In 2005, through the pages of a prestigious Italian trade magazine, I turned to readers, if they were aware of how many methods of excavating underground tunnels, with mechanized cutters, could exist in the complex world of mechanical engineering of the subsoil. Even today, after 17 years, I have received only a few positive feedbacks. Up to now, the advantage of excavating with circular mechanized cutters arises from the guarantee because we are able to know the structural stability for any type of tunnel. In practice, it has been possible for many years to create tunnels with different cross sections, rather than circular ones. In fact, it is possible to design, and at the same time excavate, tunnels with rectangular and oval sections, taking into account that, in the underground excavation, there are always stress states that are little known to most of us, even if we are technicians in the sector. New emerging problems linked to sustainability and innovation are the development of a new concept of underground excavation, developed in the last 30 years, with particular mechanized cutters to create very versatile tunnels, with different types of configurations. The 3 types of excavation machines most used and known today, for the type of soil on which they must operate, are illustrated below, in figure 01) [Silt & Clay, Sand, Gravel and Rock]

1. Some brief considerations on the mechanized evolution of metropolitan tunnel excavation technology
through a photographic sequence
by Lamanna Luigi Franco [*]
1. Introduction
In 2005, through the pages of a prestigious Italian trade magazine, I turned to readers, if they were aware
of how many methods of excavating underground tunnels, with mechanized cutters, could exist in the
complex world of mechanical engineering of the subsoil.
Even today, after 17 years, I have received only a few positive feedbacks.
Up to now, the advantage of excavating with circular mechanized cutters arises from the guarantee
because we are able to know the structural stability for any type of tunnel. In practice, it has been possible
for many years to create tunnels with different cross sections, rather than circular ones.
In fact, it is possible to design, and at the same time excavate, tunnels with rectangular and oval sections,
taking into account that, in the underground excavation, there are always stress states that are little
known to most of us, even if we are technicians in the sector.
New emerging problems linked to sustainability and innovation are the development of a new concept of
underground excavation, developed in the last 30 years, with particular mechanized cutters to create very
versatile tunnels, with different types of configurations. The 3 types of excavation machines most used
and known today, for the type of soil on which they must operate, are illustrated below, in figure 01) [Silt
& Clay, Sand, Gravel and Rock]:
- 1.1-TBM open, which are used when the excavation face and the surrounding terrain are characterized
by a high self-supporting capacity.
- 1.2-TBM SHIELDED IN OPEN MODE: the shield of this type of machine has the function of providing
active and immediate support to the ground above, on the base and on the sides of the tunnel.
These are special TBM cutters that provide mechanical support to the "excavation face" and the
surrounding ground and are characterized by their self-supporting properties.
However, this type of cutters are used in particular difficult hydrogeological situations.
- 1.3-SHIELD TBM IN CLOSED MODE [Slurry TBM]: for this type of mechanized cutters the machine shield
is in closed mode, and provides an active support on the ground above, also at the base and at the sides
of the tunnel. In this particular case, the head of the cutter [stability of the face] as well as the excavation
pre-chamber guarantee a counter pressure to the excavation face, supporting it.
This particular type of cutters is used when the excavation is in the presence of intensely fractured sands,
clays, gravels and / or rock masses, characterized by the presence of zero cohesion.
Slurry TBMs are able to guarantee the mechanical support of the excavation face, the support means of
which is constituted by a fluid technically known as SLURRY SHIELD TBM.
In these machines, the bentonite mud is interposed between the face and the excavation head and is
responsible for exerting a counter pressure necessary for the stability of the excavation. Thanks to its
thixotropic properties, bentonite mud is very suitable for this purpose.
I remember that the preparation of the mixture must be carried out on a case-by-case basis, in fact, given
the granulometry of the soils crossed, a different concentration of bentonite will be required to form a
continuous and impermeable layer on the excavation face, such as to prevent the filtration of the mud
into the ground. . This aspect is of fundamental importance as it guarantees regular pressure over time
inside the excavation chamber.

