report for tunnelling by tunnel boring machine

Tunnelling by Tunnel Boring Machine.
Dept. of civil eng. CMRIT Bangalore. Page 1
CHAPTER 1:
INTRODUCTION
1.1 HISTORY
The first successful tunneling shield which is commonly regarded as the
forerunner of the tunnel boring machine was developed by Sir Marc Isambard Brunel
to excavate the Rotherhithe tunnel under the Thames 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 using miners to dig under the shield and behind
them bricklayers built the lining. Although the concept was successful eventually it
was not at all an easy project. The tunnel suffered five floods in all. It is also
noteworthy that Marc Brunel’s son who was the site engineer went on to become
what is generally thought of as Britain’s greatest engineer, Isambard Kingdom
Brunel
Fig 1.1 Diagram of tunneling shield used to construct the Thames tunnel

Improvements on this concept were used to build all of the early deep railway
tunnels under London in the early 20th century and lead to the name ‘tube’ which is
the nickname all Londoners call their metropolitan railway and give tunnels made by
this method their characteristic round shape.
In other countries tunnel boring machines were being designed to tunnel through
rock. The very first actual boring machine ever reported to have been built is thought
to be Henri-Joseph Maus' Mountain Slicer designed in 1845 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 however it was not used, and the tunnel was finally
built using conventional methods.
In the United States, the first 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 and the tunnel had to be completed
many years later, using less ambitious methods.
We need to move on nearly 100 years when James S. Robbins built a machine 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 modern 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 use rotating grinding heads
with cutting wheels for boring through rock.

Below is an example of a tunnel boring machines which is equipped with a back hoe
whilst the cutting head has been a breakthrough on soft material the shield with a
back hoe is still a cost efficient and well utilized solution even today.
Fig 1.2 TBM with Back Hoe
1.2 OBJECTIVES
The goal of this paper is to give an understanding of what is a
TBM and to build up an awareness of the wide variety of perils TBMs are
exposed to during their utilization for a tunnel project.
And we’ll discuss the role of civil engineers in this machine tunneling work.
And how they utilizes their geotechnical knowledge for selecting the type of
TBM, & its execution. In future how maximum work will be underground & for
their tunneling how TBM will be the best.
1.3 DIFFERENT TYPE OF TUNNEL BORING MACHINE
The description of the types of TBM derive from what type of soil is being excavated
1. Slurry Machine
This is used for soils usually of varying hardness. The excavated soil is mixed with
slurry to create positive face pressure required to sustain the excavation. This is
known as a closed machine. The system for the removal of the soil involves pumping
the soil mixed with slurry to plant located outside the tunnel that separates the slurry
from the muck allowing its recirculation. See sketch below.

Fig 1.3 Main features of SSM TBM shield
2. Earth pressure Balance machine
This is a closed machine and is used usually for softer fairly cohesive soils. In this
case the positive face pressure is created by the excavated ground that is kept under
pressure in the chamber by controlled removal through the rotation of the screw
conveyor. The muck is thereafter removed by a conveyor belt and/or skips.
Fig 1.4 EPBM

3. Rock Machine
This is used for excavating rock. The rock is crushed by the cutters (often discs) and
removed on conveyors and/or skips. Cutters are specifically designed to resist hard
abrasive material.
Fig 1.5 Rock TBM
1.4 DESCRIPTION OF THE MACHINE
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. The chamber may be
under pressure (closed machine) of open to the external pressure (open machine)
Behind the chamber there is a set of hydraulic jacks supported by the finished part of
the tunnel which push the TBM forward. 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: soil/rock 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 (usually called muck by
tuneless). Depending on the type of TBM, the muck will fall onto a conveyor belt
system or into skips 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, if the pressure at the tunnel face is greater than behind the chamber a
caisson system is sometimes formed at the cutting head this allows workers to go to
the front of the TBM for inspection, maintenance and repair if this needs to be done
under pressure the workers need to be medically cleared for work under pressure like
divers underwater and to be trained in the operation of the locks.
Shields
Modern TBMs typically have an integrated shield. The choice of a single or double
shielded TBM depends on the type of rock strata and the excavation speed required.
Double shielded TBMs are normally used in unstable rock strata, or where a high
rate of advancement is required. Single shielded TBMs, which are less expensive, are
more suitable to hard rock strata.

