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Engineering Design andConstructionCourse Notes
4/25/2013
The University of Sydney
Abraham Kazzaz
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3.12 Retaining Wall at Collaroy Development .............................................................................. 87
4 Storm Water Drainage .................................................................................................................. 95
4.1 Storm Water Drainage Design .............................................................................................. 95
4.2 Construction of Drainage Systems ...................................................................................... 123
5 Pavements ................................................................................................................................... 125
5.1 What Are Pavements .......................................................................................................... 125
5.2 Types of Pavements ............................................................................................................ 125
5.3 Rigid Pavement Joints ......................................................................................................... 135
5.4 Design of Road Pavements ................................................................................................. 147
5.5 Road Construction & Perfromance ..................................................................................... 149
5.6 Road Design Example .......................................................................................................... 151
6 Environmental Engineering ......................................................................................................... 155
6.1 The Environment and Environmental Projects ................................................................... 155
6.2 Soils & Construction ............................................................................................................ 156
6.3 Nutrient Reduction Schemes .............................................................................................. 159
7 Timber Engineering ..................................................................................................................... 162
7.1 Types of Timber .................................................................................................................. 162
7.2 Natural Timber Properties .................................................................................................. 163
7.3 Engineered Wood Properties .............................................................................................. 166
7.4 Timber Design ..................................................................................................................... 168
7.5 Mixing Steel and Timber ..................................................................................................... 173
8 Crane Engineering ....................................................................................................................... 175
8.1 About Cranes ....................................................................................................................... 175
8.2 Types of Cranes ................................................................................................................... 175
8.3 Design of Crane Base .......................................................................................................... 182
8.4 Occupational Health and Safety (OH&S) and Legal Issues with Cranes.............................. 1859 Marine Engineering ..................................................................................................................... 186
9.1 Civil Engineering in Marin Environments ............................................................................ 186
9.2 Pile Driving in Marine Environments .................................................................................. 187
9.3 Pile Socketing in Marine Environments .............................................................................. 190
9.4 Underwater Concrete Placement ....................................................................................... 191
9.5 Mooring Dolphins ............................................................................................................... 192
9.6 Gladstone LNG Gas Terminal .............................................................................................. 193
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1 Introduction to Engineering Design
1.1
What is Engineering Design?is simply the process of the process of gathering requirements from clients and relevant
external parties (such as regulatory bodies) and translating these into an idea that meets these
needs. The designer is concerned with mainly human factors like:
Aesthetics,
Functionality,
Fitness for purpose, and,
Quality.
Designers are less concerned with the implementation details; i.e. construction and constructability.
is concerned with the translation of these requirements into a technical
specification (plans, detailing and construction notes). Engineering design is concerned with more
than just aesthetics and fitness for purpose, it about ensuring that the design is compliant with legal
requirements, is safe, constructible and cost effective.
A thorough understanding of construction methods is necessary to ensure that the design minimises
construction time and resources, as well as using construction materials effectively. A relatively
miniscule proportion of engineering design is dedicated to calculations (5% or less) and a fare
greater fraction (95%) involves conceptualisation, logical reasoning, industry knowledge andcommunication.
Engineering designs must meet 3 fundamental civil engineering requirements:
The designed structure must have the required ultimate capacity to stand up
to the most likely extreme events that a structure may be subjected to. The structure must
also have required strength to deal with service loads.
The structure must stand up to the elements as well as sustain the required
use without needing excessive maintenance.
The geometry of the structure must be in line with the required function as well askeeping deflections and displacements within acceptable limits.
1.2 Engineering Design Sequence
Engineering Design Process: is the formulation of a plan to help an engineer build a structure with a
specific goal. There are several stages in the engineering design process:
1. This is a rough design given to clients by the design company
which illustrates the initial idea for a project and is intended to be the basis of attaining the
client’s approval for the construction approach adopted. This design is based on no to
limited design calculations.
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E.g. Fence designer was contracted to design and build new gates for Parliament House in
Canberra. The contractor developed a simple plan with absolutely not engineering or
technical justification, using lightweight aluminium fencing with built in Para-webbing
(polypropylene webbing through the hollow aluminium sections) which would resist
dynamic loads rather than the aluminium elements. Based on this the project was approved.
The concept design was then sent to an engineer to be properly designed.
2. This is a set of plans with greater detail, intended to be provided
to the construction company by the design company if design and construct (D&C) is
selected. This design allows tenderers of construction companies to price the construction of
the design. Again no verification of the design by calculations or code compliance is
necessary at this stage. This stage is skipped if separate design-only and build-only
companies are selected by the client.
3. This is a set of plans produced by the design engineers that
have been subjected to the full design process with detailed calculation verification carriedout and code compliance which are ready for construction. This design is now given to the
construction company directly if D&C is used or alternatively given back to the client if
delivery is design-only. In the latter case the client is responsible for tendering to a build-
only company and providing the detailed design.
There are several risks associated with design which a design company will bear. These include:
Design errors,
Ambiguities, and,
Omissions.
If a separate design-only and build-only mode of delivery is selected, the design company will
demand a premium over the cost of delivering the design in order to cover the additional risks which
they bear. In addition the construction company may also demand a premium for the risk they bear
associated with ambiguities in the design and the tendering process. In addition some construction
issues cannot be isolated to just the designer or the builder and as such there will likely be a
significant risk placed on the client if things go wrong.
In contrast if a design and construct mode is selected the risks associated with design as well as
construction are borne by the tenderer company. As such a reduced premium is charged by the
builder and designer, resulting in a cost saving. In addition the risk borne by the client is reduced.
1.3
What Makes a Good Design?
For a design to be good it must satisfy 3 key requirements:
1. The final design must perform the required function for which the
project was proposed.
2. There must be some underlying problem which the project addresses and
the final design should completely solve that problem. A design which partially solves the
problem is not satisfactory.
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3. The design should efficiently use materials, resources and meet safety
requirements. It must solve the problem in the least complicated and most common sense
way.
Examples of good design include:
The wheel, one of the simplest of designs is a highly logical design taking
advantage of a pined circle, which satisfies transportation and many other needs and works
exactly as intended (spinning about its axis and allowing translation).
The tube is another example of a great design; being logical in the sense that it is
the most efficient way of moving fluids under pressure, is fit for purpose because it does
exactly that and satisfies the need for the transportation of fluids from one location to
another.
Reinforced concrete is highly logical because it combines
the compressive strength and low cost of concrete with minimal use of steel with highstrength and higher cost. It is also fit for purpose being a cheap, strong, durable and
effective construction material. Finally it satisfies the need for strong, cheap and efficient
building materials.
There was a persistent need for a large car park to service
the Opera House, Circular Quay and the Botanical Gardens. The Opera House car park met
that need very well, providing 1100 parking spaces. The car park is also fit for purpose,
servicing the needs of the area as intended without problems.
It’s design is also highly logical; making use of a cylindrical exterior to resist the pressures
from surrounding rock, a donut shape with helical floors, rather than flat floors with rampsto maximise the amount of car space and minimise space used for movement from one
floor to another, a ventilation system dug into the central rock rather than wasting car park
space, and many other logical solutions.
The Harbour Bridge satisfied the need for a link between
north Sydney and the city areas. It was also fit for purpose, having 6 lanes of traffic that
continue to ease congestion and offer a quick journey between the two areas. It is also a
highly logical solution, making use of the newly developed suspended deck steel arch
design at the time and the most durable and widely utilised construction techniques of the
time.
