Structural Failure of Precast-Concrete Span Sets Back Sydney Metro Job
Detailed forensic analysis criticizes techniques used in segment grouting
A key component of Australia’s biggest public transport infrastructure project—Sydney’s $6.3-billion Metro North West—is the subject of a critical and detailed technical report describing how an elevated viaduct span failed at a stitch joint between two precast segments during construction last September. Project officials say the affected span, which did not suffer a progressive collapse, has since been removed and its replacement fast-tracked to avoid further delays. Little additional detail was provided.
The so-called Skytrain section is one of 115 spans being constructed using the precast segmental span-by-span method, erecting a series of precast box-girder segments supported by an overhead gantry and then stressed longitudinally.
Sydney Metro Northwest, formerly called the North West Rail Link, is the first stage of Sydney Metro and will be the first fully automated metro rail system in Australia. Sydney Metro City & Southwest is the second stage. Sydney Metro Northwest is delivering eight new railway stations and 4,000 commuter parking spaces to Sydney’s growing North West region. Trains will run every four minutes at peak.
Italy’s Salini Impregilo won the $260-million contract to design and build the 6.2-kilometer project, which includes the 4.6-km viaduct and a 270-m-long cable-stayed railway bridge over a major arterial road. Construction began in December 2013. The contract grew to $298 million to accommodate the extra work required to clean up larger-than-expected quantities of contaminated land in locations along the alignment, including buried asbestos and dumped construction materials.
Last November, engineering consultant Aurecon Australasia released the results of its independent investigation for its client, Transport for NSW (TfNSW). It highlighted a combination of errors that led to the failure of Span 60. As a result, new controls have been introduced to improve the process, according to TfNSW.
The failure occurred in a stitch joint used to join two segments at the midpoint of Span 60. The use of a closure concrete pour at this location was a departure from the usual practice for match-cast segmental construction, in which each segment is cast against its neighbor to achieve a matching interface.