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**History and Evolution of Tunnel Construction**
– Tunnel construction methods have evolved over centuries.
– Innovations in tunnel construction have been driven by the need for efficient transportation and infrastructure development.
– Historical tunnel projects have contributed to the advancement of engineering and construction techniques.
– Ancient civilizations engaged in tunnel construction for various purposes.

**Cost Considerations in Tunnel Construction**
– Tunnel construction costs vary based on factors like location, ground conditions, and project scope.
– City subway tunnels can cost around 500 million EUR per kilometer.
– Motorway tunnels in Switzerland were estimated at 300 million CHF per kilometer.
– The undersea tunnel between Denmark and Germany was planned for 425 million per km in 2015.
– Cost estimation is crucial for tunnel construction planning and feasibility studies.

**Tunnel Construction Methods**
– Cut-and-cover tunnels involve excavating a trench and covering it with a support system.
– Tunnel boring machines (TBMs) automate the tunnelling process, reducing costs and time.
– Clay-kicking is a specialized method for digging tunnels in strong clay-based soil.
– Sprayed concrete techniques like NATM are used to stabilize tunnels.
– Pipe jacking and box jacking are methods used for creating tunnels under existing structures.

**Shafts in Tunnel Construction**
– Temporary access shafts are sometimes necessary during tunnel excavation.
– Shafts are usually circular with concrete walls.
– TBMs are lowered into shafts for excavation to start.
– Construction access shafts can be used for ventilation and emergency exits.
– Multiple shafts may be bored for long tunnels.

**Types of Tunnels**
– Underwater tunnels can be bored or immersed tubes and are essential for connecting land masses.
– Land tunnels are built at ground level to reduce environmental impact.
– Examples include the Bjørvika Tunnel, Marmaray, and the A2 motorway tunnel at Leidsche Rijn near Utrecht.
– Submerged floating tunnels are a novel approach that has not been constructed to date.
– Various tunnel types serve different purposes and contribute to efficient transportation infrastructure.