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Tunnelling and underground space development

ICSMGE Plenary session 3

Alain Guilloux is president of French geotechnical consultant Terrasol, which he founded with Francois Schlosser in 1979. He is also professor of soil mechanics and tunnelling at the Centre des Hautes Etudes du Beton Arme et Precontraint in Paris.

Specialising in tunnelling, deep excavations, trenchless technology, slope stability, reinforced soil and construction on soft ground, he has combined teaching with a career in practice, and has written over 60 papers. Guilloux is on the board of the French Committee of Soil Mechanics and Foundation Engineering and is a member of ISSMGE's TC28 technical committee on underground construction.

The theme lecture of this session reviews the main developments in tunnelling, focusing mainly on the last 10 years, and gives an overview of the foreseeable future developments.

These develop- ments have been largely driven by clients' desire to use underground space more intensively, sometimes in very severe ground and environmental conditions.

The report is organised according to the four steps involved in any tunnel project, but recognises the large degree of crossover and interaction between the various stages:

l Data collection, mainly ground investigations l Design of the project l Methods of construction l Monitoring Ground investigations The relatively few major developments in ground investigation are related to the following:

l The almost systematic use of insitu testing, allowing determination of more continuous ground parameters than those derived from laboratory testing l Surface and borehole geophysics for better evaluation of drilling data l Deformation characteristics (modulus), which has become necessary where the prediction of ground movements due to tunnelling is one of the major design criteria l The widespread use of ongoing investigations, performed from the tunnel face, including those projects using tunnel boring machines.

Computer technology is of course most helpful for the analysis and synthesis of the very large amount of data gathered in any tunnel project, from preliminary up to on-site investigations.

New developments have begun to widen investigation capabilities, for instance directional drilling, which can investigate up to few kilometres along a given route, or with monitoring of a TBM response, analysed in terms of ground conditions.

Design of the project This is one of the fields which has seen dramatic changes in the past decade.

One key change is from analysis based on failure mechanisms to methods based on the actual interaction phenomena, including the comparable responses in deformation of the ground and of supporting structures. The convergenceconfinement method (Panet, 1995) can be considered one of the pioneers. These include the three-dimensional effects due to the proximity of the face, by considering that the loading and deformations of support are mainly dependent on the distance between the face and the support.

Of course, computer tech- nology, together with the development of more realistic constitutive laws of soil behaviour and numerical methods (such as finite elements or finite differences methods), was one of the most important factors in enabling more accurate prediction of the real stress-strain analysis of tunnelling. Both two-dimensional models, with consideration of the face effect, or real threedimensional models are now available, which permit a relatively good analysis of the various phases of construction involved in modern tunnelling methods.

Face stability and analysis of deformations ahead of the tunnel face has also seen many developments in the past 15 years, albeit not in all ground or hydraulic conditions. These developments are linked to the widespread use of TBMs, where the confining pressure is aimed to ensure face stability and to reduce the induced deformations.

With these tools it is possible to predict ground deformation due to tunnelling (especially important in urban areas) with reasonable accuracy, with the proviso of adequate soil parameters from the ground investigation.

Furthermore, the effect of ground deformations on existing structures, which is related to the type and stiffness of buildings, has also been investigated with good results, but further work is needed to understand this strong interaction completely.

Methods of construction There have been significant developments in tunnelling methods in recent years, through both technological developments of boring equipment and better knowledge of the mechanisms involved in ground response to tunnelling.

The first major step was by Rabcewicz (1965), with the New Austrian Tunnelling Method (NATM), which originated the use of shotcrete and rock bolting. Although sometimes used in a different way than the original concept, the method is still widely applied, and many impressive projects, including in harder ground, have proved its efficiency.

In the field of conventional (or sequential) methods applied to difficult ground conditions and to large excavations, there have been a number of developments:

l Progressive evolution from 'horizontally divided sections' where the excavation of the lower halfsection often causes large deformations through ground failure due to the load acting on the vault support, to 'vertically divided sections'with arched sections formed over the full tunnel height.

l More recently (Lunardi, 1995), the use of true full-face tunnelling has become widely used, even for very large tunnels with face areas greater than 150m 2. Such methods were made possible, in difficult ground conditions, by the use of systematic ground treatment ahead of the face known as 'prelining' or 'pre-confinement' methods, such as fibreglass longitudinal bolting and grout umbrellas.

In the field of mechanised excavation, the development of TBMs since the 1980s has allowed construction of very long tunnels such as the Channel Tunnel between France and the UK, or tunnels in very adverse ground conditions (for example loose and water bearing soils in urban areas). There were also impressive advances in:

l Better control of the face pressure, either with slurry, earth pressure balance or compressed air techniques, with the help of additives (eg polymers) lBetter control of the void closure behind segments, which is a major way of limiting ground deformation l More multifunctional machines, for use in varied ground conditions, including ground where different materials occur in the face (eg from sands to clays or soft rocks) l Increase in the power and therefore the diameter of tunnelling machines, which can now be up to 15m in diameter.

Thanks to these developments, the control of deformations caused by tunnelling can be considered as solved in most cases. Techniques exist that allow a given settlement target to be reached, either by 'conventional' methods or TBMs. But it must be kept in mind that an objective of a few centimetres of settlement for a shallow tunnel has a high cost in severe conditions.

Compensation grouting, which is a relatively recent development (Mair, 1998), has developed some interest, as it does not aim to reduce the deformations at the source (around the tunnel face), but instead compensate the effects of these deformations on sensitive areas.

Monitoring Monitoring of excavations has become a specific part of tunnelling, with the difficult ground conditions and tight deformation specifications on many projects.

Despite advances in ground investigation and the design and construction of tunnels in recent times, excavation is still complicated as geology and soil behaviour remain difficult to predict.

Although monitoring methods have changed to some degree (deformations and, with less success, stresses, are measured with almost the same tools) the main changes in tunnelling practice have been the more widespread use of these measurements and the fact that they are used in real time to adapt the excavation works.

Increased computer power has been a major factor in faster data gathering and analysis. For example, compensation grouting could not be carried out with the efficiency it is used today.

Other new developments have begun to emerge, such as analysing TBM response as a tool to investigate what happened around the machine (Aristaghes,2000). This is of major interest in an area where the engineer is often working 'blind' References Aristaghes P and Blanchet V (2000). CASTBY signal aid boring - Sydney experimentation. Proceedings Inter- national Symposium IS '99 on Geotechnical aspects of underground construction in soft ground, Tokyo. Balkema Eds.

Lunardi P (1995). L'importanza del precontenimento del cavi in relazione ai nuovi orientamenti in tema di progetto e construzione di gallerie. Gallerie e grandi opere in sotterraneo 45 Panet M (1995). Le calcul des tunnels par la methode convergence-confinement. Editions Presses de l'ENPC.

Rabcewicz L and Von Slatter K (1965). Die neue Osterreischische Tunnelbauweise (the New Austrian Tunnelling Method).

Bauingenieur, Ann. 40, Vol. 8.

This theme lecture was prepared with Professor R Kastner of INSA Lyon.

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