° and 33°. Ring shear tests performed on samples compressed to normal stresses ranging from 50kPa to 100kPa and sheared to displacements ranging from 20m to 30m indicated that the friction angle increased by approximately 3° at large shear displacements. The fluctuation of friction angle that initially ranged from 31° to 33° started to increase at approximately 7m and 10m for the samples compressed to 100kPa and 50kPa, respectively. At the end of these tests the mobilized friction angle ranged from 33° to 36°. Ring shear tests also showed that particle crushing developed only in a shear zone whose thickness ranged from 4.4mm to 4.7mm (6.5 to 6.9 times the D50 of the original material). Considering a shear band thickness of 4.55mm, critical state was reached for the first time at a shear strain of 1648 % (75mm of shear displacement). As the gradation of #2Q-ROK sand evolved with shear displacement, its maximum and minimum void ratios decreased to 0.72 and 0.36, respectively (shear displacement of 11.3m). Sieve analyses combined with particle morphology analyses indicated that larger particles were likely to be crushed by shearing-off of their asperities, whereas small particles, by particle splitting.
Prezzi, Purdue University.
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