Trenchless Technologies

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B1.48 - Trenchless Technologies
WG B1.48
SEPTEMBER 2019

 Chapter 1: Introduction
 Chapter 2: Horizontal Directional Drilling
 Chapter 3: Pipe Jacking/Microtunneling
 Chapter 4: Pipe Ramming
 Chapter 5: Ploughing
 Chapter 6: Cable Rating and Bonding
 Chapter 7: Surveys and Design
 Chapter 8: Conclusions
Table of contents

Chapter 1
Introduction
B1.48 – Trenchless Technologies
September 2019

WG B1.48 Members
Country Name
Australia H. Kent (Corresponding)
Belgium B. Pelssers
France Y. Douima
Ireland E. Bergin (Convener 2013-2018)
Ireland E. Butler (Secretary)
Italy S. Chinosi (Convener 2018-2019)
Norway L. Opsahl
Norway V. Narby
Spain C. Bataller
Spain S. Fernandez
Sweden J. Hedlund
United Kingdom R. Atwell
United States J. Williams (Corresponding)

Chapter 1: Introduction
 Review the range of trenchless technologies currently available for cable
installation:
 Horizontal Directional Drilling
 Microtunneling / Pipe Jacking
 Pipe Ramming and Auger Boring
 Ploughing
Large tunnels are not included in the scope of this Working Group.
 Review the technical constraints (thermal, mechanical, civil, geotechnical and
environmental) relating to the trenchless installation of HV cable systems.
 Provide examples of where trenchless techniques have been used in the
installation of HV cable systems, highlighting the benefits and adverse
experiences in each case.
Technical Brochure Objective

Trenchless Technologies Abstract
Trenchless Technology Description
Pipes
Diameters Lengths
Pipes
materials
Horizontal Directional Drilling
The normal HDD process consists of drilling a pilot hole in the
ground, and then back reaming to open the pilot hole to take a
casing pipe.
200 mm to
1800 mm
Up to
2000 m
Casing pipe
material: HPDE
or Steel.
Conduits may then be pulled into the casing pipe and cables may
then be pulled into these conduits.
Conduits
material: HPDE
Microtunneling / Pipe Jacking
Casing pipes are installed by thrusting with hydraulic jacks at the
entry shaft at the same time as excavation takes place at the tunnel
face using a steerable shield.
150 mm to
3000 mm
Up to
2000 m
Casing pipe
material:
Concrete, steel,
mortar-filled
glass fibre
reinforced
plastic pipes.
Microtunneling: Fully automated non-accessible entry tunnelling
operations. Generally below 1200 mm diameter.
Pipe Jacking: Larger diameters requiring accessible-entry.

Trenchless Technologies Abstract
Trenchless Technology Description
Pipes
Diameters Lengths
Pipes
materials
Pipe Ramming and
Auger Boring
A steel casing is physically thrust through the ground by using pneumatic
hammers.
100 mm to
4500 mm
Suitable for
short lengths
(less than
100 m).
Casing pipe
material: Steel
Auger boring: Thrusts a steel casing whilst simultaneously excavating
the ground using a screw auger.
Limitation on accuracy and not steerable.
Ploughing
This method of cable installation basically consists of pulling a plough
through the soil and simultaneously installing a cable(s) or conduits at
the depth of the plough. Ø not
defined
Very long
installation
lengths.
Direct burial
normally
preferred.
Mainly used for voltages up to 150 kV.
Plastic pipes
possible.It is not a trenchless technology but due to the speed of installation it
has been considered in this Technical Brochure.
Tunnel boring
Excavated by machine and depending on the geology are lined by
concrete segments in the walls of the tunnel, as the excavation
advances.
Large
tunnels,
typically
above 3
meters
diameter
This TB does
not cover full
tunnel
installation.
Casing pipes
material:
Concrete
Microtunnels above 3 m diameter cannot be readily managed on site
owing to their size and weight. This is the reason for the cut-off point.
This is already the subject of TB 403 Cables in Multipurpose or Shared
Structure.

Chapter 2
Horizontal Directional Drilling
September 2019

Chapter 2: Horizontal Directional Drilling
Construction process
1- Pilot drilling between pits (launch and exit pit)
 Drill starts at the launch pit where the drill rig
is positioned.
 A steering system is used to guide the drill
bit along the planned route from the entry to
the exit.
 Different guidance systems can be used
depending on the depth of the drill and
surface conditions.
 A drilling “mud” is often used to facilitate the
drilling.
 Drill rods are linked together to form a drill
string. These will assist operations in later
steps.
 Cuttings are removed and drilling fluids are
cleaned and recycled.

2- Back reaming
 After the pilot borehole is completed, the drill bit
is removed and a back reamer is attached to
the end of the drill rods.
 The back reamer is used to enlarge the hole.
 Increasingly larger sized reamers may need to
be pulled or pushed through the borehole until
reaching the required dimension, if the hole
dimension is significantly larger than the pilot
bore.
 The type of reamer selected must be suitable
for the soil or rock type.

3- Pipeline Pullback
 After the last back ream, a high strength swivel,
smaller reamer or barrel and a pulling head will be
added.
 Depending on the installation technique selected,
the pipe, a bundle of pipes or a bundle of cables, is
pulled inside the borehole.
 It is convenient to carry out all the described
operations in a consecutive way and avoiding long
interruptions to avoid risk of collapse.

