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5. Bridge Design and Construction Options
The following bridge design and construction options have been developed for the low and high
level crossings of the Clarence River. The designs and costing are preliminary and have been put
forward for discussion purposes and to assist the development process. Sketches of the bridge
options are provided in Appendix A.
5.1 Low level bridge
The low level bridge would be approximately 900 metres in length, including 400 metres in
approaches and 500 metres over water.
5.1.1 Opening span for low level bridge
The length of the opening span for the low level bascule bridge has been matched to the existing
bridge, with a span length of 38 metres (refer Section 1.1).
For this assessment, the opening span is assumed to be a steel box girder with an isotropic steel deck
in order to minimise the weight.
As noted, mechanical and electrical designers have not been involved at this stage. Should the low
level bridge be adopted as the preferred option they would be involved once the structural conceptfor the opening span and the parameters for the operation of the opening span have been established.
Similarly technologies to those proposed here have recently be used for the Port River Bridge in
Adelaide, South Australia
5.1.2 Superstructure for low level bridge
The superstructure for a low level bridge option is dictated by the spans of the existing bridge. The
three spans either side of the opening span are 43 metres each whilst the approach spans are 22
metres. The 43 metre span is too long for precast concrete T-girders, so the superstructure for this
span and width of bridge is likely to be a single cell box girder of around 2.4 metre depth. If this
section is to be used for the main spans, construction efficiency would dictate that the approach
spans are constructed using the same section with 44 metre spans to match every second pier on the
existing bridge.
The concept design for the low level bridge has nine 43.7 metre spans (393 metres) on the northern
approach to the opening span and ten 43.7 metre spans (437 metres) on the southern approach. The
likely methods of construction for the superstructure are launched or span by span precast segmental
construction.
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Launched superstructure for low level bridgeFor a launched bridge the superstructure would be cast on the river banks and launched across the
river to the opening span in a similar manner to the bridges over the Karuah River on the Pacific
Highway and the bridges over the Murray River at Albury, Corowa and Robinvale.
Launching a bridge involves construction of a casting bed for the box girder section and a launching
pad to jack against to push the completed segments out to cast the next segment of the bridge. The
construction of the girder segments would be set up so that installation of the reinforcement and
formwork, casting, curing, stressing and launching of the segments can be undertaken on a regular
cycle. It is assumed that a 21 metre segment (half a span) could be cast on a two week cycle.
For a bridge of 700 metre length it would be preferable to set up a casting yard and launching pad on
one side of the river and launch the bridge across to the other bank. At the Harwood Bridge site, the
southern bank of the river is the preferred site for the casting yard and launch pad because it is out of
the village, and rock is reasonably close to the surface. The cost of construction of a temporary
platform for the casting and launching of the bridge would be in the order of $1 million. The
northern bank of the river has very deep alluvial deposits and the cost of setting up the launching pad
would be in the order of $2 million, due to the cost of the large deep piles to support the weight of
the casting bays and resist the lateral forces due to the jacking of the bridge during the launching
process.
The complication with the Harwood Bridge is the inclusion of the opening span in the centre of the
river, and the need to ensure that the river remains passable during construction. Accordingly, it may
be necessary to launch the bridge from both sides of the river. To launch from both sides would
entail additional costs for the set up of the casting yard and launch pads on both sides of the river. If
the two approaches to the opening span are launched simultaneously there would be an additional
cost of around $1 million for the second launching nose and launching equipment. This cost is likely
to be offset by the cost savings associated with the 3 to 4 month time saving by not having to wait for
the first approach to be completed before commencing on the second approach.
In order to save time and cost with the launching of the bridge, it may be feasible to consider
launching both approaches from the southern bank with the two ends of the box girders temporarily
anchored together with stress bars. After the bridge has been launched across to the northern
abutment, the stress bars would be released and the southern approach would be pulled back into its
final position. There are some challenges with this system and it would require launching structures
at both ends and an intermediate launching frame for the retrieval of the southern approach, but there
are potential cost and time savings that would be significant.
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The option to close the waterway to traffic and launch from one side only could also be investigatedduring the environmental assessment and detailed design, in consultation with NSW Maritime and
the local community.
Pre-cast segmental superstructure for low level bridge
The span by span pre-cast segmental construction involves placing trusses below or over the bridge
deck and opening a series of pre-cast segments into position and post tensioning them together to
form simply supported spans. Alternatively the segments can be installed using the balanced
cantilever construction method then joined with a cast in situ infill segment to form a continuous
superstructure. This system worked very successfully on the M7, as both balanced cantilever and
span by span construction, and on the Windsor Flood Evacuation route project, as span by span. It is
also currently being used in balanced cantilever construction on the Gateway project in Brisbane.