CHAPTER 2:
GEOTECHNICAL CONSIDERATIONS
2.1 DESIGN PHASE
Before design can begin, it is important to collect all the available information, data
and documents concerning the geology and the hydro-geological conditions of the
region and the underground works already known and from similar situations.
To optimize the layout of the tunnel, the following parameters have to be taken into
consideration:
• The nature of the soil / rock in the path of the tunnel (morphology, mechanical
characteristics, deform-ability, etc)
• The position of the different layers / schist (strata graphic characteristics)
• The direction of the main discontinuities (tectonic-structural characteristics)
• Petrography
• The presence of water (hydrogeology)
• The nature of the cover and highness of the overburden
• The impact of the excavation on the environment (soil / rock stress, subsidence,
hydrogeology alteration)
• Buildings, traffic systems, presence of pipes and services at the surface.
.
Geological maps or studies are often already available. These are a useful base to
define and optimize the specific investigations which will be performed to complete
the necessary knowledge.
The most common investigation technologies are:
• Core sampling
• Test in the drilled hole, borehole measurements
• Laboratory tests on samples.
The following techniques may also be appropriate:
• Air and satellite photography
• Geo-electric
• Gravimetric

• Seismic behavior investigation
• Electromagnetic radar
• Geo-radar.
The geotechnical predictions should be checked for confirmation during the
execution of the excavation by performing on-site tests and monitoring.
The geotechnical study, together with the geological and hydro-geological studies,
should allow for:
• Interpretation of the geotechnical classification of all present materials
• Determination of the conditions of stability
• Dimensioning of the reinforcement and the tunnel lining properties
• Identification of “critical” points of the excavation and relative precautions to be
taken
• Design and size of the separation plant
• Selection of the driving method.
In the specific case of tunneling in soft ground and hard rock, the following
investigations should be per-formed:
• Soil / rock identification
• Determination of the initial stress conditions
• Study of mechanical characteristics
• Study of the hydraulic characteristics.

2.2 SOIL / ROCK CHARACTERISATION
Main characteristics to be identified:
Table 1 Soil/ Rock characteristics
2.3 GEOLOGICAL REPORT
The geological report should give "predictions" of the characteristics listed in the
above paragraph for all the single layers which will be directly or indirectly involved
during the excavation.

Other information on the material, to be excavated, which should be included in the
report are:
• Mechanical resistance (measured by normal laboratory tests of compressive
strength and direct or indirect tensile strength, or measured with specific load tests,
like the "Point Load Test" or "Franklin Test)
• Hardness and abrasiveness and, especially for a mechanized tunneling project:
• Fracturing generally
• Number and dimension of the faults
• Presence of geological discontinuities
• Homogeneity/heterogeneity of the soil along the sections
• Cohesiveness (stickiness) of any clay present
• Permeability
• Potential change of the soft ground properties with compressed air.

CHAPTER 3:
BUILDING OF TUNNEL
3.1 MAJOR COMPONENT OF TBM
Figure3.1 Typical Layout of a TBM
3.2 OPERATING SEQUENCE OF TBM
The TBM moved forward as it excavated

the tunnel by extending the pushing jacks
at the back. When the advancement of the
machine reached distance of the length of
a ring, the excavation stopped and the
pushing jacks were retrieved, a concert
circular ring in form of a numbers of
segments were then put together at the
tail of the shield. The pushing arms were
once again extended in full contact with
the concert ring just erected and
excavation resumed. The cycle of
excavation and ring erection repeated as
the TBM advanced to form the lining of
the tunnel.
3.3 SURVEY METHODOLOGY
3.3.1 Pre-Construction Stage
Step 1
Identified the geographical extent of the construction works involved and designed a
scheme of survey control network to cover the area .
Fig 3.3 Tunnel alignment
Step 2
Carried out a reconnaissance survey on site to identify the known control stations
nearby and established the new survey stations.
Figure 3.2 The lining of the tunnel is formed
by a continuation built up of the rings
TUNNEL ALIGNMENT
SANKAR VIHAR
PURAM

Step 3
Set up a survey control network, the new stations were rigidly tied to the known
stations.
Figure 3.5 Survey Control Network
Step 4
Carried out field measurements of angle and distance among the stations followed by
computation of global coordinates of control stations.
All field measurements were to meet the following acceptance criteria’s before
computation was performed.
Figure 3.4
Control stations

Figure3.6 Survey field work and computation at office
1) The spread of a repeated angle measurement should not be more than 3”.
2) The spread of a repeated distance measurement should not be more than the
measuring accuracy of total station (2mm+2ppm), 5mm for the 2.5km as an example.
The global coordinates of the stations was finalized and would be made use of for
construction as primary control stations. The accuracy of the stations is better than
1:50,000
3.3.2 CONSTRUCTION STAGE
Step 1
Prior to the initial drive of TBM, secondary control station was established at the
TBM Launching Shaft at surface by transferring co-ordinates from the primary
control stations.
Figure3.7 Establishing Secondary Control Station

Step 2
Transfer the secondary control station from surface at the TBM Launching Shaft to
the tunnel control station at underground level.
Figure 3.8 Transfer of secondary control station
Transfer of control station from surface to underground
Step 4
Traversed the temporary control stations at the erected rings above the TBM back up
gantry to reach the Laser Station located about 30m behind the TBM.
Figure 3.9 Transfer of control station at the TBM back up gantry

Step 5
The Laser Station carried the coordinates from the control station shot the prism
target affixed to the bulkhead of TBM to determine the absolute spatial coordinates
(x,y,z) of the TBM at that point. The tunnel guidance system and the dual axial
inclinometers simultaneously measured the amount of rotation along the three
perpendicular axis of the TBM to determine the orientation of the heading of the
machine.
Step 6
The laser station with the built in robotic mechanism tracked the prism continuously
as the TBM advanced, updating the spatial position and the orientation of the TBM
in every 10 seconds.
Figure 3.10 Processing of captured data of the moving TBM
The system linked to the TBM control cabin (Figure 3.11), where on the screen the
positional deviation of the TBM with the Design Tunnel Axis was displayed (Figure
3.12) instantaneously in graphical and numerical formats at all times to aid the pilot
to steer the machine.

Figure 3.11 The screen display of the Figure 3.12 Control cabin
TBM and its deviation from the design path.
The extension of the Articulation Jacks allowed the TBM to turn flexibly and
advance forward in the direction of the design tunnel axis.
Figure 3.13
Figure 3.14

Tail Skin Clearance between the segment and the tail skin. The elongation of all
pushing jacks and the shield tail clearance were measured by sensors and sent to the
computer to derive the position of ring just erected (Figure 3.15)

CHAPTER 4:
SOFT GROUND TBM OPERATION
There is an increasing world-wide demand for conditioning of soil especially in
connection with the use of both Slurry Shield Machines and Earth Pressure Balance
Machines (EPBM).
Tunnel building for infrastructure projects (subways, sewers, water supply, etc) often
takes place in soft ground under urban areas. The risk of settlement and consequent
damage to structures above is high, and almost unlimited claims could arise. As a
result of improvements to both the slurry shield machines and the earth pressure
balance machines (EPBM), such risks have been reduced.
However, even the most advanced tunnel boring machine will face problems in
mixed and changing ground conditions which it cannot excavate in a safe, efficient
and economic way. Instead of costly changes and adaptation of the machine, even if
possible, it is usually simpler to treat the ground in order to provide properties which
the machine can handle.
With the Slurry Shield Machine process this can be achieved by pumping water
and/or bentonite slurry into the tunnel front and excavation chamber. The bentonite
slurry helps to maintain an even over-pressure in front of the TBM cutter head and
also acts as an aid to soil transportation by pumping.
With the EPBM technique, soil conditioning products are generally injected ahead of
the cutter head and of-ten into the working chamber and screw conveyor. By
correctly choosing these products and their composition to match the requirements of
the encountered soil and ground water conditions, they can:
• reduce stickiness of plastic clays (that can lead to blockage of muck conveying
system) by TBM both with and without a shield
• lower the angle of internal friction and abrasiveness of the soil slurry (in order to
reduce power for soil extraction and conveyance and also the wear costs)
• create plastic deformation behavior (providing an even and controlled supporting
pressure increases the stability of the face and reduces segregation and the
consequently risk of settlement)

• adjust the soil consistency to enable tunneling by EPBM
• reduce the soil permeability to minimize water ingress.
The EPBM - in comparison with the Slurry Shield Machines, makes the on-site muck
handling easier and eliminates the need for a sophisticated separation plant.
The two types of TBM are shown schematically in Figure 3.1 and 3.2.
Figure 4.1: Schematic Representation of Slurry Shield Machine
Figure 4.2: Schematic Representation of Earth Pressure Balanced Machine (EPBM)

