Table of Contents for Tunnel Construction Methods-

1. Introduction
2. Tunnel Construction Methods
   • Cut & Cover- Bottom-Top & Top-Bottom
   • Boring Machines- Earth Pressure Balanced, Bentonite Shield & Rock TBM
   • New Austrian Tunnelling Method (NATM)
   • Drilling & Blasting- Full Face, Heading & Benching, and Drag Tunnelling
   • Clay Kicking
   • Shaft
   • Pipe Jacking Method
   • Box Jacking Method

A tunnel is an underground or underwater passage for transportation. It is made in the earth by removing the soil from the proposed tunnel cross-section. Only the entrance and exit of a tunnel are open for use, and the earth covers the longitudinal section all around it.

Further, there are several uses of a tunnel. For example-

• Footway
• Road
• Railway
• Water
• Sewage
• Electrical and telecommunication cables.

However, tunnel construction is a tedious task. The biggest challenges are soil behaviour, geological conditions, lack of light and oxygen, water drainage, and working space constraint.

New York City has the world’s largest tunnel network used for railways. But the longest tunnel is in Norway- Laerdal Tunnel of 24.5 km length used as a road.

Tunnel Construction Methods

In addition, a tunnel is also expensive to construct. A tunnel is an alternative where bridge construction isn’t possible due to space constraints. Also, it is advantageous where land acquisition is problematic.

First known tunnel dates back to the 8^th Century BC. Since then, tunnel construction methods have evolved. However, maximum development has been achieved in the 20^th Century. And the credit goes to the development of new equipment, scientific approach, and better geological understanding.

Moreover, each tunnel construction method has its’ own pros and cons. Also, the speed of tunnelling, ground conditions and, economic feasibility differs for methods of tunnelling.

Cut & Cover Tunnel Construction Method

The preferable construction method for a shallow depth tunnel is the cut & cover method. In simple terms, excavate a trench along the alignment of the tunnel. Construct the sidewalls and the roof. Backfill the sides and top of the tunnel with the soil, and reinstate the surface.

Consequently, cut & cover finds application in many of the shallow depth tunnels. The method is suitable because of its’ cost efficiency at shallow depths. Moreover, it is a viable method for construction in rock and soft soils.

The side walls & the roof of the tunnel may have two construction methodologies, i.e., top to bottom and bottom to top. Therefore, the cut and cover tunnel construction method has two approaches-

Bottom to Top

It is a general method of construction. Excavate a trench along the proposed tunnel alignment with a width equal to the width of the tunnel’s outer walls. Construct the tunnel walls on a foundation at the base of the trench. Construct the roof over the tunnel walls and backfill safely.

Top to Bottom

The top to bottom approach constructs the walls and roof beams at ground level. First, excavate a shallow pit with a width equal to the tunnel outer walls width. Second, make the tunnel walls with the slurry-wall method. Third, install the roof over the walls. Lastly, complete the tunnel excavation below the tunnel roof. This method gives an advantage in early reinstatement of the road.

Advantage

1. Suitable for any shape.
2. It is a viable method in both rock & soft soil.
3. Cheaper for shallow depths.

Disadvantage

1. It disrupts the ground services like traffic.
2. The method is limited to shallow depths only. The cost increases with the depth of the tunnel.

Tunnel Construction by Boring Machine

A Tunnel Boring Machine (TBM) is a modern engineering marvel and a boon to tunnel construction methods. However, it is a costly method of tunnelling, but sometimes its’ advantages outweigh the cost.

A TBM is suitable for rock as well as soft soil. Moreover, the speed of construction is the fastest among all the tunnel construction methods. That is why most of the rapid transportation systems deploy a TBM for tunnelling.

The TBM has a cutting face that rotates to excavate the tunnel. On top of that, it has a control station, propelling system, and material conveying system. Other operations like wire-mesh erection, bolting, grouting, shotcrete etc., need additional attachments to TBM or are performed separately.

Some of the TBM types are-

Earth Pressure Balanced Shield

It stores the excavated material in the bulkhead behind the cutting face. The bulkhead balances the earth pressure of the excavated area. A conveying system takes out the spoil at a controlled rate.

Bentonite Shield

The bentonite slurry fills the bulkhead that balances the external earth pressure. The spoil mixes with the bentonite slurry and is pumped out.

Rock TBM

Disc cutters mounted on the cutter head cut the rock. The rock fails by shear. The disc cutters are 300-500 mm in diameter and made of hard steel.

Advantages

1. Overcut is lesser than 5%.
2. Fast tunnelling method.
3. No need for compressed air.
4. Cost-effective for long tunnels (>2 km).
5. No risk of face collapse.

Disadvantages

1. The high initial cost.
2. The TBM cut only a circular cross-section.
3. High operation & maintenance cost is a big challenge in hard rock.

New Austrian Tunnelling Method (NATM)

The NATM method is suitable for rock tunnelling. It utilises the flexible lining that carries the earth pressure. In simple terms, it converts the load exerting rock into the load-carrying rock. Therefore, reducing the need and cost of the lining.

