Brisbane, Australia’s third-largest city, is home to the country’s third-busiest airport with millions of passengers traveling through each year. To support future growth, a second runway was needed to increase capacity and services. The challenge was that the new runway site overlies varying depths of the compressible clay soils of the Brisbane River Delta, which can be up to 35 m deep (and 25 m thick). To protect against flooding and storm surges, the site level needed to be raised by approximately 3 m over the compressible clay soils of the delta. When a major load (such as that needed to raise the site level) is applied to compressible clay soil, the soil will consolidate and settle as water ‘squeezes out’.
Since the 1980s, Golder has supported proposed developments at the airport site with geotechnical and environmental services. Building on this long-term relationship with client Brisbane Airport Corporation (BAC), and drawing on our deep understanding of the site conditions, we have provided environmental, acid sulfate soil and geotechnical consulting services for the airport’s 360 hectare new runway site since 2005.
A crucial design consideration was the long-term settlement performance of the runway and associated infrastructure. A ground improvement solution was essential because areas of the site with thicker and deeper clay deposits could take decades to consolidate, presenting the risk of ground movement after construction, which could cause differential settlement in the runway.
As part of the work, our specialists undertook extensive site investigations and provided surcharge design solutions appropriate to the varying conditions across the site. During construction, 11 million m3 of sand dredged from Moreton Bay was placed across the site for periods of up to 3 years. The surcharge design solution also included more than 330,000 wick drains installed across the site (to accelerate water drainage and settlement), making it the largest wick drain project in Australia.
We adapted an existing calibration program and developed a methodology for probabilistic back-analysis of hundreds of settlement plates during the surcharge phase. This process delivered the high degree of confidence required in the ground improvement solution. Additional surcharge design and back-analysis was also completed for the supporting infrastructure such as roads and runway lighting.
Our team developed a GIS-based interface allowing access, both in the office and in the field, to a wide range of data from thousands of site investigation records (e.g. boreholes and cone penetrometer tests) and construction records (e.g. wick drain installations and settlement plate survey data). This increased efficiency allowed us to review data nearby to the area of consideration (that we may not have known was there).
Over the past 15 years, Golder has also provided environmental impact assessment, acid sulfate soil assessments, groundwater monitoring and modelling, and verification of sand density for the development.
The runway was officially opened in July 2020, having successfully overcome the complex, challenging site conditions.