Abraham Notes.pdf

8/16/2019 Abraham Notes.pdf

1/195

Engineering Design andConstructionCourse Notes

4/25/2013

The University of Sydney

Abraham Kazzaz


2/195


3/195

2

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


4/195

3

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.


5/195

4

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.


6/195

5

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


7/195

6

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.


8/195

7

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:


9/195

8

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:


10/195

9

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


11/195

10

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.


12/195

11

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.


13/195

12

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.


14/195

13

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


15/195

14

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


16/195

15

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





Bottom reinforcement bars


top reinforcement using L-bars

or bent ends

Top reinforcement bars


bottom reinforcement using L-

bars or bent ends

Inner and outer reinforcement

lapped at ends to form

overlapped loops around

corner vertical reinforcement


17/195

16

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


18/195

17

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


19/195

18

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


20/195

19

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


21/195

20

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


22/195

21

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


23/195

22

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.


24/195

23

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.


25/195

24

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


26/195

25

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)


27/195

26

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)


28/195


29/195

28

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.


30/195

29

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


31/195

30

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


32/195

31

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.


33/195

32

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


34/195

33

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


35/195

34

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)


36/195

35

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


37/195

36

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


38/195

37

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


39/195

38

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


40/195

39

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


41/195

40

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


42/195

41

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


43/195

42

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


44/195

43

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


45/195

44

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.


46/195

45

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

Abraham Notes.pdf

Documents

Transcript of Abraham Notes.pdf