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Council

Twentyfirst Street Bridge Replacement

Overview

Following a competitive tender process, Council has appointed experienced civil contractors, BMD Constructions as the preferred contractor to construct the new Twentyfirst Street Bridge after it was damaged during the River Murray Flood in 2022.

Council has considered a number of design options for the replacement of the Twentyfirst Street Bridge and has selected the design option that will address impediment to flow, provide road and pedestrian user improvements, provide a high level of flood immunity and water flow constraint reduction and is costed within a range that is affordable with funding assistance from the State and Federal Governments.

While Council understands the frustrations of the local community surrounding the closure time, protecting the safety of our community has always been Council’s highest priority.

It has been extremely important to ensure the works on the bridge were not a ‘quick fix’ and that the bridge would serve as a long-term solution which would be safe and resilient well into the future.

On-site works at the bridge are expected to commence in July 2024 by BMD Constructions who will employ five (5) local sub-contractors to assist with the project. The project is anticipated to take approximately seven (7) months to complete with an estimated completion date expected for early to mid-2025.

Access

The bridge is currently open to pedestrians and cyclists.

During the construction period, there will be no access during the site to ensure the safety of the public and contractors.

How is it being funded?

Council has been successful in securing funding for the detailed design of the bridge replacement and is investigating other avenues of funding to support the construction of the bridge.

The project will also receive disaster recovery funding.

Further details of the costs involved will be provided to the community as the project progresses.

The technical details

In March 2022 (pre Flood), a regular engineering assessment of the bridge was undertaken by TMK Engineering and whilst it was not in excellent condition at that time and required some remediation works, it was determined safe to use.  A copy of this report can be viewed here: Twenty First Street Bridge – Pre Flood Assessment

After the 2022 River Murray Flood, all major Council assets that were flood affected had to be assessed by an engineer prior to re-opening. TMK Engineering returned in March 2023 (post-flood) and undertook another engineering assessment, which then determined that due to damage sustained through the flood, it was no longer safe to use. This report also identified that the damage incurred was significant and the costs to remediate the bridge structure are likely comparable to replacing the bridge structure entirely.

Upon receiving this report which confirmed that the bridge was not safe to be reopened, Council couldn’t risk the public’s safety and made the decision to close the bridge to all users. A copy of the post flood report can be viewed here: Twenty First Street Bridge – Post Flood Assessment

After the determination to close the bridge, Council engaged the services of Tonkin Consulting to undertake a design and options report for a replacement bridge. This scope of work also included a secondary condition assessment on the bridge structure and road approaches.  A copy of this secondary condition report can be viewed here: Twenty First Street Bridge – Secondary Post Flood Assessment

In August 2023, Tonkin Consulting completed a Level 3 Bridge Inspection to investigate the potential short term temporary reopening of the bridge and to what type of users. This inspection identified that the Twentyfirst Street Bridge could be reopened to pedestrians and cyclists only if several access controls were implemented. Following the implementation of the access controls the bridge was reopened to pedestrians and cyclists with access to remain in place until construction commences.

In addition to the Level 3 Bridge Inspection, Tonkin Consulting also completed a Condition Assessment and Options Investigation Report which provided Council with six (6) design options for the permanent replacement of the bridge. The option selected is the most cost effective and will ensure the construction of a resilient asset that will be able to withstand future flooding events.

Further, two rounds of geotechnical drilling – which involves drilling down 15metres against the bridge abutments – were completed in late 2023. The results of the geotechnically drilling have ensured that the design is catered to the specific soil and site conditions.

When is construction due to commence?

The project is scheduled to begin in July 2024 and will be supported by five local sub-contractors.

How long will it take?

The project is expected to be finished in early to mid 2025.

How is the project being funded?

The cost of the project will be shared between Council and the Federal and State Governments. Council took the time to seek external funding to lessen the burden on ratepayers.

Will the bridge be flood resilient?

Yes, in the case of a high river, the bridge and road approaches will be accessible up to 120GL/day. Keeping the bridge open during higher flows will be assessed at that time.

After flows recede it would be promptly reopened with minimal recovery works required.

Was a temporary bridge considered?

A temporary bridge was considered, however would have impacted the construction of the permanent bridge. There was also significant cost involved with constructing a temporary bridge and the use time (12 months) did not justify the investment.

Will there be a dedicated pedestrian path?

Yes, the bridge will have a separate path for pedestrian crossing. The entire project site will incorporate safer and improved access for pedestrians and cyclists.