Cyclic response of Gulf of Mexico clay from the Walker Ridge
A significant number of offshore, nearshore and onshore structures are founded on soft clays. Both the structure and the foundation soft clay generally must withstand a combination of non-standard cyclic environmental loads, gravity loads and seismic loads. The importance of these structures, be it from a safety, economic or environmental point of view, warrants an understanding of how their foundations and the foundation soils behave under cyclic loads. When a fine-grained soil (e.g., a soft clay) is subjected to cyclic loading under undrained conditions, its structure gets progressively destroyed. The consequences of this are a progressive decrease in the cyclic shearing resistance, a build-up of cyclic-shear induced excess porewater pressure, and a decrease in the shear modulus. These three effects collectively are known as undrained cyclic degradation. For offshore soft clays, undrained cyclic degradation poses a significant hazard to infrastructure such as oil platforms, towers, pipelines, cables, and subsea or floating systems. This research used laboratory and in-situ tests on Gulf of Mexico (GOM) clay from the Walker Ridge to draw insights on the cyclic response of soft clays in general. The main objectives of this research were to (1) characterize GOM clay in terms of its index properties, compressibility and monotonic shear behavior, and compare it to other soft clays studied in the literature, (2) propose a simple semi-empirical predictive relationship for its small-strain shear modulus, and extend it to soft clays in general, (3) propose a new framework to quantify the cyclic shear behavior of GOM clay and other soft clays in terms of the cyclic shearing resistance, shear modulus and excess cyclic shear-induced porewater pressure, (4) investigate how the cyclic response of reconstituted GOM clay differs from the intact clay and, (5) investigate its post-cyclic drained and post-cyclic undrained behavior of GOM clay. The main findings of this research are described below: 1. GOM clay from the Walker Ridge area is similar to terrigenous soft clays from the US in terms of its index, compressibility and strength properties; 2. Based on laboratory bender element tests on GOM clay and an extensive literature review on the small-strain shear modulus of soft clays, simple predictive tools are proposed that could allow the seismic geotechnical practioner to estimate Gmax for a soft clay; 3. The cyclic degradation of soft clays is inherently difficult to quantify because it is influenced by a number of factors, which is why the cyclic response of soft clays has been historically described graphically. Combining the laboratory cyclic test results on GOM clay and an extensive literature review on the cyclic behavior of soft clays, a simple framework is proposed that could quantifiably describe the cyclic response of soft clays. These findings will be valuable to geotechnical engineers involved in foundation cyclic and general seismic design in soft clays. 4. Using the proposed framework for the cyclic response of soft clays, the cyclic response of reconstituted GOM clay is shown to be quantifiably different from the cyclic response of intact GOM clay. Such an insight could be helpful to the geotechnical engineer tasked with the cyclic foundation design of a structured clay. As the growing scale of the oil and gas industry pushes for construction in deeper seawaters, there is a need to characterize deep-water soft clays and to understand how they behave under cyclic loading. Unfortunately, deep-water offshore clays are notoriously difficult to sample, which has led to offshore cyclic geotechnical design to be rather conservative and the technical literature on them to be rather limiting. Insights from laboratory cyclic testing of offshore soft clays will not only augment the available technical literature on the cyclic characteristics of offshore soft clays but will, in the long term, improve the design process and lead to increased economy.
cyclic response of clays, cyclic triaxial testing, cyclic degradation of clays, Gulf of Mexico clay, reconstituted clay cyclic behavior, post-cyclic behavior of clays, bender element testing of clays, small-strain shear modulus of clays
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign