Published in June 2005 Australian Longwall Magazine

The Can, developed by Burrell Mining in the US, is a steel cylinder containing a weak cementitious fill. Used in high deformation tailgate environments, it is designed to fold in on itself while maintaining strength.

Earlier laboratory tests conducted by the US National Institute of Occupational Safety and Health had established the overall load/deformation characteristics of the Can, but the relative contribution of the steel cylinder versus the fill had not been established. In order to develop stiffer and higher capacity (or softer and lower strength) systems, this information was vital.

Conducting full-scale tests was prohibitive, so the study, with support from the Australian Coal Association Research Program (ACARP), was conducted using one-third scaled versions of the product.

The test was conducted at the University of Sydney’s civil engineering laboratory. Greg Tarrant, senior strata control engineer with SCT Operations, presented the research findings at the recent Coal 2005, AusIMM conference.

In practice, the largest “can” available has a 915mm diameter with a yield of approximately 160t. The bigger the can size the more difficult it is to handle, as well as causing possible ventilation and access disruptions in the tailgate.

Given these limitations, the ability for mine engineers to optimise support cost against other variables such as support capacity, density, size or handling, is limited.

Among other things, the testing program aimed to establish if the scaled-down versions could be used for product development and to establish design criteria regarding steel thickness and filler properties. This would allow mine engineers to dial up support properties.

One of the major outcomes of the tests was that the mini-versions were found to adequately reflect the loading behaviour of full-scale versions, making possible further product development at less expense.

The mini-can test program highlighted the greater contribution of the fill (more than 60% for 915mm diameter versions) versus the steel to the overall strength of the can.

“Since the fill is considered to be a very weak material (UCS 1.7 MPa) and has very low responsiveness to confinement, development of improved fill strength both pre- and post-yield appears to be the best avenue for product improvement,” Tarrant said.

So while can strength and stiffness can be increased by changing fill property, the can would weigh more, though Tarrant said engineers at mines indicated existing handling equipment could cope with increased weight of up to 50% of existing cans.

The unconfined fill and steel cylinder account for 63% and 35% of the total load, respectively, for a 915mm diameter can.

“The confining stress provided by the steel cylinder to the fill was found to be surprisingly low. The inferred confining stress developed in a 915mm diameter “Can” was approximately 0.05 MPa, which would account for only 2% of the total load contribution,” Tarrant said.

In conclusion, he said the scaled down tests offered a suitable first pass to assess field behaviour rather than the more hazardous alternative of trial and error in an underground situation.