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Session 4: Ground performance and building response

CONFERENCE PREVIEW

Chris Clayton, University of Southampton The session includes papers covering ground improvement techniques such as grouting, compaction, dynamic replacement, soil reinforcement and micro piles.

Other papers cover induced subsidence, the use of piles as settlement reducers and the effects of tunnel construction on adjacent foundations. Several case histories are described comparing predictions with field observations and the use of the observational method. The papers come from many countries, giving the session a truly international flavour.

The 16 papers presented illustrate the wide scope of geotechnical engineering. They can be very approximately grouped into four categories:

insitu tests and material behaviour;

ground improvement;

foundation behaviour and stabilisation; and lmonitoring and observation.

Material behaviour is covered in a series of papers on the stiffness anisotropy of London Clay;

on reinforced decomposed granite; on a novel lightweight geomaterial; and on the impact of pore fluid compressibility on soil behaviour.

Of particular interest are papers dealing with ways of improving the performance of foundations and slopes. These include permeation and fracture grouting, dynamic replacement, piling and under-excavation.

As Skempton would have reminded us, and as emphasised by the paper on under-excavation by Johnston and Burland, most of these techniques have a long history in civil engineering.

Improvement of soil properties by permeation grouting is considered in a paper by Anagnostopoulos et al and a useful case history of ground improvement using dynamic replacement is given by Wong and Lacazedieu.

In the paper by Santoyo and Ovando-Shelley on Mexico City's Metropolitan Cathedral, it is noted that under-excavation, as implemented at Pisa and in the stabilisation of various other old structures, is a corrective rather than a remedial measure.

At the Metropolitan Cathedral the differential movements resulting from very large settlements of soft clays and variable preloading by an Aztec temple complex were only temporarily halted by under-excavation. Fracture grouting has subsequently been used to improve the ground and reduce future movements.

Because the Mexico City clay is normally or lightly overconsolidated, insitu horizontal stresses are lower than vertical stresses.

Fracture grouting produces vertical sheets of stiffer material, capable of significantly improving the stiffness of the ground.

The behaviour and use of piles is covered in three papers. Russo, Viggiani and de Sanctis give a case history on the use of piles as settlement reducers for steel tanks. Kuwano et al use centrifuge tests to assess the impact of vertical and batter piles in supporting landslides.

Jacobsz et al report centrifuge testing results aimed at assessing the impact of ground loss caused by tunnelling on the bearing capacity of piles. The results are reassuring - it is concluded that for tunnels with controlled volume losses there should be relatively little reduction in base capacity, except where they are within one diameter of the tunnel or in difficult ground.

In their paper on the monitoring of piled foundations, Price and Wardle remind us that in many cases, predictions of pile behaviour are based upon simple global soil parameters which, although they may be fed into complex numerical analyses, may not provide good predictions of performance.

Without feedback on actual performance there can be no calibration of a design method and hence confidence in its predictions, for example of interaction between foundations, structures, and soil.

Indeed, in many cases the most efficient way to achieve certainty is to recognise uncertainty, adopt a flexible design, and monitor construction.

Such a situation is described in the paper on a trial excavation for the Dublin Port Tunnel, presented by Menkiti et al. Uncertainty about how to implement an observational approach to ground anchoring led to numerical monitoring and a full-scale trial, which gave confidence in a novel approach to support.

Chapman, Debble and Nicholson present a case record of the successful use of the observational method to simplify construction and reduce support of diaphragm walling, leading to a saving estimated as 5% of cost and 18% on programme.

In their paper on the subsidence of a multi-storey car park at Heathrow, Powderham et al provide an interesting case history of unexpectedly severe structural damage resulting from relatively small ground movements, suggesting vigilance and the value of monitoring even where the expected effects of ground movements are small.

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