CHAPTER 5:
SLURRY SHIELD MACHINE
5.1 GENERAL
The slurry shield method is applicable to a wide variety of grounds, from clay to
sand and gravel, with hydraulic conductivity (K) between 10–8 m/s and 10–2 m/s
under varying charges of water. However, for ground with high silt or clay content,
problem may result in the separation plant.
Slurry: The slurry (sometimes known as mud) helps remove the cuttings, maintaining
the front face and preventing settlement; it also cools and lubricates the tools. It
comprises a suspension of bentonite in water with appropriate additives. The slurry is
prepared at the surface in tanks and is circulated in the slurry feeding pipe (suction
line) to the front face in order to help remove the cuttings from the bore. It is then
circulated out in the slurry discharge pipe.
The Slurry is the vital link between the Slurry Shield and the ground, and the success
of the excavation will depend on its performance.
There are several types of polymer additives that can be used to improve the
rheological properties of bentonite slurries. The use and choice of polymer additives
is determined by the ground conditions. Polymers types include: Bio-polymers,
CMC, PAC and Polyacrylamide. The following ground conditions can be treated
with the use of polymer additives:
• Soil with high salt content. The use of polymers, particularly CMC, can make
bentonite slurries less sen-sitive to salt contamination
• Soil with heavy clay: The use of polymers will reduce the clay dispersion, as a
result, the slurry will maintain its functionality longer
• Generally, polymer additives can be used to increase the slurry yield stress and
viscosity.
5.2 PERFORMANCE REQUIREMENT
When excavating with a Slurry Shield Machine, the Slurry must be designed in
respect of concentration, viscosity, filtration, etc, to suit the type of geology and the
type of equipment used.

The primary function of the slurry is to stabilize the face. It is also required to
suspend and transport the cuttings, to lubricate and cool the cutting head, and to
reduce abrasive wear of the cutting tools.
The technical requirements of the slurry shall be specified by the contractor and will
depend on the machine being used and the geological conditions. The programmed
should also detail the additional chemical dosing and mechanical treatments
(screening, hydrocycloning) for re-cycling the slurry followed by the procedures for
discharging waste slurry according to local regulations.
Controlling the site manufacture, maintenance and treatment of the slurry to meet the
required performance requires an experienced specialist slurry engineer. The site
laboratory will run tests, at regular specified intervals, to ensure the designated
properties are in line with the slurry specification. If necessary, the slurry will be
treated, circulating via a by-pass until the parameters are fully restored in the holding
tank as well as in the circulating line.
5.3 TEST METHODS
The slurry test programmed will generally include requirements for the following
key parameters:
Table 3: Slurry test parameters

CHAPTER 6:
EARTH PRESSURE BALANCED MACHINE (EPBM)
6.1 GENERAL
A modern EPBM drive combines knowledge of three main subjects:
• Soil mechanics (pressure support and soil characteristics)
• TBM technology (cutter head design, installed force ...)
• Soil conditioning additives.
Only a good comprehension and interaction between these aspects will result in a
successful TBM drive and commercial success.
The control of face support is a major issue in EPBM tunneling. Continuous support
of the tunneling face must be provided by the excavated soil itself, which should
completely fill the working chamber. The required support pressure at the tunneling
face will be achieved through:
• Thrusting the shield forward - by means of hydraulic jacks - against the soil mass
(force equilibrium)
• Regulation of the screw conveyor-rotation (volume equilibrium).
The support pressure has to balance the earth pressure and the water pressure.
Depending on soil characteristics and the cover to diameter ratio (t/D) different types
of earth pressures are to be determined.
6.2 AREA OF APPLICATION FOR EPBM EXCAVATION
Figure 6.1 indicates typical particle size distributions for the use of EPBM, It can be
used as a guide in order to give an idea of the soil conditioning needs in different
ground types.

Figure 6.1: Soil conditioning needs of EPBM in different ground types (Boundaries
are only indicative)
6.3 PRODUCT PERFORMANCE REQUIREMENT
The following product types are intended to achieve one or more effects:
• Foam:
Maintenance of pressure, fluidizing effect for the soil, creation of a
homogeneous soil paste, permeability reduction, lowering of torque, reduction of soil
stickiness, reduction of abrasion
• Anti-clogging agents:
Mainly used for heavy clay soil
• Other additives:
Structuring effect on non-cohesive soils, stabilizing of foam or soil, water retention,
viscosity effects.
• Anti-abrasion agents: to add to very abrasive soils or rock formation, to reduce
wear of the cutting head and its tools, extraction screw.
These products should be environmentally acceptable and safe to handle with normal
site precautions.

6.3.1 FOAM
6.3.1.1 Soil Conditioning: Choice of Foam Types
The Foam type chosen should match the properties of the soil to be excavated - see
figure 6.2.
Foam type A: high dispersing capacity (breaking clay bonds) and / or good coating
capacity (reduce swelling effects)
Foam type B: general purpose, with medium stability
Foam type C: high stability and anti segregation properties to develop and maintain a
cohesive soil as impermeable as possible.
Table 4. Product types for EPBM relative to different soils (FIR values are indicative
only)
6.3.2 ANTI CLOGGING ADDITIVES
In highly cohesive ground with high clay content, the anti clogging additives can be
used to prevent the clay from clogging of the cutter head, to reduce drive torque, and
to make it possible to fill the working chamber.
6.3.3 POLYMER ADDITIVES
In some cases polymers can be added to improve foam stability or adjust the
consistency of the soil passing through the working chamber and screw conveyor. A
typical example might be in wet, sandy soils with little cohesion.

