In long, deep tunnels, it is not possible to completely investigate the ground conditions along the future alignment in advance. Unexpected ground conditions and fault zones thus have to be accepted as residual risks and overcome by suitable measures. This is also the situation at the Gotthard Base Tunnel (GBT), which is 57 km long with overburden depths of up to 2.350 m. The main contracts included items for systematic advance probing from the tunnel and provided a catalogue of suitable additional measures. These measures were intended to detect and overcome unforeseen ground conditions.During the advance of the GBT, grouting measures had to be called on in three cases. In two of these cases, a trapped TBM had to be freed using ground improvement measures (Amsteg West and Faido West). In the south drive of the Sedrun west bore, water inflow into the tunnel had to be reduced to prevent damage to the nearby Nalps dam caused by draining of too much water from the rock mass. The common experience from all three events and the resulting conclusions are described.The AlpTransit Gotthard AG is the client for the new Gotthard rail axis through the Alps, including the Gotthard und Ceneri Base Tunnels. The company is a subsidiary of Swiss Railways SBB founded in 1998 and employs about 180 employees at the head office in Lucerne and the branch offices in Altdorf, Sedrun, Faido and Bellinzona.Bei langen, tiefliegenden Tunnelbauwerken ist es mit wirtschaftlich vertretbaren Mitteln nicht möglich, den Baugrund längs der künftigen Achse vorgängig vollständig zu erkunden. Unerwartete Baugrundverhältnisse und Störzonen müssen deshalb als Restrisiko akzeptiert und mit geeigneten Maßnahmen beherrscht werden. Auch beim 57 km langen Gotthard Basistunnel (GBT) mit Überlagerungshöhen bis zu 2.350 m war diese Ausgangslage anzutreffen. In den Werkverträgen der Hauptlose waren systematische Vorauserkundungen aus den Vortrieben und ein Katalog geeigneter Bauhilfsmaßnahmen vorgesehen. Mit diesen Maßnahmen sollten unvorhergesehene Baugrundverhältnisse erkannt und gemeistert werden.Während des Vortriebs des GBT musste in drei Fällen auf Injektionsmaßnahmen zurückgegriffen werden. In zwei Fällen galt es, eine blockierte TBM mittels gebirgsverbessernden Maßnahmen freizulegen (Amsteg West und Faido West). Im Südvortrieb der Weströhre Sedrun mussten die Wasserzuflüsse zum Tunnel derart reduziert werden, dass eine Beschädigung der nahe gelegenen Bogenstaumauer Nalps, als Folge zu großer Wasserentnahmen aus dem Gebirge, verhindert werden konnte. Die gemeinsamen Erkenntnisse aus allen Ereignissen und die daraus zu ziehenden Schlussfolgerungen werden dargestellt.Die AlpTransit Gotthard AG ist Bauherrin der neuen Eisenbahn‐Alpentransversale Achse Gotthard mit den Basistunneln am Gotthard und Ceneri. 1998 gegründet, beschäftigt die Tochtergesellschaft der SBB am Hauptsitz in Luzern und an den Außenstellen in Altdorf, Sedrun, Faido und Bellinzona rund 180 Mitarbeitende.
In the Faido section of the Gotthard Base Tunnel (GBT), two open gripper tunnel boring machines (TBM) with a bored diameter of 9.4 m are each driving about 11 km of single-track tunnel. No extensive fault zones were expected in the entire section except for the Piora-Mulde. A special tunnel was driven to investigate the Piora-Mulde, from which investigation boreholes were drilled down to tunnel level. Because of the finding of sugar-grain dolomite, the tunnelling work was tendered by drilling and blasting as well as mechanically, but the tenders led to the award being based on TBM tunnelling.In the course of construction, a stretch some hundreds of metres long unexpectedly had to be bored through squeezing rock, where the temporary support was destroyed and the West TBM and backup almost became jammed. This article describes the experience from the mechanical tunnelling of the Faido section and then gives details about the intended support, the design of the TBM, the difficulties that arose and the experiences gained. ProjectContractually, the Faido section is combined with the Bodio section. After the two open TBMs had completed the Bodio section between 2002 and 2006, they were refurbished in the assembly cavern at the Faido multi-function railway station (MFS) for further tunnelling towards Sedrun. As had already been laid down in the construction contract for the Faido tunnel section, the intention was to increase the cutterhead diameter from the Bodio section of 8.8 m to 9.3 m. On account of the experience with TBM tunnelling in the Bodio section and the excavation of the MFS Faido, where very squeezing conditions were encountered in places, and the fact that hydraulically extendable overcutters did not work on the Bodio section, the decision was made to increase the bore diameter to 9.4 m. The geological conditions encountered at the TBM starting area were flatly bedded and relatively weak Lucomagno gneiss. The overburden at starting was 1,500 m and increasing to a maximum of 2,500 m. It was decided to use movable gauge cutters for an achievable bore diameter of 9.5 m. This was intended to create sufficient room for displacement and for support measures.The design of the cutter head draws on experience from the Raron-Ferden drive of the Loetschberg Base Tunnel. The cutterhead is flat and closed and fitted with
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