The Sörenberg Tunnel runs at 1,500m above sea level by the village of Sörenberg in the municipality of Entlebuch, Canton of Lucerna, central Switzerland.
Its total length of 5,203m consists of a straight north-south run of 4,654m, a curved segment (R=300m) of 273m and the final, straight stretch of 275m. A constant gradient of 4.87% runs upwards from north to south and its maximum finished circular cross-section is 3.8m dia.
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The tunnel was excavated with a Herrenknecht full-face shield TBM with a 4.6m diameter. The lining is a circular shell made of precast concrete segments reinforced with 25cm-thick Bekaert metallic fibres RC 65/60 BN.
The 150mm annular space between the lining’s extrados and the excavated periphery is filled with cement mortar in the lower section and with pea gravel in the upper section to ensure effective drainage of the tunnel.
The completed tunnel will house a continuous 1,200mm diameter pipeline for transporting gas beneath the mountains. It will also have: a gangway for small vehicles, with lateral ducts to carry the water drained from the tunnel; an antideflagration lighting system; a ventilation system to enable inspection of the tunnel and to disperse and diffuse the gases which might seep from the rock mass into the tunnel and; a grounding system running along the arch to prevent negative effects of the possible presence of wandering electrical currents and for protection against lightning strikes.
Geological setting
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By GlobalDataThe tunnel runs through heavily deformed tectonic units, namely: Schlierenflysch – fine sandstones, siltstone and argillite, with subordinate conglomerates; Melange de Sörenberg: sequence of marlstone, sandstone, siltstone, argillite and limestone and; Globigerinenmergel, consisting of marlstone, calcareous marl and marly argillite.
Hydrogeology
The tectonic units along the tunnel are, on average, composed of not very permeable lithotype (argillite and marls). However, the presence of highly permeable arenaceous strata, which forms a part of the vast Nunalpstock-Hagleren syncline, may allow an influx of water and, consequently considerable seepage in the tunnel.The presence of methane gas in the Nunalpstock-Hagleren syncline rocks has been reported, probably originating from sediment – argillite, marls, calcilutites – containing organic matter.
Geotechnical model
From a geotechnical standpoint, the geological units along the tunnel are, “structurally complex” formations, because of a marked lack of lithological and/or structural homogeneity (AGI _ Italian Geotechnical Association, 1977).
Using the AGI classification system, geotechnical complexity can be categorised as:
All the geological and geotechnical studies were prepared by Geodata which also provided assistance during construction and contributed to resolving the technical problems met during construction.
Choice of construction method
Alternative methods of tunnel excavation were considered during the planning stage. These were:
Use of a TBM and precast reinforced concrete segments emerged as the best solution with regard to security and construction time and costs.
The lining ring
The lining of the Sörenberg Tunnel is a ring of 250mm thick reinforced concrete made up of six precast segments. Two of the segments are trapezoid shaped, with the remaining four shaped like a parallelogram.
This “universal” ring is able to follow the curve on the alignment as well as the straight course without needing special rings. Each ring is rotated around its axis in relation to the previously installed one, allowing it to follow any course, keeping the contact areas between subsequent rings on one plane.
The geometric property is called “conicity”, that is, the difference between its maximum and minimum length. The definition of conicity derives from the need to minimise the shift from the theoretical axis to the actual axis of the tunnel (the sequence of axes of the installed rings), which will inevitably occur during construction, so as to always guarantee falling within the maximum outline in reference to the theoretical axis of the tunnel.
The conicity was calculated using Autocad, which visualised the ideal theoretical sequence of rotated rings to make a best fit of the curvature found on the course. However, during actual installation the sequence may turn out to be different,because of the needs related to the actual progress of the excavation equipment.
Waterproof seals
A system of waterproofing was planned to cope with an external water pressure of 3bar. It was also designed to prevent the cement mortar seeping through the joints between segments during injection.
Longitudinal connectors
In assembling the rings, correct placement is ensured by longitudinal connectors together with guidance bars. These also minimise offset in coupling seals on successive rings, and guarantee that the seals remain appropriately compressed during construction and in use.
Construction site
The construction site was established after topographical studies beginning in March 2000. The main 23,000m² site near the northern portal, is about 3km south of the residential area of Fluhli. Because of lack of space in the narrow valley, the site is divided into two areas. The first area comprises offices, accommodation modules, canteen, car parks, areas for storage and stockpiling of excavated materials, as well as the rail depot and the pipeline depot.
The second area is allocated to all operations relating to the actual progress of the tunnel. It is located close to the portal and also contains foremen’s offices, workshop, and areas for material loading/unloading, lining-segment storage, and conveyor belt deposit. An in-depth study of the layout of the facilities and service structures formed part of planning of the construction site.
A 200 to 300mm-thick reinforced concrete slab was cast over all work areas to avoid interfering with the existing Transitgas pipeline.
The site near the south entrance, close to Sörenberg village, is dedicated to the exit and disassembly of the TBM and will have to house the pipeline’s final infrastructures. Work started in this area in July 2000.
Excavation problems
During excavation, problems were encountered with water inflows of 35 litre/s, damage to the lining segments and pockets of gas.
The first evidence of inflow showed that it did not constitute a real obstacle to progress, except for the need to adapt the purification plant (located at the north portal) to the new requirements.
The discovery of a few damaged precast lining segments was definitely a bigger problem.
A systematic study of all the installed segments was initiated. This called for surveys, measurements and tests in the tunnel, back analysis calculations, detailed analysis of all the processes, and verifying the cause of every type of damage.
It was found that the main cause of damage was the absence of pea gravel in the upper parts of the void between the excavation outline and the lining. This was caused by the presence of water which facilitated the adjustment of the granular material.
By refining the pea gravel emission and constant verification of the filling operations, the incidence of damage was drastically reduced to a level considered normal in this kind of work.
A systematic verification of the filling has been conducted along the entire completed stretch of the tunnel and where material was found to be missing, new material was added. Subsequent observations showed that the cracks had closed, a sign of a correct diagnosis and remedy.
The preventive measures to deal with numerous pockets of gas in the Schlierenflysch also proved to be ex-ceedingly successful. Machine advance was automatically halted whenever gas was encountered emanating from the rock mass, even at low levels. A quick removal or diffusion of the gas to safe values was effected within brief periods.
Given the frequency of these emissions, an unacceptable delay in advance could have ensued. However, improving the ventilation at the front of the excavation equipment proved to be the winning solution. This also boosted the emission of fresh air on the cutter head to maintain the atmosphere below alarm levels. In this way the excavation was able to proceed without further problems.
Construction performance
More than 5km of the Sörenberg Tunnel advanced at an average of more than 16m/day, with 508 work shifts necessary to excavate 5,133m with the TBM.
For the first part of the tunnel at the north portal, a 35m stretch was excavated using a conventional heavy-duty hammer drill in 14 days. This worked out to an average advance rate of 2.4m/day The initial stretch of the tunnel at the south entrance was excavated (also using the conventional method) in 13 days – an average advance of 1.9m/day. The final breakthrough by the TBM took place on 11 June 2001.
Conclusions
Considering the geologic and construction difficulties encountered during boring, the excavation of the Sörenberg Tunnel was accomplished in an extremely brief amount of time. This constitutes an excellent result. A few problems were encountered during construction which prevented the achievement of an even better performance.
Understanding the cause of the cracking of the lining segments, and remedying it in as short a time as possible, was the issue which demanded the greatest commitment in this job.