CHAPTER 7:
TAIL SHIELD SEALANT
7.1 GENERAL
The sealant compound shall be designed to seal the tail end of the TBM against
ground water (fresh or marine), grout and bentonite slurry (if used). It shall have:
• Good resistance against water and grout pressure
• Good anti washout properties
• Good wear protection for the brushes
• Good pumping properties over a wide range of temperatures
• Good adhesion to concrete and metal
• Good stability (no fluid separation) in storage and under pressure
• No harmful effect to the sealing gaskets
7.2 PERFORMANCE CHARACTERISTICS
Table 5. water resistance characteristics
Other characteristics such as density, colour, odour, consistency, flash point should
be as described by the product manufacturer and should conform to their stated
limits.

CHAPTER 8:
ANNULUS GROUTS AND MORTARS
8.1 GENERAL
This chapter deals with annulus grouts for Shielded Tunnel Boring Machines, where
lining segments are erected inside the shield. In a hard rock TBM it is usual only for
an invert segment to be present. However, this chapter is deemed to be appropriate
also for filling the void beneath the segment in this case.
During the construction of a tunnel with a shielded TBM a void is created behind the
segments which need to be filled with a pressurized grout or other similar material.
Failure to do this results in ground subsidence, asymmetrical loads on the concrete
segments and possible damage or leakage through the tunnel gaskets.
This chapter is designed around performance requirements rather than prescriptive
requirements. This allows more imaginative use of materials and allows for the most
effective solution for a particular application.
8.2 REQUIREMENTS
8.2.1 Structural Requirement
Grout is the important link between the surroundings (soil or rock) and the structure (precast
segments). The reasons for using a grout can be summarized as follows:
• To prevent flotation and heave
• To prevent surface subsidence
• To prevent misalignment of the segment rings
• To bond the soil and segments into a single component.
The grout may be pumped into position either through the tail shield or through holes
in the segments and needs to provide early support in the build area.
8.2.2 Logistical Requirement

There are also logistical requirements to be fulfilled due to mixing , transporting and
placing of the grout. These will depend on the nature of the project and will vary
accordingly. However, the grout:
• Should be pump able from the mixer or agitator to the point of placing without
segregation or bleeding, irrespective of the distance or time involved
• May need to remain workable for an unspecified period (up to 24 or 48 hours) for
long or difficult delivery schedules
• May need to stiffen or set quickly to provide rapid support to (invert) segments,
or to achieve an early strength to reduce subsidence or prevent water washout.
8.3 CHARACTERISTICS
The general characteristics of the grout and the working procedure shall satisfy the
following requirements;
• In the short term the grouting procedure shall prevent settlement prejudicial to the
safety of the environment
• In the long term the grout shall be a factor for water-tightness and durability of
the tunnel.
The grout shall have the following characteristics:
• have minimum water content sufficient to allow pumping but resist segregation;
• be of a suitable consistency and workability to fill the void created during shield
advancing;
• have limited shrinkage during and after hardening;
• set or stiffen quickly, where required to avoid settlement;
• resist segregation and bleeding in order not to block lines, pumps and tail seals
(less than 1%);
• resist wash-out from water entering the void from the surrounding soil;
• provide a long term homogenous, stable and low permeability ring around the
tunnel lining.
Accordingly the following aspects should be taken into consideration:
• The grout composition and type of admixtures and additives

• The setting and rheological characteristics of the grout
• The working conditions, shrinkage characteristics and injection pressures taking
into account the results from site investigations and the location of the water table
• The long term durability and strength requirements of the grout
• Quality control procedures and testing (both in the laboratory and in the field): in
particular the volume injected for each ring compared to the theoretical volume. If
the amounts injected are shown to be insufficient or the grouting imperfect,
secondary grouting as a complementary treatment shall be performed as soon as
possible.
8.4 PERFORMANCE REQUIREMENTS
All grouts shall have the following essential requirements:
8.4.1 Single Component Grouts
Figure 8.1: Plastic Consistency for single component grouts

