St Lukes Bridge

The North Western Motorway links Auckland’s CBD to the western and north western suburbs of Auckland. As part of a major motorway upgrade in the area, the St Lukes Rd Bridge, which spans the motorway, had to be removed to allow the motorway to be widened from 3 lanes to 4 without disrupting daytime motorway traffic.

The process was:

  1. Construct new bridge to take traffic across the motorway.
  2. Demolish existing bridge
  3. Construct replacement bridge

The bridge was 44 metres long by 16 metres wide and consisted of an abutment at each end, piles and head stock in the middle, with 12 beams, each 1100mm high by 900mm wide at the top and 500mm at the bottom of the beam and a topping slab of 150mm approx. linking each abutment to the centre pier/headstock; 24 beams in total. A complicating factor was that asbestos sheeting had been used as permanent formwork for the topping slab. This spanned the 400mm between each beam as well as the hollow section of each beam.

Because of the constraints on motorway closure, between 10.30pm and 4.00am, Leighton Contractors, the principal contractor, determined that the best methodology was cutting the bridge into manageable sections, lifting each section off and transporting the concrete to a dump site.

A1 Kiwi Cutters and Drillers Ltd involvement was as follows:
Cut the bridge deck to a depth of about 130mm at the centre point between each beam. This could be done during the day as no slurry was spilt onto the carriageway.

Cut the remaining bridge deck, including the asbestos sheeting permanent form work. This was done between 11.00pm and 4.00am during a motorway shutdown. A trough, 12 metres long, 375mm wide and 500mm deep was erected on top of an elevated working platform. This was positioned directly under the cut. This trough captured all the asbestos contaminated slurry. Air samples were taken adjacent to the cutting site to ensure that all asbestos was captured. As the work was being done at night, a 3-phase electric floor saw with noise reduced blade was used. From a distance of 20 metres, no cutting noise could be heard above the ambient noise levels.

Drill 200mm diameter lifting holes through the bridge deck. There were 4 holes per bridge beam, 96 holes in total.

Drill holes 40mm diameter to a depth of 130mm. This was so that a temporary restraining structure could be installed prior to the diaphragm being cut.

Cut diaphragm. The diaphragm was 500mm wide by 1.1 metres high. This had to be cut 44 times. This was cut using a wire saw.

Attend site as the beams were removed. This was done over 8 nights. As the motorway could only be closed for 5 hours, and the crane took 1 hour approximately to set up and 1 hour to take down, leaving only 3 hours working time, it was vital that the beams could be lifted out seamlessly.

On completion of the bridge deck removal, one abutment was removed using wall saws and as the seat was 600mm thick a 1600mm diameter blade was used.

Remove the centre headstock and columns. A wire saw was used on this.

Some statistics:

  • 500 metres of 200mm deep floor sawing
  • 50m2 of wire sawing
  • 96 x 200mm diameter holes 200mm deep
  • 96 x 40 mm diameter holes 130mm deep
  • 30 metres of 600mm deep wall sawing
  • 1200 tonnes of concrete removed

Phil Houghton

Houghton Design, Nelson, Nelson,