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 ��m. 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. / Graduation date: 2003
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31795 |
| Date | 04 June 2002 |
| Creators | Butler-Brown, Jason J. |
| Contributors | Dickenson, Stephen E. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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