Improving use of PUR as a strata consolidation technique

IN 1999 an estimated 70 PUR injection operations cost the Australian underground coal mining industry a total of about $9 million, but the method was regarded as a last resort. Tim Britten of Strata Engineering outlines in this article a current ACARP project which aims to present industry with a method to to rationalise the use of PUR.

Staff Reporter

Approximately 75% of these PUR operations constituted remedial measures aimed at consolidating roadway and longwall roof strata. In many of these cases, the use of PUR was a reaction to the unacceptable roof displacement trends and the PUR itself was regarded very much as the "weapon of last resort".

Generally, the application of PUR is not planned, with the methodology adopted often as much a function and reflection of time constraints as geotechnical or even financial considerations. Given that mine site experience with the use of PUR is often limited, the operator is very much reliant on the experience of the contractor.

Unless the contractor has an intimate understanding of the nature and behaviour of the particular rock mass, any design outcomes or recommendations provided are liable to err on the side of caution, being founded on what has worked well elsewhere/previously in apparently similar circumstances.

Under these conditions, it is considered likely that the outcome is often sub-optimal, in that although the result is generally successful (ie roof control is re-established), there remains a significant degree of conservatism and waste (both physical and financial) that is not understood.

A collaborative ACARP study bringing together the relevant experiences of Strata Engineering and Wilson Mining Services is currently under way with regard to the application of PUR. The focus of the project is on gathering data related to the use of PUR in remedial roadway roof control applications, developing a database from which a methodology can be distilled and then testing that methodology. The ultimate objective is to present the results in a transparent format that the industry itself can apply to rationalise the use of PUR.

The project team considers that the best value research relates to roadway roof control applications of PUR, where a significant body of available (and very expensively acquired) industry data already exists that would give the project significant leverage; effectively millions of dollars in potentially useful research data currently resides in the filing cabinets of industry personnel across the coalfields.

The project team also considers that a successful outcome to this study would result, as a by-product, in the identification of a process that could subsequently be applied in cost-effectively developing a parallel methodology for the optimisation of the use of PUR on longwall faces.

The underground coal mining industry currently spends around $10 million a year on “large-scale”, injected strata reinforcement, which unfortunately has a significant element of “hit-and-miss” which is somewhat reflected in the varying operational outcomes. The emphasis is on "large-scale" strata reinforcement and relates primarily to ground control on longwall faces, such as pre-consolidation of fault zones and longwall recovery/stop positions.

Major falls on longwall faces can represent a significant business interruption and the recovery of these falls can be hazardous. Spending in this area is increasing as more longwall operators see it as a pro-active solution to continued production in difficult ground conditions. Cementitious grouts have seen increasing application for such purposes in recent years, but at present the industry lacks a methodology to begin to rationally assess the effectiveness of these operations. The uncertainties surrounding benefits appear to be preventing its even wider uptake.

Ground consolidation, through various means, in particular grouting, is a commonly used practice in civil engineering where the hazards and potential losses associated with failure can be just as significant as halting production on a longwall face (eg tunnel development in “running” ground or beneath major surface civil structures etc).

The uncertainty element in civil engineering applications is minimised through the use of a rigorous and proven design methodology. With regard to the "state of the art", it is apparent that no such methodology exists for the application of large-scale ground consolidation in the underground coal mining industry.

The second part of the aforementioned ACARP study will be to conduct a range of field trials to investigate various aspects of strata consolidation and ultimately produce user friendly guidelines in the form of a single "handbook"-style reference covering the range of strata consolidation techniques utilised in civil applications and collieries. One of the aims of the project is to make available, for the first time to Australian coal producers, methodologies for the rational application of these materials and techniques.

Overall the project hopes to fill a major gap in the industry technical knowledge and ultimately save users considerable expense. Currently, work is ongoing with field trials planned for a number of mine sites in New South Wales and Queensland. If any mines have further interest in this project, desire to contribute historical information, or wish to participate in a field trial please contact David Hill or Russel Frith at Strata Engineering,