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La prova di slump per determinare le condizioni ottimali di condizionamento durante lo scavo con EPB

Barla Marco Bonini M.
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Gallerie e grandi opere sotterranee
Gallerie e grandi opere sotterranee N.102/2012

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La prova di slump è di semplice esecuzione ed è stata usata da più Autori per la determinazione delle condizioni ottimali di condizionamento del terreno nel caso dello scavo di gallerie con TBM di tipo EPB. Questo articolo descrive i risultati di una campagna di prove di laboratorio effettuate al Politecnico di Torino che ha preso in considerazione terreni di caratteristiche granulometriche differenti.

This paper describes the experimental programme undertaken at the Diplab Geomeccanica of the Politecnico di Torino to investigate soil conditioning for EPB tunnelling by means of slump tests. The slump test is a simple empirical technique, usually used to measure the workability of fresh concrete, which has also been adopted for soil and foam mix by a number of Authors (EFNARC, 2005; Vinai et al., 2008). In TBM EPB tunnelling in fact, good soil conditioning allows to improve excavation production. Previous experience on laboratory testing of conditioned soil refers to simple techniques (such as the use of the slump test) or more advanced physical models where the screw conveyor element is reproduced in scale (AFTES, 2001, Yoshikawa, 1996; Bezuijen e Schaminée, 2001; Merritt e Mair, 2006; Vinai et al., 2008). The experimental programme described herewith focused on slump tests of three soils characterised by different grain size distribution curves. Tests were conducted by using commercial foaming agents (Globalfoam HG) and additives (Gloablcril46). Thanks to the different nature of the three soils it was possible to underline the effects of conditioning both on coarse- or finegrained soils, extending previous observations of other Authors (e.g. Vinai et al., 2008, Borio et al., 2011). In particular the work performed so far allowed to determine that: – for a higher concentration of fines in the soil, higher water content is needed for good conditioning; – reducing the quantity of the surfactants in the foam implies higher water content for good conditioning; – the Globalcril 46 additive is effective in increasing considerably the time for which good workability condition of fine-grained soil is maintained; – the presence of gravel reduces the area of good workability conditions, therefore the water content becomes a key issue, raising concerns for excavations below the water table. It is to be noted herewith that the experimental conditions and techniques adopted, due to their intrinsic simplicity, are not able to reproduce satisfactorily what effectively occurs in the TBM EPB excavation chamber. This is particularly true for that pertaining to the favourable effects on saturated soil conditioning of the stabilisation pressure acting in the chamber. More realistic experimental conditions may be achieved by the use of physical models of the screw conveyor as mentioned above. Despite this, the intrinsic simplicity of the slump test, combined with its low cost, encourage its use in engineering practice.