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The Swiss government was exploring ways to lay a railway line through the Alps for a long time. The idea was materialized in the year 1995 with the plan to construct the New Rail Link through the Alps (NRLA), a high-speed railway link that connected southern europe with northern europe.
The 57 km-long Gotthard Base Tunnel is a part of the NRLA project and is the world’s longest transportation tunnel. The tunnel consists of two different tubes of a single track with diameters varying between 8.5 to 9.5 m. The tubes are connected with the cross-passages at an interval of 312 m along the central axis of the tunnel.
In 1996, preliminary works for the construction of the access tunnels and the shafts were started. However, the main tunnel excavation work began in 2002 and the construction of the Gotthard Base Tunnel was completed in 2015. After finishing the railway installations and commissioning, the first commercial operation of the Gotthard Base Tunnel began in 2017.
Two multifunction stations were constructed at one-third and two-thirds distance along the central axis of the tunnel, and are located in the Faido and Sedrun sections. The purpose of these multifunction stations is to divert the trains, provide a path for emergency evacuation, and house technical infrastructure and equipment.
The Gotthard Base Tunnel is the longest tunnel with a maximum overburden of 2500 m. Most of the length of the tunnel is subjected to an average overburden of 1500 m. The Gotthard Base Tunnel is considered a civil engineering triumph as it was constructed with such a huge overburden pressure.
1. Geology of the Gotthard Base Tunnel
The following points describe the geology of the Gotthard Base Tunnel:
- The rocks encountered in the vicinity of the Gotthard Base Tunnel are gneisses, phyllites, and schist. The gneisses rock was mostly filled with the debris of soft soil.
- The thickness of the rocks varies from decimeters to decameters.
- The major part of gneisses rock was made up of kakiritic rocks (kakiritic rocks are made up of shear plane discontinuities along with the filling of soft ground material). In general, the term kakirite denotes a broken or intensively sheared rock, which has lost a large part of its original strength.
- Approximately 67% of the rocks were designated as kakirites. The rest of the 33% rocks were considered as good rocks only with hairline cracks.
- The cohesion and friction angle of the rocks were between 200-600 KPa and 250-300, respectively.
- The permeability constant of the kakiritic rocks was very low and it was in the range of 10-8 to 10-10 m/s. Thus, the possibility of seepage was significantly less.
- The ground along the alignment of the tunnel was classified into rock types based upon the shear fractures and lithology. This classification was used to describe the effect of tectonic disturbance on the rock mass.
- Six classes for the shear fractures are described in Table-1 and the lithological types of rocks are described in Table-2.
- The degree of shearing-1 represents very good rock, whereas the degree of shearing-6 represents fractured and very poor rock. Similarly, the strength and quality of rock decrease from lithological type-1 to lithological type-9.
- Mostly, the dip direction of the rocks was oriented in the north direction and are strongly influenced by shearing deformations.
|Degree of shearing||Description|
|2||Sporadic shear fractures, slickensides|
|3||Schistous and laminated rocks, mylonites, phyllites|
|4||Sheared, fractured rocks (portion of rock flour <10%, disturbed over <25% of the tunnel face surface)|
|5||Sheared, crumbly, friable rocks (portion of rock flour 10-30%, disturbed over >25% of the tunnel face surface)|
|6||Rocks with a portion of rock flour >30% and plastic consistency. It can be deformed by hand and disturbed over the majority of the tunnel face surface.|
|1||Pegmatites, amphibolites, quartzites|
|2||Quartz- and feldspar-rich gneisses, migmatites|
|4||Gneisses with a high content of mica, dolomites|
|5||Gneisses with a high content of schists|
|8||Kakirites (fault gouge)|
|9||Kakirites with high plasticity and high percentage of fines|
2. Tunneling Methods Used to Construct the Gotthard Base Tunnel
Two tunneling methods were used to construct the Gotthard Base Tunnel: Conventional tunneling method and Tunnel Boring Machines (TBM). These methods are described in detail in the subsequent section.
2.1 Conventional Tunneling Method
Construction of an underground opening using conventional tunneling comprises of the following procedure:
- Firstly, drilling and blasting is used to excavate the tunnel profile
- After that, the muck is removed using a mechanical discharging device
- Finally, tunnel lining elements are applied based on the ground conditions.
Each of the above steps is carried out in a cyclical process. The tunnel is divided into small segments and the same procedure is followed for each segment. An experienced team of tunnel workers, with the help of standard or special equipment, executes each individual cycle of the tunnel construction.