Examples of bad designs include:
The M5 tunnel was meant to satisfy the need for a motorway linking the
western suburbs and city to ease congestion. But design failed to fully satisfy this, having
insufficient capacity; having 4 lanes (2 in each direct) rather than 6. Even at the time of its
design this was insufficient to meet the needs of Sydney. As such it also did not fit the
purpose for which it was constructed.
It also did not have a logical design. The tunnel section running from Arncliffe to Bexley
should have had 3 tall stacks to expel pollution; one at each exit and one in the middle. This
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would have made smog within the tunnel impossible and the tall stacks would have
prevented the surrounding suburbs from suffer poor air quality.
Defying logical design, the tunnel was built with only one exhaust outlet 1km away from it.
This would not be sufficient to prevent high pollution concentrations in the tunnel. In
addition an elevated stack was replaced with a ground level outlet, causing the air quality inthe surrounding area to deteriorate significantly.
The poor design of the tunnel can be attributed to the way in which the project was
funded. The M5 tunnel was built using only government funds and as such the project
expenditure was limited, preventing all necessary measures being implemented in the
design. Had the project been funded as a build, own, operate and transfer (BOOT) project
more funds would have been available, resulting in a far more satisfactory design.
With BOOT construction a non-government entity designs and constructs the motorway, it
owns it for a duration of 5 or more years during which it operates it and collects tollsindependently of the government. Finally upon the completion of the contract period
ownership of the motorway is transferred to the government.
Another example of poor design is a project commissioned by a
school in the Bankstown area. A gym and several science labs were to be constructed on
the grounds of a school in Bankstown. The architect who created the initial design placed
the gymnasium on the lower floor and the science labs above it. The gymnasium had to be
without columns and as such the beams supporting the floors above had to span 25m. In
addition science laboratories have large live and dead loads due to equipment as well as
significant pluming which would require significant service hole be made in the beams. Fora span of this nature with such live loads a conventional RC beam would have to be 3m
deep and a prestressed beam, 1.5m deep. This seems like a highly illogical design with
these beams costing more than $2.4 million alone, when the entire project budget was $3
million.
The architect ridiculously designed the structure with beams of 300mm depth. There was no
way on earth that such beams could support the structure above. In addition the amount of
services that needed to be run to the laboratories would have created too many
penetrations in the beams.
A far more logical design would have placed the science laboratories on the ground floor
and the Gymnasium above. Having small rooms on the ground floor allows walls and
column to be put in, reducing the spans of beams supporting the upper floor as well as
reducing their depths. In addition the significant amount of service pipes for the
laboratories could easily be run through the ground beneath. With a light weight roof, the
beams above the gymnasium could have been made with reduced depth.
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1.4
Effect of Minor Aspects on Design
Sometimes certain trivial matters like architectural aesthetics make a design inefficient. This does
not make the design bad, but it also prevents it from being a good design. An example of this is the
Lane Cove Aquatic Centre. The design involved steel roof over the pool being supported by steel
columns at regular spacing around the exterior walls, between which masonry would be placed to
seal off the structure. The columns would have to be attached to the walls through welded rebar
connections embedded into the masonry.
The architect requested that the columns be constructed of CHS section with T sections welded
along their length against which the masonry would be built. Starter bars would have to be welded
to the flanges of these T sections to be embedded into the masonry.
This seems like an overly complicated design consuming an excessive amount of materials, labour
time, construction resources and project funds; (1) having to take a CHS and weld T sections on
either side (2) weld starter bars for the masonry. A much simpler, cheaper, faster and less labour,
material and resource intensive design would have involved using an SHS section for the column and
simply welding the starter bars directly to it. Sometime architectural aesthetics override logic and
functionality.
1.5
Communication of Design to Building Contractors
The design concept and required construction techniques are communicated to builders through a
variety of channels. These include:
These documents communicate the design of the structure in terms
of both components and materials graphically to builders. Several views including plan
(overhead), elevations (exterior side views), section (long/short) (side views cut through the
structure) and details (zoomed in images of particular elements of structure e.g. column-slab
connection) are used. It is best to show all information that builders need on the actual
drawings rather than the specifications because builders will often not read the
specifications.
There are 2 things that drawing must be in order to be good drawings:
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1. The drawings need to clearly define all aspects of the design in
such a way that only one interpretation is possible. Poor drawings have elements
which are not clearly defined or for which multiple construction interpretations are
possible. Multiple interpretations can lead to the incorrect measures being taken.
2. Drawings need to minimise the scope for dispute
between designers and builders. In avoiding dispute over drawings, both parties will
avoid legal action being taken.
Most construction errors are due to either a lack of specification (where the builder is forced
to use their own discretion) or mistakes in the actual drawings. To avoid these situation
designers should ensure there are no mistakes, have their designs checked by others and
limit the scope for contractor discretion.
These are written requirements that need to be satisfied by builders
and other contractors. They contain information about materials to be used, construction
methods as well as requirements for services like lighting and air-conditioning. It is best to
avoid putting information that builder will use in this section, but information for specialised
services contractors can be put in this section because electricians, plumbers and air-
conditioning specialists will read and comply with the information.
These include things like inspections; where an
engineering will ensure that the most critical elements of a design are correctly
implemented as well as offering a means of verbally communicating the design with
builders, checklists which again ensure that the most important parts of the design are
correctly constructed and drawing the attention of builders to these areas, and many other
related items. This channel is usually used at the beginning of a
pro ject to communicate information in two directions between contractors and the
tenderer. A request for information is a standard process which aims to collect written
information about the capabilities of a supplier. These can take several forms:
o This involved the client or tenderer inviting designers
to submit their design concepts, providing the potential contractor with some
background information about the site, requirements and other relevant
information.
o
This is a written invitation sent out to constructioncompanies to submit be part of the tender process for a particular project. A tender
design is provided to the potential contractors at this stage.
o This is quite similar to the preceding request for
information but usually used by a construction company to recruit individual builder
or services contractors. Information about the particular segment of the project is
provided to the potential contractors.
1.6
What are the Risks in Civil Engineering Construction?
Some of the construction risks include:
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Inadequate site investigation which means that corrective measures (which increase cost
and construction time) will have to be taken down the track.
Appropriateness of specification (if they are sufficiently detailed and/or correct).
Construction errors (due to design or construction workers).
Cost blow outs due to inaccurate cost estimation.
To manage these risks several strategies may be adopted:
It is important to use knowledgeable people on site to offer advice and to
use designers who are knowledgeable in construction methodologies. In this way
construction errors can be picked up and rectified immediately and designs will be in tune
with the needs of builders (avoiding misinterpretations and builder discretion).
A significant amount of risk can be reduced through the use of sub-
contractors for different parts of the project construction. Sub-contractors bear the
construction risks associated with their work rather than their employer (as would be the
case with an employee builder). In this way rectification of errors and ensuring satisfactory
quality becomes the responsibility of the sub-contractor.
Following the exact detail of the designs means that if something
should go wrong, the designer is at fault and not the builder. Deviating from the design
drawings opens up the construction company to increased risks. The cost savings from the
deviation from the design cannot offset the cost of the risks should they become reality.
An example of this is found in the case of the EG Harbord child care centre project lift shaft. A lift
shaft will always require increased depth below the ground floor. This is achieved by excavating a
rectangular lift sump to a depth of 1m or more which may be below the water table or in saturatedsoil. A lift sump floor slab is then poured at the bottom of the excavation with starter bars added
around the perimeter. Once the floor slab has set the walls are then poured up to the ground floor
slab. Because the lift well is poured in two stages a gap exists between the slab and walls. If nothing
is done water will seep through and eventually flood the sump.