2. While the excavated material, that is the excavated mud, is accumulated, before its removal, in the
"pressurized pre-chamber" [Earth Pressure Balance TBM] and subsequently sent outside the tunnel, thus
having total control of the surrounding soil and relative subsidence.
Fig. 01 – Tunnel Boring Machine, Shield & TBM
2. A sequence of photographic illustration of some mechanized cutters
With the variety of TBM mechanized cutters present on international markets, it is now possible to create
tunnels with excavation techniques in the most disparate shapes and difficult geological conditions. These
are divided into:
2.1 - Slurry Shield Machine, which are the TBM cutters SHIELDED IN CLOSED MODE illustrated above in
point 1.3).
2.2 - Earth Pressure Balanced (EPB-TBM) Shield Machine, are machines characterized by a full section
cutting head, able to guarantee the confinement and support of the excavation face, by pressurizing the
excavated material, through the excavation soil, applying a certain pressure to the face itself, and then,
suitably conditioned with foaming agents and / or polymers, injected in front of the milling head, and
inside the excavation chamber and in the "auger", used for the removal of the "excavated earth" [that is
the excavated material that is unloaded through the "screw" on a conveyor belt with which it reaches the
storage area outside the construction site where it is expected that the surfactant content [the
conditioning products] descend to the below the limits permitted by law, to be then transported to the
deposit / landfill area outside the tunnel] does not leak into the subsoil.
I would like to remind you that the use of conditioners must be studied and calibrated, case by case, in
order to guarantee optimal behavior of the material.
From this solution, the first cutters called Earth Pressure Balance shield [EPBs] were born in Japan which,
to date, represent 90% of the mechanized excavation of the tunnels dug with this type of cutter.
I point out that the "excavated material" is extracted through a pressure discharge system that allows the
material to pass from the excavation chamber, through the auger until it is released onto the conveyor
belts.
Fig. 02 – Mixed ground shield machine layout [Earth Pressure Balanced Shield Machine]
ROCK TUNNELING MACHINE EPB TBM SLURRY TBM
KAWASAKI
[today
UGITEC]

3. 2.3 - Mixed Ground Shield Machine, this type of cutter is commonly used using cutting discs according to
the characteristics of the ground that must be excavated for size and strength.
2.4 - Full Shield TBM, this type of cutter is commonly used in a wide range of soil conditions, from
fractured rocks to soft layers.
Fig. 03 – Full shield TBM layout [30 mR, articulate 6°]
2.5 - Open Gripper TBM & New Open TBM, this type of open TBM allows operators the advantages of
having a dual combination of being used as a Full SHield TBMs and / or an Open TBMs.
Fig. 04 – Open gripper TBM
Fig 05 – Open Gripper TBM layout
Fig. 06 – Open TBM layout
MITSUBISHI
KAWASAKI
[today
UGITEC]
KAWASAKI
[today
UGITEC]

4. This type of TBM is used for excavation in rock masses, with excellent mechanical characteristics and with
a medium-high self-supporting time, where it is guaranteed to work in absolute safety without any
support work, to be used in the excavation phase.
This type of cutter is compatible with a wide range of geological conditions, from hard rock to weak layer.
This type of machine is capable of digging in very difficult terrain. There are two types of TBM mechanized
cutters on the market. One is the "Double Shield TBM", which can be driven forward by the main gripper.
The other is the "single shield TBM", which is pushed forward only by the shield.
Furthermore, the "open TBM" has a simple structure, which allows the cutter to dig into the "rock" which
is very stable and "hard", very efficiently.
Fig. 07 – The "Gripper" bulkheads are shown in red in an open TBM [not shielded]
2.6 - Special Shield Machines for special cross sections
In the case of tunnels of modest length and / or small underground structures, which are usually carried
out in urban areas, the excavation works are developed with the jacking system, with a rectangular
section, which has been widely used, for several years, in the Asian area, mainly used to build service
tunnels, pedestrian underpasses, controlled crossings for underground services in urban areas and very
low traffic tunnels.
Most of the use of this type of TBM allows us to build service tunnels related to the development of major
city extensions and new urban developments.
The recent use of this type of activity has shown that service tunnels can become a useful element for the
regeneration of historic centers, moreover it also serves to ensure the transmission, to future generations,
of some urban structures that guarantee a high degree of life quality.
The use of this technique guarantees safety in the execution of the works, both for the workers and for
the buildings present, as well as for the no need to carry out further excavations for future restoration of
the main networks [such as: gas, water , electricity, etc.].
2.6.1 - APORO All Potential Rotary Cutter [Rectangular]
This type of cutter can excavate not only circular sections, but also rectangular, horseshoe and other non-
circular shapes. The cutting head has a high-precision control system, capable of maintaining the planned
excavated section along the entire length of the excavation.
GRIPPER
HERRENKNECHT