The use of pre-cast box girder segments is, however, reliant on having a sufficient volume of bridge
structures on the project to warrant setting up a box girder fabrication yard for the project. There are
some forty bridges on the project, so it may be viable to set up a plant if the contractor chooses to use
box girder segments for most of the other bridges.
5.1.3 Substructure for low level bridge
It is proposed that the bridge piers would consist of a cast in situ concrete blade column supported on
a cast in situ concrete pile cap with two rows of raked piles.
The piles would need to be in the order of 40 metres deep to found on rock or in the dense sand and
gravel layers. The piles would also need bending strength in the top section of the piles to avoid
buckling in the water and soft silt layers and to resist lateral loads such as a vessel impact. It is
proposed that the piles are composite sections either using the 550 octagonal pre-cast pre-tensioned
concrete piles with a 310UC steel H piles or steel tubes with the top section of the pile filled with
reinforced concrete.
5.1.4 Alternative designs for low level bridge
A number of alternatives were considered but have not been pursued at this stage as the additional
complexity and cost would likely make them unviable. These options include:
Extending the spans from 43/44 to 86/88 metres and using balanced cantilever construction with
concrete box girders or using steel arches or trusses to achieve the longer spans.
Using cable stays from the opening span towers to create 130 metre spans either side of the
tower. This would create an iconic structure and remove some piers from the river. Although
there are construction and design issues relating to the resolution of the forces in the towers due
to the dislocation caused by the opening span that would be difficult to overcome. Moreover,
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the creation of a cable stay bridge could have significant visual impacts on the village ofHarwood.
5.2 High level bridge options
The high level bridge would be approximately 1500 metres in length, including 1000 metres in
approaches and 500 metres over water.
5.2.1 Superstructure for high level bridge
As with the low level bridge options, the high level bridge design is dictated by the spacing of the
piers for the existing bridge. The main differences between the high level bridge and the low level
bridge are that; (1) the high level bridge is 40% longer than the low level bridge length, (2) there
could be a requirement for twin bridges at the outset in order to realise the benefits of the high level
bridge and (3) the cost of the substructure is higher than for the low level bridge due to the
significantly higher piers and the longer span lengths that may be required to address issues related
to aesthetics, and hence options that were discounted for the low level bridge become feasible for the
high level bridge, e.g. minimising the number of piers in the river.
The three construction options considered for the high level bridge use a concrete box girder section
similar to the low level bridge, with the depths varying to suit the spans.
Pre-cast segmental construction (44 metre spans) for high level bridge
The first option is to keep the piers at the same spacing as the existing bridge and to have 43 and
44 metre spans on the approaches and a 38 metre span coinciding with the opening span on the
existing bridge.
The construction could be similar to the span by span form of construction described above for the
low level bridge, using pre-cast segmental construction with a truss below the deck and opening a
series of pre-cast segments into position and post tensioning them together to form each span.
Alternatively, a truss over two spans could be used to install the segments in a balanced cantilever
sequence to provide continuous spans.
For the span by span construction a 2.4 metre depth box section would be used. For the balanced
cantilever the section could be reduced to 2 metres. The balanced cantilever method would also use
integral connections between the piers and superstructure which would reduce future maintenance.
The cost of the piers for the high level section over the river makes this option less efficient than it
was for the low level bridge and the span lengths are too short for the pier lengths. On the basis of
cost, this is unlikely to be the preferred option for this bridge.
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Pre-cast segmental construction (86 metre Spans) for high level bridgeThe second option for a high level bridge is to position the piers to match every second pier on the
existing bridge and use the balanced cantilever pre-cast segmental system for these 86 metre
maximum spans. A similar form of construction was used on the approaches to the main spans on
the Bolte Bridge in Melbourne and is currently being used for the approaches to the Gateway Bridge
in Brisbane. Based on the designs for these bridges the concrete box girder section would need to be
around 4.1 metres deep for the 86 metre span.
The bridge would be designed using an integral cast in situ pier head, which is match cast with the
adjacent pre-cast segments in order to avoid bearings at the piers and the associated complications to
the balanced cantilever construction and future maintenance. Expansion joints would be placed at
the 1/5 point on every fourth span to minimise the thermal expansion and contraction effects.
The size of the truss required to open the girders into position is significant and, depending on the
timing of major bridge projects in Australia, may need to be imported into Australia for the project.