There are different ways of dealing with this problem which defines the type of lift sump:
The gap is not sealed and the water seeps through and is dealt with by having
a pump at the bottom of the sump which daws it away. The gap between the walls and slab
which allows water to flow through also exposes the starter bar portion in the gap to
moisture which can corrode it.
o To deal with this hydrophilic water bars are used. This involves
placing donut shaped water bars over the starter bars after pouring the slab and
before pouring the walls. The water bar sits around the starter bar at the slab-wall
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interface. As water seeps through, the water bar absorbs the water, swelling up and
preventing further water from penetrating.
The gap is sealed to prevent water from seeping in, protecting starter bars
from corrosion and preventing water ingress into the sump. This can be achieved in two
ways.
o A back stop is a flat continuous strip of
plastic that has ribbing on the side in contact with the concrete to hold it in place.
The back stop is laid around the outside of the slab-wall interface prior to pouring
the concrete slab and walls. Where the back stop is discontinuous it should be
welded together to maintain continuity and keep a water tight seal.
o This is similar to the back stop but is placed in the middle
of the slab-wall interface. The concrete slab is poured first, and the centre stop
placed on top. Holes are punctured in the centre stop so that the starter bars can be
passed through. Again where it is discontinuous, it should be welded together.
The construction error involved the use of a back stop. The back stop had been put on backwards so
that it could not bond properly with the concrete. The back stop also was not welded at the point
where it was cut. Had this not been picked up during an inspection of the site, the sump would have
leaked and this could have been disastrous.
1.7 What are the Risks in Civil Engineering Design?
Some of the main risks in civil engineering include:
Design errors, A lack of specification that leads to builder discretion and possibly a construction error.
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Ambiguity in the design can also lead to conflict between designers and builders.
To deal with these risks some strategies include:
A highly knowledgeable designer knows how to ensure their designs are
constructible, lack ambiguity and specify all necessary aspects. Personal indemnity insurance reduces risks
further so that if a designer should make a mistake the cost of remedial work and
compensation is covered by the insurance provider. Further, many clients will not engage a
design contractor without proof of their personal indemnity insurance.
It is best to make conservative decision and assumptions during the
design process so that should something go wrong, sufficient capacity is still available. For
example in the design of the slab as part of Project H, the client requested that the slab be
designed for a 4.1 tonne forklift. After the design and construction had been completed, the
client had realised they miss read the weight of the forklift and in fact it should have been14.1 tonne. But with recalculation, the conservative over design of the slab meant that it still
had sufficient capacity to deal with the 14.1 tonnes.
Another example is the design of concrete using charts. It is often a good idea to add on
small amount of additional steel because steel is cheap. In this way the concrete should have
a far greater than necessary strength capacity.
This refers to doubling up and having more
than one contingency in a design. That is, providing additional redundant elements to act as
backups. For example in the EG Harbord example a belts and braces approach would involve
using both the back stop and water bar, and not just the back stop.
An example of a design error is again the EG Harbord project. The site was an old service station with
the petrol tanks removed and filled with loose uncompacted fill. In an obvious mistake the design
engineer specified that auger piers be used. These are screw like piers that are driven (twisted) into
the ground until refusal. Obviously the uncompacted ground was too weak to resist against the piers
and provide sufficient strength, refusal could be reached.
As such sufficient strength could not be reached and the piers had to be removed. This was a
massive waste of funds and it should have been obvious to the designer loose fill could not provide
sufficient strength for auger piers. Wide and deep continuous footings should have been used for
such soft soil.
Designers also often take for granted that managers and tradespeople have experience with
standard construction methods and common sense. In reality they sometimes lack one or both so it
is important that engineers do not assume that they have both. Designers need to spell everything
out with full and strict specifications.
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1.8
Important Aspects of Reinforced Concrete Detailing
There are certain projects where a large amount of reinforcement will have to be placed in a single
location with limited space. This is typically the case at beam-column intersections and bridge decks.
This can often lead to difficulties in the placement and compacting of concrete. In extreme cases this
may result in poorly compacted concrete or regions where concrete has not penetrated at all. The
first strategy to dealing with this problem is for designers and detailers to be aware of the minimum
spacing requirements between reinforcing bars.
In addition, designs with congested reinforcing can often confuse builders or builders may place the
reinforcement incorrectly leading to insufficient spacing between reinforcement. It is always a good
idea for designers and drafters to include detailed drawings and specifications of how reinforcing
steel is to be placed; making clear the spacing between bars, the order of reinforcement (i.e. which
should be top and which should be bottom steel) and the lapping and tying.
Furthermore, designers and drafters should ensure their designs simplify
construction and make the job of placing reinforcement as easy as possible to limit the potential for
error. For example in beams it might be better for the designer to use an open top stirrup with a
closer on top rather than a single piece of stirrup wire wrapped around the beam. This means that
the builder can get right into the beam, place the longitudinal steel carefully and then place the
closer on top when done. If a single stirrup wire was used, the job of placing longitudinal steel would
be awkward and would likely lead to errors in steel placement.
Another consideration is the reinforcing steel in
basement walls that will be poured directly against earth behind or walls poured directly against
existing structures. Because the vertical rebar is put in first and the rear face of the wall cannot be
access, the horizontal rebar should be drawn as the front steel. If it was drawn as the back steel,
builders could not access it in order to put it in and tie it off.
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Cases where reinforcement in a wall is to extend into
adjacent corners should be given particular attention. Having a single bar with bent ends can be
quite problematic. If the dimensions are misread (such as the internal length misread as the external
length or vice versa) then the correct placement of reinforcement will be impossible.
A better way of dealing with this situation would be to use 2 L-bars with one very long end and lap
them or to use one straight steel bar with 2 short ended L-bars. In this way the distance between the
corners may be adjusted to ensure correct positioning every time even if manufacturing dimensionsare slightly off.
Column-beam connections are another area of concern. Often the vertical bars in column are
specified to be continuous through a 90o bend into a beam. The correct method builders will use to
achieve this is to place a timber block of appropriate height at the edge of the beam formwork and
bend the vertical bars into a 90o angle by hand. Sometimes builders will be lazy and not use a timber
block or use a timber block of incorrect height. In such a case the angle may not be 90o and the
Double bent-end bar
There may be error in the
length so correct positioning
not possible
2 long ended lapped L bars
The internal distance
between the bent segments
can vary for perfect fit
2 lapped L bars with a long straight bar
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horizontal portion of the bars will not sit at the correct position in the beam formwork leading to
incorrect cover and effective depth.
To avoid this uncertain situation and the reliance on the diligence of builders it may in fact be best to
use lapped L-bars to achieve a transition from vertical to horizontal reinforcement. This way the
angle will be perfectly 90o and the correct positioning of reinforcement will be guaranteed.
If the corner of a wall is simply reinforced with inner and outer L-bar reinforcement
it can easily resist forces imposed on that want to close the walls together, but it cannot resist
opening forces. The only resistance comes from the inner reinforcing bars which have limited
concrete cover. Moderate to large opening forces will cause the concrete at the inside of the corner
to spall off, removing any resistance for the reinforcement. The walls will quite easily move apart at
the corner like the pages of a book about the book spine.
To resist the opening forces we use the 3 bar trick 3 bar laps are made. The outer reinforcing bars
are simply joined with an L-bar like the previous case, but rather than connecting the two inner steel
reinforcement bars together with a single L bar we now extend the inner steel to the outer steel and
lap each bar with the outer steel using L-bars. Hence the inner bars which resist the opening forces
will have a much greater cover of concrete so that spalling is no longer an issue and greater
resistance can be developed.