Equipment
1- Drilling rig
Drilling rigs can be classified into four equipment sizes based on their capability:
Midi-rig Maxi-rig
Mini-rig
Service tool

Equipment
2- Drilling rod
 Link between the drill rig and the drill bit, reamer or pipe.
 Main functions:
 Transfer torque to cut the hole.
 Pull back to install the pipe/cable.
 Conduit to deliver the drilling fluid to the drill bit.
 Small diameter rods are flexible and provide the most
steering and the best control for small diameter boreholes.
The drawback is that the small rods are weak and
susceptible to fatigue.
 Large diameter rods are strong and can transmit the most
power for fast drilling of large diameter holes; the drawback
is less flexibility for steering and control.

Equipment
3- Drilling bit
 Leading section on the drill rod.
 Main functions:
 Cutting.
 Steering.
 The soil type determines the design and selection of the
drill bit.
 Fluid jets are incorporated in most drill bits to aid in cutting,
cooling, spoil transport and hole stabilization.
 The drill rods advance by a combination of rotation and
thrust applied through the drill rods to the drilling bit.
 Directional control is achieved by varying the rotational
speed of the cones in the tri-cone or changing the
orientation of the bit and water pressure if a jet bit is used.

Equipment
4- Reamers
 Reamers are tools that allow the enlargement of the pilot
hole.
 The reaming tool is secured to the rod string at the exit
point.
 The reamer is rotated and at the same time pulled by the
drill rig, thus enlarging the pilot hole.
 As the reamer proceeds, the new pull rods are attached
immediately behind it to ensure continuity of connections
inside the hole.
 Depending on the diameter of the casing pipe the reaming
phase may be repeated several times.

Equipment
5- Drill guidance systems
A tracking system is used to help guide and monitor the route of the pilot borehole. The four guidance
systems commonly used are:
 Wire-Line: Uses a cable through the drill string to transmit data from the drill head to the control console in
the cab of the drill rig. it overcomes the depth limitation of the walkover system. It also has a higher degree of
accuracy as the information sent to the control console is not interrupted by the different ground conditions or
the depth of the borehole.
 Walkover Locator: Uses a battery-operated transmitter (sonda)
located near the drilling bit. This transmits a magnetic signal
picked up by a portable hand-held receiver located above the
drilling. Limited to accessible crossings, depths up to 40 m and
non electro-magnetic sources disturbance.

Equipment
5- Drill guidance systems
 MGS (Magnetic Guidance System): Uses two magnetic fields.
Earth´s magnetic field to measure the orientation of the drill bit with
respect to the North-South axis. An artificial AC magnetic field is
created by laying electric cables over the area where the drilling is
going to be performed. This “known” magnetic field is used to
determine the perpendicular distance of the drill bit. The accuracy is
jeopardised in urban area where the presence of other utilities disrupt
the magnetic field.
 GST (Gyro Steering Tool): Provides and accurate 3D (longitude, latitude and height) reference for the
position of the drill via an optical fibre connection between the tool and the control centre. It uses the GPS
position of the launch pit and continuously measures the angle of the drill with reference to the launch pit, to
control the approach of the drill towards the exit pit.

Equipment
6- Drilling fluids
Drilling fluids usually consists of a water based mixture with either polymer and/or bentonite clay (with
additives). They have four important functions:
1. Remove soil cuttings from the hole.
2. Cooling the drill bit.
3. Preventing the soil collapse
4. Lubricating the drill pipe and thereby reducing friction and drag.

Equipment
7- Mud pumps / mud mixing / recycling
The drilling mud system mainly consist on:
 Mud mixing unit: Elaborates the drilling fluid with the
required properties (viscosity, weight, etc…).
 Mud cleaning/recycling unit: Pumps the returning mud from
the HDD pit, cleans the mud and then pump it back to the
mixing tank.
Mud cleaning
system
Mud mixing systemMud pump

Equipment
8- Pipes (casing) and conduits
 The most commonly used casing pipes are made of HPDE and Steel.
 Conduits are usually of HPDE and PVC, similar to that laid in trenched installation, but with a slighty
greater wall thickness.
 The casing pipes and conduits are usually welded or butt-fused at the launch-exit pit in a continuous string.
 The pipe/conduit installation must not be interrupted or stopped to reduce the risk of getting stuck in the
drilled hole due to collapse.

Mechanical and Cable Installation
The four main techniques are:
i. Drill the hole, enlarge and condition the hole and pull the cable/s directly into the borehole. Not usually
adopted because of the risk of soil collapse and cable sheath damage.
ii. Drill the hole, enlarge and condition the hole, install a large casing pipe into the hole and pull the cables
directly into the pipe. Risk of cable twisting and difficulty of retrieval only one cable of the bundle.
iii. Drill the hole, enlarge and condition the hole, install a large casing pipe into the hole, pull a bundle of
conduits into the casing pipe and pull a cable into each conduit. The cable is more easily accessed for
the purpose of retrieval.
iv. Drill the hole, enlarge and condition the hole, pull conduits into the hole without using a large casing
pipe and pull the cables into each conduit. Less mechanical protection of the installation.
Alternatively, it may be possible or necessary to drill individual holes for each phase, one cable (with conduit) per
hole.