The viability of setting up a pre-cast yard for the box girder sections is also an issue. The cost of
setting up a pre-cast yard for the manufacture of pre-cast concrete box girder segments is in the order
of $5 million. To compete with T-Girders from an existing pre-cast yard, there needs to be a
significant area of bridges to cover the cost of setting up a pre-cast yard. The Ballina Bypass project
included around 30,000 m2 of bridge deck, which was not considered large enough to justify the use
of pre-cast box girder segments. By comparison, the duplication of the Harwood Bridge would
require approximately 20 500 m2 of bridge deck for the low level bridge and 34 500 m2 for the high
level bridge.
If twin bridges are constructed this option is likely to be viable. If only one bridge is to be
constructed the viability would depend on the market and what other projects are taking place in
Australia and South East Asia.
Cast In situbalanced cantilever construction for high level bridge
Cast in situ balanced cantilever construction is significantly slower than the pre-cast segmentalconstruction described above. It has lower set up costs, so if the bridge is not on the critical path for
the project and only one bridge is being constructed, it is likely to be the most viable solution.
The existing bridge has 43 metre spans on either side of the central 38 metre opening span. A single
span of 125 metres has been assumed for the new bridge to correspond to the three central spans of
the existing bridge. There is then an 85 metre span either side of the main span to match the next
two 43 metre spans, two 66 metre spans each side to match 6 of the 22 metre approach spans and an
end span of 44 metres.
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This 647 metre cast in situ balanced cantilever bridge would be used across the river. Theapproaches on either side of the river would then be constructed using 33 metre long precast concrete
T girder spans, as this should be the most cost effective form of bridge deck construction for the land
spans.
5.2.2 Substructure for high level bridge
It is proposed that the bridge piers would consist of a cast in situ concrete blade column supported on
a cast in situ concrete pile cap with two rows of raked piles. With the higher columns, longer spans
and integral connections to the superstructure the vertical loads and shear and bending forces would
be higher than the substructure for the low level bridge. The piles for piers in the river are likely to
be large diameter steel casings driven to rock, with reinforced concrete protecting the top section
projecting above the river bed. The concrete would provide bending strength and stiffness and
provide for corrosion protection of the upper section of the piers (i.e. that section most likely to be
exposed to the corrosive effects of the Clarence River.
5.2.3 Alternative designs for high level bridge
A number of alternatives were considered but have not been pursued at this stage because the
additional complexity and cost would likely make them unviable. These options include:
Constructing a series of 125 metre span concrete arches under the deck. Although they can bequite elegant, arches are better suited to single spans with good foundation material at each side
of the river as the resolution of the thrust forces from the arch into the foundations would be
difficult during construction and operation.
The use of cable stays to increase the spans. This was not pursued because there are no
functional reasons to increase the span beyond 125 metres and it was considered that the bridge
is high enough without adding to the height with stay towers. The cost of this option is also
likely to be quite high. The main cable stayed section of the Anzac Bridge cost $90 million or
$6 000 per square metre. The rates for a cable stayed bridge at the Clarence River are likely to
be in the order of $9 000 per square metre due to the increase in construction rates, thegeotechnical conditions on the northern side of the river and the distance of the site from the
specialist contractors required for a cable stayed bridge.
5.3 Interchange impacts
Associated with the new bridges, the Yamba Road Interchange would need to be upgraded. The
upgraded interchange would be able to use a similar layout to the existing, which would include
ramps constructed on low embankments. To facilitate the large movement of cane vehicles, a grade
separated crossing of the upgrade would be provided at Watts Lane, with the upgraded highway
passing over Watts Lane.
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The concept design process has seen comparable interchange arrangements adopted for bothinterchanges under both the low level and high level bridge options. This applies to both the Yamba
Road and Watts Lane Interchanges. Although the interchange detail would differ between the low
level and high level bridge options, the general arrangements and footprints of the interchanges
would be the same.
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6. Urban Design
An urban design analysis was undertaken to assess the relation of the high and low level bridges to
the existing bridge as well as the potential impacts on the amenity of the village of Harwood. The
analysis considered the following:
Characteristics of Harwood village and the surrounding area.
Loss of convenience or connectivity for residents.
Visual impact on the historic bridge and its setting in the landscape.
Visual intrusion within the village and quality of views from it. Extent and effects of shadowing.
6.1 Characteristics of Harwood village and the surrounding areas
The visual environment to the south of the proposed bridge is characterised primarily by wetland
vegetation, Maclean Hill, the Yamba Road interchange and other local roads. The visual
environment to the north of the bridge is characterised by the trees and buildings of Harwood Village
and the sugar mill. The existing bridge with its distinctive towers and the sugar mill are the
dominant elements in a very flat landscape (Figure 3)
6.2 Connectivity
The features of Harwood Village are illustrated on the Maclean LEP zoning map ( Figure 7) and on
an aerial photograph (Figure 8).