Properly bent bars
Timber block used
Improperly bent bars
Timber block not
used
L-bar bars used
Opening Forces
Cover will spall off and steel will
offer not resistance to opening
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The problem of opening forces also occurs in concrete stair cases where loads on the stairs cause the
stair-floor corner to want to open up. As such independent top and bottom reinforcement is
insufficient.
To avoid this problem we again use the 3 bar trick, extending the top and bottom steel at the ends
to meet the opposite steel.
The three bar trick can also be used in masonry wall corners. It is now slightly different; the inner
and outer reinforcement steel for each wall is looped about the corner where vertical steel will
usually be placed. The vertical steel passes directly through the overlapping loops.
Inner reinforcement bars
extended and lapped with the
outer reinforcement using L-bars
or bent ends
Cover will spall off and steel will
offer not resistance to opening
Cover will spall off and steel will
offer not resistance to opening
Bottom reinforcement bars
extended and lapped with the
top reinforcement using L-bars
or bent ends
Top reinforcement bars
extended and lapped with the
bottom reinforcement using L-
bars or bent ends
Inner and outer reinforcement
lapped at ends to form
overlapped loops around
corner vertical reinforcement
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1.9
Movement Joints
Movement joints tend to be over used in most concrete structures. They often add unnecessary cost
and waste labour time. There is even scepticism about the extent to which movement joints are
mandated by the concrete code; being required every 25m for horizontal non-prestressed concrete.
This seems extremely excessive when contrasted with Continuously Reinforced Concrete Pavements
(CRCP) used beneath roads which require a movement joint every 15km. The use of additional
reinforcement can easily alleviate the need to use a movement joint.
Some of the typical movement joints used in column/wall supported slabs include:
Movement joints are used here at the interface between a slab and
column. A corbel is created on one side of a column which involves creating a flat topped
projection with significant reinforcement placed within it. A polymer based material is
placed onto the corbel and the slab rested on top of this. The slab resting on the corbel is
free to move independently of the column. This may be repeated at the next column to the
left for the slab on the left. The main issue with this approach is issues with the sliding
surfaces where too much friction may prevent motion or excessive movement occurs
allowing the slab to slide off the corbel.
Here the movement joint is created by pouring the column supporting
slabs in two sections. Essentially the column is split in two with the portion supporting the
left slab independent of that supporting the right stab. A small gap exists between the two
columns. A split column is usually achieved by pouring the first column section, then placing
a polystyrene strip, pouring the second column segment and then dissolving the strip with
acid to leave a 10mm movement gap. The two column sections can flex in opposite
directions; outward or inward (closing the gap). This is a better solution as there are no
sliding problems.
Polymer movement joint allows
independent motion
Corbel
SlabFixed Connection
10mm gap made through
polystyrene strip that is
then dissolved away
2 Colum Segments can bend
independently allowing slabs to
move freelSlabFixed Connection
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1.10
Reinforced Concrete Pit Construction
An example of the construction of a reinforced concrete pit can be found in the Orana sewer pump
pit project. A deep pit had to be built in order to house a sewage pump beneath the ground.
Reinforced concrete was selected as the construction technique due to its versatility and low cost.
There were several steps in its design and construction.
The first step in the design process for this sewer pit was to determine how the earth pressure acting
on the walls would be accounted for. To simplify the problem two approaches could be used:
This
involved only considering the earth pressures acting on the walls in a plan view. The
maximum earth pressure, which would be that acting at the very bottom, was assumed to
be uniformly distributed along the height of the walls. Due to the connectivity of the walls
negative moments would be generated at and near the corners (bulging out) and positive
moments acting in the middle segment of the walls (bulging inwards).
This
involved only considering the hydrostatic pressure acting at the middle of the walls insection view and assuming this was distributed throughout the length of the wall. Moments
considered would only be positive, ignoring the effect of having corners joining the walls.
Peak Uniform Earth Pressure
Bending Moment Distribution
Bending Moment Distribution Peak Uniform Earth Pressure
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The better approach, and the one actually used by the designers was the plan view pressure
distribution. This is because the section view would have ignored the effect of rigid corner
connections which would have meant insufficient reinforcement on the outer faces of the wall
would have been put in, possible leading to cracking.
Note that because the concrete was directly in front of the earth, the outer face of the concrete did
not require formwork. Only the inner (front) face of the wall had formwork applied to it as the
excavated soil face behind could support the concrete and provide it with the required shape.
The next issue that had to be addressed was uplift. Because the surrounding soil was saturated, if
the weight of the concrete was less than the weight of the removed water displaced, the structure
would float up and the surrounding soil would fill the space beneath, effectively ejecting the
structure form the soil. In addition a safety factor of 2-3 is employed so that the concrete structure
must weight 2-3 times more than the displaced water.
If this is not the case then concrete flares have to be used. This involves extending the edge of the
slab a small distance beyond the outer edge of the wall to create a lip all the way around the
structure. The lip grabs hold of the soil above (which has a triangular shape) increasing the weight of
the structure in the soil.
But over time the soil will flow around the flares and they will stop working. To prevent this, a lip is
created at the top of the wall around the exterior perimeter. This upper lip holds a small amount of
soil in place above the flare, stopping the complete flow of soil and keeping the flares effective.
Concrete Drop: The maximum drop for concrete pour is about 1m. Hence for deep walls like
the ones in the pit, the concrete has to be poured in through a tube down to the region
where it will occupy.
Vibrator Space: Also it must be ensured that during the design phase that sufficient space is
left between reinforcement to fit a concrete vibrator in so that air bubbles can be removed.
Flare
Soil segment
bearing down on
flare
Top lip retains a
portion of soil
above the flare
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Durability: It must be ensured that sufficient concrete cover is provided at the faces of the
wall and slab to protect the steel from the environment. The cover depends on the
environment. For example the cover for this pit which will be exposed to sewage (highly
corrosive) will have to be very thick while for concrete exposed in air the cover is relatively
thin.
Pour Sequence: In terms of pour sequence there are two ways of pouring the slab and walls:
This involves pouring the slab first, possibly with a
back water stop or centre water stop put around the edge and inserting starter bars
with water blocks. If it is decided not to use water stops (i.e. wet sump) a sump
needs to be created at the bottom of the slab. This involves creating a rectangular
depression in the middle of the slab into which water flows and can be pumped out.
Once the slab has set and cured the front face formwork is attached to the slab and
the walls poured.
This involves having a shutter (form) at the top of the
slab as well as the front face of the walls. These forms are attached together at their
intersection. Together they form what is known as a floating shutter. Now if
concrete was poured in the shutters would simply float up and not give the concrete
the required shape. To stop this happening anchor bolts are drilled down into the
bedrock and grouted in before the placement of the shutters. A bolt is screwed on
the top (this will hold the slab shutter up), the slab shutter seated on top of this and
a second bolt screwed on (this will stop the slab shutter floating up).
For the information of the reader this pit could have also been constructed in several different ways:
This involves pre-casting the entire concrete
pit structure off-site, transporting on site and simply placing it into an excavated hole,
Sump
Floating Shutter Front face wall
Form rests on slab
form
Top Face Slab
Form
Anchor driven
into rock to resist
uplift
Nut holds up top
face slab form
Nut resists uplift
Excavation Face
E x c a v a t i o n F a c e
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covering the sides back up and compacting the fill soil. This is a highly cost effective and time
saving solution which works well where there is no existing piping. But if there is existing
piping, this cannot be used. In-situ cast concrete is better for situations where there is pre-
existing piping because it can be cast around them.