As a general rule, the internal diameter of a duct should be (minimum) 1.5 times de cable outer diameter.
Factors that must be taken into consideration when installing the cables:
• Compliance with cable manufacturer´s for permissible pulling tensions;
• Length and profile of entire cable section avoiding cable joints on the trenchless section;
• Type of transition at entry and exit pits;
• Friction factor between cable type and duct.
• Whether the pipe is filled with water and if buoyancy applies.
A typical cable system may be the following:

Site Setup / Working Areas
The drilling site should be carefully chosen by first surveyinn the area to identify its boundaries:
 Legal issues, including landowner agreements;
 Environmental exclusions zones and vicinity impact (noise, traffic cuts, etc…);
 Adequate space for the working area;
 Preparation of the land by levelling/grading to allow access and to made stable the area;
 Water supply that will be required for the construction;
LAUNCH SIDE:
 Entire drill rig spread (Rig, Control Cab, pipes, etc…);
 Slurry Mixing Tank and Cutting Separation Equipment;
 Storage and Site Offices.
 Launch pit;
 Pit or containers for capturing waste materials;

Site Setup / Working Areas
EXIT SIDE: Space is needed for the exit pit and a pit or containers for capturing drilling fluids and spoil from
the drill.
 Exit Slurry Containment Pit;
 Product Pipeline (Space for laying out the conduits);
 Drill pipe;
 Construction Equipment;

Advantages and Limits of HDD Technique
Advantages
Safety: HDD avoids pitfalls since trenches are not necessary.
Convenience: HDD are less inconvenienced by traffic backups, dust and uneven pavements.
Conflict Reduction: Increasingly congested utility corridors and easement make it very difficult to place cable or
conduit by open trenching.
Route Selection: Drilling may allow for different or shorter routes.
Reduced Environmental Issues: Job-site excavation is minimised as is also the risk of excavation and
disposing of soils that may be contamined.
Cost Savings: HDD may provide significant savings due to faster installation time, less backfill materials used,
traffic control issues, pavement removal, separation from other services, reduced spoil handling and trucking
costs especially in an urban environment.
Well performance in a range of ground conditions including silt, sand, clay and solid rock formations. HDD has
excellent steering capabilities.

Advantages and Limits of HDD Technique
Limits
The necessary hole diameter exceeds the capability of the available equipment.
The depth of installation does not allow enough cover to prevent fluids frack-out.
Long segment/section lengths between pit locations may mean deeper HDDs and bigger cables, this may
lead to a less cost-effective solution.
Limitations imposed by length, geology, drilling rig capacity, space for site set-up, electrical concerns, cable
manyfacturing lenghts, etc.
Maximum allowable pulling tension on the cable depending on the lenght of the HDD.
HDD does not perform well in locations with gravel soils, boulders, and compact stone layers. Sandy soils
with high water table or rocky soils not for the proper directional control needed.
Risk of drill head stuck because of ground geology and the drilling fails.
Where the ground contains voids or sinkholes it may be very difficult to successfully steer the drilling head
HDD are limited by bends and radius. HDD are also limited by the entry and exit angles. A minimum lenght is
needed to get to a certain safe depth under a object.

Chapter 3
Pipe Jacking/Microtunneling
September 2019

Chapter 3: Pipe Jacking/Microtunneling
Pipe jacking is generally referred to as microtunnelling below 1.2 metres diameter.
1- It is necessary to construct a thrust and reception pit. The thrust pit shall provide a reaction against the
jack and a reception pit of sufficient size for removal the tunnelling machine is required.
2- A tunnel boring machine is used to excavate the route for the pipe from the thrust pit to the reception pit.
3- Hydraulic cylinders/jacks normally power the thrust system. The jacks are interconnected hydraulically to
ensure that the thrust from each is the same.
4- The thrust system push the casing pipes through the ground behind the tunnel boring machine and at the
same time as the excavation is taking place.

Jacking Lenghts
In small diameters (up to a meter) lenghts up to several hundred
meters are achivable-
In larger diameters lenghts up to 1 km are possible.
Many interrelated and variable factors influence the lenght
which a pipe jack can be installed:
- Geology of the soil to be tunnelled: Stability and friction.
- The self-weight and stenght of the pipes.
- The diameter of the pipe.
- The type of excavation method.
- Jacking reaction.
In order to reduce the total required jacking force on the full pipe, intermediate jacking stations are
frequently used between the launch pit and the tunnelling machine.

Jacking loads
The tunnel boring shield is designed to produce a small overcut in comparison with the external diameter of
the pipe. By injecting a lubricant in this annulus, the pipe can be jacked freely through a fluid médium and
considerable reductions in jacking forces are achived.
The loads required to jack the pipe forward are mainly function of frictional forces built up around the pipe.
These forces depend on the type of ground, the shear stress of the ground (friction angle), the depth of the
ground water, the length and diameter of the pipe. These variables should be assessed by an experienced
geotechnical engineer.
As a guide, frictional forces are generally between 0.5 and 2.5 Tonnes/m2 of external circumferential area.
The use of lubricant injection techniques can reduce frictional forces to as little as a 0.1 Tonnes/m2 .
Jacking loads must be resisted by a jacking reaction built up within the thrust shaft.
Spoil removal
For microtunneling below 1.2 m diameter the spoil removal is inbuilt into the system (screw auger or slurry pipe).
Other systems spoil removal used in larger diameters are auger or belt conveyer to wheeled skips.

Equipment
1- Microtunneling Machines
Fully guided microtunnelling machines are remotely controlled from the surface. They have an internal
diameter below 1.0/1.2 m and access by personnel is not permitted. This machines are generally of two types:
1.1- Pressurised Slurry Microtunneling Machine
A slurry system uses water based fluid (slurry) to transport excavated soils from the tunnelling machine to the
surface where the excavated soil is removed from the slurry enabling the re-use of the slurry for further
excavation.
When slurry tunnelling process are used slurry separation plant will be required.

Equipment
1- Microtunneling Machines
1.2- Auger microtunnelling machine
A ‘full-face’ tunnel boring machine in which the excavated material is transported from the face by a balanced
screw auger or screw conveyor. The face is supported by excavated material held under pressure behind
the cutter head in front of the forward bulkhead.
Pressure is controlled by the rate of passage of excavated material through the balanced screw auger or
valves on the screw conveyor.