CHAPTER 9:
HARD ROCK TBM OPERATION
9.1 GENERAL
Tunnel boring machines (TBM) have been used in hard rock for several decades and
a great deal of experience has been gathered. A distinction is made between open
gripper-type TBMs with conventional methods of rock support and shield machines
with a tubing-type (segmented) lining. However, the excavation process for the
drilling head is identical for both processes. A cutter head, rotating on an axis which
coincides with the axis of the tunnel being excavated, is pressed against the
excavation face; the cutters (normally disc cutters) penetrate into the rock, locally
pulverizing it at the contact surface between the ring and the rock, creating intense
tensile and shear stresses. As the resistance under each disc cutter is overcome,
cracks are created which intersect and create chips. Special buckets in the cutter head
allow the debris to be collected and removed to the conveyor belt.
The open TBM uses its gripper system to support itself laterally on the tunnel walls,
so that the driving force is brought to the drilling head. The shield TBM has presses
which support themselves longitudinally on the tubing lining, enabling the forces to
be conveyed to the drilling head in the direction of driving. Hard rock is normally a
problem for cutters. The cutters become damaged and/or heavily worn and the
penetration rate is reduced. Frequent cutter inspections and changes reduce the utility
time of the TBM. Great efforts are constantly being made to increase the quality of
cutters. Much can be achieved by improving the steel quality in the rings and
increasing the size of the cutters.
The heat that is generated in the cutters by working on hard rock increases cutter
wear, and may lead to an increased occurrence of cutter clogging which further
increases wear on the cutter heads. Wear on the cutters results in higher costs and
increased TBM downtimes. To counteract this problem, chemical products have been
developed to reduce, clogging, abrasion and wear on the cutter head.

An additional problem in dry hard rock is caused by the production of dust due to the
fine particle size (chips) of the material excavated by the TBM.
This part of the guideline only covers ways of reducing cutter wear and improving
the dust control in hard rock TBM applications. Other hard rock problems are not
covered by this guideline.
Fig.9.1 Schematic Representation of a Hard Rock Tunnel Boring Machine
9.2 WEAR
9.2.1 Cutter disc Wear in Hard Rock TBM Tunnelling
In hard rock TBM tunneling, one of the most important economic factors is related to
cutter wear. This is in part caused by risks involved in the interpretation accurately of
site geological data, hence the difficulties in predicting accurately in advance cutter
replacement frequency. High cutter wear not only leads to high cutter replacement
cost, it also increases TBM downtimes and reduces TBM advance rates. Replacing
cutters is a time consuming process and invariably brings the TBM to a standstill.
The main factors that affect cutter wear are:
• Cutter characteristics
• Properties of rock (strength, hardness, abrasively, quartz content)
• Effect of Water
• Temperature.

9.3 ROCK ABRASIVITY CHARACTERISATION
The abrasivity of rock is affected by a number of parameters related to the rock. The
main ones are de-scribed below.
9.3.1 UNCONFINED COMPRESSIVE STRENGTH (UCS)
UCS is the most basic strength parameter of the rock, which gives an indication of
rock, bore ability. It is easily determined on cored rock samples, using standard test
method, for example, ASTM D2938. UCS is calculated by dividing the maximum
load at failure by the test sample’s cross section area:
σc = F / A (1)
Where:
σc – Unconfined compressive strength
F – Maximum failure load
A – Cross sectional area of core sample.
9.3.2 INDIRECT TENSILE STRENGTH (TENSILE SPLITTING STRNGTH)
Indirect tensile strength provides a measure of the toughness of the rock as well as its
strength. Standardized test methods are available, for example, ASTM D3967. It is
measured by applying a load perpendicular to the axis of the core sample. It is
calculated according to the following formula:
σt = (2 * F) / (π * L * D) (2)
Where: σt – Indirect tensile strength
D – Diameter of core sample
F – Maximum failure load
L – Core sample length.
9.4 APPLICATION REQUIREMENTS
Current state of the art hard rock TBMs are equipped with a water spraying system.
This normally includes 1 water supply line to the cutter head, a splitter box and small

