Master of Engineering Science
Civil and Environmental Engineering
Professor Abouzar Sadrekarimi
The cyclic behavior and liquefaction behavior of reconstituted samples of a local silica-carbonate sand are examined by performing monotonic and cyclic simple shear tests in addition to bender element shear wave velocity measurements. Shear wave velocity was found to vary with effective overburden stresses by an average power of 0.25. Maximum shear modulus (Gmax) was also computed from the shear wave velocity measurements and a correlation was developed between Gmax, effective stress and void ratio. Small strain and cyclic strain dependent shear modulus of the soil were determined, and shear modulus reduction curves were obtained. The results show a slower degrading stiffness and damping ratio, compared to an average proposed for sands. A critical state line of the samples was developed by performing drained and constant-volume monotonic simple shear tests.
Cyclic resistance, shear wave velocity and rigidity factors of the sand samples were evaluated with the critical state approach. Circular, elliptical (oval) and figure-8 bi-directional cyclic load patterns were applied, in addition to mono-directional shearing modes and cyclic response of loose (25% relative density), medium dense (45% relative density), and dense samples (65% relative density) were investigated. Wide ranges of the generated excess pore water pressure ratio (ru) were observed. Changing patterns of ru values were assessed based on the cyclic shear strength of multiple tests and number of cycles prior to liquefaction. Also, the test results showed up to 35% reduction in cyclic resistance of bi-directionally loaded samples relative to mono-directionally loaded ones, which is the least for an elliptical cyclic pattern and the most for a circular cyclic pattern. Bi-directional cyclic results are found to be dependent on the loading pattern, and confining stress, and under-estimated by currently proposed coefficients.
Critical state analysis was used for evaluating the cyclic resistance of the samples under different cyclic load patterns and the results were correlated to shear wave velocity parameter of samples. In addition, liquefaction susceptibility of the samples was evaluated and compared to previous research studies based on bi-directional cyclic tests, which suggests that current procedures may overestimate cyclic resistance of the soils based on shear wave velocity measurements.
Moreover, the behaviour of samples was investigated for multiple liquefaction occurrences by applying a second cyclic load to specimens after the occurrence of a first liquefaction. Densification of samples due to repeated cyclic load application was observed and evaluated. The cyclic resistan ce of samples was assessed in consecutive liquefaction events. Results show a general decreasing resistance for loose samples which is more significant for mono-directional load patterns rather than bi-directional patterns. The resistance changes of medium dense samples from first to a second liquefaction is negligible and almost consistent for all load patterns. For dense samples, there is a distinction in the behaviour of samples under mono-directional shear patterns and bi-directional patterns. Although the majority of samples get more resistant to a second liquefaction under mono-directional loads, more complicated behavior was observed for dense samples as well as medium dense samples in large consolidation stresses under bi-directional cyclic loads. It was found that when applied shear loads get larger, the resistance of the samples decreases (becomes more liquefiable); inversely, when shear loads get smaller, the behavior changes to an increasing resistance and less liquefiable response. Pore water pressure generation of the samples was also studied. The results showed the quicker development of pore water pressures during first cyclic loading compared to the second cyclic loading stage for almost entire cyclic load patterns regardless of soil initial relative density. However, pore water pressure generation was found to be dependant on initial consolidation stresses.
Mirbaha, Keyvan, "Bi-directional Cyclic Behaviour and Liquefaction Analysis of a silica-carbonate sand" (2017). Electronic Thesis and Dissertation Repository. 5035.