As underground mines across Australia recover from the impact of recent rainfall and subsequent flood levels, safety authorities are now warning mine operators to consider the potential impact of water ingress from surface accumulations, directly or via aquifers, in volumes miners have not experienced before.
This was recently demonstrated in Queensland, where a coal mine nearly had a fatal accident after water gushed into an underground mine, forcing three miners to climb nearby equipment to escape.
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 underground 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 co-operation 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.
"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," Michael Salu, technical executive for PB Energy, Mining and Industry 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 potential sources of surface water that could find their way into the underground mining environment.
They also need to consider the impact of overlying or underlying adjacent mine operations, any nearby infrastructure and their water safety 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.
"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," Salu said.
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 mine geology is of critical importance.
"Most known bulkhead failures have been through the surrounding strata or along the strata/bulkhead interface.
"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.
The effect of sustained water or rain over a period of time effecting a coal mine was shown in Victoria, where the Princes Highway saw a partial collapse in the Gippsland region after water undermined an underground mine.
"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.
According to Whitfield, it has been put to the test in underground mines for the past five years, with consistent results.
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.
"Our technical services team has subjected our products to stringent testing protocols over the years. 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," Whitfield said.