CSIRO researchers Baotang Shen, Habib Alehossein and Brett Poulsen together with Mine Subsidence Engineering Consultants’ Arthur Waddington have been working on subsidence control using coal washery waste for the past two years.

Pre-trials at West Cliff, Moranbah North and Mandalong have provided invaluable data to formulate the best conditions for an overburden grout injection operation and to move towards a full-scale trial.

By the end of the trial and with a little bit more work, the researchers hope they can offer the coal industry a proven cost-effective and environmentally friendly subsidence control technology as well as coal washery waste disposal.

Currently ground subsidence can dramatically increase mining costs where major surface structures, facilities and natural environments need to be protected from ground movements.

In addition, mine subsidence has become a major community concern, particularly in New South Wales.

Subsidence under sensitive surface features is often controlled by leaving large blocks of unmined coal behind – sterilising the resource and increasing mining costs because of production loss and longwall relocation.

Remedial and mitigation measures to manage damage caused by subsidence can also often be very costly.

In the researchers’ method of using overburden inter-strata grout injection, fine waste material (flyash) is injected into the overburden bed separation openings caused by longwall mining while the mining is taking place. The injected material fills the bed separation gaps and hence reduces the propagation of the overburden movement to the surface.

According to Shen, this technology has significant advantages over other subsidence control methods, like backfill.

Operations are conducted on the surface through boreholes and therefore do not interfere with underground mining operations.

The technology uses waste material as the injection material so reduces the need for surface waste disposal and the associated environmental protection issues.

The method can also prove to be highly economical when compared with using large coal barriers to reduce the environmental impact.

Like most technology though, there are some disadvantages. This method can only reduce the subsidence by up to 60% depending on the geological, geotechnical and mining conditions and it requires surface access for drilling vertical boreholes.

Some mines are better suited to overburden grout injection too, which the researchers were able to find out from their field monitoring, laboratory simulations and numerical modelling.

Geotechnical and numerical modelling was carried out at West Cliff colliery to address the key technical issues of a full-scale trial, including determination of the safe and effective grout injection horizons and quantitative assessment of the effects of the protective improvements by grout injection on the stability of surface features.

Next the researchers went to Moranbah North and Mandalong to carry out technical feasibility assessments based on the new and existing measurement data at these mines and findings from West Cliff.

A cost analysis of grout injection operations in different site conditions to highlight the benefits and costs of the technology for each site was conducted and, finally, the feasibility of using a wide range of coal washery wastes as the injection material was studied.

Based on these results, a set of conditions that favours overburden grout injection was found:

• Mining depth is greater than 200m
• One or several strong and thick competent strata exist in the overburden at a distance greater than 50m above the mining seam
• Strata in the immediate overburden are soft and impermeable
• Grout materials (fine coal wash rejects or flyash, water) are locally available and can easily be transported to site
• The protected surface features are located in a limited area in each longwall panel.

Shen said the cost of the injection for one operation was $A1-7 million depending on the mine condition, distance to coal processing plant, surface environmental requirement and the protection target. This cost, however, is compared with more than $14 million for leaving a 70m-wide and 300m-long coal pillar as an alternative subsidence control method.

While the technology does not offer better subsidence reduction when compared with backfill (about 80% reduction) or leaving a large coal pillar (up to 100% reduction), Shen said in many cases a 50% reduction in subsidence was sufficient to protect the surface infrastructure from damage.

The next steps in the research are to conduct a full-scale trial of the method at an operating mine, evaluate the effect of subsidence protection and improve the technology based on the trial results, if necessary.

Then the technology will be applied on one or several mines as part of the mine’s routine subsidence management plan.