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Session 2: Foundations

CONFERENCE PREVIEW

Rab Fernie, Cementation Foundations Skanska Foundations can be on the boundary of the half space (pads, rafts and footings) or within and part of it (pile anchors and plates).

While the keynote lecture to Session 2, by Jamiolkowski, Randolph and Zdravkovic, tends to concentrate on the former, five out of the eight papers concentrate on the latter.

History, internationalism, analyses, close observation, monitoring and the pragmatic solving of small and large scale problems are addressed. Such a grouping would have delighted Skempton.

Compare the evolution of soil mechanics foundation practice as described in Powderham and Peck's paper on the Chicago Auditorium - a structure completed around 1890 - with the numerical and physical modelling employed by Yang and Peck to validate the as yet uncompleted Rion Antirion Bridge in Greece and revel in the connecting author - Peck.

The Chicago Auditorium is a seminal building in the development of soil mechanics, with its soft clays and heavily loaded footings. It moved 300mm by the time it was built, has moved a further 400mm in over 100 years, yet remains serviceable. It works because the farsighted designers understood the need to provide ductile structural elements.

The designers of Rion Antirion have many more tools at their disposal to forward predict soil structure behaviour and on this site they need them. In deep water on soft ground, in a seismic region with large spans, predicted high (geometric and) impact loadings and scour potential, the bridge needs best modern practice to validate proposals.

The paper concentrates on the benefit of piling to foundation piers, on rotational and translational loadings behaviour and shows the power of modern numerical and physical (centrifuge) modelling to illustrate and define foundation behaviour.

The observational method, a driver in modern soil mechanics research (and its funding), and is well illustrated in the practical papers from Brazil by Remy and China by Xie et al.

The Brazilian paper describes the forensic detection of piping as a cause of failure in non-cohesive fill, fed by water from decaying bamboo drainage pipes and its dramatic consequences. It looks at the use of close monitoring and control of parameters in assessing the efficiency of drainage and grouting remedial works.

In the half space, piles jacked into soft ground and occupying 1.5% to 2% of the building's footprint area, result in considerable lateral displacement (heave).

The Hang Zhou Chinese experience of closely monitoring large scale sites aids prediction of the effects of pile jacking and allows assessment of control measures.

Measures included controlling the process, ie the speed and placement geometry, and damping displacement by introducing relief holes and ditches.

The UK paper by Clark et al on wall pressure with the paper by Rampello et al from Italy on pile groups also illustrates how 'process and previous works' affect the ground.

The wall pressure paper uses results from elegantly calibrated spade cells which show how short term (and intermediate) wall pressures are closer to the active pressure conditions than the 'at rest' condition.

They illustrate how this arises from the stress relief set up from placement of the embedded walls (in this case contiguous bored pile walls) and confirm, for stiff overconsolidated clays not previously well characterised in UK, similar observations to that for other wall types in differing ground. Economy in construction can immediately be foreseen, with possible long term benefits.

Bored piles in the half space below a power station in Italy allow a great many interesting conclusions to be drawn.

The half space consists of over 20m of settling waste overlying soft peaty then silty, then stiff over consolidated clays. The development of negative skin friction on the upper sleeved piles is noted. The importance of relative shear stiffnesses is recognised and the single verse group pile behaviour explored.

The well monitored and calibrated site allows numerical analyses comparisons to be made with actual behaviour and sufficient attention is paid to soil and structural stiffness to feel some confidence in results.

Off-shore work has been a modern driver to geotechnical engineering, bringing new problems and new insights to soil mechanics. This is well-illustrated by papers from work in Ireland by Lehane et al and in Japan by Akai and Tanaka.

The Irish paper reports on the group behaviour of piles under single direction cyclical tension loading and follows on from some thorough work on load application rates, amplitude and cyclical degradation of pile groups.

The model testing on a five pile group in the well calibrated Belfast Sleech powerfully illustrates the difficulties in extrapolating single pile results (cyclic degradation) without this sort of research.

