Underground » Strata Control and Windblasts
Timber chocks are used on a systematic basis to support longwall tailgate roadways by 13 Australian longwall operators. At least five other operators use timber chocks on an as-needed basis. Typically, it requires about 80 acres of Australian hardwood forest to supply enough timber to chock one tailgate roadway. Chocks are usually of 4 or 6 point construction which results in only between 15 and 30% of the timber in a chock being utilised to carry load.
Australian hardwood timber is under the focus of conservationists and is becoming scarcer and more expensive at a time when hardwood timber chocks are becoming more widely used in the mining industry. Accordingly, in 1995 the Australian Coal Association Research Program (ACARP) awarded a research grant to the School of Mining Engineering at the University of New South Wales to develop design and performance criteria for timber chock constructions in order to optimise their use and cost effectiveness.
The Stage 1 timber chock research was very successful. It produced a set of design guidelines for chock construction, with the cost of the research being recouped in 12 months by the savings made at one operation alone. In the process, the research identified a number of areas worthy of further research for reasons of improving safety, reducing costs and improving chock performance. In 1997, ACARP agreed to fund research into three of these areas. During the process of undertaking this research, new chock support systems developed in the USA began to appear on the Australian market. One of these, namely the Link-n-Lock chock, appeared to offer a range of benefits over traditional chocks and to address many of the safety and performance factors identified in the Stage 1 research. ACARP approval was granted to modify the research program to include an evaluation of these chocks. This presented an opportunity to undertake cost effective full scale testing of chocks at the USBM/NIOSH establishment in Pittsburgh, USA and to gain a point of reference with USA research into timber chock performance.
The Stage 2 ACARP research has confirmed that:
- Chocks constructed from 25 and 50mm thick elements fail at approximately 70% of the failure load of the 100 and 150mm thick elements.
- Up to the point of failure of the 25 and 50mm thick elements, there is no significant difference in the response to convergence of chocks constructed from 25, 50 or 100mm thick elements.
- Chocks constructed from 150mm thick elements have a similar ultimate strength to the 100mm thick elements. However, they have a significantly lower resistance to load, thereby permitting more convergence to occur.
- There is more potential for chocks comprised of thin elements to be constructed 'out of plumb' and so suffer eccentric loading.
Two series of full scale testing were undertaken in the USA. Traditional Australian 1.2m square 4 pointer chocks were evaluated against both 1.2m square and 1.0m square Link-n-Lock chocks. Two grades of Australian Blue gum were tested in the trials, namely structural grade 4 and landscape grade.
In general, it was found that the Link-n-Lock chocks were distinguished from the 4 pointer chocks by:
- A doubling in stiffness in the linear elastic range.
- A trebling in load capacity in the linear elastic range.
- A steady state yield characteristic in the linear plastic range.
Specifically, it was established that the cost per tonne of support provided by a Link-n-Lock chock is effectively half of that provided by a 4 pointer chock of equivalent dimension and timber.
Laboratory testing has proven an effective means of evaluating chock performance. However, it must be appreciated that because the load-displacement behaviour of a chock is sensitive to the loading rate and loading regime (load controlled, displacement controlled or a combination of both), values derived from tests must be treated as relative and not absolute. They must also be related to the species and the structural grading of the timber under test.