3. DESIGN AND THE CONSTRUCTION
3.1 General description
The design is based on a cable-stayed structure with
three pylons. The 80m high Central pylon is shorter
than the two outer pylons, which are 110m (North
pylon) and 125m (South pylon) high. sandstone. Piled foundation options were considered
but direct (spread footing) foundations using an in-
situ reinforce concrete foundation were selected as
the preferred solution.
This design for the bridge was selected from a wide
range of options and it brings maximum benefits for
users and local people and has a minimal impact on
the estuary and its surrounding environment. Each footing is 4.5m high and up to 22m in diameter.
For their construction double skinned cofferdams
were built using around 300 steel piles driven into the
riverbed.
Including the approach viaducts on each side the
bridge is 2,250m long with a river span of 1,000m. Inside each cofferdam a mass concrete base slab was
cast onto which a cage consisting of 190 tonnes of
steel reinforcing bars was assembled.
3.2 Temporary Access Bridge
Work started on the bridge in mid-2014 with two
temporary access roads being built across the salt-
marsh.
A 1km temporary trestle bridge with a precast
concrete deck around 9m wide was built across the
estuary to provide construction access. Up to 140
steel piles were driven to a depth of around 16m to
support the temporary access bridge.
3.3 Cable-Stayed Bridge Construction
3.3.1 Pylon Foundations
Vertical steel reinforcement bars were then fixed into
the centre of the cage to form the base of the pylon
shaft. 1,400m 3 concrete was poured to form the
pylon foundations.
The weathered rock provides a high bearing capacity
although careful assessment was required to validate
the strength and stiffness at each foundation location.
Therefore, during construction of the spread footings
and subsequent bridge pylon and superstructure
construction, the settlement and rotation of the
foundations and ground movement was monitored.
The ground conditions comprise a relatively shallow
thickness of alluvium overlying variably weathered
Figure 7: The North cofferdam with its newly completed pylon base
Figure 6: The concrete pour on the North cofferdam
3/2017