There are three main roads in Harwood:
River Street, which extends parallel to the bank (about one kilometre long within the village).
Harwood Mill Road.
The northsouth Morpeth Street (west of the existing highway).
The Harwood Mill complex at the eastern end of River Street is the main development in the village.
Morpeth Street starts at the riverbank in a small commercial area where a punt would have landed
prior to the construction of the bridge and is one of the oldest streets.
As can be seen from the various photos (Figure 8 to Figure 10), the Pacific Highway, including the
bridge, its approach roads and associated buffer zone cut through the centre of the village, passing to
the rear of the school. However, as is evident from the photograph taken along River Street and the
layout of the village streets, the Highway and bridge are neither functionally nor visually divisive.
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Figure 7 Harwood Village Zoning (from Maclean LEP)
Figure 8 Harwood Village, north side of Clarence River.
1(a) Rural Agricultural
Protection
1(t) Rural Tourist
1(w) Rural (Waterways)
2(a) Residential (Low
Density)
3(a) Business Zone
4(a) Industrial zone
5(a) Special Uses
Harwood Mill complex
River Street
Morpeth Street
Mill Road
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Figure 9 View of Harwood Bridge from River Street, Harwood
Figure 10 View of Harwood Bridge from River Street, Harwood
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6.3 Visual context
6.3.1 Low level bridge
The low level bridge would use a bascule lifting mechanism for operating the opening span. Figure
11 shows the operation of a two-leaf bascule bridge, although a single leaf bridge may ultimately be
preferred to reduce costs. The assessment of whether a one-leaf bascule or a two-leaf bascule bridge
would be preferred for an opening bridge would be determined as part of the detailed design process,
should the low level bridge emerge as the preferred design solution.
The use of a dark grey finish on the concrete would reduce the visual impact of the bridge. Noise
walls near the end of the bridge and on its approaches may be required and have been assumed in
this preliminary visual assessment (refer to Section 6.4).
Figure 11 Low Level Bascule Bridge in Open Position
A low level bridge has a low profile that is aligned with the existing bridge and, as it would be at the
same level as the existing bridge, it would require minor embankments for its approaches. This
would minimise visual impacts on adjoining properties and the potential overshadowing that would
be created by a larger / taller structure.
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6.3.2 High level BridgeA photomontage for the high level bridge is provided in Figure 12. To achieve a 30 metre clearance
height in the centre, the bridge would have long approach spans which would impact on the views
from the surrounding properties. The use of a double cantilever structure for this bridge may be
more elegant, although this could entail more significant visual impacts, which would need to be
assessed prior to proceeding with this option.
Noise walls near the end of the bridge and on its approaches may be required and are assumed in this
preliminary visual assessment (refer to Section 6.4).
Figure 12 High Level Bridge with 44 metre Pier Spacing
.
6.4 Impacts of alternative bridge options
6.4.1 Visual
A low level bridge would have limited visual impact on the village, much of which could be
ameliorated through appropriate screening, e.g. trees, located particularly along the northern banks of
the Clarence River.
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The high level bridge would have a very strong visual presence on the river and there would bedifficulties in providing a form that was sympathetic to the existing bridge.
Embankments and approach structures to a high level bridge would be visible above cane fields and
out of scale with the surrounding streets and buildings.
While alternative design configurations could reduce the number of piers and spans needed to cross
the river, these changes would not alter the relationship of a high level bridge to the existing bridge,
the floodplain and the surrounding village area.
Within the village a high level bridge would be far more visually prominent than a low level bridge.The quality of views of the historic bridge from the village would be compromised by a high level
bridge.
6.4.2 Shadowing
A new low level bridge would increase the amount of shadow immediately adjacent to the existing
bridge. At approximately 25 metres above River Street, the shadow effects of a high level bridge, as
can be seen in Appendix D, are likely to be more widespread.
The new high bridge, located on the east side of the existing bridge, would only affect a few houses
and the church, on late winter afternoons.
6.4.3 Constraints on future growth
For both options an interchange is proposed for Watts Lane, thus the footprint required for each
option, north of Harwood, would be similar.
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6.4.4 Summary
The following table sets out and compares the potential impacts noted above.
Table 1 Comparison of Potential Impacts
Issue Option 1Low level bridge
Option 2High level bridge
Visual Impacton historicbridge & itssetting
Avoids visual conflict with existingtowers (see Figure 10).
Form would be sympathetic to thehistoric bridge.
Severe visual conflict with the formof the existing bridge (see Figure11).
Quality of views of historic bridgewould be compromised.