The wall around the slab at the bottom of the pit could
have been made out of reinforced concrete filled masonry. But the problem with this is that
the interface between adjacent block is not water tight which would allow water from the
surrounding soil to flow in, reducing the effective cover of the reinforcement relative to the
case of an equivalent reinforced concrete wall. These interfaces would have to be filled up
with water proof grout. In general this solution is not as durable as the reinforced concrete
options.
This solution involves fabricating a fibre glass shell that has the same
dimensions as the interior of the pit off site and transporting it on site. The shell is then
placed into an excavation which is actually bigger than it. The cavity around the fibre glass
shell is then filled with concrete so that the interior shell is encapsulated in a protective and
strong layer. Ribbing all the way around the shell along its sides grips the highly viscous
concrete, preventing it from floating up out of the concrete. This option results in a fully
water proof pit.
Each of these options was found to be unsuitable, making in-situ cast reinforced concrete the most
suitable construction method. There were existing pipes which made precast concrete an unviable
solution. Also the lower durability of the reinforced masonry in a highly reactive environment like a
sewage pump made it unsuitable. Finally the Fibre glass option would be too expensive to cast and
transport on site.
2 Tunnel EngineeringTunnels are underground passages that allow vehicles, air, fluid or other matter to be
transported under buildings, roads, rivers or hills. Examples of tunnels include:
Fibre Glass Shell
Ribbing
Concrete
Encapsulation
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M5
Eastern Distributor
Lane Cove Tunnel
North Side Storage Tunnel (used to divert waste water from North Shore waste water
treatment plant during periods of excessive rain, for later processing)
Cross City Tunnel
Harbour Tunnel
Tunnels have 2 components:
This is the region of the tunnel between the opening and some distance
down until the tunnel geology becomes uniform. These usually take the form of toughs or
slots cut through unstable earth material so that the beginning of the tunnel is open air, or
cut and cover sections where an open excavation is conducted, the tunnel formed in the
open pit and then covered again with fill.
This is the main body of the tunnel which is excavated by tunnel
boring machine (TBM) or road header.
Standards for the design or urban tunnels are set by PIARC (Permanent International Association of
Road Congresses) the world authority for road construction.
2.1
Opera House Car Park
As mentioned earlier the Opera house Car Park was an outstanding design. It sits within the
sandstone rock of the Bennelong Point cliff, directly beneath the Botanical Gardens. The
construction of the car park relied upon tunnel engineering techniques rather than conventionalopen cut excavation and back fill upon completion because the botanical gardens above could not
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be disturbed. It has a pedestrian access tunnel dug through the bottom of the cliff to give access to
the Opera House, and 2 vehicular access tunnel (one of entry and another for exit) through the same
side, joining up with Macquarie Street.
The floors have a double helix shape the first of its kind, with one spiral being for clockwise motion
to go down and another for anticlockwise motion to move up. 4 tunnels have been drilled through
the central rock core to link the upward and downward spirals at various locations. Each has an
angle offset so that the 4 tunnels do not lie directly parallel above one another as this would lead to
weakness in the rock above the lower tunnels.
Two other ventilation tunnels were excavated; an air intake tunnel was excavated through the cliff
face to the exterior while an exhaust tunnel was dug at a lower level, ending in an exhaust shaft
reaching the surface a fair distance beyond the cliff face.
The car Park outer shell was excavated using a flat arch for the roof and straight vertical walls down
to the foundations. The flat arch (crown) was excavated using a road header with low cost bulk
excavation for the remainder. The construction techniques used in the opera house car park andother projects are discussed below.
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2.2
Flat Arch
The roof (crown) is the key feature of the Opera House Car Park. It is almost flat with a span of up to
19m, which is actually the most appropriate shape for horizontally bedded strata. The flat arch
design is also appropriate because the surrounding rock is Hawkesbury sandstone which has
strength ranging from 15MPa to 40MPa; sufficient to enable the roof to support itself without
additional material.
The roof of the flat arch cavern is not supported with formed concrete, but is rather self-supporting
with the aid of tensioned rock bolts (Macalloy bars) that extend up to 7.5m and un-tensioned rock
bolts (un-tensioned dowels) that extend up to 4.5m embedded in the rock around the arch. The net
result is that rock layers are fixed together so that they acted as a single linear arch. This reason for
this is addressed by the concept of crown support.
The roof rock bolted roof surface is covered with a steel mesh, held in place beneath the rock bolt
plates and sprayed with shotcrete. This reinforced shotcrete layer prevents the dislodgement of
loose rock in regions between the rock bolts. The crown of the tunnel was excavated using a road
header because of its versatility in navigating tight turns and avoiding the noise and vibrations
associated with blasting. The crown was made with a 19m span and a height of just over 5m in order
to fit a caterpillar excavator for the rest of the excavation job.
2.3
Crown Support
If an opening with a curved roof is excavated into a jointed rock mass as
pictured, the loose material will want to slide out and fall from the arch.
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Alternatively if the rock mass is composed of thin horizontal beds, then there is a risk that these
beds will separate (delaminate) and chunks will crack off and fall down.
This process will continue until stresses are redistributed such that all rock is placed in compression
and no further rock fall is possible. This is associated with an increase in height of the arch, with therocks ending up in a stalled state.
The arch line formed is called the natural arch and the distance between the natural arch and the
excavated opening depends upon the type of ground and the span of the arch. That is, the more
jointed and softer the ground is or the wider the span, the greater the distance will be. The rock
above the natural arch line is referred to as the supporting zone while the rock below is called the
supported zone.
This refers to the encasement of the excavation with an in suit poured
reinforced concrete lining. The reinforced concrete around the arch supports the loose material and
directs the loads down to the ground beneath the excavation. The rock above relies upon the
concrete for its stability. The concrete lining is continuously poured. This is achieved using a form
that travels on wheels, so that concrete can be poured, allowed to set and then the form is
transferred and further concrete poured before the initial segment cures. This technique results in a
fully waterproof tunnel that is almost always used for railway tunnels. Little to no deflection of the
roof occurs over the long term.
Excavated Arch
Natural Arch
Supported Zone
Supporting Zone
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Although called temporary this crown support will last a very
long time. But over the long term it will continuously experience deflection increases, leading to the
name temporary. It does not provide water proofing and as such drainage will have to be
considered.
It is based upon the exploitation of the natural arch to support the artificial arch created by the
excavation. This is achieved through the use of rock bolts that extend into the supported arch zone.Rock anchor holes are drilled all the way around the arch extending some distance into the
supporting region. Rock anchors are then placed into the holes and the portion of the bolt within the
supporting zone is grouted. The rock anchors grab hold of the natural arch, using it to hold up the
supported zone.
Each rock anchor is capable of holding up a triangular region of rock surrounding it. Having the
anchors spaced closely together ensures that these supported zones overlap. A small triangular zone
between bolts remains outside of this zone and if no action is taken will likely fall out. To deal with
this, steel mesh is placed beneath the rock anchor plates and is subsequently shot created. This thin
layer of reinforced shotcrete helps to support that small triangular zone between anchors.
In horizontally bedded laminated rock, tension bolts can be used to make the distinct layers act as a
single piece of rock. Essentially the rock layers in the roof behave like a conventional concrete slab.
Reinforcing mesh embedded in shotcrete supported by the rock bolt plates takes on the role of
tensile reinforcement, preventing the cracking of the rock in tension. The tensile forces are
transferred to the steel mesh through the rock bolts. This scenario is designed in an almost identical
fashion to a conventional concrete slab.
Zone not
supported by
anchors
Natural Arch
Zone directly by
anchors
Supporting Zone
Zone not
supported by
Reinforce
Shotcrete
Reinforcing mesh
(blue)
Zone directly by
anchors
Shotcrete(orange)
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Another case in which rock anchors are often required involves tunnel excavations with a gradient in
horizontally layered bedrock. Steep gradients will cut across bedding planes and create wedges of
rock that are highly unstable, potentially cracking off their beds and falling down. Rock anchors are
used to hold the wedges in place and prevent their collapse.
