The Illawarra’s new $49 million Sea Cliff Bridge arcs over the ocean as part of the Grand Pacific Drive tourist route between Sydney and Wollongong. From an engineering perspective, the project was challenging. The 900m, cliff-hugging section of road between Clifton and Coalcliff had a daunting history of rock falls and embankment failure. In 1983, an expert study condemned it as an “intolerable risk” to public safety.
Instead of calling tenders, the NSW Government invited the state’s largest road construction companies to form teams and submit proposals to the NSW Roads and Traffic Authority (RTA). This process, unprecedented for a NSW road project, produced an alliance between the RTA, Barclay Mowlem , Coffey Geosciences and Maunsell Australia . Barclay Mowlem’s engineering manager Peter Stewart, reckons that the location was arguably one of the most challenging sites to put any road through. “We identified early on that, irrespective of the geotechnical expertise applied, the long term stability of the slopes could never be guaranteed”, he says. The preferred solution combined an innovative curving, off-shore bridge with geotechnical stabilisation treatments. The bridge is a first for Australia – it combines two different bridge construction methods, a balanced cantilever bridge and an incrementally launched bridge. Ross Abraham, alliance systems manager, says the bridge had become essential as rockfalls cascaded onto the old road. “It’s a really unstable piece of real estate,” comments Abraham. Between 1996 and 2003, more than 50 rock falls occurred, a boulder measuring 15m3 crashed onto the road and an embankment collapsed, causing a crack about one metre wide.
Sea Cliff Bridge – 665m long and 41m above sea level at its highest point – has vastly reduced the risk, creating a classic driving route in the process. The bridge has a hollow core, giving engineers easy inspection access. Abraham says the interior will be inspected regularly. Construction took two and a half years to complete and used over 12,000m3 of concrete, during which time the road was closed. “When the job was in full swing we had about 90 people on site, with four to five workplaces going simultaneously,” says Abraham. Four tower cranes were used for the cantilever and some 10m of coastline was reclaimed to provide access to revetment walls, rock armouring and pier construction areas.