Visual impactin village & onviews from it
Limited visual impact on village and canpredominantly be ameliorated throughscreening.
Less visually prominent than high levelbridge.
Very strong visual presence on theriver.
Would be visible above cane fields.
Out of scale with surroundingstreets and buildings.
Shadow effects Would increase shadowing immediatelyadjacent to existing bridge.
At about 25m above River Street,shadow effects would be morewidespread (see Appendix D).
Would only affect a few houses, andon late winter afternoons, wouldaffect the church.
Constraint onfuture growth
Limited impacts. Limited impacts.
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7. Traffic and Accessibility
7.1 Existing traffic and accessibility
In 2004, 10,000 vehicles per day, including 2,000 heavy vehicles (20%), crossed the Clarence River
at the Harwood Bridge. By 2021, the average daily volume will be almost 14,000 vehicles. The
peak hourly flow on an average day is approximately 7% of the daily volume, although the 30th
Highest Hourly Volume, which is more representative of busy holiday times, is closer to 10% of the
annual average daily traffic volume.
Based on information available for 2001 to 2007, the bridge is currently opened on average justunder thirteen times per month. Table 2 shows that whilst usage declines through the colder months
of the year, there is not an obvious pattern to the openings (also refer to Appendix E).
Table 2 Harwood Bridge Opening 2001 to 2007
Month 2001 2002 2003 2004 2005 2006 2007 2008
January Not avail 22 15 14 24 16 20 13
February Not avail 13 10 11 7 10 13 8
March Not avail 14 16 6 14 17 11 11
April Not avail 13 17 15 22 18 20 11
May 10 13 20 5 21 20 20 16June 8 6 17 10 14 12 16
July 13 11 13 6 15 8 12
August 17 10 15 7 10 15 8
September 18 7 8 7 14 11 16
October 18 9 6 10 9 15 13
November 12 12 14 14 8 13 9
December 15 8 10 8 7 12 13
Total 111 138 161 113 175 167 171 59
7.2 Low level bridgeA low level bridge would be at the same deck height as the existing bridge, and would therefore need
to have the capability to open to allow yachts and other vessels access to and from the river west of
the bridge. It is not expected that the frequency or duration of bridge openings would change due to
the presence of a second bridge.
The specifications for the Port River Bridge in Adelaide includes the following maximum times with
regards to the times to operate traffic control and safety equipment, open the road bridge and close
the road bridge:
Lower traffic gates: 7 seconds.
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Lower traffic barrier gate: 7 seconds.
Disengage span locks: 6 seconds.
Open movable bridge: 70 seconds, including time for ramping and seating.
Close movable bridge: 70 seconds, including time for ramping and seating.
Engage span locks: 6 seconds.
Raise traffic barrier gate: 7 seconds.
Raise traffic gates: 7 seconds.
This totals 90 seconds for each of the opening and closing cycles. It would be expected that similar
performance could be obtained from a new opening bridge at Harwood, although ultimately the
impact on traffic movement would be dictated by the volume of maritime traffic, and hence the
length of time for which the bridge was required to remain open. Although upgrading of the opening
mechanism of the existing bridge may marginally reduce opening times, this is unlikely to have a
significant influence on the overall efficiency of the opening/ closing cycle.
As outlined in Chapter 8, an opening time of 20 mins has been assumed in the economic analysis.
This is a conservation estimate, since an opening time of 10 minutes was recorded during a site
inspection undertaken on 30 October 2007.
7.3 High level bridge
In traffic terms, a high level bridge would not need to be opened to allow boat traffic to pass beneath
it, which provides advantages in terms of traffic efficiency. The bridge would, however, require
relatively steep grades for its approaches. A grade of 4.5% over a distance of 500 metres would be
required to reach the top of the bridge. Although the Austroads Guide to Traffic Engineering
Practice (Part 2 Roadway Capacity) indicates that a section of steep grade less than 800 metres in
length does not require specific analysis, the actual effect may be a reduction in capacity of as much
as 30%, taking into account the passenger car equivalencies suggested by Austroads for heavy
vehicles on longer sections of steep grade. While forecast volumes for the bridge are well within
even the reduced capacity, the high level bridge would have a lower capacity in comparison to the
low level option. Travel speeds, particularly for trucks, would be marginally lower for the high level
bridge option. However, the impact on overall travel times would be relatively small.
7.4 Local traffic
Local access arrangements would be expected to be comparable for the high and low level bridge
options. The current proposal is to provide an interchange at Watts Lane to provide access under
both the Class A and Class M scenarios. Whilst the ramp configurations may differ depending on
the bridge option, both bridge types would provide the same fundamental connections and levels of
accessibility for local traffic.