The increasing use of TBMs requires improved ground investigation. Pilot galleries can be used for a large scale ground investigation and have other beneficial effects for the main tunnel excavation. Examples of successful pilot galleries and of setbacles are given. The stability of the tunnel face influences strongly the selection of an adequate tunnelling method. The assessment of the stability at the tunnel face and behind and, when required, its improvement are discussed.
L'utilisation croissante de tunneliers necessite des reconnaissances de terrain ameliorees, Des galeries de reconnaissance servent à l'investigation du terrain à grande echelle tout en produisant d'autres effets benefiques pour l'excavation du tunnel. Des exemples de galeries de reconnaissance reussies et moins reussies sont donnes. La stabilite du front du tunnel influence fortement la selection d'une methode de construction adequate. L'evaluation de la stabilite du front et de la section courante du tunnel et, en cas de besoin, l'amelioration des conditions sont discutes.
Die zunehmende Verwendung von TBM erfordert verbesserten Baugrund-Aufschluβ. Probe- oder Pilotstollen dienen dem groβmaβstablichen Aufschluβ und erweisen sich auch sonst fuer den Tunnelvortrieb vorteilhaft. Die Standsicherheit der Brust beeinfluβt die Wahl einer geeigneten Tunnelbaumethode maβgebend. Die Voraussage der Standsicherheit der Brust und des laufenden Tunnelquerschnitts und, sofern erforderlich, eine Verbesserung der Bedingungen werden diskutiert.
For underground works of a certain volume, decisions on the envisaged construction method must be made long before the start of the excavation. Modifications during the works are either impossible or, at least, costly and time consuming. These decisions concern a variety of fields:
method of excavation, whether by drill and blast or mechanical (backhoe, rock hammer, roadheader, TBM);
full face excavation or subdivision of the section;
tunnel support, in particular the allowable distance between the face and the support, need or not of a face support, to mention but the most important ones.
Contrarily to structural engineering projects such as buildings or bridges, the available information about the ground conditions for a planned tunnel excavation remains limited, particularly for deep tunnels. Investigations by boreholes and geophysical survey are often restrained to the portal zones and few deep boreholes. Therefore the forecast of the geological profile and of the rock mass stability around the tunnel is affected by considerable uncertainty. Effective help for decision making can be provided by the construction of a pilot gallery, sufficiently long before the start of the main tunnel works. Recently developed risk analysis codes allow the quantification of the geological-geotechnical risks and of their incidence on cost and duration of the tunnelling works, for a given construction method. When this analysis is carried out for various potential construction methods, their relative sensibility to the geological-geotechnical risk can be compared (Dudt and Descoeudres 1994).
The construction of pilot and investigation galleries, rather limited to exceptional cases for a long time, has been considerably augmented with the increased use of TBMs. In fact, whereas unforeseen adverse geological" conditions impair tunnelling by conventional methods more or less severely, they may definitively stop an excavation by means of a TBM. Galleries preceding the excavation of the main tunnel are generally multipurpose constructions: First of all, they serve to investigate the geological conditions at a large scale, thus minimising the related risk for the main tunnel excavation. Field tests and long term observations can also be carried out in pilot galleries, providing more reliable geotechnical parameters then cored boreholes. This was already emphasised more than 30 years ago by Pacher (1964) who explicitely proposed deformation measurements in pilot galleries as a means to investigate the rock mass behaviour and to design the tunnel support.