5. Fig. 08 – Variation of APORO Cutter excavation section. System used in Japan in 2008 for approximately
577 m of excavation
2.6.2 - MMST Multi-Axial Rectangular
This type of TBM, called MMST, was built in 2002, in Japan, in Kanagama Prefecture to excavate a 1.080 m
long tunnel.
Fig. 09 – H 7,85 X W 3,19 - Multi-Axial rectangular Slurry Shield Machine.
The TBM illustrated in the following photo is equipped with double cutting heads for excavating a horizontally long
rectangular section and with double telescopic cutters for cutting rectangular corners. The surface coverage of the
earth varies from a minimum thickness of 2.6 m, to a maximum coverage of 4.76 m. This underpass, having a length
of 102 m, was built in Japan, in 2016, in the Prefecture of Giza.
KAWASAKI
[today
UGITEC]
KAWASAKI
[today
UGITEC]

6. Fig. 10 – H 4,69 X W 7,29 - Multi-Axial rectangular Slurry Shield Excavator
2.6.2.1 - Other type of rectangular TBM cutters
Currently, this rectangular excavation method is used when there is a series of soil layers and weak rock layers and,
through, with ground pressure balancing. In fact, this type of tunnels require, as mentioned above, a very low surface
overload.
In China, in 2015, the CREG company built the largest rectangular TBM in the world [height 10.42 m x width 7.55 m]
used for the excavation of an underpass tunnel in the project of line 11 of the underground Tianjin.
Fig 11 – Illustration of particular cutters on rectangular TBM cutters
2.6.3 - Horizontal & Vertical
In the past, the excavation of sections of subway stations were done using Cut-and-Cover tunneling methods. Due
to the impact that was created on above-ground activities and on the large numbers of criticalities of underground
structures, under congested urban areas, the H&V shield machine, illustrated in the following photo, is widely used
to simultaneously excavate the tunnel of the subway and at the same time the platform area of the station.
The characteristics of this first TBM H&V shield was made in the world by the Japanese company Kawasaki and is
equipped, in parallel, with an articulation mechanism [called the H&V mechanism], which enables the construction
of large tunnels with uniquely shaped sections, and which makes high-precision positioning control possible. This
H&V TBM is equipped with four types of particular cutters positioned on the same surface.
Fig. 12 – Slurry Shield Machine
HITACHI
ZOSEN
[today
UGITEC]
CREG
-
WIRTH
MITSUBISHI
[today
UGITEC]
KAWASAKI
[today
UGITEC]
MITSUBISHI
[today
UGITEC]

7. 2.6.4 – DMB Shield Machine
2.6.4.1 - The only feature of this TBM-DMB model is that this machine allows the excavation of two independent
parallel tunnels, and in the same direction.
As I said, these two TBMs can be separated to carry out the excavation with different directions. Each shield
continues to dig as if they were two separate and independent machines after their divergence.
Fig. 13 - Kawasaki, Dual-mount branching H&V Shiel machine and Mitsubishi, a machine for continuous
vertical and horizontal excavation
In summary, the characteristics are those that, after excavation, the TBMs in the tandem position, with
upper and lower shields, can independently excavate in different directions, both for design reasons and
for divergences arising from the subsoil.
This type of machines can also perform the excavation of tunnels in curves, even very narrow ones, with
a minimum radius of 15 m [articulation angle: max. 13 ° for both left and right directions] in both tandem
and independent positions .
2.6.4.2 - Horn method [a machine for continuous vertical and horizontal excavation]
A single machine can be used continuously to excavate a vertical shaft from above the ground, and then
a horizontal shaft. This type of machine also has a function for excavating a vertical shaft with caissons or
underground walls, which would be required with conventional methods. This means that the site
required for constructing a vertical shaft is smaller than it was in the past
2.6.6 - DPLEX Earth Pressure Balanced
Shield machines for special cross sections [non-circular shields]. With this type of mechanized cutter by
changing the cutter head to a shape other than a circle, various cross section cutouts are possible.
UGITEC
-

