Slopes: can centrifuge models contribute to improved design?

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• Professor Malcolm Bolton, Head, Geotechnical and Environmental Group, CUED
• Friday 08 May 2009, 16:30-17:30
• Engineering Department - LR6.

If you have a question about this talk, please contact Zelda Stuck.

Geotechnical engineering research can span from science (How does the ground behave?), through technology (How can we intervene to improve things?) to the formation of Codes of Practice (What is best practice?), and ultimately to the development of national policies (What is sustainable, and can we afford it?).

The failure of slopes can have a severe impact on local communities and regional transport corridors. Examples include mining waste heaps, and road or railway embankments. Two strands of centrifuge model research on the stability of slopes will be described.

The first relates to the catastrophic failure of uncompacted fill slopes subjected to heavy rainfall, which are often described as liquefaction flowslides. Examples include the 1966 catastrophe in Aberfan, Wales, which killed 144 people, and a number of failures in Hong Kong which used to take an annual toll of lives until their Geotechnical Engineering Office took the matter in hand. Centrifuge models conducted here by GEO engineers seconded as MPhil students cast doubt on “static liquefaction” as the most probable mechanism, and suggested an alternative. Of course, new ideas are not easily digested by those who have been successful applying older ideas. But it is nevertheless the role of the University to suggest new guidelines based on new thinking.

The second strand of research relates to the creep and progressive failure of clay slopes, especially relevant to road and railway embankments. The accepted mechanism of progressive failure is the fatigue-like growth of a shear rupture along which the soil strength has dropped from its peak friction angle for dilatant shearing to its residual friction angle for sliding on a polished surface. The average soil strength along the failure surface at the point of a delayed failure of this sort is generally found to be an intermediate value – given by the critical state friction angle for turbulent shearing. Adherents of the peak-residual concept regard this as a coincidence. Centrifuge tests conducted by Andy Take in an atmospheric chamber indicated a quite different mechanism, in which a large region of overconsolidated clay beneath a slope could incrementally soften towards its critical state angle of friction every time it had to mobilise any more than that value in a succession of wet and dry seasons. This naturally points to the critical state angle as the appropriate design value. This new evidence has proved to be controversial, and the work has only recently been approved for publication subject to minor revisions. The main contribution to good design may be clarity of thought.

The relevance in the field of new mechanisms found in a laboratory can always be challenged. And the question of whether practise and policy can respond to research insights always remains open. But the drive to simplify and verify our understanding of geotechnical engineering remains a strong motivation.

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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