Early faulting ID saved costly rail excavation

The early identification of significant faulting, has allowed the repositioning of an underground railway station in Sydney, before design work started.

So the biblical parable about the merit of building on rocky ground has been given an added dimension in geotechnical work undertaken by Douglas Partners engineering geologists David Och and John Braybrooke and others, in the company's Sydney office.

The work has developed further information on the series of significant geological faults which run through the greater Sydney area, including the centre of the Sydney CBD.

Faults previously believed to be confined to the Sydney CBD can now be demonstrated to extend over distances of at least 20 to 30km and appear in suburbs such as Harbord, Ryde, Homebush, Watsons Bay and Heathcote.

“Similar faults have also been identified in Kangaroo Valley to the south and the Central Coast to the north,” the geologists say.

The faulting is the result of the break up of the Tasman Sea about one hundred million years ago.

While geologists and engineers have known about individual fault locations within the Hawkesbury Sandstone for many years, at least four discrete fault zones crossing the Sydney CBD are known.

The faulting is much more extensive than previously believed, potentially affecting sites previously considered to be unaffected by faulting.

The faults affecting the Sydney CBD area have potentially significant cost and risk implications for both deep foundation developments, typically in the CBD, and for major infrastructure schemes, the firms says in its latest newsletter.

“The fault zones are relatively linear and sub-vertical extending from ground level to depths of many hundreds of metres or more and can be up to 40 to 50m wide. At the surface they are identified by 'weak', crumbly or fractured rock with limited load bearing capacity for building foundations,” David Och said.

He added, “Over the years Douglas Partners has provided advice to clients on a number of development sites affected by these geological faults. Often their presence had not been identified until after excavation had started, which sometimes required costly and time consuming remediation works.

“This highlights the importance of an appropriate site investigation prior to both design and construction to avoid the risk of costly and time consuming remediation, once site work has started.”

More information: David Och or John Braybrooke, Douglas Partners, 02 9453 5558.

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Early faulting ID saved costly rail excavation

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