Port Bouvard Bridge
The Port Bouvard Bridge was opened to traffic in September, 1993 after a thirteen month construction period. It spans the Dawesville Channel, a new waterway construction near Mandurah, 100 km south of Perth. The 200 m wide, 1 km long channel was a very large environmental project to connect the Peel/Harvey Inlet, a shallow inland body of water, to the ocean. It is designed so that flushing through the channel increases salinity and removes nutrients from the Inlet, which have entered the system from fertilisers and other causes on adjacent rural land through the Harvey River system.
Thiess Contractors Pty Ltd under a contract with the Department of Marine and Harbours Western Australia carried out the works, including the Bridge.
Where the channel cuts the Old Coast Road heading south towards Bunbury the bridge was constructed, and for this part of the project the Commission was jointly issued by the Department of Marine and Harbours and Main Roads Western Australia. Thiess chose Bruechle Gilchrist and Evans Pty Ltd as their bridge consultant. The bridge contract with Thiess was negotiated on a “design and construct” basis.
The key dimensions of the bridge are an overall length of 360 m, six main spans of 47 m, end spans of 38.5 m and a navigation clearance above water of 19 m. A sketch showing a cross section and elevation of the bridge is shown in figure 1.
For a project of this size, the bridge is unusual in that it consists of two "I" beams rather than a conventional box structure. The bottom flanges of the "I" beams incorporate dual use paths and maintenance walkways. Views looking towards the southeast and northeast are shown in figures 2 and 3 respectively; Figure 3 clearly shows pedestrians walking along the lower flange of the nearer beam.
There are no diaphragms. Instead, concrete straps connect bottom flanges over the piers and at mid span. The format was chosen after a careful study of aesthetics, to suit the Contractor’s construction method and provide ease of access for services, which are many, and future maintenance. The absence of diaphragms allows an unbroken run for services as well as access along the maintenance walkways. It also made the construction simpler as the straps were easy to add after the main structure had been cast.
The Contractor decided on incremental launching as the preferred construction method. The perceived advantage with this method was in cost effective construction because the system essentially establishes a site precast operation without the disadvantages of transport and erection. As well, it enabled the bridge to be built to a high standard – an important consideration in this location with a small labour force in a relatively short period of time. Substructure construction occurred concurrently.
The particular section chosen proved very simple to cast and a weekly cycle was maintained throughout the building of the superstructure. Major services were installed as the bridge was launched, as were handrails and guard rails. However, final alignment of these was not carried out until the bridge was completed.
With lower level dual use paths for cyclists and pedestrians, vertical webs are an advantage, as they do not encroach on usable space, as do the sloping walls of a box. Another advantage of the open web construction is that gas pipes could be located on the inside flanges, whereas with a box they need to be external because of the risk of explosion.
Each internal pier consists of a pair of elongated truncated cone columns flaring to a short inverted truncated cone capital at the top. As the piers are such a dominant visual element of the project extensive use was made of computer imaging and small models. The use of computer imaging and models, together with architectural input enabled subtle changes to be made to the pier profiles to enhance the visual effect. The top of the piers, with the inverted cone, allowed generous size for the setting of temporary bearings, the location of jacks for permanent bearing installation and also provided for very good maintenance access if it is ever required in the future.
The size of the pile caps was dictated by the loads and overturning moments of the structure, whereas the shape to a large extent was dictated by ship impact provisions for a 300 tonne vessel travelling at 8 knots. As the pile caps have a pointed sharp end, direct impact from a vessel travelling in the normal direction is unlikely.
The pile caps on land are each supported by twenty Franki piles, 550 mm in diameter and 13 m long. Those in the channel are each supported on four 1.5 m diameter bored piles founded in the siltstone, approximately 20 m below sea level. The abutments are on spread footings.
Both piers and pile caps were constructed from concrete consisting of 65% blast furnace slag, 35% OPC with a 7% silica fume replacement of cementitious material. The mix was chosen for its high chloride resistance and its low thermal gain during the hydration process.
The Port Bouvard bridge was the first major bridge structure built in Western Australia for a government agency on a “design and construct” basis.
Structural efficiency, ease of construction and visual appearance were major considerations in the solution that evolved with its unique twin "I" beams and open web construction.
The bridge was built to a high standard within the programme construction time and within budget.
Author:
This article was written by Mr Ernie Evans, principal of Bruechle, Gilchrist and Evans.
