Bridges

The New Bridgewater Bridge (NBB) Project is Tasmania’s largest ever transport infrastructure project. The NBB project involves construction of a four-lane, 1.28 kilometre long bridge over the River Derwent with new interchanges at Granton and Bridgewater, a shared path for cyclists and pedestrians, and a navigation clearance consistent with the Bowen Bridge, all aimed at improving traffic flow and connectivity.

The ground conditions along the bridge alignment encompass a particularly challenging and highly variable geological zone. To produce a robust and reliable foundation for the new bridge, the main contractor, McConnell Dowell (MCD) in conjunction with their geotechnical designer, Tonkin & Taylor (T&T) opted for a design of large diameter bored monopiles up to 2.5 m in diameter and maximum lengths of 89.7 m.

The highly variable geotechnical conditions on the bridge alignment, included low-strength rocks (4-5 MPa) and very high strength basalt flows (up to 200 MPa), resulting in the construction of the monopiles with some of the largest piling rigs and equipment available in Australia and internationally. Two Bauer BG45 piling rigs were used for the bored holes, along with large impact and vibratory hammers for driving the upper steel casings.

This paper commences with a brief description of the pile details and the two solutions adopted for the marine construction access in the deep section of the river channel and the lower-lying southern mud flat zone. The paper proceeds to describe the full piling procurement and construction process, which included the construction of three test piles, using Osterberg cell equipment to validate the design approaches, assumptions and construction methodologies adopted.

Further discussion elaborates on the various challenges faced and the mitigation measures implemented during the piling campaign.