1. William Lawrence Geowork Engineering
Recent Secondary Extraction Experience at Cook Colliery in a Multiseam Environment
William Lawrence
Geowork Engineering

2. Most of the mining activities at Cook up to the mid-2000s was concentrated in the upper Castor Seam. There are extensive secondary extraction workings ranging from various forms of partial pillar extraction to longwalls. The majority of the workings are sealed, i.e. a methane rich and oxygen deficient atmosphere, that we would like to avoid connecting with. Caved secondary extraction areas are characterised by high stress abutments around panel or goaf edges and remnant pillars within extracted areas. This abutment stress is transmitted through to the lower Argo Seam workings.
Castor Seam Workings
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

3. Argo Seam Workings (Castor superimposed)
Recent and current mining activities are concentrated in the lower Argo Seam. Sealed Argo workings are shown. Non-caving secondary extraction methods, such as floor stripping and pillar sumping have been used. Cook does not use MRS or timber to facilitate secondary extraction.
Argo Seam Workings (Castor superimposed)
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

4. Argo Seam Major StructureN
Mining at Cook has always been dominated by the presence of complex geological structure, particularly reverse faults and thrust zones. Structure is the major restriction on the method of secondary extraction and the size of panels. The in situ major principal horizontal stress orientation is typically NNE to NE, and does have an impact on roadway conditions as depth of cover increases.
Argo Seam Major Structure
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

5. Lithology
The immediate roof of the Argo seam is a interbedded siltstone and sandstone sequence with occasional mudstone and thicker sandstone units. Uniaxial compressive strength may vary from 15 MPa for the mudstones, to 20 to 25 MPa for the interbedded material to 40 MPa or more for the sandstones. In terms of CMRR the immediate roof varies from the high 30s to the low 40s. Depth of cover in current mining regions varies from about 170 m to 250 m. Interburden thickness to the overlying Castor Seam and workings varies from 14 m to 22 m. The Argo Seam can be up to 4.8 m thick. To maximise secondary extraction by taking floor coal and to provide protection against a weak mudstone floor, the roadway development mining horizon is to the stone roof.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

6. CMRR – South Argo
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

7. CMRR – North Argo
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

8. North Argo – 305 Panel
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

9. Continuous Haulage System
ABM25 with nominal 5.5 m x 3.8 m roadway profile
Flexiveyor continuous haulage
Cook also utilises more conventional roadway development systems consisting of a Joy 12CM12 single-pass miner with shuttle cars. Sumping secondary extraction is by a narrow-head Joy 12B miner. For all its strengths, the current continuous haulage setup at Cook can only develop minimal size pillars, being limited by the number of cars. First working pillar size is a limiting parameter on method and extent of secondary extraction.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

10. Roadway Development Authority To Mine
Minimum primary support is specified by an Authority to Mine. Support density ranges from a 4-bolt pattern (Green) to an 8-bolt pattern (Blue), occasionally with additional cables (Red). The minimum installed primary support does consider secondary extraction requirements. Primary support is intended to be sufficient to maintain strata integrity during roadway development and secondary extraction. Minimum primary support can be dependent on roadway orientation, geological structure and Castor abutments.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

11. Trigger Action Response Plan (TARP)
Actual installed support is controlled by a Trigger Action Response Plan (TARP, Figure 7). Strata support above the specified minimum would be required in zones affected by geological structure and overlying Castor workings.
Trigger Action Response Plan (TARP)
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

12. Method of Secondary Extraction
Recent secondary extraction in Argo workings has utilised floor stripping and pillar sumping. This method of extraction is not unique to Cook. Secondary extraction is done on the retreat, where a narrow-head continuous miner excavates unsupported sumps into first working pillars or solid. Sump dimension are nominally 3.6 m wide and up to 12 m deep. Both double-sided and single-sided sumping is shown. In some cases floor stripping is done after sumping. Pillar remnants are required to be stable, i.e. no caving and interconnection with overlying sealed Castor workings before panels are sealed. We are currently required to maintain strata integrity after secondary extraction and for the life of the panel to allow inspection and access in all roadways to the inbye end of the panel. This requirement does present geotechnical challenges in a weak roof environment, as it has been customary to consider inbye areas as waste with no access.
Method of Secondary Extraction
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

