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Underground Excavation is Here to Stay
Photo courtesy: Herrenknecht Asia Ltd
Sub-terrain excavation and drilling to provide sustained connect for utilities, in urban
spaces, is the essence of the challenge. It is adequately met by using micro-tunnelling
and trenchless technologies.
P
ower, water and gas supply;
modern high-tech electronic
communications; Internet highways and connectivity linkages;
provision of geo-sensitive alarms and
signalling systems; safety connectivity;
sewerage disposal and waste management
facilities and many other such utilities
are the undisputable essentials of modern urban and infrastructure development
in the country.
The above utilities need to be provided under the surface, in order to make
them robust, reliable and sustainable,
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under the most adverse climatic conditions and undisturbed from the numerous activities on the surface like traffic
and other activities. Most developed
countries have established norms, standards and laws governing the provisions
of various utilities. However, in India,
despite availability of sub-terrain drilling
technologies, the implementation of the
same has been highly self-discretionary
so far. Going further, driven by the government’s impetus to modernising the
utilities across the entire space of infrastructure and its plans to develop Smart
Cities in India, it is safe to state that
micro-tunnelling and trenchless construction is the need of the hour.Therefore, the government is bound to bring
in regulatory statutes, making it mandatory by law in the country.
Prashant Mehta, Director, Kem
Tron Technologies, reinforces this view
by commenting, “The oil & gas sector
has planned to install 2,500 km of pipeline in the coming two-three years.With
the strict environmental norms, there is
a huge scope of using the recycling units
for the river/highway/canal crossings to
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Photo courtesy: Kem Tron Technologies
this equipment has been present for a
long time, it is only now that a whole
range of new markets have opened up,
and the growth story is expected to
continue for a long time to come.
Coming to specifics as related to
micro-tunnel boring machines, Anand
Dwarkanath, South Asia Representative (Utility Division), Herrenknecht Asia Ltd, explains the challenges
and the need for mitigation. He states,
“Micro-tunnelling, as with all tunnelling,
comes with construction risks that need
mitigation during construction planning
phase, equipment configuration and also
during construction phase.The equipment
supply business for micro-tunnelling is
unlike other construction equipment
businesses such as supply of excavators,
dumpers, tower cranes, etc.”
There is a major scope for trenchless equipment in the market.
be installed where the use of bentonite
slurry is required for operation. It has
become mandatory to use our equipment
for the same to avoid any environment
issues and pollution.”
Challenges and solutions
To begin with, India has a long history of evolution of infrastructure development spanning over many centuries.
At every stage, depending upon the
comprehension of the implementers,
available engineering, technologies and
work methods, new systems were built
many times over the existing facilities,
and the older ones were made redundant
and discarded in an as-is-where-is condition.There was no standardised approach,
nor were the same documented, and the
people involved are not alive anymore.
This is one of the most significant and
complex challenges involved in sub-terrain infrastructure development.Take for
example cities like Kolkata, Mumbai,
Chennai, New Delhi and other old cities
on one side, and our first known national highway— Sher Shah Suri Marg — which
was built much before the British era.
Secondly, the geological uncertainties
of unexplored sub-terrain conditions
throw up many unforeseen, unpredictable and unprecedented challenges
during construction.
Modern imaging and soil investigating
methods provide some relief. However,
depending on the dimensions and shapes
of small underground excavation required,
different micro-tunnelling and trenchless
equipment provide the best possible
solutions in today’s world. In India, although
“Micro-tunnelling comes with
construction risks that need
mitigation during construction
planning phase, equipment
configuration and construction
phase.”
Anand Dwarkanath,
South Asia Representative (Utility Division),
Herrenknecht Asia Ltd
Micro-tunnelling option
Dwarkanath, an expert in tunnelling,
defines the term, “Micro-tunnelling is
defined as a tunnelling method during
which an unmanned tunnel boring machine is controlled remotely in pipe
jacking from a control container on the
surface. The process often involves
active ‘face pressure’ control to keep
the pressure in the machine excavation
chamber balanced against the ground
pressure and water pressure faced
during excavation.”
