Undrained response of loose gassy silty sands
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AbstractGassy soils are commonly found in the ocean seabed in the form of gas-charged sediments and are known to be widely distributed throughout the world. Perturbations to the sediment could result in instability of the gassy soil matrix and result in flow liquefaction. Although gassy soils behavior has been studied by various researchers, more emphasis has been on clay mixed soils or dense soils and it appears that there is no published research data on the effect of gas on loose silt-sands. The main objective of this thesis was to determine whether loose gassy silt-sands experiences strain softening flow liquefaction behavior under monotonic loading conditions at high degree of water saturation levels (i.e. Sr> 95% ). The triaxial system used for monotonic tests was modified from an unsaturated stress path triaxial system. The ultrasonic wave measurements were taken by using an ultrasonic apparatus. The results show that the monotonic undrained loading tests on loose saturated sand and silt-sand specimens confirmed silt-sands are more susceptible to liquefaction, compared to pure sand. The study also confirmed loose gassy sands and silt-sands are more susceptible to liquefy and experience strain softening behavior than saturated sand and silt-sand mixtures when the degree of saturation is above 95%. The results indicate that the critical state lines on e-p' space for saturated and gassy pure sand possess a shape similar to sand with fines and using the intergrain state concept it was observed that the critical state lines could be collapsed to one unique line. For both saturated and gassy sand and silt-sand mixtures, the research confirmed that critical state lines on the q-p' space passes through the origin and the inclusion of fines appeared to rotate the critical state line in the anti-clockwise direction. The instability line analysis for saturated and gassy silt-sand mixtures manifested as a straight line passing through the origin and the instability zone expanded when fine content was increased. A larger instability zone for gassy soils relative to saturated soils was observed. P wave behaviour during bubble nucleation showed that P wave velocity is sensitive to the wave amplitude. Compression wave studies on monotonic undrained shearing tests indicated P wave data points followed a path similar to the undrained stress path.
Bibliography: p. 342-343