 
This Project involved the design & construction
of a 3-span 60 m double lane bridge, and the realignment of
the existing road to suit the new bridge position. The location
of the bridge over a tidal river led to the innovative use of
large spans and pre-cast units to overcome the tidal affect,
and minimise environmental impact.
The position of the new bridge necessitated
the installation of steel piles, to found one abutment and the
2 piers. Geological conditions dictated that the piles be driven
to a depth of 16.0 m below natural surface level. Stubbs Constructions
Pty. Ltd completed this portion of the project in-house, with
tidal conditions only allowing work at low tide.
Due to the position of the northern pile cap within the river, a project specific
cofferdam was constructed. Due to the large tidal movement in the river, a
dam wall height of 3.4 m was necessary. This item had a two-fold use; firstly
to retain the water and provide a safe working environment; and secondly to
act as the formwork for the pile cap, which eliminated the need to construct
formwork within the dam. With such a large head of water, the problem of water
leaking into the dam was overcome by the installation of a sacrificial steel
plate into the bottom of the dam.
In-situ abutments, with integral wing walls,
and piers were installed, however the innovative use of pre-cast
cross heads shortened the construction period.
It was decided early in the design process
to use pre-cast concrete “I” beams for this project.
This concept utilised large spans due to the low weight of the
beams and therefore minimised the number of piers required.
Transport of the 20 m long beams to site was completed with
2 beams in convoy, in specialty built cradles on extendable
semi-trailers. For the installation of the 18 tonne beams, the
largest crane in the state was utilised. This 150 tonne crane
installed each beam in a single lift. Within each 20 m span
4 “I” beams were incorporated. In total the 12 beams
required for this project were manufactured in 12 weeks.
In consultation with Boral Construction
Materials a project specific concrete design mix was developed.
The S60 Mpa mix allowed the removal of the beams from the mould,
before post-tensioning, 36 hours after commencement of the pour.
The innovative use of pre-cast concrete
deck panels, developed by Stubbs for this project, eliminated
the difficult process of traditional formwork. Each of the deck
units was pre-cast at our Burnie pre-cast facility, then transported
to site.
Once on site, the units, designed as sacrificial
formwork only, were installed over the top of the “I” beam
to form the soffit, with the entire structural deck load incorporated
in the in-situ topping.
Poor access around the site led to the
decision to launch the beams for the second span from the newly
constructed first span. For this process to work the new bridge
was designed to support the 150 tonne crane, which was positioned
on grillage constructed from 300 UB steel beams, which distributed
the weight over all 4 beams in the span.
The concrete beams, once delivered to site,
were installed directly from the trucks. Installation of the
4 beams in each span was completed within 4 hours.
The roadworks portion of this contract involved the construction of 1,000 m3
of embankment, supply and installation of 2,500 m2 of sub-base and base material,
and 251 Lm of new DN200 mm water main.
The environmentally sensitive area within
which construction took place necessitated the development of
a project specific Environmental Management Plan. The EMP called
for the use of several techniques to minimise impact. These
included oil booms to trap any spill from machinery, sediment
fences to minimise ingress of solids into the river system,
run of management, and solid waste disposal and recycling.
|