He adds,“Simply put, micro-tunnelling
Micro-Tunnelling
Applications
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Water supply pipelines, gravity
sewer installation with different
pipe materials such as vitrified
clay, RCP, steel, composite, polycrete or even DI pipes
River crossings for oil & gas pipes
Railway crossings for multi-utility applications such as high-voltage cables
Casing pipe installation for multiple carrier pipe applications
Pedestrian subway installation
using pipe-arch method
Sea-water intake and effluent
outfall applications for power
plants and process plants.
- Anand Dwarkanath
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Tunnel Alignment Methods
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Electronic Laser System (ELS) comprising an active laser target (installed
in the machine), a laser source (installed in the shaft), and a measurement
wheel (length encoder) for pipe drive length measurement, together gives
accurate straight or constant inclination tunnels for recommended distances up to 200 m.
From tunnel length of 200 m to 400 m, length increases may begin to refract
the laser beam upwards as a result of climatic conditions in the tunnel, thus
affecting vertical deviation (grade) measurements. Therefore, in this case,
millimetre level machine grade control is then achieved by a Hydrostatic
Water Leveller (HWL) along with ELS. The HWL is a pressurised water
tube that enables monitoring the elevation conferred pressure difference
at the machine and shaft end. The difference in pressures indicates the
actual elevation change of the machine.
For over 400 m drive lengths, the laser beam turns weaker and diffused to
a higher spot diameter that also ‘dances’ on the target. Therefore, a gyro
device (which is a north-seeking gyrocompass) is used instead of a laser
system.The gyro-compass is mounted on the machine to determine accurately the horizontal deviation of the latter. The elevation is checked with
the help of the HWL system. Gyro is also used for curved drives.
Operationally though, having advanced guidance systems does not absolve
a contractor from having their surveyor crew personnel to do checks every
day on the jacking shaft end. This is mainly to ensure that there have been
no shaft movements due to high jacking forces, resulting in laser beam shifts,
etc. For example, the laser mounted in the shaft should always be checked
in relation to the survey points.
- Anand Dwarkanath
equipment are miniaturised versions of
their large diameter counterparts, except
that they involve ‘pipe-jacking’ in contrast
to segmental lining. In other words,
lower diameter shields, cylinders, slurry
pumps, excavation transport components
such as slurry pipes (for slurry machines),
screw conveyors (in case of EPB machines)
are all smaller in size compared to their
large diameter counterparts.”
Mehta defines the same, “Micro-tunnelling is a tunnelling process where the
workforce does not routinely work in
the tunnel. Micro-tunnelling is currently the most accurate pipeline installation
method. Line and grade tolerances of
one inch are the micro-tunnelling industry standard. This can be extremely
important when trying to install a new
pipeline in an area where a maze of
underground utility lines already exists.”
He adds, “Micro-tunnelling is an underground digging technique used to construct a tunnel for laying utility pipes
lines like sewage, gas, water etc. Micro-tunnelling is a process that uses a
remotely controlled Micro-tunnel Boring Machine (MTBM) combined with
the pipe-jacking technique to directly
install product pipelines underground
“There will be major growth in
trenchless equipment in the near
future with the kind of infrastructure
planned by the present government.”
Prashant Mehta,
Director, Kem Tron Technologies
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in a single pass. This process avoids the
need to have long stretches of open
trenches for pipe-laying, which causes
extreme disruption to the community
at large.”
What micro-tunnelling entails
Dwarkanath explains, “Diameter
difference also results in lesser number
of cutting tools dressed on the cutting
wheel/cutting head and creates the
important difference of the location of
the operator who sits on the surface
outside inside the Control Container
for micro-tunnelling machines, whereas
he is inside the tunnel for large diameter machines.”
He adds, “In addition, the tunnel
lining method of precast segment installation necessitates a whole different set
of equipment to transport, feed and erect
the segments inside the tunnel in case
of larger machines. In the case of pipe-jacking, these are usually clay, reinforced
concrete pipes and steel pipes that are
successively jacked in short lengths behind
the machine, thus keeping the entire
tunnel in constant motion until the
machine reaches the exit shaft.”
Thus, micro-tunnelling is different
from segmental lining wherein the tunnel
is static and each ring’s position is fixed
as soon as it is erected. A large diameter
tunnelling machine needs a segment
erector to erect pre-cast lining, whereas the small diameter micro-tunnelling
machine needs a ‘jacking station’ located
at the entry shaft to push the pipes into
the tunnel.
