Types of Tunnel Construction

Tunnel construction is a complex field that bridges the disciplines of engineering, geology, and construction. Tunnels are essential infrastructures that facilitate transportation, utilities, and communication by providing passage through mountains, beneath cities, and under bodies of water. The construction of tunnels is a subset of underground construction, and the methods employed vary depending on several factors, including ground conditions, groundwater conditions, tunnel dimensions, depth, and intended use. This article explores the various types of tunnel construction, focusing on the three basic methods: cut-and-cover tunnels, bored tunnels, and immersed tube tunnels.

Overview of Tunnel Construction

Tunnels are constructed in various types of ground, ranging from soft clay to hard rock. The choice of construction method depends on factors such as:

- Ground conditions: The type of soil or rock through which the tunnel is to be excavated.

- Groundwater conditions: The presence and behavior of groundwater, which can impact the stability of the excavation.

- Tunnel dimensions: The length and diameter of the tunnel, which influence the choice of equipment and construction technique.

- Tunnel depth: The depth of the tunnel below the surface, which affects the method of access and the need for ground support.

- Logistics of support: The planning and management of materials, equipment, and labor to maintain tunnel stability during construction.

- Final use and shape: The intended purpose of the tunnel, such as transportation, utilities, or pedestrian access, which determines the design and layout.

- Risk management: The assessment and mitigation of potential risks, including ground instability, water ingress, and environmental impact.

Types of Tunnel Construction

Three primary types of tunnel construction are commonly used: cut-and-cover tunnels, bored tunnels, and immersed tube tunnels. Each method has its unique advantages and challenges, and the selection of the appropriate technique depends on the specific project requirements.

1. Cut-and-Cover Tunnels

Cut-and-cover is one of the simplest and most traditional methods of tunnel construction, typically used for shallow tunnels. The process involves excavating a trench, constructing the tunnel within it, and then covering it with a support system strong enough to bear the load of whatever is built above the tunnel.

a. Bottom-Up Method

In the bottom-up method, a trench is excavated, and the tunnel is built within it. The ground may require support to prevent collapse during excavation. The tunnel structure can be made of in situ concrete, precast concrete, precast arches, or corrugated steel arches. In earlier times, brickwork was commonly used. Once the tunnel is constructed, the trench is carefully back-filled, and the surface is reinstated.

The bottom-up method is widely used in urban areas where the disruption to the surface is acceptable or can be minimized. It is also preferred for constructing large underground structures such as metro stations, where extensive excavation is required.

b. Top-Down Method

The top-down method is an alternative approach where side support walls and capping beams are constructed from the ground level, often using slurry walling or contiguous bored piling techniques. A shallow excavation allows for the construction of the tunnel roof, usually made of precast beams or in situ concrete. The surface is reinstated early in the process, except for access openings. Excavation then takes place beneath the permanent tunnel roof, and the base slab is constructed.

The top-down method is advantageous in scenarios where minimizing surface disruption is critical, such as in densely populated urban areas. By reinstating the surface early, roads, utilities, and other surface features can be restored quickly, reducing the impact on the surrounding community.

Applications of Cut-and-Cover Tunnels

Cut-and-cover tunnels are commonly used in urban environments, particularly for transportation infrastructure such as metro systems, road tunnels, and pedestrian underpasses. One notable example is the Canary Wharf tube station in London, which was constructed using the cut-and-cover method. The station's large excavation created an underground space that has been likened to a cathedral due to its impressive size and scale.

2. Bored Tunnels

Bored tunnels are constructed in situ, without removing the ground above. This method is typically used for deep tunnels and is particularly suited to urban environments where surface disruption must be minimized. Bored tunnels are usually of circular or horseshoe cross-section and can be constructed using various techniques, including the New Austrian Tunneling Method (NATM), tunnel boring machines (TBMs), and tunneling shields.

a. Tunnel Boring Machines (TBMs)

TBMs are large, complex machines designed to excavate tunnels through a variety of ground conditions. They are equipped with rotating cutting heads that grind away the soil or rock in front of the machine. As the TBM advances, it installs precast concrete segments to form the tunnel lining, providing immediate support to the surrounding ground.

TBMs are highly efficient and can be used for long tunnel drives with consistent ground conditions. They are particularly suited to urban projects where minimizing surface disruption is essential. However, TBMs require significant upfront investment and are best suited to large-scale projects where their cost can be justified.

b. New Austrian Tunneling Method (NATM)

The New Austrian Tunneling Method, also known as the Sequential Excavation Method (SEM), is a flexible approach to tunnel construction that adapts to varying ground conditions. NATM involves the excavation of small sections of the tunnel, followed by the immediate application of shotcrete (sprayed concrete) to stabilize the surrounding ground. This process is repeated in stages until the entire tunnel is excavated and supported.

NATM is particularly useful in complex ground conditions where the tunnel's stability needs to be continuously monitored and adjusted. The method allows for real-time modifications to the support system based on the behavior of the ground, making it a versatile choice for challenging projects.

c. Tunneling Shields

Tunneling shields are mechanical structures that provide temporary support to the tunnel face during excavation. The shield is pushed forward as excavation progresses, and the tunnel lining is installed within the shield to provide permanent support. Tunneling shields are often used in conjunction with compressed air or slurry to control groundwater and prevent the collapse of the tunnel face.

The use of tunneling shields is particularly advantageous in soft ground conditions, such as clay or sand, where maintaining the stability of the tunnel face is challenging. Shields can be used for both circular and non-circular tunnels and are often employed in urban environments where ground movement must be carefully controlled.

Applications of Bored Tunnels

Bored tunnels are commonly used for transportation infrastructure, including subway systems, railway tunnels, and road tunnels. They are also used for utility tunnels, such as those carrying water, sewage, or telecommunications lines. The use of TBMs and tunneling shields allows for the construction of long, deep tunnels with minimal surface disruption, making bored tunnels the preferred choice for many urban projects.

3. Immersed Tube Tunnels

Immersed tube tunnels are constructed by sinking prefabricated tunnel sections into a body of water and laying them on or just beneath the waterbed. This method is typically used for tunnels that cross rivers, estuaries, or other bodies of water.

a. Construction Process

The construction of an immersed tube tunnel begins with the prefabrication of tunnel sections in a dry dock or casting basin. These sections are then sealed and floated to the tunnel site, where they are carefully positioned and sunk into a pre-dredged trench on the waterbed. The sections are connected underwater to form a continuous tunnel, and the trench is back-filled to secure the tunnel in place.

Immersed tube tunnels require precise engineering and construction techniques to ensure the proper alignment and connection of the tunnel sections. The method is highly effective for underwater crossings where other tunneling methods may be impractical or prohibitively expensive.

Applications of Immersed Tube Tunnels

Immersed tube tunnels are commonly used for road and rail crossings of rivers, estuaries, and other bodies of water. Notable examples include the Øresund Tunnel between Denmark and Sweden and the Fort McHenry Tunnel in Baltimore, Maryland. The method is also used for utility tunnels, such as those carrying water, sewage, or power lines across bodies of water.

Conclusion

Tunnel construction is a critical component of modern infrastructure, enabling the efficient movement of people, goods, and utilities through challenging environments. The choice of construction method depends on a range of factors, including ground conditions, tunnel dimensions, and the need to minimize surface disruption. Cut-and-cover tunnels, bored tunnels, and immersed tube tunnels each offer unique advantages and challenges, making them suitable for different types of projects.