Undrained dynamic response of fully and partially saturated sands through dynamic simple shear test device with confined pressure

Liquefaction induced seismic hazard is still a major problem in geotechnical engineering. Sand boils, liquefaction induced settlements and bearing capacity failures create damages for both human life and structures. At this concern, various liquefaction mitigation techniques have been improved. Altering the fully saturated condition in loose sands to partial saturated is one of the recent liquefaction mitigation techniques that is being developed. Partial saturation is induced by air entrapment inside the voids. In this method, liquefaction resistance of loose fully saturated specimens can be increased by decreasing degree of saturation of the specimen. This study aims to investigate undrained dynamic response of fully and partially saturated sand specimens. VJ Tech brand dynamic simple shear with confining pressure (DSS-C) testing system was used in this research to simulate field conditions in the laboratory. The testing system was established in Geotechnical Earthquake Engineering Laboratory of Civil Engineering Faculty in Istanbul Technical University. Initially, the working mechanism of DSS-C testing system was studied comprehensively. Clisp Studio software program was used to perform the tests. The test setups include; saturation, isotropic consolidation and undrained cyclic loading liquefaction tests. The best specimen preparation methods were examined for both fully and partially saturated sand specimens by using an unconfined membrane. Dry pluviation, wet pluviation and moist undercompaction methods were performed for fully saturated sand specimen preparation. Moist undercompaction method was chosen as the most suitable technique for this research to get loose, undeformed and fully saturated sand specimens for undrained dynamic loading tests. Secondly, partially saturation was achieved by generation of gas bubbles inside the voids. Gas bubbles were created by using sodium percarbonate powder. The partially saturated sand specimen preparation methods were examined in two sections, which were preparing sand-chemical mixture and preparing water-chemical mixture. The best specimen preparation method was chosen as preparing water-chemical mixture and wet pluviation of dry sand into this mixture to get loose and undeformed partially saturated sand specimens at various degrees of saturation. Several undrained cyclic loading strain controlled tests were performed on fully and partially saturated sand specimens. During the test program, change in effective vertical stress and excess pore pressure generation with time and shear stress-shear strain behavior of the fully and partially saturated specimens were determined. Excess pore pressure generation in fully and partially saturated sand specimens were compared under different shear strain levels at 15, 50 and 100 number of cycles. Furthermore, the effect of effective vertical stress, degree of saturation and back pressure on excess pore pressure generation in partially saturated sand specimens were investigated. The results demonstrated that the liquefaction resistance of partially saturated sand specimens increases by decreasing degree of saturation, decreasing back pressure and increasing effective vertical stress. Although the excess pore pressure generations are lower in partially saturated sand specimens compared to fully saturated specimens, it was observed that eventually the excess pore pressure ratio (ru) reaches to 1.0 in partially saturated sand specimens, contrary to the published results in large sand specimens tested, where ru stabilizes at maximum values less than 1.0. This difference in small specimen tests can be attributed to the escape of oxygen bubbles from the short samples, resulting in increasing degree of saturation as cycling continues. Therefore, the behavior in small specimen tests should be only examined for low number of cycles. This research was funded by TUBITAK (The Scientific and Technological Research Council of Turkey) under the grant No: 213M367 and project title of ''Dynamic Response of Sands Mitigated by IPS (Induced Partial Saturation) under New and Existing Structures''. The research results were used to improve the project goals. Additionally, this study will give a contribution to next literature researches to determine the undrained dynamic response of fully and partially saturated sands through dynamic simple shear tests with confining pressure also to improve Induced Partial Saturation (IPS) method as a liquefaction mitigation technique against liquefiable soils.