Equipment
2- Pipe Jacking excavation systems and cutter heads
2.1- Tunnel Boring Machine
The typical tunnel boring machine with a shield having a rotating cutting head.
Various cutting heads are available to suit a broad range of ground
conditions.
2.2- Earth pressure balance machine (EPBM)
A ‘full-face’ tunnel boring machine in which the excavated material is transported
from the face by a balanced screw auger or screw conveyor.
The face is supported by excavated material held under pressure behind the
cutter head in front of the forward bulkhead. Pressure is controlled by the rate of
passage of excavated material through the balanced screw auger or valves on
the screw conveyor.
When slurry tunnelling techniques are used EPBM or pressurised slurry systems
are required.

Equipment
2- Pipe Jacking excavation systems and cutter heads
2.3- Pressurised slurry
A ‘full-face’ tunnel boring machine in which the excavated material is transported from the face suspended in
a slurry. Various cutting heads are available to suit a broad range of ground conditions.
When slurry tunnelling techniques are used EPBM or pressurised slurry systems are required.
2.4- Cutter Boom Shield
Cutter boom shields are essentially open face shields with mechanical means of
excavation. The cutter shield is more suitable in higher strength soils, marls and
some rock types. The excavation may proceed with a slight overcut to the shield
diameter circumference in firm ground. Alternatively, the shield can be used to
trim the under excavated face.

Equipment
Pipe Jacking excavation systems and cutter heads
2.5- Backacter Shield
Backacter shields are essentially open face shields with mechanical means of
excavation. The backacter shield is suitable in semi-stable to stable soil with high
cohesion values. The excavation may proceed with a slight overcut to the shield
diameter circumference in firm ground. In loose ground conditions, consideration
should be given to a protective hood on the top leading edge of the shield and
ground breasting boards to seal the face or sand tables.
2.6- Open Hand Shield
It should be noted that for health and safety considerations, in particular hand–arm vibration hand
excavations should only be considered for short lengths where no alternative mechanised excavation system
or alternative construction method is practical. If it is considered necessary to install significant lengths by
hand a rigorous risk assessment should be undertaken. Open Hand Shield must be equipped with at least
four steering jacks spaced around the lead. The shield should be of a diameter slightly greater than the
outside pipe diameter to allow it to be steered and provide an annulus for lubrication. The aspect ratio of a
diameter to length of a shield can be critical to its steering ability.

Pipe Jacking Pipes
1- Concrete Pipes and Glass Reinforced plastic (GRP)
Standard diameters are in the range of 450 mm to 2400 mm and typically lengths are from 1.2 to 2.5 m.
The joint design includes capacity for joint deflection and draw. Flexible joints must be proven to be
watertight at given draw and deflection limits.
2- Clay Pipes
Standard diameters are in the range of 150 mm to 700 mm and typically lengths are from 1 to 2 m.
The benefits of clay pipe include chemical resistance and longevity.
3- Steel pipes
Steel pipes of varying lengths can be used for the installation of cables. Factors such as welding time and pit
size should be considered when determining the length of each individual pipe.
Due to increased cable losses, steel pipes would only be used in the instances when cables are installed in
bundles, and not individual pipes for each phase.

The overall duration of the construction of a pipe jacking / microtunnel is similar to that of a HDD in many
ways. The most important parameters are:
• Geological conditions of the underground environment;
• Size and depth of launch and exit shafts and size of the working area;
• Diameter and length of microtunnel;
• Average progress rate by thrust is about 10 m/day; It can vary from 30 m/day in very soft soils (chalk) to 4-5
m/day for harder soils (clay, hard marl);
• Laying and connecting of cables outside the microtunnel;
• Reinstatement of the shafts and the working area;
The use of cementitious grouts to fill space between cable and pipe/conduit has been largely discarded in
pipe jacking / microtunneling installations on account that its use has minimal advantage in increasing
cable rating. One must also consider the difficulty in ensuring that the filling material penetrates along the
entire conduit and the difficulty to remove/replace cable/s in filled conduits/pipes.

Site Set Up / Working Areas
The size of launch and exit working pits/shafts generally depends on the requirement to install the jacking rig,
the length of the jacking pipe and the space required to remove the excavation machinery on completion of
the drive. Depth, site requirements and ground conditions will also influence the choice of pit/shaft. Pits/shafts
can range from 2.4 – 4 m in diameter or can be constructed to meet specific site requirements.
Around these pits/shafts an area must be available for storage of material, decanting of mud, machines,
cranes, control and other equipment.
A range of working pit/shaft construction methods can be used:
- Segmental lining;
- Pre-cast or cast in-situ caissons:
- Sheet piling or secant piling;
- Shallow trench sheeted or timber supported excavation;
- Battered excavation;
- Ground anchorages;

Advantages and Limits of Pipe jacking/Microtunneling
Advantages
Microtunnelling is a versatile technique that can achieve a very high degree of alignment accuracy
(usually with a deviation of less than 20 mm over 100 m).
Many microtunnelling methods have been designed that are able to deal with a variety of
ground conditions. The drilling heads can be designed to crush boulders with a diameter of up to 20 % of
the machine diameter and for tunnelling through hard rock.
Microtunnelling can reduce the length of a link compared to open trench, since it could avoid
longer deviations, thereby reducing associated labour costs and personnel risks. Unlike standard
trenching methods, a large increase in depth typically only results in small increases in relative cost.
Microtunnelling requires less cover to sensitive structures than HDD due to its slurry pressure control.
Microtunnelling shafts can be placed closer to the edge of the item to be passed under, thus reducing the
length of the tunnel significantly compared to a HDD installation. HDD installations by their nature require
entry and exit pits positioned in locations that allow gradual approach of the HDD under the “item to be
passed”.

