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Multi-directional simple shear response of soils

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Increasing interest in submarine slope stability, especially related to seismic loading, was the motivation for the development of a new multi-directional simple shear device. Submarine landslides can involve vast areas and considerable depths. A number of the failures takes place on slopes inclined 5 or less and typical soil profiles consist of almost parallel layers of normally consolidated clay. The submerged slope under earthquake loading can be modeled using simple shear conditions to describe the response of the soil. Most experimental results for fine grained soils concentrate on one-dimensional response, both for monotonic and cyclic tests. In addition, experimental information on marine clays mostly comes from marine deposits that are now onshore and easily accessible, such as Boston Blue Clay and San Francisco Young Bay Mud; however, these deposits have a different stress history than offshore deposits.

The new multi-directional simple shear device developed by Dr Biscontin and her students allows loading along three independent axes, two perpendicular horizontal directions to allow any stress or strain paths in the horizontal plane, and a third in the vertical direction. The device is also fitted with a pressure vessel providing the ability to control cell pressure and backpressure independently, in a manner similar to triaxial systems, for truly undrained tests. The experimental program focused on sample of clay from the Green Canyon in the Gulf of Mexico. The goal of the investigation was to study the effects of anisotropy, and multi-directional loading in particular, on the monotonic and cyclic response of normally consolidated Gulf of Mexico clay.

The test results were used to develop and validate constitutive models. Even if simplistic, the SIMPLE DSS formulation can handle the generation of excess pore pressure as a by-product of stress-strain-strength relationships that require a small number of basic parameters. Examples of predictions and comparisons with test data will illustrate the potential of the SIMPLE DSS model. Different scenarios for slide initiation will be identified and predictions of slope response analyses with a one-dimensional finite element code implementing SIMPLE DSS will be presented. Modelling challenges for multi-directional loading will be presented.

Finally, an experimental program testing steel ballotini in simple shear was paired with discontinuous element method (DEM) simulations for validation. The goal of this program is to provide conditions as similar as possible during all the phases of the physical and numerical processes. This allows us to be confident that the information obtained at the particle level from the simulations is reliable because backed by the macroscopic validation. The simulations, validated by the experimental results, provide invaluable information on the micro-mechanical response of granular materials.

This talk is part of the BPI Seminar Series series.

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