Concerns over Hobart's Bridgewater Bridge structural integrity

There are fresh concerns over the structural integrity of Hobart's new Bridgewater Bridge, which one expert who worked on the project suggested could eventually lead to a need to reduce traffic flow or speed limits.

The concerns stem from the design of the concrete piles that form the foundation of the $786 million bridge and the "level of uncertainty" over the make-up of the riverbed.

The bridge builder and an advisory group of engineers who worked on the project were at odds over the extent of the issues, with one of the engineers describing the problems as "intolerable".

Documents seen by the ABC highlight concerns from the Technical Advisory Team, which was employed by the Department of State Growth to check that the bridge was built to contract by designer and builder McConnell Dowell (MCD).

MCD is also part of Constructure Joint Venture, one of the parties competing for the tender to build the Hobart stadium at Macquarie Point.

The documents show MCD and the advisory team disagreed over the pile design's suitability for the varied ground conditions at the River Derwent crossing.

There were concerns the piles could not provide enough support for the bridge piers to meet the contract's standards, which could lead to damage to the bridge beyond the defects period.

The department ticked off on the bridge design in 2022, despite the issues, on the condition that MCD later addressed the concerns to the state government's satisfaction.

But one of the engineers from the advisory team has chosen to speak to the ABC, saying news broken by the ABC in February of a large crack in a pier caused by incorrect modelling may indicate that the more widespread issues raised by his team were not resolved.

Chosen design was an 'intolerable' risk

Geotechnical specialist Derek Pennington said his advisory team was let go by the Tasmanian government in 2024 due to the standstill between the team and MCD over the piles — in his belief, to avoid project delays.

"I would have to say it's the most complex ground that I've dealt with in my 35-year career."

The state government and Department of State Growth did not comment on why the advisory team was stood down.

Dr Pennington says he does not have fears for any sudden, significant damage to the bridge but it is possible there may be a need for more management of loads on the bridge, such as through reduced speed limits and traffic.

The department said survey markers were attached to the bridge piers during construction to help with monitoring any movement.

"The bridge is safe and it has not experienced any unanticipated movement," the department spokesperson said.

Builder didn't understand site's ground conditions, engineer says

The new four-lane Bridgewater Bridge opened in June 2025, three years after construction began.

As Tasmania's largest infrastructure project, it replaces a previous lift-span bridge, which was coming to the end of its life, showing signs of wear, including sinking support piles after nearly 80 years of operation.

"It's entirely going to be down to the designer's choice of how to interpret putting a structure into the ground conditions that exist at that particular crossing," Dr Pennington said.

"But in the very first meetings we had with the team as technical advisor, we thought: 'how are we going to deal with having single piles with the level of uncertainty that there appears to be in the understanding of ground conditions'."

Piles are concrete posts injected down into the earth, providing support for bridge piers and making up the foundations of the overall bridge.

Dr Pennington said bridges could also use multiple piles at different angles, which was "more conventional for these deep estuarine conditions".

"If you've got … a pile cap with multiple piles and rake piles, you can add extra driven piles into the foundation system, you can incline them to get more lateral stability," Dr Pennington said.

But MCD's design for the new bridge used single vertical piles drilled between 30 and 88 metres below the river's surface.

The technical advisory team were concerned that the design of the piles and supporting calculations could not adequately support each of the bridge's 42 piers.

One reason for this was because they believed MCD had misunderstood the conditions at the bridge site, with some of the ground being more brittle and therefore inappropriate for the single-pile design it was pursuing.

"So that was another issue with the single mono-pile, is that it had to be vertical and the ground conditions were such that in certain parts of the crossing, there was 40, 50 metres of ground you would call very soft and not able to provide much lateral support," Dr Pennington said.

In the documents, in response to the opposition, MCD maintained its modelling was accurate and sufficient.

These specific concerns were put to the Department of State Growth, along with the question of whether they were resolved after the advisory team departed, to which it did not respond directly.

Builder MCD declined to comment and referred questions to the department.

"The structural designs were reviewed and checked by a proof engineer and the overall process was reviewed and verified by an independent verifier," a State Growth spokesperson said.

"These activities were undertaken with the oversight of the Department of State Growth's project team with support from the department's consultant technical advisors," they said.

"Additional verification was obtained on a range of technical matters from the contractor and its design engineers, the proof engineer and the independent verifier."