Impact of Strain Rate on the Shear Strength and Excess Pore Water Pressure Generation of Clays and Sands

Svoboda, Jenna S.

Graduate Thesis Or Dissertation

Impact of Strain Rate on the Shear Strength and Excess Pore Water Pressure Generation of Clays and Sands Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/cz30pt15n

Abstract

The purpose of this study was to investigate changes in shear strength and excess pore water pressure of unsaturated clay and dense sand subject to increased strain rates. Consolidated undrained (CU) and unconsolidated undrained (UU) triaxial compression tests were performed on specimens of compacted clay performed at axial strain rates ranging from 0.1 %/min to 14.5 %/min at degrees of saturation ranging from 100% to 75%. In addition, CU tests were performed on saturated, dense sand at axial strain rates ranging between 1.1 %/min and 220 %/min, and consolidated drained (CD) tests were performed on oven-dry sand at axial strain rates ranging from 1.1 %/min to 4.4 %/min. Results from the CU tests on saturated clay (degree of saturation equal to 100 %) show an increase in undrained shear strength of 13.8% and a decrease in the magnitude of positive excess pore water pressure with increasing axial strain rate. These findings are consistent with results from classic studies on normally consolidated soils by Casagrande and Shannon (1948) and Richardson and Whitman (1963) as well as compacted soils by Olson and Parola (1967). The undrained shear strength of unsaturated compacted clay also increases with increasing axial strain rate as well as increases in matric suction. However, the excess pore water pressure at failure measured for unsaturated tests at a higher strain rate first increased from the saturated value at a low suction and then decreased at a higher suction (lower degree of saturation). The rate of increase in the shear strength of unsaturated clays having suction values up to 140 kPa (degrees of saturation greater than 75%) was found to be less than that of the clay under saturated conditions. UU tests on compacted clay at different initial compaction water contents confirms the trend of shear strength increase with increased strain rate and lower degree of saturation. Overall, the results from these tests support the hypothesis that rate effects in clays occur due to the difference in the hydraulic conductivity of the soil, which affects the rate of drainage of excess pore water pressure, and the axial strain rate, which affects the rate of generation of excess pore water pressure. The difference in these two rate effects leads to a decrease in the positive excess pore water pressure at failure for faster axial strain rates, which causes the effective stress to increase within an undrained clay specimen. The results from the CU tests on saturated sand show an increase of 33% in undrained shear strength and a decrease in the magnitude of negative pore water pressure at failure with increasing axial strain rate. However, CD tests performed on dry sand indicate that the shear strength at failure does not change with increasing axial strain rate. These observations indicate that the rate effect in saturated dense sand likely occurs due to an increase in the amount of dilation with increasing axial strain rate, which affects the magnitude of negative pore water pressure. Similar to the clay specimens, the lower magnitude of negative excess pore water pressure at failure at faster axial strain rates leads to an increase in effective stress in an undrained sand specimen.

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