Limits
Higher initial capital cost than HDD, as the equipment is more complicated.
Microtunnelling can have difficulties in soils containing boulders with sizes greater than
30 % of machine diameter due to the inability to crush them.
Microtunnelling is unable to make rapid changes in alignment or level.
Auger type microtunnelling machines are usually limited to tunnelling less than 3 m below
ground water levels.
During thrust boring, if the cutting head becomes damaged, and if its diameter is larger than
the casing pipe, it may be impossible to withdraw it, resulting in costly and time consuming
rectification works.
The greater risk is frack-out of the lubricant injected along the jacked pipe. This tends to
happen when extreme pressures are used particularly when frictional forces have increased
to the point that the jacking might be stopped. It is possible to heave/hump the surface with
a TBM but this is an extremely rare event.

Limits
Pipe jacking below the water table in unstable ground can and often does lead to catastrophic ground loss
and damage to the structures above.
With microtunneling in extremely soft soils it is difficult to control alignment and grade
without soil improvement. The TBM cannot develop enough side force using its articulation steering system
to deflect the TBM in the correct direction. In larger TBMs (>1.5m), their weight is so great that they tend to
sink 50-100 mm in spite of best efforts to steer them.
Rock microtunneling is limited normally to the rock hardness being less than 200 MPa due to the small
size of the cutting tools used on the cutter head.
Microtunnel boring machines must be 1.5 m or larger O.D. to have “face" access to the back of the
cutter head in order to change the cutting tools. This is critical when microtunneling in granitic rock where
disk cutters can fail or wear out in as little as 10 m of tunnelling.

Chapter 4
Pipe Ramming
September 2019

Chapter 4: Pipe Ramming
Pneumatic percussive casing thrust
1- The site set-up consists of a launch pit with a strong and flat laying basement, normally concrete, which
holds a double T steel slide strong enough to hold the casing weight and size. This slide is set up to be
perfectly aligned with the required bore path.
2- Next steps are reinforcing the entry hole edge and installing a lubrication line. Lubrication is obtained
by pumping water and polymer, which create both internal and external films on the casing during ramming
installation.
3- The hammer/ramming tool is physically connected to the casing to be installed. The ramming tool
has to be perfectly aligned, so as to correctly direct and discharge all the impact energy along the casing.
4- Once the first casing section has been pushed into the ground, subsequent casing sections can be
welded on and rammed until the requested exit point is reached.
5- Once the installation is done, or after every section has been installed, the casing shall be internally
cleaned of all debris. For a diameter, smaller than 0.7 m the cleaning is done with air and/or water, while for
bigger diameters manual cleaning by jet washing is employed.

Auger boring
This technology consists of casing installation by pushing the casing and simultaneously drilling the ground
through a rotating drilling head. This is an open drilling head and all spoil evacuation is done by internal
auger, which transports it out of the boring.
This technology allows drilling installation for a casing up to 1.4 m.
The thrust chamber size is determined by the auger unit size, but also by pipe section length to be installed.
For pipe sections of 3 m we need to think of a thrust chamber size of 9.5 m x 4 m.

Piercing tool
Piercing tools or moles can be used for small diameter pipes installations up to 180 mm.
The new pipe can be installed directly if connected to the piercing tool or it can be pulled in at a later time if
the hole is stable.
The tool is placed and aligned by eye sight and the bore accuracy can be considered reliable for lengths
between 10 m and up to 40 m.
Because the installation process is percussive, the application of such technology is only suitable for
installation depths at least 10 times the utilized tool diameter. Failure to comply with this requirement may
cause ground heave.

Down the hole hammer
The new pipe is pushed simultaneously through the ground using a hammer tapping on the digging face.
The disintegrated material (normally rock) is then extracted and transported externally by a screwdriver
system.
Straight bores can be drilled at any angle (including horizontal) at depth and length up to 100 meters.
The main components of the system are:
• An internal pilot drill, which drills the central part of the hole and
guides the drill string.
• A circular outer crown shoe to the pilot bit to which the pipe sleeve
is welded.
The drilling takes place by means of rotary-percussion with the
possibility of advancing the pilot bit separately from the drilling shoe.
These systems are able to handle casing pipes from 76 mm up to
1200 mm.

Advantages and Limits of Pipe ramming
Advantages
Minimal design effort needed
Cheaper than microtunnelling or HDD
Pipe Ramming shafts can be placed closer to the edge of the item to be passed under, thus reducing the
length of the tunnel significantly compared to a HDD installation. HDD installations by their nature require
entry and exit pits positioned in locations that allow gradual approach of the HDD under the “item to be
passed”.
Limits
Only suitable for very short uncomplicated installation.
Pipe ramming is not steerable technique.
Pipe ramming is unable to make rapid changes in alignment or level.
Limitation on accuracy.

Chapter 5
Ploughing
September 2019

Chapter 5: Ploughing
1- The cables or conduits are installed in a direct buried environment.
2- Possible to include backfill sand, protective polymeric cover strips and/or warming tapes simultaneously
with cable/conduit installation process.
3- The plough works best in soils that can be easily displaced and reused as fill. The mole plough pushes the
spoil removed by the plough back into the opening as the cable/conduit is fed into the opening. This method
is not considered as trenchless when the soil is hard and a cutter chain of disc has to be used.
The ploughing method is mostly used for the laying of lightweight cables and pipes.

Equipment
The equipment is composed of (left to right):
- Conveyor belt feeding controlled backfill into hopper.
- Hopper for feeding controlled backfill in ploughed opening
in ground and around cable/s being laid.
- Plough for opening the ground and feeding cable/s into the
ground.
- Tracked excavator for pulling the plough.
- Cable drums.
Note: The controlled backfill may be required because:
a) The ground contains stones that could damage the cable.
b) The in-situ soil has a high thermal resistivity.

