Nexus’ hitch is ‘unprecedented’
A TOOWOOMBA geologist says the geological feature that delayed construction of the Toowoomba Second Range Crossing is “unprecedented”.
Last week, Nexus Infrastructure flagged additional work after ground movement was identified in the geological layers below an embankment at Ballard.
Dr Edwin Willey, honorary senior lecturer at the University of Southern Queensland, said the formation came about after a basalt magma flow encountered a “grassed, alluvial flat” between 17 and 20 million years ago.
The flow, encased in a thick, bubbly skin, made no contact with the ground as it rode on a cushion of steam arising from the soil.
While under normal circumstances, the lava’s base would flatten and stretch against the ground, the steam prevented that happening and the bubbles were preserved, frozen in the solid base.
“This is anomalous,” Dr Willey said. But that’s not all.
The lava kept moving forward and made contact with the soil beneath as it cooled and solidified, shrinking and forming fractured hexagonal columns from the base of the flow, up into its centre.
The columns tilted along the developing fractures as the lava pushed forward, and the base of the flow jammed.
“The still-plastic lava and the underlying alluvium, fluidised by vaporised water, are the weak points,” Dr Willey said.
“They resolve the forces, and fracture the solidifying base. The advancing flow and the underlying alluvium rise and override the jammed section.
“It forces the once horizontal contact into a vertical position, pushing it some 10s of metres upward.
“Such a feature has not yet been described in the geological literature.
“It is a complex geometry of weak and strong rocks; a slope stability conundrum,” Dr Willey said.