Underground » Strata Control and Windblasts
Australian coal deposits commonly have coal seams in close proximity, however over recent times, underground mining has typically concentrated on the single seam mining of the most economic seam. The proximity of seams within the Hunter Valley in particular, lends itself to multiple seam underground mining. Mining of two to four seams within the Hunter Valley is a likely scenario of new mines, and in the future there are also opportunities for multi seam operations in the Bowen Basin. This study was undertaken as a strategic review to assess the issues and planning approach for new multi seam operations. This was undertaken as a combined study of overseas experience and a more detailed analysis of the specific geotechnical issues at a site within the Hunter Valley.
A number of multi seam layout options have been evaluated together with a review of partial extraction of the chain pillars to reduce their impact.
A technical steering committee was established with industry representatives providing input on logistical issues which may arise with various geotechnical options available. Significant input was received on the issue of chain pillar overcutting.
The multiple seam layouts evaluated were:
- Vertical stacking whereby panels are superimposed vertically below the extracted seam above.
- Indented stacking whereby the lower panels are superimposed vertically but are indented such that they are contained inside the goaf of the overlying panel.
- Offset panels whereby the gateroads of the lower panels are located under goaf and the layout is not directly related to the geometry of the overlying panels.
The study is targeted at layouts progressively extracting lower seams rather than extracting overlying seams.
Overseas experience utilised in the study was primarily from the UK and USA.
The background experience obtained from the overseas review has provided a general framework in which to undertake a design overview, but did not provide sufficient detail to allow its application to specific field sites.
In order to improve the design approach, computer modelling of layout options has been applied to provide an understanding of the ground behaviour within the multi seam panels. Computer modelling has progressed to the point where realistic simulation of caving and fracture within the strata can be obtained. The computer simulations have been applied to an assessment of the chain pillar interaction effects, subsidence, stress concentration zones and gateroad conditions in lower seams.
A number of planning issues were highlighted within the study. These included:
- Chain pillar strength and load will be dependent on the strata strength surrounding the coal seam. Failure of the strata within the interburden is common in sections of weak to moderate strength strata. Such failure will reduce pillar loads and increase subsidence.
- Stress concentrations under pillars and goaf edges may extend 4 pillar widths below and above pillars. Roadways located in these stress concentration zones are likely to suffer increased deformation, reduced driveage rates and have higher support requirements. Roadways located greater than 4 pillar widths below a seam are unlikely to be significantly affected by the overlying panel geometry during development driveage. Goaf within these stress concentration zones above pillars will undergo additional consolidation.
- Surface subsidence from multiple seams will be an accumulation of each equivalent single seam subsidence profile, chain pillar compression and additional goaf consolidation above underlying pillars. Subsidence is therefore likely to be greater and more complex than the addition of single seam subsidence profiles. Empirical predictions should be seen as initial approximations based on idealised ground behaviour.
- Vertical abutment loading is transferred within the goaf of overlying seams as they are undermined, however the overall layout geometry will influence the goaf and pillar loading characteristics as overlying seams are undermined. Overlying chain pillars may increase their loading immediately prior to being undermined, however this is dependent on their strength and confinement conditions in the overlying goaf.
- Longwall rib and roof deformation under chain pillars are typically increased relative to equivalent depth single seam operations. It is likely that fracture will extend well ahead of the faceline and supports will need to be designed to control unstable roof and rib conditions. Experience overseas and limited local experience indicates that this is achievable and typically does not have significant detrimental impacts on face stability if adequate control measures are planned.
- Longwall supports used in multi seam operation should be designed for the worst case situation of pillar loading conditions. They will need to have a high priority on forward face and rib control.
- Concept planning for multi seam operations may be undertaken using empirical pillar strength and subsidence characteristics, however planning at a feasibility to operational level requires computer modelling of the strata section and mine geometries. Detailed characterisation of the strata within the total mining section is an essential requirement to undertake realistic mine planning.
Geotechnical guidelines for a multi seam planning process are provided within the report.