For trefoil laying, the cables must be bound together and conduits must be installed in bundles.
Note with this technique it may be difficult to install mechanical protection slabs and warning tapes, so
the installed cable may be more liable to damage at a later stage. In the most of the countries, due to
electrical regulations, mechanical protection slabs may be required above cables.
As for any other laying operation, special case should be given to cable temperatur during laying.
Achievable depths for ploughing extend to 2.0 m.
It´s necessary to have a very clear route without obstacles in order to use the ploughing technique.
One design developed to cater for the external environment is to use a grooved outer serving in order to
improve mechanical protection and enable the cable to be buried directly in mortar or in the native
ground. Another possible design is to have a shock absorber layer underneath the outer serving.

The metallic sheath and cable serving design may also be adapted to provide a lightweight cable design,
which will allow longer cable lenghts to be transported and installed.
Factors that need to be taken into consideration for system and cable design:
a. Lightweight construction;
b. Serving;
c. Rating;
d. Bonding/máximum standing sheath voltajes.
Site Set Up / Working Areas
For MV cables a working area of 6 m wide would be required along the route and for HV cables up to 10 m
may be required. This is necessary to handle all equipment including the possibility of 3 cable drums.

Advantages and Limits of Ploughing
Advantages
Minimal invasiveness by vitue of its limited "trench".
No need for top soil stripping, the soil strata remains in situ.
Rapid mobilisation and setup.
Limited reinstatement.
The ducts/cables are installed at the required depth of cover, as this is set by the plough setup.
Environmentally, the ploughing method is less invasive.
No trenches are left open which pose both a healt and safety risk.
The speed of installation is far in excess of traditional open cut techniques thus, saving time and reducing
cost. In optimum ground, the plough is able to install in the region of 500 m per day.
Low cable tension is imposed on the cables as they just roll off the drums and don´t need to be pulled.
Crop loss through arable land and land damage is kept to a minimum.

Advantages and Limits of Ploughing
Limits
Space is required along the entire route during installation process for all the required equipment.
The ground must be relatively level. Slopes higher than about 25º are difficult to manage.
The installation has to be interrupted if other utilities in the ground are interfered with the routing of a plough.
The cable serving may be damage during or after installation by small stones.
It can prove difficult to install 3 cables at the same time.
It can be difficult to plough with the very large cables/drums required for high voltage cables.
The native backfill may not be suitable either thermally or mechanically and imported backfill may be
required. In this case a lot of equipment has to be managed at the same time – backfill truck, conveyor belt,
hopper for backfill, plough, pulling excavator, cable drum/s.

Chapter 6
Cable Rating and Bonding
September 2019

Chapter 6: Cable Rating and Bonding
1- Depth of the installation
 Greater depth generally results in lower ratings in buried cable installation.
 Transient thermal ratings for high depth installations will be different from those for typical trench depths in
terms of response to temperature variation.
 The current rating of shallow cables is influenced by daily, weekly and even yearly load variations. These
effects are not so profound for cables installed at deep depths.
 The IEC Standard 60287-2-1 contains a statement about very deep installed cables: ‘for cable circuits
installed at laying depths of more than 10 m, an alternative approach for calculating the current rating is to
determine the continuous current rating for a designated time period (usually 40 years) by applying the
formulae given in IEC 60853-2. This subject is under consideration’.
 Various papers have been published on ratings of cable installed at depth. A good summary and guidelines
ca be found in IEEE Dorison et. Al. (2010) Ampacity Calculations for Deeply Installed Cable PAS 2010.
 The guidelines in CIGRE Technical Brochure 640 “A guide for rating calculations of insulated cables”
consider the cable crossing through various soil layers with different thermal properties.

2- Separation between phases
Depending on depth and rating required it may be necessary to increase the separation between the phases
in order to permit a greater heat flow into the surrounding medium.
3- Bonding
It is necessary to ensure that the bonding design is suitable for purpose.
HDD installations are normally quite long and the phase separations may be high. These conditions may
result in very high sheath induced voltages under normal and short circuit operating conditions. In the
case of solid bonding and cross bonding, any increase in separation between phases will lead to
magnetic imbalance, circulating currents and may lead to a reduced ampacity rating in the circuit.
It will be necessary to ensure the outer serving design can tolerate these voltages and also to ensure that
national limits for standing voltages, if they exist, are not exceeded.

4- Pipe materials and losses
Steel pipe/s may have magnetic losses that will reduce the rating of cable systems.
Close triangular spacing with optimization of separation from pipe wall can reduce the losses.
IEC60287-1-1 provides empirically-derived equations for calculating the Joule losses in steel pipes normally
associated with pressurised pipe-type cables.
5- Conduits material losses
HPDE or PVC conduits are similar to the conduit in open trenched sections.
HPDE thick-wall conduits reduce the rating of cable systems.
Spacers can be designed to hold the conduits in the optimum position in the pipe to improve rating.
Usually conduits are not filled and only have air inside them. The thermal resistance might be high
considering as a worst-case approximation stationary air when modelling the behaviour of such a
conduit.
IEC60287 and TB640 “A Guide for Rating of Insulated Cable (Dec 2015)”.

6- Pipe/conduit filling
It´s important to consider the thermal resistivity of the HDD pipe and conduit filling material.
Solid, solidifying or fluid filling materials may be used. In the former situation – a solid – heat transfer from the
power cable is governed by conduction. This means that the heat transfer can be modelled with the means,
as described in IEC 60287-2-1.
The filling pumpability and shrinkage properties must be considered to avoid any air space between installed
conduit and/or pipe. HDD length and profile are key factors.
Horizontal – water – closed at both ends
A horizontal cable system in a perfect horizontal water filled pipe, closed at both sides, can be considered in
the same way as above, though the properties of the fluid are of course significantly different. Possible axial
heat transfer in the water-filled conduit in addition to the radial heat transfer should be considered.
Non horizontal
Will be axial heat in addition to the radial heat transfer. This means warmest locations are expected to be
near the higher sides of the pipes and the heat transfer may be governed by convection and radiation, which
are strongly temperature and geometry dependent.

