The Gotthard Base Tunnel (GBT) was inaugurated in 2016. The 57-km long railway tunnel connects northern to southern Europe, enabling passenger and goods transport to reduce travelling time by one hour between Zurich and Milan. It is the world’s longest railway and deepest traffic tunnel and the first flat, low-level route through the Alps.
The primary purpose of the Gotthard Base Tunnel is to increase transport capacity through the Alps, especially for freight, notably on the Rotterdam–Basel–Genoa corridor. A more specific objective is to shift freight volumes from heavy goods vehicles (HGV) to freight trains to reduce the environmental damage caused by HGV significantly.
The Gotthard Base Tunnel mainly consists of two single-track tunnels connecting Erstfeld with Bodio. It is part of the New Railway Link through the Alps (NRLA) project, which also includes the Ceneri Base Tunnel further south (opened in 2020) and the Lötschberg Base Tunnel (opened in 2007) on the other main north-south axis.
Two interesting figures about the Gotthard Base Tunnel construction are outlined below:
- Impact of tunnelling on arch dams
In the Surselva region, three arch dams and hydropower reservoirs are located almost directly above the new GBT, approximately in the middle of the tunnel. The height of the concrete arch dams varies between 117 m for Santa Maria, 127 m for Nalps and 153 m for Curnera.
In 1978, the driving of an exploratory tunnelling gallery had adverse effects on the arch dam of the Zeuzier reservoir in the Alps, causing significant settlements of up to 13 cm. Well aware of this, tunnelling engineers already started surveying the area four years before tunnelling began in the region.
- Tunnelling challenges in squeezing rocks
Engineers had to face challenging tunnelling conditions in squeezing rock, in the Sedrun section, in the geological section of Tavetsch Intermediate Massif North (TZM North), due to poor rock quality with low strength, squeezing properties and an 800 m thick overburden, and additionally in the Faido section, with the contact zone between the Leventina and the Lucomagno gneiss formations, under an extremely thick overburden exceeding 2,000 m.
2. Impact of GBT tunnelling on arch dams
2.1 The Zeuzier dam case history 
The 156 m high and 256 m long Zeuzier arch dam was built between 1954 and 1957 on the Lienne, a right-bank tributary of the Rhône in the central part of the canton of Valais. The basin is closed by a limestone rock lock, deeply cut by a narrow gorge (Figure 5).
During routine checks in the autumn of 1978, a slight deviation of the deformations from their usual value was observed. The dam appeared to be moving upstream. The cause of this abnormal behaviour was initially sought in the weather conditions in the current season, which turned out to be warmer than usual. However, at the beginning of 1979, when successive control measurements were performed, it became clear that the phenomenon had intensified and that differences in deformations were now totally significant; they could in no way be explained by exceptional heat conditions or inaccurate measuring. Immediately, as the level was already being lowered due to the facility’s regular operation, the order was given to drain the basin quickly.
Similarly, the driving of the exploratory gallery for the planned Rawyl road tunnel was stopped. As the reservoir level was lowered, the deformation of the structure progressed upstream. Geodetic measurements showed that subsidence had occurred in the basin-shaped area, causing a convergent inclination of the two banks which resulted in the closing of the valley (Figure 6). This convergence movement wedged the dam and caused it to deflect upstream; it was precisely this deformation that monitoring pendulums had been detecting for some months. The movements, which were extremely rapid at the beginning, slowed down progressively and were almost exhausted after 6 to 8 years.
The total settlement at the dam can be estimated at 13 cm (Figure 6) and the closing of the valley at 7.5 cm; the deflection of the dam reached 12.5 cm upstream at the keystone.
To explain the origin of these movements, various hypotheses, such as tectonic dislocations, have been put forward. Although the tectonic or seismic assumption may quickly come to mind, since it is well known that the central Valais region is subject to a certain amount of activity of this type, it cannot be denied that the real reason for these movements was an entirely different one. The extreme intensity of the movements at the beginning and their progressive and extremely regular exhaustion over the years seemed to indicate that they were independent of any seismic activity, which was minimal and did not vary significantly either during the period preceding the construction or during the construction or operation of the dam.
