Date of Award
Master of Science
Earthquakes are natural calamities that occur as a result of sudden release of strain energy stored in fault planes. Earthquakes have been observed to cause huge damage to infrastructures and lives. Earthquakes result in development of fissures, abnormal or unequal movement of foundations, and loss of strength and stiffness of the soils. Liquefaction is attributed as a major cause for the loss of strength and stiffness of soil during earthquakes. In the past, liquefaction was attributed only to coarse-grained to medium-grained sand and was extensively studied but the fine-grained soils were generally considered as non-liquefiable. However, from observations during recent earthquakes, fine-grained soils having low plasticity (plasticity index (PI) <20) have experienced ground failures due to liquefaction or large deformations. Moreover, laboratory experiments show that not only saturated cohesionless soils but also fine-grained soils may liquefy if certain criteria are met. One of the parameters which influences the liquefaction characteristics of fine-grained soils is its plasticity. This study may become helpful in understanding the effect of plasticity on liquefaction resistance of fine-grained soils. The objective of this study were to investigate the (1) effect of plasticity on pore pressure built up and deformation characteristics of fine-grained soils, and (2) effects of cyclic shear stress on liquefaction resistance of fine-grained soils. A total of 24 tests were conducted using a stress controlled cyclic triaxial testing machine on identically prepared specimens at an initial effective confining pressure of 5.0psi. The plasticity index (PI) was varied from non-plastic (NP) to 14.53. Sil-Co-Sil #40, a non-plastic commercial silt (product of US Silica Company) and EPK Kaolin clay (product of Edgar Minerals Inc.) were used as base materials. These materials were mixed in different proportions to obtain desired plasticity index. Out of the twenty-four tests, eleven tests were conducted on clean silt samples. Among the tests on clean silt samples, four tests were conducted on specimens having a post consolidation void ratio of 0.74 to 0.76. Further, six tests were conducted on specimens having a post consolidation void ratio of 0.74 to 1.04 by using a cyclic stress ratio (CSR) of 0.2 and 0.25. Seventeen tests were grouped to study the influence of plasticity on liquefaction characteristics of fine-grained soil. The PI of specimens tested ranged from non-plastic (NP) to 14.53. Each of the specimens with a definite PI was tested at an initial confining pressure of 5.0 psi using a CSR of 0.2, 0.3, and 0.4. The results obtained from the tests were used to compare the effects of plasticity on liquefaction characteristics of fine-grained soils. Based on the limited tests conducted, it was observed that plasticity index had distinct influence on the cyclic strength of the samples. It was found that CSR required to cause a pre-determined strain in a given number of loading cycles reduces as the plasticity index increases from non-plastic (NP) to 3.46, but increases for soils having PI greater than 3.46. Moreover, the liquefaction resistance decreases with the increase in cyclic shear stress for all soils regardless of plasticity indices (PIs). The critical PI value corresponds to 15% of EPK clay content in the specimen which gives a PI of 3.46.
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