3,29
-

2,89
MITSUBISHI
Horn
Method
[today
UGITEC]

8. Fig. 14 - Earth Pressure Balanced DPLEX TBM [eccentric multi-axis] Shield Machine with special circular section
excavation cutters
3. Conclusion
Through the brief photographic illustration, within this memory, we have seen that today's underground
tunnel excavation techniques, using TBM, begin to demonstrate that their field of application will be much
more intense than today, through a very different reality, in the immediate near future.
Today there are a series of TBMs, suitably designed, which have the capacity to overcome a wide range
of different geological formations and all this also requires a quick requalification of our sector companies,
their technicians, the chemical industries that produce support materials [see the conditioning and
consolidation of the land in the excavation phase], and of the designers who, in my opinion, all of them,
even them, will have to retrain to face in the immediate future, the challenge of new technologies that
are already rapidly changing the world [has already changed].
Fig. 15 - If you do not rely on expert technicians in the sector, you risk having great uncertainties and
confusion in your head not knowing how to choose, to solve the problem you are facing, among the
vast range of TBMs on the market, which would seem to be all alike
An epochal transformation is taking place in this sector too, difficult to make understood in all its aspects, because
it is not just a technological question, but something much more radical, which will bring mankind ever closer to a
sort of "hybridization" with machines, with artificial intelligence, robotics and with the development of the network
[today we are already at 6G] of a hyper-connected world in which nothing and no one will have to escape.
Requalifying the design also means having the real knowledge of the "useful life of the materials and of everything
we are going to create with new structures" which, these [structures] must be identified in the "operational limit
states", through an adequate modeling in able to guarantee the "non-degradation", and an adequate statistical
quantification of the variables on all the "materials", which will be used, at an "environmental" level.
UGITEC
-

7,67
MITSUBISHI
[today
UGITEC]

9. Fig. 16 – Full range solutions for underground space development
We must begin to document, for each structure, that we will build in the immediate future, with a so-
called "birth certificate", since we are all perfectly aware that, as regards the existing structures, normally
there is currently no useful information. and detailed, issued within a real "certification" but, on the basis
of the quantification of the future data collected, we will have to start, immediately, to have a qualified
and reliable response to any information during the useful life of any structure , as are the galleries.
Furthermore, it is of vital importance, not only in metropolitan areas, to dig new support tunnels such as
technological ones [new sewers, new aqueducts, gas transport, technological systems, etc.], but to dig new
tunnels in areas mining, to go deeper and deeper, using new robotic TBMs [which have the possibility of
allowing tunnel excavation, underground extraction and at the same time crushing the rock in a few hours],
through the intelligent electronics it uses inside of these [the TBMs] of microprocessors, in search of "new
products" [Rare Earths], necessary in the field of renewable energy, to extract "new raw materials", which are
today the integral part of our life of all days and for which their request is constantly increasing.
4. Privacy Policy
This blog is not a newspaper or advertising as it is updated without any periodicity. It cannot therefore be
considered an editorial product under the Italian law n ° 62 of 7.03.2001. Some images included in this
post are taken from the internet and, therefore, considered to be in the public domain; if their publication
violates any copyright, please notify us by email. They will be removed immediately.
[*] Luigi Franco, LAMANNA
Independent Technical Consultant in the sector of Tunnelling, Mining and Underground Technology
President of the Fondazione Internazionale di Centro Studi e Ricerche, ONG
132, via dei Serpenti, 00184 ROMA, Italy, U.E.
Email: lamannaluigifranco1@gmail.com
Note:
I find it necessary to point out that the Japan Tunnel Systems Corporation and Mitsubishi Heavy Industries Mechatronics Systems, Ltd. integrated
their respective shield tunneling machine businesses, which resulted in the establishment of a new company called JIM Technology Corporation
on October 1, 2016.
Furthermore, I still point out that, in the year 2021, Kawasaki Heavy Industries Ltd. and Hitachi Zosen Corporation integrated the TBM business
and the newly formed "Underground Infrastructure Technologies Corporation" UGITEC.