The old Beaumonts Bridge in Manurewa has long been an accident black spot and had come to the end of its serviceable life.

Manukau City Council commissioned Opus to design a realignment that included extra clearances for future rail upgrades and electrification.

Project Planning and Development

Construction activities were planned so the existing bridge remained operational until the new bridge was constructed and ready to be used. This allowed load bearing piles to be bored, poured, and the abutment capping beams to be cast before the deck double hollow core beams were laid in place. Traffic was then diverted from the old bridge directly onto the temporarily unsurfaced deck.

The demolition of the old bridge allowed the completion of the bored piles for the wing walls. The vertical alignment had to address the fact that the new bridge was 1.4 metres higher than the old one. The additional height (to allow for the future electrification of the rail network) meant that the ground level at the approaches had to be significantly raised.

The new bridge and realignment of Great South Road is an example of excellent design, construction and collective proactive team input. With an innovative methodology and a design based on 48, 20 metre long deck beams – manufactured off -site and placed over a live railway – the project demanded critical attention to detail and precision. The construction site was environmentally sensitive due to the presence of contaminated material and the proximity of open surface water channels discharging into the adjacent waterways. The Contractor proposed an innovative construction methodology for constructing the road approaches using ‘rotten rock’ fill over a clay liner to contain contaminants and reduced pile lengths in response to geotechnical conditions. Both methods delivered savings in excess of 10 percent.

The old Beaumonts Bridge, also known as the Mahia Rail Overbridge, on Great South Road in Manurewa, was built in the 1920s and did not meet modern safety requirements. Tight corners on the approaches meant that cars had to slow down to 35-40 km/h and as a result the bridge was an accident black spot. While side protection and signs helped to reduce the accident rate, further improvements were required to provide the necessary degree of safety on this busy regional arterial route which is also a priority transportation emergency route.

In addition the width of the old bridge was inadequate for the volume of traffic. Great South Road has average daily traffic volumes in excess of 20,000 vehicles per day that travel over Beaumonts Bridge, with five percent of traffic being heavy vehicles. Great South Road is also the Auckland Oversized Load Route – the identified route for vehicles that are too heavy or too large to travel on the Southern Motorway. Due to the structural constraints of the old bridge, these vehicles had to cross the railway line by means of an adjacent level crossing that catered solely for that purpose.

 

Critical success factors

  • Demolition of the existing bridge in 24 hours with minimal impact on railway operations and minimum disturbance to the adjacent residential properties (existing road bridge, redundant old road alignment, southern sector of existing overweight/ over dimension bypass).
  • Realignment of approximately 300 metres of Great South Road (including footpaths, cycle lanes, removal of bus stop, tie-ins to side roads).
  • Construction of a new bridge over the North Island Main-Trunk Railway Lines (NIMT) with minimal disruption to rail operations, full compliance with Kiwi Rail safety requirements and no safety incidents.
  • Construction of the bridge works to very stringent tolerances for rail clearances and road levels to minimise the impact on adjacent intersections and property accesses.
  • Protection/relocation of major utility services along the Great South Road corridor and rail corridor with minimum disruption and damage (includes Manukau Water watermain, Telecom fibre -optic cables, Police fibre-optic, Vector electricity cables, installation of new utility services, stormwater drainage).
  • Provision of residents’ access throughout the works (extension/alteration of driveways, provision of service access to No.s 269 – 273 Great South Road).
  • Construction in an environmentally sensitive area due to the presence of contaminated material on site and the proximity of open surface water channels discharging into the adjacent waterways. As a result the ARC Resource Consents contained very restrictive conditions and construction methodologies to ensure that the management of the contaminated material did not cause any environmental harm.