Research Stay of Ting-Wei Wu

    During the last two months of the year 2018, I would like to thank Dr. Antonio Cattaneo to host my stay at the Ifremer (Institut français de recherche pour l'exploitation de la mer in Brest/Plouzané). A new project to study about the “seismic strengthening” behavior of saturated Ottawa Sand 20/30 was initialized from this secondment.

    Seismic strengthening is the increase of sediment’s strength due to densification after earthquake events that are not strong enough to cause failure. This hypothesis can be simulated in the laboratory by measuring the increase of undrained shear strength after cyclic loadings and drainage. The study of undrained shear strength is important because it is directly relevant to slope stability analysis. The outcome will, for example, improve the understanding of the initiation (or no initiation) mechanism of submarine landslides under earthquakes.

    Together with my co-supervisor Dr. Nabil Sultan, we developed a testing procedure that utilized load-controlled undrained cyclic triaxial tests in anisotropic conditions with different initial relative densities and confining stresses. After imposing seismic loadings without making a specimen fail and subsequently draining the specimen with sufficient time to represent drainage after an earthquake, the increase of its shear resistance could be measured. We aimed to demonstrate the threshold between failure and strengthening under cyclic loadings, document the density increase after drainage, and eventually discover the trends of seismic strengthening of Ottawa Sand 20/30.

    It was very technically demanding to conduct cyclic triaxial tests, and all the steps needed to be handled with a lot of care in order to get good data quality. Eventually, 49 tests were done in these two months. It has been observed that anisotropy (the presence of initial static shear stress) is beneficial to liquefaction resistance, though this does not necessarily mean that the sample is very resistant against seismic shaking. If a significant cyclic amplitude makes the effective stress state come close to the critical-state, "strain failure" criteria can still be reached. Axial strain can still accumulate in the compression side to 5% or more with a nearly constant rate (plastic strain accumulation). More details will be demonstrated in the EGU General Assembly in 2019.

    Since this was my first project to do triaxial tests, I was not skilled at all in the beginning. I am very grateful to the great supervision and support I received during this stay. Most importantly, I would like to thank SLATE for funding this two-month research stay at the Ifremer. It was a great opportunity for me to widen my network as well as to get immersed into another kind of research topic and working environment through this stay in a foreign country.