diameter pipe work (12 mm) to the different injection points on the cutter head.
Sometimes the injection points are equipped in addition with sprinklers in order to
create a fine water spray. The use of Anti-Wear Additives requires a change or
update of this water spraying system:
• Installation of foam generator
The foam generator can be manually controlled but fully automatic ones are
preferable because failures can be easily detected from the TBM control room.
• Installation of rotary coupling for both water and foam injection
The rotary coupling ensures that 1 foam gun is directly linked to only 1 or 2 injection
points on the cutter-head. This enables the setting of different foam parameters to
defined injection points, which is necessary to obtain increased foam quantity on the
cutter head periphery. The rotary coupling has to have as many foam lines as the
foam generator, plus additional 1 or 2 water lines. This makes it possible to inject
both water & Foam on the cutter head through different injection points.
• Installation of bigger diameter pipe works (minimum 25 mm)
If the existing pipe work to the injection points is less than 25 mm, the foam will be
unstable and decomposes back into water & air before reaching the injection points.
• Additional installation of foam injection ports and pipe works
The water sprinklers destroy the foam. Water spraying systems need to be shut down
during the operation with foam.
The dimensioning of the number of foam guns is recommended as follows:
• TBM diameter <3m: 1 gun
• TBM diameter >3m and <5m: 1 - 2 guns
• TBM diameter >9m: 4 – 5 guns
9.5 DUST CONTROL
The chapter concentrates on the dust formed by the excavation process and not from
other processes during the tunneling operations such as rock support.

9.5.1 DUST FORMATION
In hard rock TBM tunneling, the cutters located on the cutter head rotate
continuously under a strong thrust force on the rock face causing the rock to crack
and form chips, allowing excavation to take place. As cutters impact on the rock, and
during its continuous rotation on the rock face, rock dust is formed, which rapidly
becomes air-born due to high air turbulence. The air born dust soon find its way
through any openings, forming dust clouds in the TBM working area just behind the
TBM cutter head. Rock chips are normally transported by conveyor belt to the
surface. Transfer points are also locations where rock dust can become air-born and
create high dust concentration.
High concentration of fine dust in the TBM working area and in the area behind it
can be a cause of eye and respiratory irritation to the TBM operators. In the case of
silica dust, it may also cause silicosis. Dust there-fore represents a serious long term
health risk. For this reason, there is strict legislation in some countries limiting the
maximum dust level permitted in working areas.
High dust concentration also has a negative effect on working conditions and
productivity. The performance of high tech electronics on the TBM can also be
affected. Surfaces that are covered in a layer of dust may become slippery increasing
the risk of personal injury.
Additionally, high dust concentration can reduce the visibility of drivers and other
workers, increasing the risk of accidents.
9.6 DUST CONTROL TECHNIQUES
9.6.1 VENTILATION
The quality of the air in the working area should not endanger the safety or the health
of the workers. For this reason, proper ventilation should be provided throughout the
work place. For dust control, the appropriate dimensioning of the ventilation system
is of great importance. To prevent the dispersion of airborne dust while using
TBM’s, stone breakers, conveyor belts and at muck transfer points, extraction
(suction) ventilation should be installed as close as possible to the points of dust

generation. The extracted dust should then be filtered out and deposited through a
suitable collection sys-tem.
9.6.2 WATER SPRAY
Water sprays are also used to help reduce dust. Water spray wets the surface of
broken rock, preventing dust formation, as wetted fine particles normally adhere to
the rock surfaces. This requires adequate distribution of water spray nozzles on the
cutter head, and a sufficient quantity of water. In order to minimize dust formation, it
is important to ensure that the water spray continuously wets out all the rock surfaces
in the breaking process. The timely wetting of rock chips during the breaking process
is necessary, as once the dust is airborne, water is relatively ineffective at capturing
it. Damp airborne dust may give problems in the extraction ventilation and dust
filtration equipment.
One of the other disadvantages of water spray is that the high water jet velocities will
create additional air turbulence that can contribute to the creation of more airborne
dust.
9.6.3 FOAM SPRAY
Foam is one of the most effective ways to reduce dust in hard rock TBM excavation
but should always be used in combination with extraction ventilation. The foam is
injected through special ports located on the cut-ter head, and spreads out rapidly to
cover whole rock face. The thin films of the foam bubbles wet out broken rock (like
water spray), so reduce air born dust formation. Unlike water, foam attracts dust
particles and also has strong staying power, forming a continuous matrix in the voids
of the excavated rock. This forms a virtual seal which captures and blocks out the
dust that would otherwise have become air born on the rock face side.

CHAPTER 10:
RISKS WHILE TUNNELLING
Elements of risk exposure during excavation works are several.
The most important ones are:
• Submersion by water;
• Fire and explosion;
• Difficulties due to geotechnical external factors :
- Damages due to tunnel collapse or detachment of rocks
- Damages due to unexpected geological conditions;
• Difficulties due to an inappropriate choice of the machine;
• Difficulties due to the inexperience of the operator;
• Difficulties due to the choice of the tunnel alignment;
• Difficulties due to machinery breakdown,
• Breakthrough location.
We will go through all of them. We would like to comment that some of the events
described not necessarily are losses recoverable under the “All Risks” Policy or its
section covering the TBM this will depend on the extent of cover purchased.