Note that in all case the stability of the excavation is dependent on the ratio of excavation span
(width of the arch) to the depth of cover rock above the excavation ceiling. The greater the depth of
rock above the more stable the excavation will be for a given span. If the depth of cover is too small,
such that insufficient rock material lies beyond the natural arch to support the imposed compressive
loads, then the excavation will become unstable and other means of strengthening will be necessarysuch as permanent support excavation or complete removal of the rock matter above. As a guide the
span of the Opera House Car Park was 19m with in excess of 7.5m of cover which is one of the
highest span to cover ratios in the world.
In order to select the appropriate rock bolt it is necessary to determine
the load carried by each rock bolt. In the case where rock fragmentation is small, we find the weight
of rock bearing down on from both the previously mentioned triangular region directly supported by
it and the section of rock supported by the shotcrete.
Hence the region supported by each anchor is almost rectangular; extending from midway between
it and the adjacent bolts on either side in the plane of the arch and longitudinally along the length of
Wedges
may fallRock anchors with
reinforced
shotcrete installed
to prevent wedge
Shotcrete
(orange)
Reinforcing mesh
(blue)
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the area of the sliding surface is and the angle between the normal to the sliding surface and therock bolts is .
The factor of safety for this case is taken to be between 1.5 and 3 again. If the load bearing capacity
of each rock anchor is then the number of rock anchors required is given by the followingequation.
To rationalise this, the force acting down the sliding surface (inducing sliding) is the component of
weight in the sliding direction (factored up) , less the friction force caused bythe component of weight acting normal to the sliding surface and less the cohesive force(due to the wedge and surrounding rock being one rock mass) .
The force acting up the slope per anchor due to the full capacity of the bolts being exploited is
factored component of the tension force acting in the bolts up the slope added with thefriction force due to the component of tension acting normal to the slidingsurface . The product of this with the number of bolts provides the total resistance force tosliding.
Rock anchors come in a variety of forms and can be installed in a
number of ways. But the key items that are common to all rock anchors are the threaded anchor bar
at the exposed end, on to which an anchor plate, dome washer and bolt are attached. The bolt is
screwed onto the anchor bar, providing support to the dome washer and in turn the anchor plate, as
well as providing a means of tensioning the anchor bar.
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The dome washer, butts up against the bolt and anchor plate, with the curvature allowing the bolt,
washer and bar arrangement to be at a different angle to the plate. This allows the plate to sit flush
with the rock face, even when the anchor is not perpendicular to the excavation face. The anchor
plate then makes contact with the rock face, allowing the compressive force to be distributed over a
larger area, as well as providing a means of supporting the steel mesh for reinforced shotcrete.
Note that in cases where additional support is required, conventional rock anchor plates will be
substituted with W-straps. These are corrugated strips which expand the supported area relative to
ordinary square plates. They can be lapped to create a continuous support strip or used in isolation.
They are usually used where the rock strength is low.
Some of the types of common anchor systems include:
The process of installation involves drilling a hole of the required
depth. The rock anchor is then prepared by attaching spacers to it which are hollowed out
circular disks that maintain the anchor bar in the centre of the hole. A 10mm grout tube is
Anchor bar
Dome washer
Anchor boltAnchor plate can
move around
relative to the
anchor bar, bolt &
Washer
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then wrapped around the anchor bar starting from it tip and working its way to the plate
end. A small hole in the plate allows the grout tube to pass out of the anchor shaft. The rock
anchor is then put into the hole and grout pumped into the tube. Grout will then flow out of
the end and will fill the shaft from the top down.
If the rock anchors are to be tensioned the segment shaft in the supporting zone is grouted
only. If grout was extended into the supporting zone tensioning the bolt would not put the
supported zone into compression. But some anchor rods come with a smooth sheathing
with a greased interior around the bar in the supporting zone so that the entire shaft may be
grouted and the bar in the supported zone can still move freely.
Alternatively if the anchor is to remain un-tensioned then unsheathed anchors are used and
the entire shaft is filled, with the emanation of grout from behind the anchor plate indicatingthis.
Rock anchor
plate
(washer) with
hole for grout
tube
Grout tube
Rock anchor boltGrout Pumped in
Grouted portion of
shaft
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Note that the bond length refers to length of anchor bar embedded into grout which can
bear load, while the fee length is that which is un-grouted or has a sheath surrounding it.
The maximum bearing capacity of a rock anchor is dependent upon the diameter of thegrouted shaft , the length of the bar segment which is bonded within the supporting zone and the adhesion strength of the bar in rock .
Once the grouting or adhesive has set and cured, the anchors may then be tensioned by
tightening the anchor bolt against the anchor plate .
This involves a slightly different approach to the installation of rock
anchors. After drilling anchor holes, one or more sausage-like resin bags are inserted all the
way to the end of the shaft. The anchor bar is then forced into the hole with the tip burst the
plastic bag and releasing the resin contents. The resin then set within 8 seconds bonding the
anchor bar to the surrounding rock. The bearing capacity equation for grouted anchors can
also be applied to chemical anchors. Once the adhesive has set and cured, the anchors may
then be tensioned by tightening the anchor bolt against the anchor plate.
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The anchor bars used in this case are quite
different to those in the preceding sections. The resistance of the bar to pull-out comes from
the friction between it and the surrounding rock in the anchor hole. After drilling the anchor
hole, the Swellex bar is placed into the shaft. It has a hollow horse-sho shape. Grout is then
pumped into the hollow body of the anchor, causing it to expand and engage with the rock
in the shaft. As the grout sets the pressure remains high, maintaining pressure between the
anchor bolt and the rock and creating sufficient friction to hold it and the supported rock in
place. Once the grouting has set and cured, the anchors may then be tensioned by
tightening the anchor bolt against the anchor plate.
2.4
Sets for Crown Support
Sets are used to provide support to highly unstable regions of tunnels constructed using the
temporary support system where rock anchors are incapable of providing sufficient support for the
crown rock. This is usually the case when rock in the crown has two or more degrees of freedom,
capable of falling directly down and moving horizontally. This usually happens when the crown
changes height abruptly or where two tunnels meet.
An example of this scenario can be found in the Opera House Car Park where the access, linking and
ventilation tunnels meet the cavern. In the following illustration the chunk of rock at the top of the
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access tunnel, adjoining the caver crown is highly unstable. It can quite easily fall down or move left.
Because rock anchors are unsuitable we now have to use an alternative crown support system called
sets. These consist of closely spaced steel UC (I section) frames composed of several sections welded
together which are linked together with steel packing (steel plates) which provide mutual support
between adjacent sets and between which reinforced shotcrete is placed to provide support to loose
material. Because there is a gap between the rock face and the sets, reinforced shotcrete is used to
fill this space. Sets are only used for short distances due to their high cost; only supporting the highly
unstable rock segment.
There are two ways in which sets can be constructed:
These are composed of two I section segments welded together
at the apex of the arch, forming an arch section which is in turn supported on vertical I
section segments parallel to the walls of the excavation and terminating at the base of the
tunnel. This type of set is used when the vertical rock walls of the excavation have
insufficient strength to support the arch load down to the base of the tunnel.
