Report: APP CMHS Project 1




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3.1.10. Strata Control

Current Technologies

Typical mine strata control management process in the Australian underground coal mines includes the following major components:

  • detailed site-specific geological and geotechnical characterisation using standard strata classification systems such as Rock Mass Rating (RMR) and Coal Mine Roof Rating (CMRR)

  • geotechnical assessment and numerical modelling

  • design of mine layout and mining sequence

  • design of strata support and reinforcement systems

  • strata control performance monitoring.

Various strata control technologies including both software and hardware have been developed and applied in Australia to support the mine strata management. Key examples of the technologies are as follows:

  • numerical modelling technique developed by CSIRO and Strata Control Technology (http://sctaust.com)

  • monitoring devices such as wire extensometers, sonic probe extensometers, Tell-Tale

  • hydrofracturing stress measurement and CSIRO HI stress measurement and monitoring techniques

  • high strength rockbolts and cable bolts

  • mobile roof bolters.

In Australian mines, a strata management plan is required to identify, assess and control geotechnical hazards with the above technologies.

Strata control in underground coal mines must be considered on a number of fronts: longwall extraction, gateroad stability due to induced mining stress, development roadway excavation and long term roadway stability. It must be remembered that strata control is not just the design of installed roof supports or longwall hydraulic supports, it begins with the analysis of geological and geotechnical information in order to adequately design panel and barrier pillars that provide the overall support requirements for a mine. So an understanding of the depth, rock characteristics, stress magnitude and direction and planned mining methods is required in order to model the resultant mine design – pillar size and geometry, roadway dimensions, barrier pillar requirements and panel geometry.

Bolts and Self-Drilling Bolts

The support of underground roadways in coal mines is fairly consistent in the initial installation of chemical anchored roof and rib bolts. The differences are in the number, length, grade and spacing of such bolts. This too, is based on the seam and mining characteristics. With the noted move toward automation and recognition of the reduction of roof bolt effectiveness due to gloving of chemicals, single pass, self drilling, post grouted bolts are being refined and gaining recognition. Typical analysis of characteristics – tensile strength ~34 kN – similar to current X Grade bolts, yield strength is ~315 kN compared to 240 kN, shear strength and elongation are essentially the same, process steps are roughly halved for the self drilling system (source Strata Control Technology). This technology is being developed by Strata Control Technology , Strata Engineering, Hilti and Sandvik.

Specific technical information can be found in Australian Coal Association Research Program Report C11028 (http://www.acarp.com.au/abstracts.aspx?repId=C11028).

An innovative support strategy for highly stressed and/or weak roof conditions is that of spiling prior to excavating the road way. Spiles or bolts of designated lengths are installed above the roof design profile at a low angle (5o). The roadway is then mined as normal and roof support installed.

Long Tendon Supports

Leading on from the standard roof bolting approach, mines with longwall operations or in high stress areas will also invariably install some form of long tendon – cable bolts, flexibolts, megabolts or other similar proprietary systems. Again these are widely available and not unique to any particular mine. It is more appropriate to interact directly with geotechnical engineers and manufacturers to determine requirements and capabilities. Suppliers include Dywidag, Jenmar and Megabolt.

Application Sites

Effective strata control is a requirement of every underground coal mine and therefore has universal application, though the potential solutions will vary dependent upon conditions. Examples of comprehensive strata management can be found at Xstrata’s Beltana mine in New South Wales, and BMA’s Broadmeadow mine in Queensland.

Technology Gaps/Needs

There are ongoing needs for technological improvements in the following areas:

  • prediction and planning of strata control in multi, deep, thick seams

  • subsidence assessment and reduction in particular mining under urban areas or significant infrastructures

  • continuous real time strata monitoring and data interpretation

  • rapid and automated roof bolting, skin support materials and rapid placement method

  • optimisation of longwall support capability to Australian strata conditions.

Mines will continue to increase in depth with the associated increase in vertical and horizontal stresses. The research and technology improvement in all facets of strata control needs to be continued and disseminated. Current innovations and knowledge are broadly sufficient for the majority of mining conditions in Australia, although roof falls on longwall faces or gate roads continue to be the major production interruption. However, the push for higher production with larger equipment in more difficult conditions continues to draw the technology forward and provide innovative strata control techniques.


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