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Engineering in Sediments

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If you have a question about this talk, please contact Lois Salem.

Our civil infrastructure of buildings, roads, railways, tunnels and pipelines is mostly carried on, or within, uncemented sediments, i.e. soils. Such soils may be mapped by a geologist as “solid” such as the Gault clay or London clay, or “drift” such as the Fenland silts or the terrace gravels. They may even be artificial, such as embankments or simply urban detritus. The geotechnical engineer is taught to treat them on their own merits as a melange of ceramic fragments which represent our cheapest construction materials. They are taught to classify them and test them mechanically with a view to substantiating design calculations which are intended to ensure that whatever soils are encountered on the site of a project, they should more or less “stay put” after construction, continuing to support the facility for perhaps a human lifetime. Geologists take a broader perspective in time and space and become aware that soils, and even rocks, endlessly transform and shift around, whether man intervenes or not. They learn of all the natural influences – tectonic, gravitational, fluid, thermal, chemical and biological – that cause all this transformation and mobility. Geologists know that a human lifetime is a pathetically short period, and that the apparent permanence of our facilities is illusory.

These differences in perspective lead to difficulties in communication and comprehension when geologists try to assist engineers in the planning and execution of large projects. But all knowledge can be valuable, if its significance can be recognised. The talk will introduce two geotechnical issues that illustrate the need for engineers and geologists to be better aware of each other’s discipline. First, there is the question of the mobility of pipelines laid on the deep ocean bed for the purposes of recovering oil from even deeper reservoirs. Here, engineers would do well to have considered the possible practical implications of bioturbation in the formation of these clay sediments. Second there is the issue of the mobility of clay slopes, variously referred to in geological literature as solifluction or slope creep, which can affect natural slopes, cuttings or embankments. Here, geologists would benefit from knowledge of the critical state friction angle, and its role in marking the boundary between conditions which are stable and unstable. These illustrations may provide the grounds for some continuing discussion and debate.

This talk is part of the Sedgwick Club talks series.

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