Unstableregion
requiring sets
Cavern Crown
Access Tunnel
Highly Unstable rock wedge likely to fall
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Here the rock is strong enough to be able to transmit
the compressive forces from the arch segment of the set down to the tunnel base. Sets are
now terminated in recesses cut into the rock walls at the level of the spring line. The string
line is an imaginary horizontal line that links the point at which the tunnel goes from arch to
vertical walls in a horse shoe tunnel or the halfway point of a circular tunnel.
2.5 Headings and Bulk Excavation
Tunnels and underground structures excavated using a road header or drilling and blasting are
usually excavated in several stages. First the crown of the tunnel is excavated which in turn isexcavated in several stages. The central portion of the crown is excavated creating a small tunnel
referred to as an initial heading. The initial heading is then expanded through further excavation to
produce headings on either side which form the crown. In the case of the Opera House Car Park, the
crown was excavated using a road header in 3 stages. First a central heading was made and this was
expanded on either side to produce the crown. The central height of the crown was made in excess
of 5m to enable subsequent low cost excavation equipment to be brought in.
Cavern Crown
Access Tunnel
Highly Unstable rock wedge held up by sets
Vertical legs of sets
Arch segment of sets
Packing
Reinforced shotcrete between sets
Vertical
legs of sets
Arch segment of sets
welded at arch peakand to set legs
Spring line
Setsembedded
into rock
at spring
line
Rock carries
vertical
compressive
forces
Sets only form an arch
(welded together atapex)
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Once the crown has been excavated the tunnel may be dug deeper. This involves successively
expanding the base of the tunnel with what are known as benches. The tunnel is expanded to a
certain depth and once a sufficient bench length has been achieved another bench is begun. The
process is repeated until the tunnel base is reached
Because road headers are extremely expensive to operate ($135/cubic metre) it was decided that
the road header should only be used be used for the crown of the main car park cavern, the
ventilation tunnels, access tunnels and the 4 linking internal tunnels. The remainder of the
excavation; the bench excavation, would be undertaken using low cost bulk excavation instead
($80/cubic metre). The bulk excavation was carried out using a bulldozer fitted with an impact ripper
and an excavator fitted with a hydraulic impact breaker to break down the rock.
2.6
Structural Elements in Underground Structures
It is always a good idea to ensure that a structure built underground is independent of the
excavation face. Engagement of a structure with the rock surrounding it can be quite problematic.
For example if a concrete structure like the Sydney Opera House Car Park was permanently fixed to
the vertical walls of the excavated cavern at multiple locations along its height then the concrete
would want to shrink beginning soon after and continuing for a long time after construction, but
because the rock is already consolidated, the shrinkage is prevented.
This places enormous stresses on the structure that may in fact cause it to crack or deform
significantly. It is far better to make the structure free from the surrounding walls and roof, letting itsit directly upon foundations at its base. In this way the structure can shrink and move
Initial HeadingHeading is
expanded
Crown is
completed
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independently of the surrounding rock, avoiding excessive shrinkage stress and not placing
additional stresses on the rock faces around it.
The circular building was also built as 4 quarters to allow for motion of the structure under the
subsidence of the footings. Movement joints where put in at the interface between the independent
sectors of the building.
2.7
Tunnel VentilationThere are two methods of ventilating a tunnel during construction:
Here ventilation ducting supplies fresh air to the construction area
and exhaust air is allowed to escape by itself through the tunnel and out through the
opening. This is a very cheap option because the positive air pressure means that collapsible
plastic sheet ducting can be used. But the main problem with this system is that dust is
distributed down the tunnel making the air quality pour behind the excavation works.
Here ventilation ducting carries exhaust air from the excavation
area under negative pressure and allows fresh air to flow from the opening through the
tunnel. Because the ducting is under negative pressure flexible plastic ducting cannot be
used. Prefabricated steel ducting has to be used which is quite expensive. But this systemhas one important advantage which is that dust is sucked away immediately from the
excavation area and not allowed to flow down the tunnel. The air quality in the tunnel is
much better than the preceding case.
Teeth Engage
with Rock Face
S t r u c t u r e w a n
t s t o
s h r i n k b u t
c a n
’ t
Independent
of Rock Face
S t r u c t u r e f r e e t o
s h r i n k
Ducting
Ducting
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There are also 4 types of tunnel ventilation system used during service. These include:
This is suitable for small tunnels with low traffic volume which
is often the case in rural areas. The flow of traffic is used to drive fresh air in and expel
exhaust air out. The air flows in the direction of the traffic. Many of these tunnels have
mechanical fans which are activated during a fir to increase the ventilation flow rates.
This is similar to natural ventilation, but now
mechanical fans are added in to help drive the air along the length of the tunnel at a much
faster rate. No ductwork is necessary as fans are simply attached to the ceiling. These are
usually used in rectangular tunnels where there is insufficient space to run ductwork or short
circular tunnels.
Long tunnels with longitudinal ventilation are often compartmentalised. This means that at
regular intervals along the tunnel air intakes and exhaust extraction outlets are put in. This is
done to ensure that the concentrations of pollutants like carbon monoxide do not reach high
levels as would be the case with non-compartmentalised tunnels. Sometimes the air intake
and exhaust are put in close proximity to each other, as is the case with the M5 tunnel.
When the intake and exhaust are put closely together short circuiting occurs. Rather thanthe exhaust outlet sucking air from the left portion of the tunnel and the intake blowing air
down the right of the tunnel, the air brought in by the intake is directly taken by the exhaust
without flowing down the tunnel.
E x
a
s t
I t
Fresh air should flow
this way but doesn’t
Exhaust air should flow
this way but doesn’t
Air short
circuits
Compartment 1 Compartment 2
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To avoid this, the ducting for the exhaust and intake are extended a great distance
longitudinally along the length of the tunnel to avoid the short circuiting problem.
Here a single duct carrying mechanically driven air
is placed either above a suspended ceiling over the roadway or below the roadway slab. It is
referred to a partially ducted because either the supply or exhaust air is ducted but not
both. There are two possible scenarios:
o Here fresh air is forced into the tunnel through a cavity
beneath the roadway slab. Regular openings in the duct along the length of the
tunnel allow the fresh air to enter the roadway envelope. The exhaust air then
travels along with the traffic through the roadway envelope to the exits of the
tunnel.
o Here exhaust air sucked out through a cavity above the
suspended ceiling. Regular openings in the duct work allow the exhaust air to be
sucked from the roadway envelope. Supply air then travels along with the traffic,coming in from the tunnel exits.
This is referred to as fully ducted ventilationbecause both the supply and exhaust are ducted in the tunnel. Fresh air is forced into the
E x
a
s
I t
Fresh air now flows
this
Exhaust air now flows
this way
Extended ducting
prevents short
circuiting
Compartment 1 Compartment 2
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tunnel through a cavity beneath the roadway slab and exhaust air is sucked out through a
cavity above the suspended ceiling. Regular openings in the ducts along the length of the
tunnel allow the air to be circulated.
Rather than circulating longitudinally along the length of the tunnel, air flows transversely
along the tunnel, which means air has a shorter distance to travel and can be circulatedmore frequently. This ventilation system is used in longer tunnels that have large amounts of
air that need to be replaced or heavily travelled tunnels that produce high levels of
pollution.
The Opera House Car Park in fact used a similar system, having ducted supply and exhaust air. Ducts
were produced by excavating notches in to the rock around the exterior surrounding rock and
interior rock wedged at centre of the structure, at regular intervals and stretching the entire length
of the structure. These were then covered with a perforated concrete membrane.
The excavated vent shafts were not lined with the excavated face becoming the envelope for the
vents. The shafts at the centre of the structure were used as exhaust vents (called exhaust risers)
and the shafts around the exterior were used as supply vents (called air droppers). Air would flow
along the Air would flow from the exterior edge of each floor towards the centre of the structure.
