Canterbury Underpass
Pedestrian Bridge Design
Source: TfNSW Online; Mount Ousley Interchange Artistic Impression
Detailed design of a pedestrian bridge underneath the existing canterbury road bridge and parallel with the Cooks River in Canterbury. The bridge is a 3-span continuous reinforced concrete “trough” with a maximum span of 24m and a total length of 60m. The underpass structure is unique in that it is designed to remain operational under a king high tide river level despite the walkway surface being below this. The parapet walls are designed to retain this water. The Bridge design was completed in May 2024 with Construction completed in December 2024.
By delivering precise and coordinated BIM models, CSS facilitated the development of a construction-ready design that met all project milestones during the pre-construction phase. As construction progresses, the groundwork laid by CSS's collaborative efforts and innovative approach continues to support efficient project delivery and alignment with stakeholder expectations. The Mount Ousley Interchange is poised to become a significant infrastructure achievement, reflecting CSS's commitment to innovation, technical excellence, and collaborative project delivery.
Technical innovations achieved
Structure regularly retaining water and submerged under minor flood events:
One of the key design requirements was to ensure that the structure remained operational during king tide events. Due to the clearance required for the cyclepath envelope to clear underneath the existing Canterbury Road bridge, the finished surface level had to be below the king tide level. This introduced additional complexity into the design including buoyancy, waterproofing and provision for pump-out under larger flood events. These challenges were overcome through a number of inclusions such as: designing the piled foundations to resist uplift, the deck parapet walls were designed for water retention, the deck structure was designed to resist the actions associated with a flood event buoyancy forces (reversal of the normal gravity forces), additional waterproofing measures at construction joints, design of a pump out pit to de-water the bridge under other flood events where the bridge becomes submerged.
Drainage design improvements:
The reference design suggested new stormwater infrastructure which included some high risk and expensive works such as boring an extra pipe under Canterbury Road and building a number of extra pits near existing structures. CSS developed a solution to utilise the existing stormwater outlets from Canterbury Road and direct this to one of the new pits needed for the underpass, utilising a pit already needed for the design rather than constructing others. In addition to this, the drainage on the underpass was optimised to utilise a single low point (rather than two from the reference design) and diverting this stormwater and flood water to just one pit that required pumping (rather than two from the reference design). The new design reduced the number of pumps required from four to two.
Benefits to the client
Improved constructability through removal of piles in inaccessible areas.
Cost savings through various improvements including: reduction of required pump out pits, drainage optimisation, superstructure material optimisations in concrete and steel quantities etc
Reduction in embodied carbon by 40%
Embodied Carbon - Materials substitution and quantities optimisation to achieve 40% reduction in embodied carbon
CSS undertook a detailed Embodied Carbon assessment using CSS’s in-house Carbon Calculation Tool and reported these findings at each design milestone. In undertaking the assessment, CSS identified early on that there were several opportunities to reduce the embodied carbon impact and overall construction cost. This was done by optimising the structural design to reduce quantities and substitute materials where carbon intensive materials were proposed. Some of the design changes that CSS implemented included:
Removing micropiles from the design from 20 to 0. These micropiles were particularly carbon intensive as they utilised stainless steel casings.
Reduction of the number of bored piles from 20 to 8. Of the remaining 8 piles, the length of the steel casings in the design were significantly reduced.
Removal of stitch pours in the construction sequence of the main bridge structure. This greatly reduced the complexity of construction, it reduced the additional steel rebar quantities required due to lapping and removed the use of stainless steel reinforcement as proposed in the reference design.
The concrete volumes in the main bridge cross section were optimised with an overall reduction in concrete volume in the superstructure despite the much larger spans.
Handrails were changed from stainless steel to hot dipped galvanised structural steel to further reduce cost and embodied carbon on a whole of life timescale despite the reduced design life.
CSS was able to achieve a reduction in embodied carbon of 40% when compared with the original design that CSS received.