The large diameter machine has no
requirement for bentonite clay slurry to
be injected surrounding the tunnel (in
the annular gap), whereas micro-tunnelling equipment above 1 m diameter (pipe
internal diameter) tend to have bentonite injection stations affixed in every 15
or so pipes that disburse bentonite in
the annulus in order to reduce ground
friction while pipe jacking.
Micro-tunnelling created pipelines
could be seen as ‘moving’, ‘floating’ pipelines that should be ideally jacked uninterrupted until the machine reaches its
destination shaft. Thereby the risk in
micro-tunnelling — pipe jacking is also
higher even though drive lengths are
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Why machines get stuck
underground
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Hard rock condition
Soft soil collapse
Very high discharge of underground
water
Improper tooling.
shorter than in large diameter projects
(100 m to 250 m average).
Mehta mentions, “It is remotely operated small diameter tunnelling and is
achieved by jacking pipes of concrete,
or other suitable materials, behind a
tunnelling machine, from a launch shaft
into a reception shaft.”
A typical micro-tunnel equipment
spread consists of an MTBM matched to
the expected subsurface conditions and
the pipe diameter to be installed; a hydraulic jacking system to pipe jack the
pipeline, a closed-loop slurry system to
remove the excavated tunnel spoil; a
slurry cleaning system to remove the spoil
from the slurry water; a lubrication system
to lubricate the exterior of the pipeline
during installation; a guidance system to
provide installation accuracy and an
electrical supply and distribution system
to power all of the above equipment.
Top side equipment used to support
the tunnelling operation usually includes
a crane, pipe driving and dewatering
equipment for shaft construction; backhoe and front end loaders for shaft
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excavation and spoil handling, and truck
transport for equipment movement.
Operational challenges &
mitigation
As mentioned earlier, micro-tunnelling entails what we might describe as
a moving and floating tunnel that under
ideal conditions, should have a fairly
smooth passage from the entry shaft
to exit shaft. It is a time-bound methodology where interruptions and waittimes, more than what would be reasonable, could translate quite rapidly to
a failed drive, whereby the machine and
the pipeline behind it can no longer be
pushed further. In other words the drive
is “stuck”.
Following from this potential eventuality, these are all the risk-mitigation
measures that need to be observed in
order to ensure a successfully completed drive. Having said that, drive completion is also not the only measure of
success. Equally important is the accuracy of installation, effect of the passage
of the TBM on nearby ground strata
thereby eliminating non-permissible
levels of ground disturbance, settlements
and the counter-measures taken such as
annular grouting. Also important is site
organisation efficiency that allows contractors sufficient width in time as well
as cost savings, to meet the above operational challenges.
In short, micro-tunnelling operation-
Mitigating operational challenges
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The main job of the operational person would be laying the pipelines
within the tolerance limit.
Practically, a lot of demanding situations are faced at the jobsite.
Detailed geological study should be conducted before the start of the
tunnelling process to identify the ground conditions.
When crossing railway lines, gas and power lines tunnelling process should
be carefully monitored so that existing infrastructure will not be affected.
The pipe is attached behind the machine, after every pipe advance, a new
pipe would be joined at the back side, forming a continuous tunnel. Here
the whole tunnel is pushed by a jacking rig which has limited capacity.When
the tunnel length increases, so does the frictional force. To overcome the
frictional force along the line, lubrication fluid should be pumped on the
periphery of the pipe. This helps to reduce the jacking load.
With through knowledge of the area where work is to be undertaken,
(through municipal and other official help of layout drawing of existing
facilities), certain hurdles on the job are taken care of.
- Prashant Mehta
On surface support
facilities
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Mud recycling unit
Mud mixing/preparation tank
Shaker Baskets with wedge type
screens
Bentonite pumping unit
Centrifugal pumps
Cranes and lifting tools
DG Set for power supply
Jacking Frame
High pressure water jet pumps
Prashant Mehta
al challenges are not all about the equipment. In fact, a major aspect of success
in micro tunnelling operations is site
preparation, readiness, efficiency of site
logistics etc., that are called in to support
the pipe-jacking process. Delays caused
due to diesel supply, mobile crane availability, lack of readiness of jacking shafts,
receiving shafts, lack of jacking pipes,
inter-jack stations etc., can lead to a drive
failure irrespective of how good the
machine is.
