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Session 3: Slopes and embankments

CONFERENCE PREVIEW

Edward Bromhead, Kingston University IfSkempton had never published anything except his Rankine Lecture, that vital contribution to the development of slope stability engineering would given him a secure place in the geotechnical hall of fame.

However, he did so very much more, even in the limited area of geotechnics that is the topic of this session, and contributions have both individually and collectively picked up on themes that have long been identified with Skempton's research.

It is difficult to elicit all the highlights in a short review.

However, as a recent convert to Quantitative Risk Analysis methodologies, I found the paper by Lacasse et al particularly fascinating. The QRA approach provides a framework within which the whole of conventional geotechnics can fit, but it is essentially probabilistic in nature, which makes the paper by Chowdhury et al of relevance.

De Mello and colleagues also touch on risk analysis fleetingly. I was uncomfortable with their terminology, and believe that what they describe as a 'congenital malformation' is not, in fact, a defect. The issue of seepage forces versus pore water pressures was elegantly treated by Cedergren (1967) and in several places since, and in my view was more fully understood by Bishop than by Terzaghi, not vice versa.

A second area in which I have also converted my views in recent years relates to 3D analysis. This conference includes a paper on 3D in limit equilibrium (myself and Peter Martin) and one on continuum methods and 3D (Sainak).

There is no doubt that for firsttime failures in slopes, continuum methods have the potential to provide the analyst with great insights. Kovacevic et al have done this magnificently, complementing Potts et al's classic 1999 paper in Geotechnique, and theirs is a deserving keynote paper in the session.

Liao & Chen provide detail on some horrific failures in dipslopes in Taiwan, and the extent of slope instability in Italy is highlighted by the papers in this session from Italian authors, notably Urcuioli and Picarelli, Santaloia et al and Sciotti and Calabresi. Kaczynski's discussion of the problems of slope stability in the Polish Carpathian foredeep are on a more UK-relevant scale.

Four papers in this session relate specifically to railways:

O'Brien et al deal with embankment failures and my own reconsideration of the earliest account ofa railway cutting failure in the annals of the ICE. Since the slopes in question are old, these papers also bear on historical engineering works and practices.

Two other papers relate to pore water pressures in infrastructure embankments in general (but mostly to railway embankments) and consider the measurement of pore pressures (Ridley et al) and the influence of permeability in modelling the effects of cyclic, ie weather-driven, pore water pressures (Nyambayo et al). Take and Bolton have also considered the variable groundwater problem, through centrifuge modelling.

The work of the Naples geotechnical community has provided great insights into the behaviour of slow-moving mudslides and other slides under oscillating groundwater regimes. Urcuioli and Picarelli have further developed their methodologies in their fascinating contribution.

When in 1957 Skempton and Delory picked up on Haefeli's (1948) simplified analysis for slopes and applied it to slopes in the London area, they were making a tentative entry into the field of engineering geomorphology.

It took Skempton's Rankine Lecture enunciation of the importance of residual strength and Hutchinson (1967) to refine Skempton and Delory's work, but thereafter, understanding of the mechanics of landscape formation was to be an important development in geotechnics allied to geomorphology. Papers in this field at this conference include those by Urcuioli and Picarelli, Santaloia et al, Sciotti and Calabresi, and Kaczynski.

Skempton's investigations into the properties of stiff clays, include not only his Rankine Lecture but also his work on other discontinuities in stiff clays, memorably published with Petley in 1969.

Sciotti et al have explicitly furthered this understanding in their paper and the issue of the strength of stiff fissured clays is central to, or figures prominently in, papers by Kovacevic et al, Kaczynski, Cornforth, and myself.

Apart from his f=0 paper in the 1948 Rotterdam ISSMFE and a brief note on the effect of side friction on slides in his 1985 paper on residual strength, Skempton appears to have left developments in slope stability analysis technique to his colleagues Bishop, Morgenstern and Sarma.

Sarma has produced an interesting approach to the finding of a critical slip surface location in limit equilibrium analysis. I am confident such an approach is essential in applying limit equilibrium methods to the firsttime failure problem in 3D and believe this paper will be recognised as a pioneering first step.

Of course, in a continuum method, the critical slip surface 'finds itself', and thus the paper by Sainak shows that way. My paper with Peter Martin uses a 3D limit equilibrium technique to reanalyse a landslide which had been successfully stabilised a quarter century ago: 3D analysis remains the exception rather than the rule, and of course, even using 2D analysis, a successful result can be obtained.

Finally, Blight's paper introduces the problem of the failure of slopes in solid waste and mine waste, a field that is of increasing importance worldwide.

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