The air droppers and air risers were then linked to ventilation tunnels linking them to the air intake
and outlet at the bottom of the cliff.
Air droppers
excavated into
exterior rock
Air risersexcavated into
interior rock
Flow of air
Flow of air
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When excavating ventilation tunnels the key parameter is the area of the tunnel. Because the
excavation and support of the crown of a tunnel is extremely expensive it is far better to have a
narrow but tall ventilation tunnel rather than a wide and short tunnel.
The factors affecting the design of ventilation systems and the size of air compartments include:
Allowable concentrations of pollutants like carbon
monoxide, nitrous oxide and other harmful gases.
i.e. the number of cars per metre/kilometre of the tunnel.
The slower cars move the more ventilation that has to be provided. This
is linked to the traffic density.
i.e. the mix of cars and truck.
This refers to the condition of vehicles passing through the tunnel.
Older vehicles will tend to require better ventilation due to higher emissions.
Steeper tunnels will cause cars to burn more fuel and emit more
exhaust so more ventilation will be required.
The area of the tunnel determines the maximum air
velocity in the tunnel. Air will move faster through a tunnel with smaller area, meaning that
the air will be circulated faster and replenished more quickly.
There is generally no requirement for ventilation of a rail tunnel to control air quality. There is
however, a need for a ducted fire product extraction system which is able to target a particular
location.
In conjunction with semi and full transverse ventilation systems, single point
extraction is also used to increase the airflow potential in the event of a fire. This involves creating
additional exhaust vents and outlets with fans which only activate in the event of a fire, helping to
suck away toxic smoke.
Alternatively some of the newer ventilation systems are capable of providing sufficient extraction
potential without the need for additional extraction ducting and outlets. This is achieved through
oversized extraction ports, ducting and exhaust outlets combined with fans whose capacity can be
significantly scaled up. This option is in fact far cheaper.
Ventilation
Tunnels with
same area
Cheaper More expensive
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2.8
Tunnel Portal Zones
Portal zones of a tunnel often have insufficient cover relative to the driven zone and as such
different techniques have to be used to excavate and stabilise this tunnel segment. Some of the
common techniques used to excavate and support the portal zone include:
Here the rock/earth over the portal zone is simply
removed creating a trough like entrance to the tunnel with high walls which are usually
reinforced concrete lined and there is no cover. The open excavation continues until a point
is reached where the rock/earth cover is deep enough for the stability of the tunnel. From
here on in tunnel boring or road heading begins.
If the unstable layer of rock or soil cannot be removed due to there being
an existing structure above or for example in the case of the cross city tunnel exit in the
domain protected trees being on top then the canopy tube technique needs to be used.
The process involves using thick walled hollow steel tubes being driven into the soil above
the crown of the tunnel to be excavated or drilled and inserted if rock material is present.
These tubes usually extend 15m and are placed close together in an arch formation around
the crown of the tunnel, referred to as a pipe canopy. In soil and soft rock the tubes are
jetted into the ground. This involves forcing high pressure water down the end to the tube,
driving away soil matter and allowing the tube to move deeper into the ground.
The ground matter beneath the tube is then progressively excavated, with sets with legs (as
previously described) placed beneath the tubes to support them. Typically 1m of tunnel is
excavated and a set put in before excavation continues and the process is repeated.
Pipe
Canopy
Thick-wall
Steel Tubes
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If the portal zone and unstable material extends beyond one length of steel tubing, then the
tubing is inserted at an upward incline relative to the tunnel excavation level so that a new
array of tubes can be inserted at the end of the first. The process is repeated until the driven
section is reached.
The tubes are hollow with perforations along their length. Once put in, concrete is pumped
down the tubes to penetrate the surrounding ground and give it some stability, as well as to
strengthen the tubes and provide them with additional rigidity.
This is a simple technique of constructing shallow tunnels or portal
segments where a trench is excavated (open cut excavation)before or after putting in side
walls, the tunnel casing (usually reinforced concrete) constructed and then covered up
again with fill. This technique is usually used for soft soil conditions. There are two methodsof constructing such a tunnel:
o Here an open cut trench is excavated all the way to the
foundation of the tunnel after creating supporting walls for the earth matter on
either side using slurry wall technique or contiguous bored piling. The tunnel base,
walls and roof are then constructed within the supporting walls using in situ cast
concrete or precast concrete segments. Once the tunnel is complete, the trench is
then carefully filled and the surface restored to its original condition. This technique
is used where disruption of the surface is not a problem.
Tubes driven in to
where crown will be 1m Excavated
Set put in
Process is repeated
Set put in
Pipe
Canopy
Sets supporting
pipe canopy
Inclined tubes to allow
subsequent tubes to be
insertedSets
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o This method is used where disruption of the surface is to
be minimised. The process starts with the creation of side walls for the tunnel that
extends from the surface or just below the surface to the foundations of the tunnel.
The walls can be made using the slurry wall technique or contiguous bored piling.
Once the walls are in place the soil within the tunnel area is excavated down to the
bottom of the tunnel roof (not all the way down to the foundations). A tunnel roof is
then poured in such a way that it is supported by the adjacent walls. Once the
tunnel roof slab is complete, the trench is covered up again and the surface is
restored. The tunnel beneath is then excavated without disturbing the surface any
further. The tunnel floor slab is poured and the tunnel made ready for use.
Note that when such tunnels are created, the soil surrounding them will inevitably be saturated.
Water ingress into the tunnels means that pore water pressure in the area above the tunnel will
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drop. This induces subsidence of the ground above. An excellent example of this was the Easter
distributor tunnel beneath Surry Hills.
2.9
Driven Section Tunnelling
There are two techniques commonly used for the construction of the driven section of a tunnel:
A road header is an excavation device used to excavate short tunnels, segments of tunnel with sharp
turns or small cross section tunnels in hard rock. This piece of equipment is easy and quick to
procure and has a moderate service cost. It is composed of a rotating bit (a single bit rotating
perpendicular to the excavation face or two bits rotating parallel to the excavation face) with sharp
spring loaded picks which pick away at the rock surface.
The road header bit is attached to a boom which can be moved up and down to cut the height of the
cross section of the tunnel. The boom is then attached to a cabin with a crawler undercarriage (like a
dozer) which allows it progress forwards and turn. By rotating on its tracks (tracks stay stationary
while chassis rotates) the road head is able to cut from left to right.
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Attached to the front of the cabin, below the boom is a pilot platform (an inclined wedge shaped
pan) which covers the front tractors. As the bit cuts away rock material, debris falls onto the pilot.
Large sweeping arms moving side to side on the pilot push the debris towards the centre of the pilot
into a hole in the pilot.
Beneath the hole is a conveyor belt that that extends some distance behind the rear of the roadheader. The conveyor belt carries the debris away to the rear of the road header. It is also inclined so
that the cut material can be loaded directly into a dump truck. The conveyor is flexible, being able to
bend a significant amount as it continues to work (much like baggage conveyors at the airport which
can navigate tight turns) allow the road header to make tight turns in the tunnel
A tunnel boring machine is piece of equipment used to excavate tunnels in variety of mediums
ranging from hard rock through to soft muddy soil which have a circular cross section. Tunnels
produced using TBM’s can range in size from 1m using a micro-TBM through to over 19m using a
large scale TBM. TBM’s are typically used for long tunnel construction (in the case of large scale
TBM’s) due to their low running costs, but have a high procurement cost and long procurement
period, making them unsuitable for small tunnels. Micro-TBM’s can be efficiently use for both short
and long tunnels and have a significantly lower excavation cost than alternativ