Title : A Comprehensive Analysis of Tunnel Boring Machines: Design, Performance, Applications, and Recent Technological Advancements
Author: Dr. shankar sikkeri
University: : Visvesvaraya Technological University Karnataka
ISSN :
Volume: 01 Issue: 02
Publication Year: JULY 2026
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ABSTRACT
1. Abstract
A tunnel boring machine (TBM) is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through hard rock, sand, and almost anything in between. Tunnel diameters can range from a meters (done with micro-TBMs) to 19 metres. Tunnels of less than a metre or so in diameter are typically done by horizontal directional drilling rather than TBMs.
Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional 'hand mining' in soil. A TBM has the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, difficult to transport and require significant infrastructure. The biggest is built by Herrenknecht AG of Schwanau, Germany to dig the 57km Gotthard Base Tunnel. It has a diameter of 19 meters.
2. Introduction to tunnel boring machines
A tunnel boring machine (TBM) typically consists of one or two shields (large metal cylinders) and trailing support mechanisms. At the front end of the shield is a rotating cutting wheel. Behind the cutting wheel is a chamber where, depending on the type of the TBM, the excavated soil is either mixed with slurry (so-called slurry TBM) or left as is. The choice of TBM type depends on the soil conditions. Systems for removal of the soil (or the soil mixed with slurry) are also present.
Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which push the TBM forward. The action here is much like an earthworm. The rear section of the TBM is braced against the tunnel walls and used to push the TBM head forward. At maximum extension the TBM head is then braced against the tunnel walls and the TBM rear is dragged forward.
Behind the shield, inside the finished part of the tunnel, several support mechanisms which are part of the TBM are located: dirt removal, slurry pipelines if applicable, control rooms, and rails for transport of the precast segments. The cutting wheel will typically rotate at 1 to 10 rpm (depending on size and stratum), cutting the rock face into chips or excavating soil (muck). Depending on the type of TBM, the muck will fall onto a conveyor belt system and be carried out of the tunnel, or be mixed with slurry and pumped back to the tunnel entrance. Depending on rock strata and tunnel requirements, the tunnel may be cased, lined, or left unlined. This may be done by bringing in precast concrete sections that are jacked into place as the TBM moves forward, by assembling concrete forms, or in some hard rock strata, leaving the tunnel unlined and relying on the surrounding rock to handle and distribute the load.
While the use of a TBM relieves the need for large numbers of workers at increased pressure, a caisson system is sometimes formed at the cutting head. Workers entering this space for inspection, maintenance and repair need to be medically cleared as "fit to dive" and trained in the operation of the locks.
3. History of TBM
The first successful tunnelling shield was developed by Sir Marc Isambard Brunel to excavate the Thames Tunnel in 1825. However, this was only the invention of the shield concept and did not involve the construction of a complete tunnel boring machine, the digging still having to be accomplished by the then standard excavation methods.
The very first boring machine ever reported to have been built was Henri-Joseph Maus' Mountain Slicer. Commissioned by the King of Sardinia in 1845 to dig the Fréjus Rail Tunnel between France and Italy through the Alps, Maus had it built in 1846 in an arms factory near Turin. It basically consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel. Unfortunately, the Revolutions of 1848 irremediably affected the funding of the project and the tunnel was not completed until 10 years later, by using also innovative but rather less expensive methods such as pneumatic drills.
In the United States, this boring machine to have been built was used in 1853 during the construction of the Hoosac Tunnel. Made of cast iron, it was known as Wilson's Patented Stone-Cutting Machine, after its inventor Charles Wilson. It drilled 10 feet into the rock before breaking down. The tunnel was eventually completed more than 20 years later, and as with the Fréjus Rail Tunnel, by using less ambitious methods.
In the early 1950's, F.K. Mitry won a dam diversion contract for the Oahe Dam in Pierre, South Dakota, and consulted with James S. Robbins to dig through what was the most difficult shale to excavate at that time, the Pierre Shale. Robbins built a machine that was able to cut 160 feet in 24 hours in the shale, which was ten times faster than any other digging speed at that time.
The breakthrough that made tunnel boring machines efficient and reliable was the invention of the rotating head, conceptually based on the same principle as the percussion drill head of the Mountain Slicer of Henri-Joseph Maus, but improving its efficiency by reducing the number of grinding elements while making them to spin as a whole against the soil front. Initially, Robbins' tunnel boring machine used strong spikes rotating in a circular motion to dig out of the excavation front, but he quickly discovered that these spikes, no matter how strong they were, had to be changed frequently as they broke or tore off. By replacing these grinding spikes with longer lasting cutting wheels this problem was significantly reduced. Since then, all successful modern tunnel boring machines have rotating grinding heads with cutting wheels.