CHAPTER 11:
SETTLEMENT
Tunnel construction by TBM will cause settlement. This settlement is a result of
ground loss into and around the TBM, commonly known as “face loss”, and this is
measured as a percentage of the theoretical tunnel bore volume (% face loss). Face
loss occurs during construction owing to stress release of the surrounding ground
during the excavation phase and over excavation of the tunnel.
11.1 PREDICTION OF SETTLEMENT
The most common form of assessment for likely settlement is the semi-empirical
method based on a 2-dimensional approach transverse to the tunnel. This method
approximates the settlement trough to a Gaussian curve. For TBM tunneling this is
usually sufficient to establish the potential settlement that can be expected. The
profile of the trough will depend on a number of factors such as tunnel diameter,
tunnel depth, face loss and the settlement trough width factor (a factor that is
dependent on soil type and condition).
It should be remembered that settlement does occur in 3-dimensions, so the “bow-
wave” ahead of the tunnel needs to be considered. This curve is approximated to a
cumulative probability curve.
Where multiple tunnels occur (for instance in a metro system with tunnels for each
direction of train travel) the effects of the tunnel construction are considered to be
cumulative, and the curves can be superimposed.
For non-TBM tunnels with complex configurations of tunnel construction it is now
fairly common to undertake complex numerical analysis to assess likely ground
movements.
The area affected by tunneling induced settlement is known as the zone of influence.
For TBM tunnels the zone of influence is centered along the centerline of the tunnel,
and as a rule-of-thumb extends to a distance approximately equal to the depth of the
invert below ground level, on either side of the centre line.

11.2 PREDICTION OF DAMAGE
The factors that can lead to damage in buildings are generally rotation, angular strain,
relative deflection, deflection ratio, tilt, and horizontal strain.
Table 7: Classification of Building Damage
Table 8: Damage Categories
The strain is calculated by approximating buildings to being a deep beam located on
the ground surface. This beam is then analyzed for hogging as it assumes the shape
of the settlement curve using Bending theory. This bending causes strain in the
building, leading to cracking, differential settlement, and eventually structural
failure.

CHAPTER 12:
CONCLUSION
The invention of the tunneling machine has revolutionized tunneling
history indeed it had revolutionized the creation of spaces under our
cities allowing metro systems, water and sewage systems, and
underground cable networks, all to be built in a safe and sustainable
manner.
History has taught us that each development of a new machine, which
will eventually result in progress of the tunneling industry, may present
short term challenges to the underwriter.
TBMs are very varied and their suitability for different soil conditions
means that the correct choice of machine and the level of experience of
the operators is critical in their successful use.
Commercial considerations and pressures on the different parties
involved in the choice of machine may affect the risk levels an
underwriter may face.
Closer cooperation between the tunneling machine suppliers, contractors
and insurers should allow insurers to develop in the future methods of
clearly differentiating the levels of risks involved in insuring these
machines. More exchange of information about losses will allow insurers
to more closely match the industry's perceptions of the level of risk.
By the very nature of the conditions in which a TBM works, it will
always be a relatively high risk piece of equipment that needs to be
underwritten by specialist underwriters with knowledge of the tunneling
industry. The lack of enough accurate statistics to date does not allow
this type of equipment to be underwritten using standard statistical
insurance methodology.

BIBLIOGRAPHY
1. Training, which I did in tunneling, on the metro site of Delhi, which was under
construction under Shanghai Urban Construction Group (SUCG), joint venture
with L&T.
2. Andrew Hung Shing Lee, “ENGINEERING SURVEY SYSTEM FOR TBM
(Tunnel Boring Machine TUNNEL CONSTRUCTION)”, Hong Kong ISBN
978-56-56-65-55
3. Kolymbass, 2005 “TUNNELING & TUNNEL MECHANICS”, 2005, ISBN
456-23-59-20-12
4. EFNARC Association House, "SPECIFICATION AND GUIDELINES FOR
THE USE OF SPECIALIST PRODUCTS FOR MECHANISED TBM IN SOFT
GROUND AND HARD ROCK"., 99 West Street Faranham, Surrey, UK ISBN
111-419-23-45-71-83
5. M. Cigla, S. Yagiz & l.Ozdemir,"APPLICATION OF TUNNEL BORING
MACHINES IN UNDERGROUND MINE DEVELOPMENT" ISBN 298-826-
67-91-55
6. Prof. jian ZHAQ “A GENERAL OVERVIEW ON TUNNEL BORING
MACHINES”, rock mechanics and tunnelling. ISBN 925-45-20-88-51-59-61

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