At Junee Shire Council’s meeting last Tuesday, they voted to endorse a proposed Dirnaseer Road Underbridge design associated with the Inland Rail Illabo to Stockinbingal (I2S) Project.
Council noted that the proposed bridge design has appropriately considered structural, hydraulic, operational, maintenance and road safety requirements relevant to Council infrastructure and the local road network.
Council received a report that aimed to provide engineering advice to Council regarding the proposed underbridge and to seek Council endorsement.
Inland Rail has undertaken a Safety in Design (SID) review process and prepared concept and general arrangement designs for the proposed structure. The material provided outlines the bridge configuration, hydraulic considerations, construction considerations and operational safety requirements associated with the proposed crossing.
The proposed structure consists of:
• A piled substructure including bored pile foundations.
• A two-span supported superstructure utilising 1515 Super-T girders.
• Reinforced soil wall (RSW) abutments.
• Maintenance walkways and bridge handrails.
• Associated reinforced concrete box culvert (RCBC) livestock underpass.
The bridge has been designed to maintain the existing Dirnaseer Road alignment beneath the new rail infrastructure while also accommodating flood and hydraulic requirements.
The proposed arrangement demonstrates consideration of:
• Structural adequacy.
• Flood resistance and hydraulic performance.
• Scour protection.
• Rural access requirements (livestock underpass).
• Operational and maintenance access.
• Road safety considerations.
• Construction risk mitigation.
While several detailed design and construction matters will continue to be refined through future project stages, the current bridge arrangement is considered acceptable from a Council engineering perspective and suitable for endorsement.
The bridge is intended to carry the proposed rail corridor over the existing Dirnaseer Road alignment while maintaining local road connectivity and access for residents, landholders, agricultural users and service vehicles. Council has received the Safety in Design workshop materials and the general arrangement drawings for the proposed structure.
The design documentation identifies the following primary bridge components:
Two-span bridge arrangement with two spans of approximately 23 metres, totalling 46m, consisting of:
• Prestressed concrete 1515 Super-T girders.
• Reinforced soil wall abutments.
• Bored pile foundations.
• Maintenance access provisions.
• Handrails and maintenance walkways.
• RCBC livestock underpass adjacent to the structure.
A Safety in Design workshop has been conducted to identify reasonably foreseeable hazards associated with the construction, operation, and maintenance of the structure, and to implement mitigation measures to reduce risk so far as is reasonably practicable (SFAIRP).
From an engineering perspective, the design appears to have undergone an appropriate level of risk identification and engineering assessment for the current stage of project development.
Council said the use of piled foundations is a suitable engineering solution for the crossing. It provides an appropriate level of structural stability under both operational and flood loading conditions. The two-span arrangement effectively minimises the number of in-channel structural elements and reduces potential hydraulic obstruction beneath the structure.
Council said the inclusion of maintenance walkways and access provisions is also considered appropriate from an operational and maintenance perspective.
The proposed bridge infrastructure forms part of the Inland Rail project and is not proposed to be funded by Council. The ongoing maintenance of the bridge asset will be the responsibility of the Rail network.
Potential financial implications for Council primarily relate to:
• Protection of council road assets during construction.
• Road reinstatement requirements.
• Future inspection and interface management responsibilities.
• Minor ongoing maintenance interactions with the structure.
The bridge has reportedly been designed in accordance with relevant flood-loading requirements, with scour-protection measures proposed where required.
Importantly, the structure has been configured to maintain flood conveyance beneath the crossing while avoiding unacceptable impacts to the existing road corridor.
The proposed scour protection measures and drainage considerations are well considered and are necessary components of the final design.
The bridge design maintains the existing Dirnaseer Road corridor beneath the structure and avoids significant road realignment requirements.
This is considered a positive design outcome as it minimises impacts to:
• Existing traffic movements.
• Rural property access.
• Agricultural operations.
• Road reserve impacts.
• Construction staging complexity.
A minimum vertical clearance of 5.76m is not expected to unreasonably limit vehicle access for standard rural traffic or legal heavy vehicles.
Access may still need to be managed for oversized loads or unusually high agricultural machinery. This would, however, be considered consistent with normal rural road network requirements.
The structure also incorporates bridge furniture and roadside safety elements intended to improve operational safety outcomes beneath the crossing.
Council moved ahead with the project as per their last meeting.
Jack Murray
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