Kansai airport, 5km offshore from Osaka, built on 33m of fill and (mainly) completed by December 1991, has settled by about 11m to date. It is built over soft sea deposits >150m thick. A hyperbolic settlement model of this material combined with a probabilistic approach to predict a 50 year settlement of about 14m to 15m.

Observation and quality monitoring means this site has and will have much to teach us now and in the future.

Under the fourth topic of stiffness characteristics, a paper by Leung and Ng examines the inherent anisotropy of the small strain stiffness of decomposed tuff and alluvium, using bender elements transmitting in different directions across a sample.

The decomposed tuff shows significant inherent stiffness anisotropy, whereas the stiffness of the alluvium is isotropic; a difference that they attribute to the role of soil structure.

Ismael, Ferreira and Fahey present a new experimental set-up for measuring small strain stiffnesses, claiming a number of advantages over bender elements.

They then show how cement type influences the stiffness anisotropy of cemented carbonate sand. For example, while gypsum cement increased the initial stiffness anisotropy of the untreated soil, calcite cement removed the initial anisotropy and both Portland cement and epoxy cement reversed the initial stiffness anisotropy.

Cavallaro, Maugeri and Ragusa performed resonant column tests and cyclic loading torsional shear tests on a cohesive soil from Sicily, to investigate the variation of shear modulus and damping ratio with shear strain amplitude.

This was done as part of an earthquake-resistant design for restoration and strengthening of an historic military airship hangar.

Tika et al describe resonant column tests showing the influence of fines content on the shear modulus and damping ratio of sands. Meng and Rix present an alternative to conventional resonant column tests or torsional shear tests, which they claim is better suited to examining the influence of strain rate on dynamic properties such as shear modulus and damping ratio.

Their non-resonance technique can be used over a wide range of frequencies, whereas the resonant column test and torsional shear test are restricted to relatively high frequencies and relatively low frequencies respectively, which may not be representative of field applications.

Brown and Richardson describe the development of a hollow cylinder apparatus specifically intended for studying the cyclic stress-strain behaviour of dry granular materials in the context of wheel loadings on pavements and rail tracks, while Naughton and O'Kelly used a hollow cylinder apparatus to investigate the stress-induced anisotropy of Leighton Buzzard sand under generalised stress states.

Of the remaining papers that do not fit easily into the four topics, two, by Santamarina and Cho and by Clayton, Theron and Vermeulen consider the interesting subject of the influence of particle shape on mechanical behaviour.

Trzcinski, in his paper, describes use of a software package for SEM-based quantitative analysis of soil microstructures, which provides information on the areas, perimeters, dimensions, shapes anisotropy and degree of orientation of solid particles and of pores.

Cui et al describe single and double oedometer tests on samples from loess deposits along the Paris-Lille TGV route, investigating the susceptibility to wettinginduced collapse compression.

They evaluate various existing collapsibility criteria for loess soils, and conclude that only one of the criteria gives a reasonable estimate of the collapse susceptibility of their samples.

They also present microstructural investigations of the loess samples before and after wetting, which indicate that collapse was associated with compression of the larger pores. Another paper by Eliadorani presents a comparative study of liquefaction and instability of sand.

Two final papers deal with issues relating to hydraulic properties and transport phenomena.

Hird and Srisakthivel describe a programme of laboratory research investigating the performance of a self-boring permeater intended for measurement of vertical and horizontal permeabilities in the field.

They found that self-boring permeater measurements performed in large samples of natural Oxford Clay gave permeability values substantially higher than from conventional permeability tests on small samples (a difference that they attributed to the role of soil macro-fabric). They also reported no evidence of smear effects or cavity collapse with the self-boring device.

Pivonka and Smith emphasise that permeability and transport of dissolved ions are not only dependent on pore morphology (porosity, pore size distribution, tortuosity) but also depend on the concentration of ions in the pore fluid, the permittivity of the pore fluid and the distribution of electrical charge on the solid particle surfaces.

They present a unified description of various transport phenomena within the framework of electro-hydrodynamics and develop the fundamental governing equations. They demonstrate successful numerical application to mass transport through an electrically charged slit opening.

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