To explain further, the term ‘flexible lining’ refers to the deforming ability of the lining. It stays in contact with the rock and deforms as per the rock deformation. Also, the flexible lining includes steel ribs, boulder-filled wire mesh, rock bolts, and shotcrete.

At the same time, sophisticated instrumentation is installed in contact with the rock surface that monitors the rock displacement.

NATM is also known as sequential excavation method and sprayed concrete technique.

Advantages

1. Useful in complex geological strata.
2. The lining is most optimised.
3. Can construct curvilinear shapes (e.g., elliptical and oval)

Disadvantages

1. Incorrect monitoring of rock movement may lead to failure.

Drilling & Blasting Tunnelling Method

The drill and blast technique is a traditional method for tunnel excavation in rock. It involves the following repetitive steps for tunnel excavation.

1. A Jackhammer or drilling equipment drills the holes in the face of the tunnel. The hole numbers, spacing, and depth are pre-calculated. Mark the points on the tunnel face for drilling.
2. Load the explosive into the drilled holes. Connect all the explosive holes to a detonator that detonates the blast.
3. Clear the tunnel, and conduct the blast.
4. The excavated material is removed by payloader and a dumper or through a conveying system.

Line the newly excavated portion with wire mesh and shotcrete. Other options for lining are concrete lining, structural lining, and precast lining.

Drill & Blast Method has three execution methodologies.

Full Face

The complete cross-section is excavated at once. It is a suitable method for hard rock that can carry the stress and doesn’t collapse under pressure.

Heading & Benching

The heading & benching method excavate into two parts. First, cut the tunnel’s top portion that creates a bench. Later, the second set of equipment cuts the bench. It is possible in large cross-section tunnels only.

Drift Tunnelling

For tunnels of large cross-sections or weak rocks, excavate a small section of the tunnel that subsequently widens to the complete cross-section. The position of the drift, further classification is possible into the top drift, bottom drift, centre drift and side drift.

Advantages

1. This method may excavate any shape and size.
2. Suitable for medium to hard rock.

Disadvantages

1. The vibrations generated during a blast might disturb the nearby buildings.
2. Continuous blasting is not possible. The interval between two successive blastings depends upon the mucking. Hence, the construction is slow.
3. A small-sized drift cannot accommodate large machinery.

Clay Kicking Tunnel Construction

The clay kicking method was developed in England for tunnelling in hard clay strata. The technique involved a clay-kicker who lies on a 45-degree inclined plank and kicks the clay. A digging tool is attached to the clay-kicker’s feet.

Further, the digging tool has a cup-shaped end which the clay-kicker inserts into the clay with his feet. Later, the clay-kicker manually turns the cutting tool to scoop out the clay off the face. Subsequently, the other persons transport the excavated clay out of the tunnel.

Advantages

1. Free of vibration and sound.
2. Can excavate small cross-section tunnels.

Disadvantages

1. Manual excavation is relatively slow as compared to mechanical excavation.
2. Only applicable in hard clay strata.

Shaft Tunnel

A shaft is a vertical tunnel made in the ground. It is circular in section and lined with concrete walls. A shaft facilitates the start and end of deep tunnels. For longer tunnels, there are multiple intermediate shafts along the length.

Also, a shaft helps lower TBM up to the tunnel level.

Advantages

1. It makes the approach to the tunnel level.
2. The shaft provides ventilation and lighting to the tunnel.
3. The evacuation through the intermediate shaft is possible in an emergency.

Pipe Jacking Tunnelling Method

It is a trenchless method. The pipe jacking method excavates a circular shape up to 3.2 m in diameter. A pipe having an internal diameter equal to the tunnel cross-section is pushed into the soil by hydraulic jacks. Therefore, the method is convenient for providing lining and casing behind TBM.

Further, a soil cutting tool is at the front cross-section of the pipe. The cutting tool cuts the soil slightly more than the cross-section. Therefore, reducing the friction between the ground and the pipe.

Later, an auger pressured air or water jet excavates the soil inside the pipe. In addition, the earth removal intervals depend upon the segment installation and the ease of pipe installation.

Advantages

1. The method is free of vibration and sound.
2. An active navigation system accurately places the pipe into the position.

Disadvantages

1. The installation length is limited to only 30 m.
2. The cross-section diameter is limited to 3.2 m only.

Box Jacking Tunnelling Method

The box jacking method is similar to the pipe jacking method. However, the box jacking method uses a square or a rectangular section instead of a circular cross-section.

The box section has a larger width as compared to the pipe. The width may be as large as 20 m.

A cutting head at the front face of the box cuts the soil and moves it inside the box rather than pushing it outwards. Hence the pressure on the box reduces.

Advantages

1. The method constructs the large tunnel below existing roads and railways.
2. The box may excavate square, rectangular, and arched sections.

Earthwork is an integral part of tunnelling. Learn the basics of earthwork and its’ quality control.

Retaining walls are often built at the portal of the tunnel. Read about the retaining wall types here.

Read about the Aerial Survey & how drones are transforming construction.