4. Methodology : Construction & working principles of TBM
All modern tunnel boring machines developed in the recent times work on the principle of New Austrian Tunneling Method (NATM). The New Austrian Tunneling method (NATM) was developed between 1957 and 1965 in Austria. It was given its name in Salzburg in 1962 to distinguish it from old Austrian tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher. The main idea is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself.
The NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The NATM is not a set of specific excavation and support techniques.
5. There are seven features on which NATM is based:
5.1. Mobilization of the strength of rock mass - The method relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support. Primary support is directed to enable the rock to support itself.
5.2. Shotcrete protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying a thin layer of shotcrete immediately after face advance.
5.3. Measurements - Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes.
5.4. Flexible support - The primary lining is thin and reflects recent strata conditions. Active rather than passive support is used and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock bolts, wire mesh and steel ribs.
5.5. Closing of invert - Quickly closing the invert and creating a load-bearing ring is important. It is crucial in soft ground tunnels where no section of the tunnel should be left open even temporarily.
5.6. Contractual arrangements - Since the NATM is based on monitoring measurements, changes in support and construction method are possible. This is possible only if the contractual system enables those changes.
5.7. Rock mass classification determines support measures - There are several main rock classes for tunnels and corresponding support systems for each. These serve as the guidelines for tunnel reinforcement.
Based on the computation of the optimal cross section, just a thin shotcrete protection is necessary. It is applied immediately behind the Tunnel boring machine, to create a natural load-bearing ring and therefore to minimize the rock's deformation. Additionally, geotechnical instruments are installed to measure the later deformation of excavation. Therefore a monitoring of the stress distribution within the rock is possible.
This monitoring makes the method very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work, e.g. by crevices or pit water. Such (usual) problems are not solved by thicker shotcrete, but the reinforcement is done by wired concrete which can be combined with steel ribs or lug bolts.
The measured rock properties lead to the appropriate tools for tunnel strengthening. Therefore in the last decade NATM was also applied to soft ground excavations and to tunnels in porous sediments. The flexible NATM technique enables immediate adjustments in the construction details, but this requires a flexible contractual system, too.
According to Scientist E.Brown, the key features of the design philosophy refer to:
• The strength of the ground around a tunnel is deliberately mobilised to the maximum extent possible.
• Mobilisation of ground strength is achieved by allowing controlled deformation control of the ground.
• Initial primary support is installed having load-deformation characteristics appropriate to the ground conditions, and installation is timed with respect to ground deformations.
• Instrumentation is installed to monitor deformations in the initial support system, as well as to form the basis of varying the initial support design and the sequence of excavation.
When NATM is seen as a construction method, the key features are:
• The tunnel is sequentially excavated and supported, and the excavation sequences can be varied.
• The initial ground support is provided by shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails, spiling).
• The permanent support is usually (but not always) a cast-in-place concrete lining.
Some experts note that many of these construction methods were used in the US and elsewhere in soft-ground applications, before NATM was described in the literature.
Figure 3.1: Reconditioning of TBM's grinding wheels
In an article of 2002 Romero states the major difference between the viewpoints of design and of construction: The deformation of the soil (rem.: at soft-ground tunnels) is not easily ‘controlled’. Therefore it can be concluded that the excavation and support planned for sequentially excavated, shotcrete-lined tunnels utilises NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.
6. Application of TBM in civil engineering
TBMs are usually used for tunnel boring for urban tunneling or near surface tunneling. Urban tunnelling has the special challenge of requiring that the ground surface be undisturbed. This means that ground subsidence must be avoided. The normal method of doing this is to maintain the soil pressures during and after the tunnel construction. There is some difficulty in doing this, particularly in varied rock strata (e.g., boring through a region where the upper portion of the tunnel face is wet sand and the lower portion is hard rock).
TBMs with positive face control are used in such situations. There are three common types: Earth pressure balance (EPB), Bentonite slurry (BS), and compressed air (CA). The compressed air method is the oldest, but is falling out of favour due to the difficult working conditions it imposes. Both types (EPB and BS) are clearly preferred over open face methods in urban environments as they offer far superior ground control.
When tunnelling in urban environments other tunnels and deep foundations need to be addressed in the early planning stages. The project must accommodate measures to mitigate any detrimental effects to other infrastructure.