At that time, studies were being carried out for a road tunnel to be built under the Zeuzier region between the cantons of Valais and Bern (Rawyl tunnel). Findings from geological and hydrogeological surveys convinced the engineers that the tunnel construction would almost certainly face severe difficulties due to high groundwater inflow. It was also feared that the groundwater inflow could be directly linked to the Zeuzier basin, which is only 400 m above the tunnel.
As an exploratory gallery was being built during the autumn of 1978 and the beginning of 1979, huge amounts of water flowed through small faults, reaching hundreds of l/s and peaks of up to 1000 l/s. At that time, the driving face of the gallery was more or less in the area of the dam; the settlement process began at that precise moment and intensified rapidly, causing the above-mentioned disorders and deformations (Figure 6). Work on the gallery was finally stopped in March 1979.
According to the hypothesis retained by experts , the Dogger bedrock (limestone) contained a captive water table that had been released. The Dogger seemed hydraulically connected to the Rawyl gallery via secondary faults. Water inflow occurred through localized open faults in the last part of the gallery. It was assumed that each fault corresponded to a rock compartment which was more or less independent of neighbouring compartments from a hydraulic point of view.
Of all the mechanisms considered, the drainage by the Rawyl gallery of the captive water table included in the Dogger formation was the one that matched best – and even perfectly – the observations made as it did not contradict any of them.
The exceptional event at Zeuzier brought to light a critical phenomenon, which had been generally neglected until that time, i.e. the settlement of rock mass following a decrease in pore pressure. This phenomenon was confirmed many years later by the precision levelling carried out in the 1990s over the Gotthard Pass and through the Gotthard road tunnel. Surface settlements of up to 12 cm were determined, which were the result of water drainage during the construction of the road tunnel .
2.2 Monitoring strategy for GBT
As early as 2000, i.e. four years before this area was undercrossed, a comprehensive geodetic monitoring system had been installed. The idea behind this was to constantly observe movements in the area, e.g. abutments of arch dams and dam crests alongside specific points in the lateral topography and general configuration of the area.
The following strategy was pursued by the client ATG (AlpTransit Gotthard AG):
1. Acquire the necessary basic knowledge to be able to recognise and handle any hazard in good time by taking suitable measures:
- Set up a large-scale measuring system to automatically monitor ground deformations throughout the year
- Perform annual precision levelling on a large-scale network
- Develop refined theoretical principles and numerical models and ensure continuous calibration and further development.
2. Define suitable construction measures during tunnelling:
- Define a catalogue of suitable exploratory measures to identify any risk of water ingress in good time
- Record any water ingress systematically
- Define a catalogue of suitable injection measures for sealing the rock mass
- Provide suitably skilled staff and specialist installations to handle accidents
2.2. Results of measurements
The data obtained made it possible to detect natural surface deformations as early as four years before undercrossing the arch dams. It provided clear evidence that seasonal fluctuations in the mountain water levels can cause reversible surface deformations in a fissured rock mass.
Based on these findings, statically permissible dam deformations were defined for the Nalps and Santa Maria arch dams: maximum widening of 20 and 40 mm, respectively, maximum closing of 90 and 100 mm, measured in the middle of the dam at the dam crest.
The concept of „net dewatering“ has been introduced by planning engineers. It is based on the observation that there is a correlation between the gross and net amount of water taken from the rock (gross: the total amount of water, net: due to tunnel construction) and the volume of the surface settlements. It was then possible to determine the maximum permissible amount of water flowing into the tunnel in a given section and a given period without causing damaging surface deformations.
A maximum permissible dewatering volume of 456,000 cubic metres was determined for the Nalps dam for every 500 m of excavation length achieved in 12 months by applying the „net dewatering“ concept. During excavation work near the Nalps arch dam, water inflows of 13 l/s were measured in the western tunnel, and after two months, water inflows of 7 to 8 l/s could still be measured. This inflow was too high and had to be reduced to 3 l/s with appropriate sealing measures (grouting), which took 100 working days. In May 2005, the effective valley closing was measured in the Nalps North cross-section where the arch dam was. The maximum crest movement out of the valley was determined subsequently: about 10 mm, constant from mid-2007 to mid-2009. During blasting, settlements could already be detected 500 to 1,000 m before the tunnel face. In 2010, when TBM driving was carried out from the south, it was possible to detect another closing of valley flanks with a maximum horizontal crest movement of up to 19 mm out of the valley. After tunnel excavation, the highest rise in settlements only reached 1,000 m.
