Published in March 2008 Australian Longwall Magazine

The new testing method has been developed through Coffey Mining principal geotechnical engineer Wouter Hartman's modification of a shotcrete beam testing device.

Traditional devices measured shotcrete beams for compressive strength (the capacity of a material to withstand axially directed pushing forces). The new device can now not only measure a shotcrete beam for compressive strength when it is cast, but also shear strength (the strength yield or structural failure).

"Shear strength in shotcrete is a very important aspect because you can now determine at a very early age – within eight hours – the actual strength of a beam," Hartman said.

In the past shotcrete could only be tested after 3 to 7 days – and not for shear strength.

Shotcrete testing has been done in the past for quality assurance – where an operation would see if the concrete applied on the walls and roof was of consistent strength throughout the process. Now with the new device testing can also be used to judge whether the concrete is aligned with the mine's mechanism of failure or mine design.

The new device also has knock-on benefits to roadway development rates.

"A mine engineer, manager or deputy can now say they have a ground control methodology which agrees with the mine design and they can with confidence take the next cut. That is where we will improve the actual development rates," Hartman said.

Hartman added that the device could also have ramifications for shotcrete guidelines in Australia, specifically for contracts between mines and contractors.

"We can see a change where a specific design criteria needs to be met by a contractor and that would be set in concrete in their contract.

"These new developments in shotcrete end product testing will ultimately change the way shotcrete quality is managed on mine and civil tunnelling sites and how it is defined in contractual agreements."

As part of the research, Coffey has been engaged by Barminco Contractors to assist in the completion of an industry guideline or code of practice for shotcrete application in Australian mines.

The shotcrete beam testing device can be bought directly from Coffey or used on site by Coffey consultants. However, Hartman said if a mine buys the device it was imperative to understand fully how the device can be used and what it will be used for.

The development of the testing technique was influenced by Hartman's experience of a fatality involving shotcrete failure 13 years ago. The incident was caused by a combination of shear and tensile failure on a relatively thin layer of applied shotcrete.

Hartman started his career as a geologist in South Africa before he moved into rock mechanics in the country's deep gold and platinum mines. He moved to Australia and worked on the development stage of Illawarra Coal's Dendrobium mine, before doing some work at Rio Tinto's Kestrel longwall. Hartman has been working as a consultant for eight years, with the past two spent with Coffey Mining.