7. A classic case study example – The Euro Tunnel
The Channel Tunnel, also known as the portmanteau Chunnel, is a 50.5-kilometre (31.4 mi) undersea rail tunnel linking Folkestone, Kent in England with Coquelles near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point it is 75 m (250 ft) deep. The Channel Tunnel has the longest undersea portion of any tunnel in the world. The Seikan Tunnel in Japan is both longer overall, at 53.85 kilometres (33.5 mi) and deeper, at 240 metres (790 ft).
The tunnel carries high-speed Eurostar passenger trains, Eurotunnel ro-ro vehicle transport that are the largest in the world and international rail freight trains.[4] In 1996 the American Society of Civil Engineers identified the tunnel as one of the Seven Wonders of the Modern World.
Ideas for a cross-Channel fixed link existed as early as 1802 but the eventual successful project, organised by Eurotunnel, began construction in 1988 and opened in 1994. The cost overran predictions by 80%, and concessionaire Eurotunnel overestimated tunnel traffic and has met financial difficulty. Fires have disrupted operation of the tunnel. Illegal immigrants and asylum seekers have used the tunnel to enter Britain, causing a minor diplomatic row over the siting of the Sangatte refugee camp, which was eventually closed in 2002.
Eleven tunnel boring machines working from both the UK and France cut through chalk marl to construct two rail tunnels and a service tunnel. The vehicle shuttle terminals are at Cheriton (part of Folkestone) and Coquelles, and are connected to the British and French motorways.
8. Advantages of TBM
8.1. Major cost items
Major cost items affected by this excavation method and considered in this analysis, are:
• Cost savings resulting from qualitative advantages of mechanical excavation
•TBM excavation rates four to six times higher than manual excavation or earth moving equipment’s excavation
• Over break (cost of delivered concrete, wastage and labor to fill over break)
• Labor crew costs
• Lower infrastructural equipment costs (drills, jumbos, muckers, cars, trains, TBM, etc)
• Lower associated supplies (drill bits, TBM cutters, blasting agent)
• Elimination of temporary construction structures (access audits and/or shafts)
8.2. Operational advantages
There are a number of inherent advantages to tunnel boring which are difficult to quantify yet have a considerable impact on the outcome of excavation rates and costs. These are difficult to value in a tender. These advantages cannot fully be appreciated except from firsthand tunnel boring excavation experience.
Some of these are:
• Structural stability and safety at the face and work area
• Continuous (non-cyclic) operation
• Consistent, less skilled and easily trained operations (labor is assigned to limited tasks that are repetitive, become
Routine, and may even produce competition among the laborers)
• Safer and more pleasant working environment than in D&B
8.3. Structural stability and safety advantages
The stability and safety at the excavation face, heading, work area and the lifeline out of the tunnel has always been of great concern in tunneling. Current technology and tunneling techniques provide the ability to provide safe and stable conditions in all areas.
There is an inherent level of stability in having a tunnel boring machine at the heading, supporting the work area or blocking possible inrush of materials into the work area. For example, many sudden failures at the heading occur as a result of an excavation blast initially in rock, transgressing into unstable material, soil or a fault zone. The final blast in competent ground triggers the deteriorating ground conditions and a full blown collapse or tunnel inrush may occur even under the watchful eye of the engineer (eg – second Tienlun Power tunnel).
The avoidance of blasting and the stabilizing effect of a full face machine has often been immeasurable, yet striking. The major zones of concern are at the tunnel face, ahead and around the TBM cutter head and in the work area.
Figure 6.1: A TBM breaks open a wall after tunneling is complete
9.Disadvantages
• Difficult to construct.
• Assembly is a complex issue
• Cost effective
• Heavy machinery
• Consumes large space for working
• Time consumer
10. Conclusion
Tunnel boring machines have surely leveraged the engineering skills of today’s world and have proven a new level of construction skills. These machines have not only increased the speed of constructing newer tunnels at a faster pace, but have also increased the rate of infrastructural development. Newer technologies like dual tunneling methods with TBMs have really taken the the field of civil engineering to a newer level of excellence. The true advantage of TBMs lie when these machines are put to use on a large scale to establish newer, safer and advanced tunnels. Despite challenges such as high capital investment, complex maintenance, and dependence on geological conditions, the advantages offered by TBMs far outweigh their limitations, particularly in long and technically demanding tunnel projects. Their ability to reduce construction time, improve worker safety, and ensure high-quality tunnel alignment makes them an indispensable component of modern underground engineering. As global demand for sustainable transportation, urban infrastructure, and underground utilities continues to grow, Tunnel Boring Machines are expected to play an increasingly important role in future construction projects. Continued research and technological innovation in automation, artificial intelligence, digital monitoring, and energy-efficient systems will further enhance TBM performance, making them a cornerstone of next-generation tunneling and infrastructure development.
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