Increasing attention is being focused on the problem of submarine slope stability. The global economy requires a better, wider reaching infrastructure, able to quickly distribute resources, such as oil and gas, from the point of production to the points of use. An expanding network of telecommunication cables cross the ocean floors and breakages caused by slope failures would interrupt or delay the flow of information around the world. Large submarine slides can cause tsunamis, which can be devastating for the coastal communities in the wave's path. This research contributes to the understanding of failure mechanisms for submarine slopes in soft marine sediments subjected to dynamic loading. One of the primary goals is to develop an extensive database of soil behavior in simple shear, specifically targeting offshore clays from the Gulf of Mexico and off the coast of West Africa. The experimental results are obtained using a new advanced testing device, which has been specifically constructed for multidirectional loading at a wide range of frequencies. The testing is used to characterize the response of the soils subjected to different initial and drainage conditions, and a variety of loading paths, replicating more closely the effects of earthquakes or storm waves. The importance of strain rate on the behavior of soft clays is also investigated. The test results are essential for the refinement and validation of a constitutive model, able to replicate the response of the marine clays, and implemented in a finite element program for the analysis of submarine slope response. The importance of the different factors contributing to the triggering of failures is investigated through parametric studies. The predictive tools are validated through the study of selected case histories. The broader impacts of this work include increasing awareness among students and the general public of how earthquakes affect soils, and how ground shaking is also changed by the soils. Undergraduate students are involved in building visualization tools targeting users at different levels of ability, using the model developed by the graduate students. The activities also involve development of educational materials integrating research into the teaching of math and science, and the training of teachers, through a Research Experience for Teachers Program and in collaboration with the South Texas Rural Systemic Initiative. The research is introduced in courses at the undergraduate and graduate level. The PI, in partnership with the Forth Worth Museum of Science and History, is developing an exhibit on natural hazards.
The interest in the offshore environment has been increasing in the last decade. The global economy requires a better, wider reaching infrastructure, able to quickly distribute resources, such as oil and gas, from the point of production to the points of use. An expanding network of telecommunication cables is also crossing the oceans floors to keep us connected. Finally, offshore wind and wave energy have lately started to be considered as viable options for decreasing our reliance on fossil fuels. Submarine landslides can cause substantial damage to our offshore infrastructure and are possible triggers for tsunamis, which can be devastating for the coastal communities on the wave’s path. Earthquakes are the second most important triggering mechanism and account for approximately one third of submarine slope failures. The project developed a new laboratory testing device for soils to apply complex loading paths that can replicate more closely the effects of earthquakes and other cyclic processes such as waves. The experimental component of the project was a comprehensive characterization of the response of Gulf of Mexico clay with the newly developed device, as well as other methods. The results clearly show the importance of including the effect of the slope in any analysis. These findings have important implications for the stability of the slope, predicting that forces acting downward in the slope direction will need to mobilize less deformation to reach peak strength and initiate failure. Additionally, sloping ground subjected to two-directional loading, such as circles and figure-8s, may accumulate large strains and fail even with minimal generation of excess pore pressures, which are generally understood as a necessary component for failure to occur. The response of clays to multi-directional loading is very complex and the current framework of interpretation may fall short of explaining the observations. The other thrust of the project was focused on developing a constitutive model which could describe the response of clays in multi-directional simple shear. An existing model was modified and expanded to handle the accumulation of plastic strains and the generation of excess pore pressures during cyclic loading. It was also very important that the model was able to capture the effect of the slope (i.e., anisotropy) on the response of the soil. The cyclic multi-directional simple shear tests presented an additional challenge because of the difficulty in defining load reversal points. The modifications provided a reasonable agreement between model simulations and experimental results. An investigation of the response of particulate materials in simple shear was added to the project thanks to a graduate supplement, which then led to a Graduate Research Fellowship to the doctoral student involved. The goal of this component of the project was to assess internal mechanisms of interaction among particles by comparing boundary measurements from experimental results with Discrete Element Method (DEM) simulations. For this purpose, actual specimens were assembled using steel ball bearings and virtual specimens were created by replicating the experimental procedures as closely as possible. Tests were carried out with the new device and then the results were used to validate numerical simulations. Preliminary results suggest that a more effective contact model may be needed to describe the inter-particle interactions. According to the simulations, when a specimen is subjected to monotonic loading in one direction, the particles actually displace sideways in the direction normal to the loading. This indicates the importance of considering multi-directional shearing to assess the response of granular materials in conditions that are more representative of actual shearing in the field, for example during earthquakes. The project has been particularly successful in providing opportunities and a supportive environment for students. The project supported three doctoral students, two of which are women and both are currently assistant professors. Master students (one female) associated with the project continued their studies towards a PhD. Six undergraduate students worked on the project over the years. Of these, three were women and two men were Hispanic. Four continued their studies and obtained graduate degrees. The PI hosted four high school teachers in her research group over two summers through a Research Experience for Teachers (RET) program in the College of Engineering. Her commitment to reaching out to K-12 teachers and students also extended to informal activities in the local schools. The PI provided inspiration for other students outside her group and was able affect real change within her department by leading the creation of a program specifically devoted to training and supporting future academics.