7- Thermal Resistivity (TR) of the Soil
Different soil layers with different thermal properties may be crossed during horizontal directional drillings.
Interesting issues regarding thermal resistivity properties are discussed in CIGRE Technical Brochure 640, A
Guide for Rating Calculations of Insulated Cables:
- Non-homogeneous thermal properties by using finite element or conformal mapping technique.
- Multiple soil layers that may assist in removing heat in a longitudinal direction.
8- Distributed temperature sensing (DTS) systems
Distributed Temperature Sensing systems have special significance in trenchless technology where soil
conditions, including temperature, may be difficult to predict. The following CIGRE Technical Brochures
covers cable rating calculation using Thermal Monitoring:
-TB606 Upgrading and Uprating of Existing Cable Systems
-TB247 Optimisation of Power Transmission Capability of Underground Cable Systems using
Thermal Monitoring.

9- Drying of the soil
The design of the installation shall consider the possibility of drying out of the soil at the external surface of
the pipe. This might result in a thermal runaway causing cable insulation failure, if the system is heavily and
continuously loaded.
Drying out of the soil can be expected to start at a continuous temperature of 50 ºC depending on the soil
characteristics.
10- Temperature of the soil/environment
The temperature at the upper surface layers (depths 0.2 – 1.0 m) varies over time depending on the sun’s
heat, wind, and air temperature.
The amplitude of the fluctuation in the soil temperature reduces with increasing depth and is nearly absent
from roughly 7m in depth (depending on soil thermal properties).
At the typical depths HDDs reach (>10m) any daily, weekly or seasonal variation in the ambient temperature
is thus absent, while at the entry and exit points of an HDD, these variations do have their effect on the
current rating of a power cable. This is described in more detail in TB 640, A Guide for Rating Calculations of
Insulated Cables

Chapter 7
Surveys and Design
September 2019

Chapter 7: Surveys and Design
General surveying
Surveying consists in obtaining all the information of interest for the design of the trenchless solution and
evaluate the impact on the environment during the execution of the works. It normally includes the following
scope:
- Topography: Necessary to prepare profile drawings and define the occupation of the working area.
- Use of existing mapping, underground cable/pipe locators, desktop consultations, trial pits, site visits, etc.
for the location of other services.
- Identification of type of environment: rural, urban, protected spaces, traffic, water resources, etc.
- Geophysical investigations: Non-destructive methods that provide subsurface details by measuring certain
physical properties and interpreting them. The most common geophysical methods are Electrical
Resistivity Tomography, Ground Penetrating Radar and Seismic Tomography.
- Geotechnical investigations: Study of ground conditions.

Geotechnical
Trenchless Technologies can be performed in most soil types, but the cost and complexity is strongly affected
by soil conditions and variations in soil conditions along the borehole.
The number of geotechnical boreholes will depend on the homogeneity of the soil conditions in that area, but
a common practice can be taken at potential launch and exit pits for lengths up to 100 m at a minimum. For
longer operations, would be taken every 100-150 m.
The depth of the boreholes should be at least 5 m lower than the planned alignment.
Geotechnical boreholes should not be made directly above the proposed alignment, to avoid potential frack-
out during construction. All boreholes should be properly backfilled and sealed.

Geotechnical
A desk study should be carried out, assessing the available literature, maps, aerial photographs, utility plans
and existing site investigations. The desk study is essential to help understand the broader geological and
geotechnical issues, and should be used to determine the scope of any intrusive investigations.
SOIL ROCK
Soil type and classification Depth and extent of rock
Standard penetration tests (SPT) Rock strenght and hardness
Particle size distribution analysis Abrasiveness (Cerchar Abrasivity Test)
Water table variations (Piezometers) Total Core Recovery
Mosture content Rock Quality Designation (RQD)
Porosity/Permeability
Atterberg limits
Soil thermal analysis

Design
- Soil thermal resistivity: Can vary along the trenchless profile:
- Moisture content: higher moisture lower resistivity. Can be better at greater depths due to aquifers.
- Soil density: Greater density results in lower resistivity.
- Soil type: Important to perform site specific thermal resistivity measurements at various depths and
locations.
In situ and laboratory TR measurements should be performed. Cigre Working Group B1.41 “Long term
performance of soil and backfill systems”.
- Stability of the soil: Settlement and unstable soil conditions present a risk to the effectiveness of trenchless
operations.
- Settlement: is a process where soils decrease in volume. Should be considered when crossing below
an infrastructure, in particular HDD.
- Unstable soils: Difficult to stabilise and maintain the drill borehole (very sandy soil conditions).
Solutions like washover pipe, cashing installation or forward reaming are used in HDD operations.
Stabilisation by grouting with cement in Pipe jacking/microtunnelling operations.
- Cavities: Difficulty to follow the desired route and loss of the drilling fluid during the trenchless
construction.

Design
- Rock hardness: The hardness of soil has an impact on the effectiveness and progress rate of the drill.
- Ground water: Water table level will help decide on the thermal resistivity and should be considered on
excavation stability (including pits) and if it can be altered during drilling operations.
- Acid sulphate soils: Are naturally occurring soils that are formed under waterlogged conditions. The
exposure of these soils to air causes sulfites to react with oxygen to form sulfuric acid. This acid could
create corrosion of metallic pipes.
- Non homogenous soils: Impact on the type of drilling bit (wear and tear) and the overall steerability of the
bit for HDD.