In contrast, laser guidance systems
that are a feature of micro-tunnelling
and pilot tube guided auger boring result
in straight lines, smooth bores and tight
gradient control. Drive accuracies better
than ±1 inch are normal.
The ground strata and its interplay
with the pipe-jacking process, accumulation of fines in the annular gap, leaking
tunnel pipes (bentonite leaks), site delays
and interruptions that would in unstable
ground end in the ground ‘catching’ the
pipeline are broad reasons that equipment
get stuck in the tunnel.
Sometimes, parts of the pipeline can
be freed up by injecting large volumes
of bentonite or additives surrounding
identified areas of the pipeline and the
jacking can continue. This is where advanced bentonite injection station systems
come into play, on their own. The ones
from Herrenknecht are normally installed
every 15 m of the pipeline and the bentonite can be injected in time as well as
volumetric measure.
Other mitigating measures a site
project manager could take, is to invest
in inserting an Inter-jack Station (IJS)
for introducing supplementary pushing
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Used equipment options
Trenchless Method
Horizontal Directional Drill (HDD) equipment is used for underground
construction. It is a trenchless method of installing a pipe in the ground at
variable angles using a guidable drill head.The technology is a preferred trenchless method for installing underground pipes, cables (OFC and power cables)
ranging from large size pipeline river crossings to small diameter cable conduits
and railway signalling cables. HDD is a proven, environment-friendly method,
which apart from being cost-effective, also imposes minimal disruption to the
community and surrounding infrastructure.
Depending on pipeline installation parameters such as length, diameter,
pipe material, geotechnical strata information, depth of pipeline, ground water
levels etc., it may be possible to choose alternative methods to micro-tunnelling. One can even consider methods that are close to micro-tunnelling such
as the Pilot Tube Auger Guided Boring Method. Competitive trenchless methods including Horizontal Directional Drilling (HDD), pipe bursting, pipe ramming etc., can also be looked at.
However, the pros and cons, capabilities and limitations of trenchless methodologies should be studied in detail before any method is finalised.
Pilot tube auger boring, for example, could be an economic accurate gravity sewer installation solution for small diameters in the range of 150 mm to
600 mm.The advantage is in ease and economy of operation. Also PVC, HDPE
pipes can be installed in this way. Moreover, there are now ways to install pipes
at these diameters in up to 20 Mpa strength ground using our front steer
system in hard rock up to 140 Mpa using front steer with air hammer up to
100 m with high accuracy. However, there are bound to be some limitations
to technology use.
On the other hand, HDD is suited for pumped sewerage and water supply
pipelines, but is not considered a suitable method for gravity sewers. The drill
hole is much bigger than the pipe it carries. In homogenous ground, it is difficult to ensure a smooth bore, resulting in undulating inverts, and drill bit deflections often result in a deviated path lying between point A and B.
- Parth Patel, Director, Apollo Techno Equipments
force available for use along the pipeline.
Factors inducing IJS installation are
primarily the total length of the tunnel
influencing the sum of ground friction
expected along the pipeline, along with
other predicted ground strata-related
risks such as soil/rock permeability resulting in loss of bentonite to the nearby ground, presence of structures above
or near the pipeline such as a concrete
foundations, manholes etc., that can weigh
on a pipeline and increase the overall
jacking forces.
However, if the pipeline refuses to
move despite all good efforts, then the
only practical option usually is to excavate a rescue pit and pull the machine
out with a mobile crane. It may be
possible to convert the rescue pit into
an intermediate shaft/manhole and drive
into it from the other end based on
project design.
“Apollo Techno Equipments is the first
in the country to manufacture
Horizontal Directional Drill
equipment.”
Parth Patel,
Director, Apollo Techno Equipments
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While it is true that second-hand/
used equipment are often available, they
are also often poorly maintained or not
in working or warranty-worthy conditions when they put up for sale by an
owner/contractor. Often, the equipment
is sold ‘as-is’ and this is not convenient
for the buying party from India, who
would have greater expectations of
performance from the equipment set.
This is especially true for projects with
risky tunnel layouts, topography, location
(either in hinterland or built-up areas)
and most importantly where geotechnical conditions (ground strata) are
complex or less predictable. From a
cost perspective too, prices for ‘as-is’
equipment could be unrealistically high
in relation to its condition.