The advantage of the conventional tunneling method over other tunneling methods is the quick adaptability in the design of the tunnel in case of an adverse situation. Thus, the conventional tunneling method is preferred in highly varying ground conditions with existing infrastructure.
2.1.1 Equipment Used for Conventional Tunneling
The following standard set of equipment is required for conventional tunneling:
- Drilling jumbo is used for making blasting holes, rock bolt holes, and for releasing the air pressure
- Road header is used in cases where blasting is not permissible
- A lifting platform is used for allowing the workers to reach the crown and side faces of the tunnel
- Lifting equipment is required for placing the steel sets
- Loader or excavator machines are used for loading excavated muck onto dump trucks
- Dump trucks are required for hauling excavated rock outside the tunnel
- Shotcrete manipulators are required for applying the wet or dry shotcrete
2.2.2 Auxiliary Construction Technologies
If adverse rock conditions are encountered during the excavation of the tunnel, auxiliary construction technologies are used together with conventional tunneling methods. A few of the auxiliary construction technologies are explained below:
- Grouting: Consolidated grouting, pressure grouting, fissure grouting, and compensation grouting are used to reduce the collapse of ground in case of tunneling through very soft soils.
- Technologies were used to stabilize the ground and to enhance the rock ahead of the actual tunnel face, such as pipe umbrella, forepoling, ground freezing, horizontal jet grouting, etc.
2.2 Tunnel Boring Machines (TBM)
TBMs are generally used for excavating a circular profile for a tunnel. A TBM can work under varying ground conditions from very soft rock to very hard rock.
The most common procedures used for tunnel construction using TBM are explained below:
- A rotating cutter wheel is used to excavate the tunnel profile.
- After excavation, the excavated muck is removed using a mechanical discharging device.
- Finally, the support systems are placed to reduce the convergence of ground. The support system includes concrete lining segments, rock bolts, steel arches, shotcrete, and steel meshes.
The advantages of the TBM over conventional tunneling methods are discussed below:
- Tunnel construction using TBM can save construction time because of the higher advance rates.
- If the ground condition is not varying much, then the advance rates can be doubled.
- TBM is used to achieve a constant shape of tunnel cross-section throughout the tunnel length.
- TBM is very useful for the construction of longer length tunnels.
- TBM is used for projects with good accessibility
To read more about the working and operation of TBMs, click the link:
2.3 Criteria for Selection of the Excavation Method
Excavation methods for tunneling are selected based on the type of project depending on the following points:
- Safety and health requirement of workers handling the tunneling task
- Environmental aspects
- Need for future extension
- Design criteria of the tunnel
- Construction cost
- Construction schedule
- Involvement of third parties
- Obstruction due to existing facilities such as roads, buildings, bridges, pavements, and railway crossings
- Influence zone of the foundation of existing structures
The construction of the Gotthard Base Tunnel was segmented into five sections and the excavation methods were selected for five sections. The length of each section ranged between 6 km to 15 km.
The central section of the Gotthard Base Tunnel was named Sedrun and it consists of a 6 km length of the tunnel. From the Sedrun section, the excavation had to be started in both the northern and southern directions with the help of an 800 m deep shaft. The rock condition in the Sedrun section was varying from very good rock to very poor rock.
Also, the overburden pressure was more than 1 km with a high squeezing potential. Additionally, the influence zone of the concrete arch dam was obstructing the southern tunneling drive. Under these circumstances, the engineers decided to choose conventional tunneling excavation method for the Sedrun section.
Out of five, the other four sections were Amsteg (11.4 km), Erstfeld (7.1 km), Bodio (14.8 km), and Faido (12.2 km). The rock condition and other boundary conditions were good for these four sections. Thus, the selection of the excavation method was decided based upon the construction time and cost. Tunnel Boring Machines (TBMs) were selected for the excavation of these four sections. The main reason behind the selection of TBM was to reduce the construction time.
The Gotthard Base Tunnel was constructed to provide a faster and more reliable rail link between northern and southern europe.
The Gotthard Base Tunnel is the world’s longest railway tunnel.
Conventional tunneling methods and tunnel boring machines were used to construct the Gotthard Base Tunnel.
The rocks encountered in the vicinity of the Gotthard Base Tunnel are gneisses, phyllites, and schist. The gneisses rock was mostly filled with the debris of soft soil.