Underground » Strata Control and Windblasts
This project addressed and evaluated a basic hypothesis, namely that the early injection of material into open bedding planes in the roof of a mine roadway that would substantially adhere to the rock, would have significant reinforcing benefits, the impetus for the project being published claims regarding the magnitude of adhesive bonds associated with various injectable materials.
The project evaluated adhesive bond strengths between various injected materials and different rock types via (a) the development of a repeatable and readily useable laboratory testing methodology and (b) the injection and subsequent coring of a mine roadway roof. The outcomes of the laboratory test work has been to demonstrate that actual bond strengths with various strata types are much lower than the published claims upon which the project was founded. The highest adhesive bonds were found with PUR (in the order of 2 MPa), followed by silicate resin (0.8 to 1 MPa), phenolic resin (in the order of 0.8 MPa) and finally cementitious grout (0.4 to 0.7 MPa). The bond-weakening impact of clay-coating on bedding planes was also established for the injection trials and subsequent roof coring.
The fact that the actual bond strengths found from the testing phase were significantly less than the published data upon which the project was based resulted in the proposed large-scale field trials being unable to proceed. The reason was that the only viable injection trial would need to involve the use of PUR and it was felt that due to its various downsides (over and above simply cost), it would not be an approach that would never be adopted by operating mine sites as a replacement for long tendons.
It was realised that strata injection would only ever be an addition to the use of long-tendons, the obvious method of application being as part of post-grouting operations. This the led to consideration of the possible strata control benefits of reverting to a bottom-up grouting process whereby migration along open fractures is possible, as compared to top-down grouting using thixotropic grout or resin, where such migration is far less likely, if at all.
A search was undertaken for any field trial or mining evidence that may support the proposition that the injection of roof strata during tendon or cable post-grouting may provide substantial benefits, as suggested by the general theory that the project was founded upon. Both a un-published finding from a NSW mine site and a published finding from Crinum in QLD supported the contention that the injection of bonding material into open roof fractures has roof stability benefits. More importantly, these two examples indicate that it is potentially of benefit across a wide range of roof conditions, from moderately deformed but still reinforceable, to highly deformed and substantially failed requiring some form of suspension or reconsolidation.
The conclusion drawn by the project is that the permeation of material into the roof that will form an adhesive bond with the roof strata at the time of tendon post-grouting, has substantial potential stability benefits. Consequently, the industry should possibly re-consider the use of top-down or bottom-up grouting of tendons in conjunction with the various other relative advantages and disadvantages of the two methods.
The project also identified a developing technology, namely the use of silicate resin for tendon postgrouting, that may provide further strata control benefits if it can be used in a bottom-up grouting approach. This is particularly the case when considering the installation of long tendons as part of the development cycle and the various difficulties and limitations of post-grouting such tendons in-cycle using cementitious grouts. This may be an area warranting further investigation if and when that technology becomes available to industry.