Managing water inrush after floods

INCIDENTS of water inrush at underground mines in Queensland and New South Wales have underscored the role of engineered water-retaining bulkheads and dam walls as part of a water management plan, says PB energy, mining and industry technical executive Michael Salu.
Managing water inrush after floods Managing water inrush after floods Managing water inrush after floods Managing water inrush after floods Managing water inrush after floods


Lou Caruana

As underground mines across Australia recover from the recent rainfall and subsequent flood levels, safety authorities are warning mine operators to consider the potential impact of water ingress from surface accumulations, directly or via aquifers, in volumes not experienced before.

While recommended water management strategies include the installation of water-retaining bulkheads and dam walls, a range of variables needs to be considered to determine the structural design parameters, particularly in addressing unknown potential conditions.

Following the development of a water-resistant, single-application shotblast product, Aquacrete’s mine support team, in cooperation with independent structural engineers PB, has developed a design basis that has been applied in the construction of water-retaining structures at underground mines around Australia, Salu said.

“Our observations and experiences in designing water-retaining structures with Wet-Repel consistently confirm that the most significant factors influencing bulkhead performance are the interface of the bulkhead with the surrounding strata as well as the potential magnitude of the hydraulic pressure to which they may be subjected,” he said.

“To determine the maximum overpressure requirement, mines need to consider not only the water to be stored in the impoundment behind the bulkhead, but also to evaluate potential additional sources of water. Mines currently affected by increased rainfall and flood conditions need to consider all sources of surface water that could find their way into the underground environment.”

Mines also needed to consider the impact of overlying or underlying adjacent mine operations, and their water management plans.

“While it may be difficult to establish the impact of unknown conditions, it is essential in determining the bulkhead design to ensure long-term structural integrity,” Salu said.

“With an appropriate factor of safety applied to the engineering calculations, a bulkhead can be designed to maximise effectiveness in addressing a range of underground conditions.”

As sustained water pressure poses the risk of leakage through fractures and strata surrounding the bulkhead, the strata in the immediate roof, floor and ribs needs to be assessed in detail and should include all strata that can be affected by a change in hydrologic conditions.

According to Aquacrete managing director John Whitfield, selecting a product that can be keyed into the surrounding geology is of critical importance.

“Most known bulkhead failures have been through the surrounding strata or along the strata/bulkhead interface,” he said. “This is due not only to the fact that water pressure is not always constant, but also because sustained water pressure over a period of time can lead to softening of the surrounding strata.

“Wet-Repel’s ability to bond well to hard rock and coal, as well as its low water permeability and high compressive strength, has certainly demonstrated the impact that these factors have on long-term structural integrity of bulkheads.

“Wet-Repel has been put to the test in underground mines for the past five years, with excellent results. Our clients consistently report on the performance of Wet-Repel over an extended period of time.”

As mines address changes to their operational environments, it is important that unknown conditions be carefully considered. While the occurrence of underground mining incidents directly attributable to inundations has been limited in the past, engineered design of bulkheads to withstand sustained and significant water pressure is now vital, Whitfield said.

“Our technical services team has subjected our products to stringent testing protocols over the years,” he said. “In addition to live blast testing, our shotblast products have undergone tests to determine compressive strength, over-pressure ratings, diffusivity properties and water permeability ratings.

“As our clients are able to obtain independent engineering design and certification for their installations, they have the assurance of knowing that their bulkheads are designed to address their individual site conditions – a feature that has become increasingly important with the changes in mine environments.”