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 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. The 57 km-long Gotthard Base Tunnel is a part of the NRLA project and is the world’s longest transportation tunnel.
In 1996, preliminary works for the construction of the access tunnels and the shafts were started. on the other hand, 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, offer 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.
The following points describe the geology of the Gotthard Base Tunnel:
Two tunneling approaches were used to construct the Gotthard Base Tunnel: Conventional tunneling approach and Tunnel Boring Machines (TBM). These approaches are described in detail in the subsequent section.
construction of an underground opening using conventional tunneling comprises of the following 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 machinery, executes each individual cycle of the tunnel construction. Each of the above steps is carried out in a cyclical procedure.
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.
The following standard set of machinery is needed for conventional tunneling:
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:
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:
The advantages of the TBM over conventional tunneling approaches are discussed below:
To read more about the working and operation of TBMs, click the link:
Tunnel Boring Machine: Working of the Tunnel Construction Giant
Excavation methods for tunneling are selected based on the type of undertaking depending on the following points:
The construction of the Gotthard Base Tunnel was segmented into five sections and the excavation approaches were selected for five sections. The length of each section ranged between 6 km to 15 km.
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 central section of the Gotthard Base Tunnel was named Sedrun and it consists of a 6 km length of the tunnel. The rock condition in the Sedrun section was varying from very good rock to very poor rock.
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. Also, the overburden pressure was more than 1 km with a high squeezing potential.
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 approaches 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.
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