Graduate Thesis Or Dissertation
 

Cyclic triaxial testing of low- to moderate-plasticity silts

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v118rh95k

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  • This project report presents a laboratory investigation of the dynamic behavior of saturated alluvial silty soils from sites in Oregon and Washington. The focus of the study was to document the liquefaction susceptibility, post cyclic strength, and volumetric strain behavior of the silt soils based on cyclic, undrained triaxial compression testing. A cyclic triaxial testing apparatus with computer control and data acquisition was assembled, calibrated, and used to perform undrained cyclic triaxial testing and post cyclic testing on undisturbed and reconstituted specimens. The results of this investigation are compared with the undrained cyclic triaxial testing data on silty soils published by others. The influence, of grain-size distribution, plasticity index, and overconsolidation ratio (OCR) on the dynamic behavior was identified. Several cyclic resistance curves were prepared that show the cyclic resistance for the silts for OCR values of 1 to 2.5. The cyclic resistance curves observed in the laboratory likely overestimate the in-situ cyclic resistance of the material due to sample disturbance during sampling, transport, and testing. It was observed that OCR has a significant influence on the cyclic resistance of silt soils. Cyclic resistance was observed to increase with increasing plasticity and percent passing the U.S. Standard Number 200 Sieve and percent finer than 2 μm. It was observed that excess pore pressure measurements recorded at the transducer for fine-grained soils subjected to rapid loading may not accurately represent the actual pore pressures of the soil. Therefore, it is proposed that strain criteria be used, rather than excess pore pressure generation, to define initial liquefaction for fine-grained soils. Post cyclic undrained strength test data shows that the silts are dilative under compressive loading in the triaxial apparatus. Peak strengths were not observed due to the dilative nature of the silty soil tested. Therefore, post cyclic undrained strengths were strain based. The strain based strengths were compared with relationships developed by Baziar and Dobry (1995) and Ishihara (1993) and were found to have a higher residual strength than the sandy soils. Unusually high S[subscript u]/p' ratios were also recorded for the silt soils. This observation highlights the need to obtain post-cyclic strength at a consistent strain. The post cyclic volumetric strain data was compared with the findings of Ishihara and Yoshimine (1992). Plots of volumetric strain versus maximum axial strain were created. These plots were then used to establish a relationship between post cyclic volumetric strain and the factor of safety against liquefaction. The volumetric strain behavior of the silt was observed to be very similar to sand at relative densities of 40 to 80 percent.
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  • File scanned at 300 ppi (Monochrome, 256 Grayscale) using Capture Perfect 3.0 on a Canon DR-9050C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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