Cobbold Gorge Glass Bridge

Abstract

The glass floored bridge at Cobbold Gorge in north west Queensland is believed to be Australia’s first glass bridge, spanning 13m across the gorge and 17 m above the water. Located approximately 500km by road from Cairns, the remote gorge is located on Cobbold Creek, named after pioneering Gulf pastoralist Francis Cobbold, who held numerous large cattle stations in the region. The gorge is Queensland’s youngest gorge at only 10,000 years old with its narrowness an indicator of its extreme youth. The water is slowly eroding through the ancient landscape with undercuts evident within the gorge. These features highlighted the need for careful selection of the crossing location and geotechnical design to anchor the bridge to the escarpment. A drone survey and 3 dimensional model were used to select the site and confirm the required bridge span. The site is inaccessible to vehicles making materials delivery challenging and prohibiting access for conventional construction equipment (cranes). Given the logistics of constructing across a natural gorge in a remote area inaccessible to vehicles, the design and installation of this project required detailed planning and ongoing consultation between the Project Manager, structural engineer, steel and glass fabricators, helicopter operator and the client to ensure a successful outcome. The project team had to consider innovative construction techniques and careful materials selection to deliver a project restricted to what could be lifted by helicopter. A heavy lift helicopter capable of lifting 1.5 tonnes was mobilised for the lift of the main components. Due to cost and machine availability, a lift plan was developed in advance to minimise time on site to one day. This required the components to be packed in lift size bundles then secured for flight. The glass panels weighing approximately 300 kilograms could not be installed until other elements were completed, long after the helicopter had left site. An assembly plan was created by the project team at the start of the design process to safely construct the works on site. This included temporary A frames supporting an aluminium I-beam assembled on site to create a girder trolley system across the top of the bridge. For the concrete abutments, concrete was batched 1km from the site and delivered by a smaller helicopter using an underslung hopper. The quantities per load were limited to the weight able to be delivered by that machine. The expert pilot was able to achieve 5-minute cycles completing a pour in approximately 1 hour. Positioning the 120kg cross members and 300kg glass panels 17m above the gorge was a key project risk and a trial build of the bridge and gantry system was completed in the workshop to confirm and refine the system prior to delivery to site. This project could only be delivered with the exceptional team commitment and planning highlighted by the pre-assembly in the workshop to test the build process. The build of the glass bridge has allowed the owners to boost their visitors experience and support their eco-tourism business.