Environmental
In many cases the use of trenchless techniques instead of open trenching will contribute positively
towards workplace safety, the interface with the public and the local and wider environment.
- Sestitivity/Heritage of areas: This plays a role in determining the feasibility of the drill an may pose problems
from planning perspective.
- Future works: The long-term future for the area needs to be considered during the design stage of any
project.
- Mud recycling: Drilling mud is reused throughout the operation and a return path must be provided for the
drilling fluid.
- Groundwater and aquifers: Contamination and pollution of groundwater must be prevented. Biodegradable
fluids are available in the market.
- Traffic management: The traffic must be minimized in the vicinity of entry and exit points by supplying a
return path overland or via a second drill with a smaller duct size.

Environmental
- Waste management: The contractor has to have a waste management plan and system to avoid any wasted
material entering into the local environment. When polluted soil is encountered, it should be removed
and disposed of in the prescribed manner with all necessary licenses and permits in accordance with
local and/or national laws and regulations.
- Nature conservation during submarine cable projects: In order to protect the coast line and protect the cable,
a trenchless installation is often carried out from behind the natural area to an appropriate place in the surf
zone.
- Archaeology: In general, trenchless techniques are the preferred methods for installing cable/s where
archaeological sites have been noted. This is as a result of less surrounding soil being removed during
drilling and the drill path can be planned to go deeper under these sites. Local authorities or actors must
always be consulted in advance about the rules and regulations.
- Site access: Site access must be provided for the entirety of all the works.
- Clearing the area (trees): A full survey of the required area should be conducted prior to being works. This
may require the clearance of trees/vegetation and this should be considered at the planning stage.

Environmental
- Noise during excavation: A noise management plan will deal with controlled and uncontrolled noise coming
from the site during drilling. A system for noise monitoring should be identified in the work plan and all
relevant regulations should also be referenced in the plan.
- Water quality:. A detailed protocol for water quality monitoring should be developed to ensure continuous
compliance with pre-agreed standards throughout the construction period. Where drilling is being
undertaken near watercourses, inspections must be carried out to ensure that water quality is maintained.
- Drain: Water run-off should be considered, if necessary, so that excess water is channelled to a suitable
area, at least 50 m from any existing watercourses. This will increase the likelihood of settlement of solids on
site and avoid any contamination of watercourses.
- Corrosion: The impact of the surrounding ground conditions with regards to pipe corrosion (i.e. if metallic
casing pipe/s are being used) should be taken into account during the design process.
- Dust pollution: There is a risk of cement, dust, and air pollution during a trenchless installation and
this should be assessed and minimised. Dust screens and water sprays may be required to
contain/minimise the amount of dust in the air.

Health and Safety
- Electro-magnetic field (EMF): National regulations relating to EMF should be considered with respect to
the cable circuits being installed. Normally EMF generated from trenchless installation is less relevant
compared to that generated by a system installed with a traditional open trench installation technique,
since the installation depths are deeper.
- Vibration and subsidence: In case of an incident occurs due to vibrations or subsidence, the following
actions must be carried out immediately:
• Stop all activity on site;
• Evacuate the site;
• Temporarily prop the structure (if possible to do so safely);
• Put corrective measures in place;
• Evaluate alternative options for completing the task

Maintenance and Removal/Repair of Asset
The preferred practice for installing cable in HDD is in a conduit which may be unfilled or filled. In the event of
a fault on a cable in a conduit the normal practice is to pull the cable out and pull a new cable in. For filled
conduits, this is dependent on the ability to remove the bentonite with water sprays. If it is not possible to
remove the cables, then they may be left in-situ. In the case of oil filled cables if being left in-situ it may be
necessary to drain and treat the cables. In an unfilled conduit, it should be possible to remove the cable, if it
has reached the end of its life or it needs to be replaced.
Alternatively, the installation of a spare conduit may be considered at the initial design stage.
In case of microtunnel/pipe jacking, once the cable installation is completed, no maintenance will be required.

Chapter 8
Conclusions
September 2019

Chapter 8: Conclusions
It is not really possible to draw any conclusion on which is the preferable method of installation for High
Voltage cables. This Technical Brochure addresses the factors to be considered when deciding which
trenchless technology (HDD, Pipe Jacking/Microtunnelling, Pipe Ramming or Ploughing) is best suited to the
circuit installation. In addition to these installation methods one must also consider:
 Direct laying
 Laying in ducts
 Submarine laying on or in the sea bed or in ducts in the sea bed
 Laying in a large tunnel
 Laying on a bridge
Each one of these may be possible and the final decision on which one to adopt for any particular route will
be decided on the basis of risk, cost, programme, environmental impact, licensing/permitting, engineering
suitability, operational aspects including availability, ability to cater for future expansion, reparability and end
of life access/recoverability.
Conclusions

Chapter 8: Conclusions
To minimise risk, it is essential that as much information as possible is gathered before the chosen installation
takes place. Notwithstanding, this does not guarantee that the installation will be successful.
The increasing demand for the development of projects in urban areas make that the trenchless techniques
are increasingly an essential complement to traditional techniques. As a result, the continuous experience
improvement in trenchless technologies makes them more and more reliable.
Conclusions

Copyright © 2018
This tutorial has been prepared based upon
the work of CIGRE and its Working Groups.
If it is used in total or in part, proper
reference and credit should be given to
CIGRE.
Disclaimer notice
“CIGRE gives no warranty or assurance
about the contents of this publication, nor
does it accept any responsibility, as to the
accuracy or exhaustiveness of the
information. All implied warranties and
conditions are excluded to the maximum
extent permitted by law”.
Copyright &
Disclaimer notice

Trenchless Technologies

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