The above would provide some
insight as to why surplus equipment
lying around elsewhere doesn’t automatically translate into a great purchase
opportunity for Indian contractors. In
the end these equipment need to ripped
apart in a qualified facility, critical items
need to be checked for the life-worthiness, hydraulic and electrical parts need
replacing, the equipment needs to be
put together and tested to be working
together as a unit worthy enough of
re-warranty. It is for this peace of mind
that a buyer should be willing to part
with his precious dollars.
The question of going in for an imported or local make is a completely
different one. This usually relies on the
ability of the equipment manufacturer
to source a niche, repetitively reliable
supply chain that helps them determine
a high build quality.
On the other hand, India has been
waiting since long on independency of
indigenous Horizontal Directional Machines for laying telecom OFC, power
cables, gas line connectivity, water and
sewer pipes, and other utilities. Indian
contractors now have better opportunities to procure machines in the least
possible time with the availability of spare
parts and engineer support to contractors across the nation.
Patel says, “Apollo Techno Equipments
is the first in the country to manufacture
Horizontal Directional Drill equipment.
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Advantage Trenchless
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Pipes can be installed either in straight lines or curved to avoid any obstacles.
Working area is confined to the points of entry and exit only, causing less
disruption and aiding.
No disruption to road, river or rail traffic and less impact on landscapes.
Short, medium or long lengths of pipe can be installed without the need
for intermediate pits.
Horizontal Directions Drilling is a much quicker process and in many
cases has lower overall costs due to fewer workmen required.
It’s unaffected by surface obstacles.
Can be used to install pipes in changeable ground system.
Flexibility (can be used anywhere).
Less soil contamination (reduces damage to valuable resources).
Optimises safety and hygiene at work.
Reduces impact on environment.
-Prashant Mehta
tions and are also put through more
number of work cycles. The models on
offer for Indian markets are A400 to
A800 with pulling force ranging from
12 to 20 tonne and torque from 5,500
to 6,500 NM, thereby providing a viable
alternative to Indian buyers against
using used equipment.”
Market size & drivers
Micro-tunnelling equipment is ver-
Photo courtesy: Apollo Techno Equipments
The world-class Horizontal Directional Drilling equipment is ideally suited
for the unique Indian working conditions.
The company offers various types of
HDD equipment. Apollo Techno Equipments’ HDD machines are the perfect
testimony to the ‘Make in India’ capabilities, offering unmatched performance.
Every aspect of the machine has been
made for India where such equipment
need to take on tough working condi-
Apollo Techno Equipments’ HDD machines are the perfect testimony to the
‘Make in India’ capabilities.
64
satile and suitable for various grounds,
covering many different cities of India.
Their biggest use is for creating underground sewerage and water transmission pipelines.
In many cases, these sewerage pipelines are gravity flow based, therefore
high accuracy in gradient is required.
Micro-tunnelling machines with their
advanced guidance features have a big
role to play in such projects.
Smart Cities are also by definition
meant to be environmentally sustainable cities. Therefore it goes with the
assumption that infrastructure such as
sewer lines, cable lines, multi-utility
pipelines and water pipelines are already
laid out to cover the needs of future
generations right from the beginning.
Once the roads have been surfaced and
infrastructure by way of above-ground
structures is cemented, re-laying and
re-digging defeats the purpose and
creates huge environmental costs. We
have several examples of this situation
all over India and abroad.
From a futuristic angle, investors
(government or private) can create
tunnel infrastructure that can be rented out to interested parties in the
future. This may require slightly larger
pipes or one large pipeline crossing
the city with tapping points in between.
That is why railway corridors have
been crucial when it comes to cutting
across already built-up infrastructure
in cities like Mumbai. Private Internet
providers etc, have benefited from
these arrangements in the past. Smart
Cities would substantially drive demand
only if the planners take into account
the capabilities of modern micro-tunnelling equipment.
Mehta says, “There is a major scope
for trenchless equipment in the market.
Almost all authorities are required to
adopt this technique for the safety of
the utility installed and also minimum
hindrance to the above-ground services. There will be major growth in this
field in the near future with the kind of
infrastructure planned by the present
government.”
EI
The future lies underground!
- SHANKAR SRIVASTAVA
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