According to Seedsman, horizontal stresses in coal seams are substantially less than the vertical stresses, which suggests that a reconsideration of various geotechnical issues, including stresses, coal failures and ribs, should be factored into planning of longwalls in thick seams.

“In Australia, the conventional wisdom is that the horizontal stress is approximately twice the vertical stress,” he said. “This remains the case for non-coal strata. For coal, the general assumption should be reversed – the vertical stress is at least twice the horizontal stress.”

Consideration of a range of geotechnical issues suggests the following planning issues for planning thick seam longwalls:

- the preferred alignment of gate roads is controlled by the joint structure in the coal and not the orientation of the major horizontal stress.

- It should be possible to use place change methods if a massive coal horizon can be used as the working section roof.

- Coal ribs will be poor at depth because of the concentration of vertical stresses in the sides.

- Because of their height, coal ribs will present a hazard if the roadways are aligned sub-parallel to structures in the seam.

- The option of changing the shape of the roadway to modify the induced stress field is not readily available. Roadway width is set by the available machines and increasing the height increases the rib risk.

- If vertical stresses increase long-term stability of roadways may be affected.

- The horizontal stress concentration at the maingate corner (assuming it develops) is not a major concern, as the increase in stress will tend to stabilise the roof.

- Horizontal stress reductions in the tailgate are of great concern. The original stresses roof stresses are low, so a lesser decrease is needed for the roof to go into tension.

Ross Seedsman is the principal of Seedsman Geotechnics Pty Ltd. Email: sgpl@hotkey.net.au