Located about 1 km north of the Lukmanier pass summit, the Santa Maria dam is significantly more distant from the tunnel alignment than the Nalps dam, with a horizontal distance of about 2.5 km. A widening of the valley had to be expected due to the subsidence trough caused by tunnel excavation. Based on model calculations and experiences at the Nalps dam, the Santa Maria dam was undoubtedly liable to “suffer” from TBM excavation carried out from the south.
Finally, the strategy chosen for the Santa Maria dam was the observational method, as all experts were convinced that it would not be possible to remedy the expected, albeit minor, water ingress in the tunnel with reasonable effort. In addition, all structural measures on the dam had become part of the emergency measures to save costs. In the event of dam deformations, it was, therefore, necessary to define possible structural measures for each level of risk. Various criteria were used for this purpose: the tunnel advance status, the development of large-scale deformations and local deformations at the dam.
As expected, settlements, shifting and valley widening took place at the arch dam between 2010 and 2011. With settlements of about 9 to15 mm, a 5-to 8-mm widening could be observed. A permanent 8-mm widening at crest height was predicted for 2015.
Long-term deformations estimated by using a model calculation from 2013, then corrected with values measured in 2015, produced the following results (for the Curnera dam, it appeared that no significant adverse effect could be determined due to greater distances):
|Dam||Max. absolute settlements||Max. differential settlements||Max. closing|
|Nalps||62 ± 5||15 ± 2||20 ± 2|
|Santa Maria||12 ± 3||5 ± 2||16 ± 2|
3 Squeezing rock: challenges during excavation
Squeezing rock challenges were expected in the following 2 zones:
- In the Sedrun section, in the geological section of Tavetsch Intermediate Massif North (TZM North), due to poor rock quality with low strength, squeezing properties and a 800-m thick overburden,
- in the Faido section, with the contact zone between the Leventina and Lucomagno gneiss formations, under an extremely thick overburden of more than 2,000 m.
According to geologists, extreme difficulties were to be expected during excavation work taking place in these 2 areas.
The first issue was the quality of the rock at TZM North where kakiritic and heavy cataclastic rock formations alternating with hard schistose gneiss were to be expected on a tunnel length of 1.45 km. The overburden was approximately 800 m thick.
The second challenge was posed by a section where the mountain overburden exceeded 2,000 m, and the rock had begun to squeeze due to unloading caused by excavation. This phenomenon intensified especially north of the Faido multifunction station (MFS) in the transition zone between the Leventina gneiss to the Lucomagno one.
2.2 Mitigation measures at TZM north (Sedrun)
The excavation of the two single-track tunnel tubes at TZM North was executed in full-face cross-section by conventional tunnelling using drill and blast and by mechanical excavation using a hydraulic hammer. In addition to the basic installation, a suspended tunnel drive was used for the large cross-section of up to 13 m diameter in squeezing rock sections.
The tunnel face was supported by 12 m long self-drilling grout anchors and thick mesh-reinforced shotcrete lining. The length of an excavation cycle varied between 1.00 and 1.33 m. Depending on rock conditions, deformable TH-44 steel arches (Bochumer Eisenhütte system) were placed every 33 cm, 67 cm or 1.00 m. About 22 IBO self-drilling grout anchors of 8 m length per running metre were used. Average deformations in the cross-section were 25 cm, while asymmetric deformations reached 85 cm. Altogether 940 to 1450 metres of tunnel were driven in squeezing rock.
2.3 Mitigation measures at Faido MFS
In the western tube, the drive had to pass through one of the two fault zones in the Lucomagno gneiss. Deformations could not be stabilised despite a circular cross-section and the use of HEB180 steel arches and a 40 cm thick concrete lining. After nine months and 160 metres of drive, work had to be stopped. With a larger diameter (12 m) and a flexible lining with TH arches, anchors, shotcrete and lining slits, approx. 125 m of tunnel were driven a second time. In total, radial deformations of up to one metre were recorded.
 FGU Fachgruppe für Untertagbau, Dokumentation SIA D 0222, Swiss Tunnel Congress 2007 – Fachtagung für Untertagbau
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