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Session 1: Soil behaviour, characterisation and modelling


Simon Wheeler, University of Glasgow In the two keynote lectures Chandler, de Freitas and Marinos will speak on Geotechnical characterisation of soils and rocks: a geological perspective; while Jardine, Gens, Hight and Coop will cover Developments in understanding soil behaviour.

Another 28 papers assigned to the session theme are included in the proceedings; eight will be presented at the conference.

Chandler, de Freitas and Marinos first review the areas of site description they consider need further research: material properties, particularly those of boundary layers (of individual particles or of larger bodies of soil or rock);

the geological history of sites and the soils and rocks they contain;

and the content of ground investigations designed to provide data for numerical analyses.

They then concentrate on appropriate frameworks for characterising the engineering behaviour of soils and rocks, particularly the Sensitivity Framework for soils and the Geological Strength Index for rocks.

Jardine, Gens, Hight and Coop first review developments related to the principle of effective stress, as applied to saturated and unsaturated soils. They then provide reviews of recent developments in four key areas where Skempton had earlier made important contributions:

lcompressibility and soil structure lshear strength of stiff clays lshear strength and yielding of soft clays lstiffness characteristics.

Most of the papers in the session relate to these four topics.

The dominant theme of nearly all papers in the session is the influence of the structure of natural soils on mechanical and hydraulic behaviour; a topic of great interest to Skempton and one to which he made many significant contributions.

Under compressibility and soil structure, a paper by Paul, Barras and Mein shows how the geotechnical characterisation of estuarine clays can be related to geological history.

The authors emphasise the importance of whether the history is of rising or falling sea level, and illustrate this with two UK sites, Bothkennar on the Forth and Lockham on the Humber, lying north and south of the isostatic hinge line.

Sills et al present research on sedimentation of soils from suspension and subsequent selfweight consolidation. They emphasise that effective stress history does not uniquely determine void ratio for a given soil;

for example the void ratio corresponding to zero effective stress (transition from a suspension to a soil) is highly dependent on factors including time and deposition rate. Layering of sedimented soils (in terms of void ratio) can therefore be caused simply by fluctuations in deposition rate, without any variation of sediment type.

Field data from three projects in Japan showing the influence of secondary compression (creep) on the long-term settlement of marine clays are presented by Aboshi. The data, covering more than four decades in one example, suggest creep does not continue indefinitely at a linear rate in a conventional semi-logarithmic plot against time (C adecreases with time).

Under the second topic of shear strength of stiff clays, five papers relate to aspects of residual shear strength.

Marui and Tiwari show how the residual friction angle f9 res isrelated to clay mineraology, while Tuladhar, Marui and Tiwari present research showing the significant influence of pore fluid salt concentration on f9 res (increasing salt concentration produces an increase of f9 res but the scale of the effect is critically dependent on clay mineral type).

The latter may be of crucial importance, as leaching (perhaps concentrated along fissures) may lead to a significant reduction in residual strength. In their paper, de Mello, Sadowski and Nieble show low values of residual strength (f9 res less than 15infinity) for samples taken from layers of quartz-biotite-schist lying between fresh granite-gneiss in Brazil.

They attributed this low residual friction angle to very strong schistosity or textural anisotropy in the quartz-biotite-schist (due to intense quartz grain flattening and to selective dissolution of interstitial quartz grains from the matrix due to differential weathering).

Papers by Lemos and Vaughan and by Fearon, Chandler and Bommer cover fast shearing to a residual state in the ring shear apparatus after previously shearing to a residual condition at a slower rate.

Both groups of authors show the possibility of either positive or negative rate effects (where the final fast shearing residual strength can be either higher or lower than the preceding slow shearing residual strength).

They attribute a positive rate effect to the development of turbulent shear during fast shearing (a return to a critical state condition from the previous slow residual condition corresponding to laminar shear). Fearon, Chandler and Bommer demonstrate that a negative rate effect can occur at very high displacement rates when free water has access to an undulating shear surface.

Shimizu presents direct shear box tests on two different weathered Japanese pumice soils showing shear strength is enhanced by the natural structure of the soils (samples tested at the same void ratio but with the natural structure partially damaged by previous isotropic compression showed lower values of strength).

Similarly, Baranski examines the influence of soil structure on the mechanical behaviour of Polish glacial tills (by testing undisturbed and reconstituted samples), while Picarelli and Olivares emphasise the influence of fissures (an aspect of soil macrostructure) on the mechanical behaviour of fissured clays and clay shales.

On the third topic of shear strength and yielding of soft clays, Karstunen and Koskinen present an elasto-plastic critical state constitutive model for soft clays (S-CLAY1S) incorporating both anisotropy (including changes of anisotropy caused by plastic straining) and destructuration (progressive degradation of inter-particle bonding during plastic straining).

They demonstrate the performance of the model by showing simulations of multi-stage triaxial stress path tests performed on undisturbed and reconstituted samples of Murro soft clay from Finland.

Similarly, Liu and Carter present an evaluation of the Sydney Soil Model (SSM), an elasto-plastic constitutive model for natural clays and sands incorporating destructuration and also limited plastic straining ('sub-yielding') for stress states inside the main structural yield surface. Model simulations are compared with laboratory test results for Emmerstad clay (a sensitive Norwegian marine clay) and dense Cambria sand.

A final paper related to soft clays is by Rivera-Constantino, Juarez-Badillo and Villa-Escobar.

They present some improved empirical equations used to fit both pre-peak and post-peak stress-strain curves and pore pressure-strain curves from undrained triaxial shear tests performed on undisturbed samples of Mexico City clay.

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