13. Strata Conditions After Sumping
Fenders are a critical issue in maintaining strata stability during and after secondary extraction. Apart from fenders and installed primary support, no other methods such as props or mechanised breaker line supports are used to control and maintain strata stability. Thin, indicator props are shown that are used to show roof to floor convergence. Pogo-sticks with a measuring strip are also utilised. Low-level and non-progressive floor heave after sumping is a common occurrence and is inferred from the prop in the foreground.
Strata Conditions After Sumping
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

14. Pillar Design Primarily Empirical Odd shaped pillars Castor abutment
Numerical modelling also required for certain situations
UNSW empirical pillar strength formulation
Minimum remnant pillar FoS of 1.5
Fenders do not contribute to pillar strength
Assume pillar side has been stripped
Fender stability can not be assessed using this empirical formula
Odd shaped pillars
Castor abutment
Extent of roof falls
Relatively simple models are usually sufficient
Fender and small W/H ratio pillars cannot be assessed by any currently available pillar strength formulation. Fender strength can be evaluated if coal strength is known and is usually done numerically.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

15. Displacement Discontinuity Numerical Modelling (LaModel)
LaModel is a multiple seam numerical model that can calculate vertical stress and yield in both Argo and Castor workings. Its strength is in assessing average pillar vertical stress. Evaluating fender stability is more difficult. Evaluating both fender and remnant pillar stability in the one model is not possible.
Vertical pillar stress with no overlying Castor workings
Vertical pillar stress with overlying Castor workings
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

16. Finite Element Numerical Modelling (Phase2)
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

17. Fender and Roadway Stability
General comments
Sumping can be done under poor roof conditions if primary support is installed to TARP standard
This includes fault-affected strata and Castor abutment
However, sump spacing is increased under known faults and Castor abutment
Some areas are just avoided!
Cleat orientation does affect fender stability
Generally, a “double-whammy” is avoided, e.g. fault and Castor abutment
Falls may occur in unsupported sumps
This is the big one!
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

18. Sump Spacing (Fender Width) Design
This design chart is only applicable for no Castor abutment. Now, you will notice that I haven’t included depth on the design chart. You may speculate that I don’t want to give away too much. Any bright spark could infer a possible depth range from data provided earlier. The reason that I have not included depth is to urge caution. Any design chart like this would be site-specific. It depends on many parameters, e.g. coal strength, coal characteristics such as cleat and brightness profile, fender height, roof strength, and installed support to name a few.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

19. Secondary Extraction Roof Stability Issues
Sump spacing design chart is a hybrid empirical and numerical method based on Cook experience.
Yes, there has been inbye supported roadway roof falls of no consequence before it became an issue. These provide excellent data points.
Even with the design chart there is no guarantee of inbye roadway stability after secondary extraction. Additional coal is being taken without supplementary support. Argo roof is relatively weak.
Stability around the current sump must be ensured, and protocols (TARP) are in place for this. The crew must not take or complete a sump if conditions change.
Even in the early days of Argo sumping, it has always been the intention to maintain inbye roadways, so please don’t get the impression that minimal fender size was initially the go. There has never been a roof instability issue when taking a sump in the Argo Seam. All inbye roof falls have occurred days or weeks after sumping. There has been a HPI incident concerning front of fender stability.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

20. Recent KM02 Roof Falls
To finish off, we have had a timely (for this conference at least) and interesting series of related inbye roof falls.
Note:
Castor workings
Argo faults
Increased sump spacing C15 to C18 and D19 to D20, although poor execution for the later
Chronology of events
Characteristics of C16/17 fall (photos) – fenders stable
Pillar stability
Characteristics of D19/20 fall (photos) – Castor abutment
Stress redistribution in the Castor
This area is being sealed, and an inbye egress roadway has been driven outbye of these falls. This strategy of periodic sealing if inbye roof falls occur may be adopted as standard practice. Apparently it may be cost-effective. There are always going to be zones of poor strata conditions that can be sumped, but with increased potential of inbye roof falls.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

21. KM02 C16/17 Fall
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

22. KM02 D19/20 Fall
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

23. Sump width MRS Caving Sealed Castor workings FLAC3D
To finish off, we have had a timely (for this conference at least) and interesting series of related inbye roof falls.
Note:
Castor workings
Argo faults
Increased sump spacing C15 to C18 and D19 to D20, although poor execution for the later
Chronology of events
Characteristics of C16/17 fall (photos) – fenders stable
Pillar stability
Characteristics of D19/20 fall (photos) – Castor abutment
Stress redistribution in the Castor
This area is being sealed, and an inbye egress roadway has been driven outbye of these falls. This strategy of periodic sealing if inbye roof falls occur may be adopted as standard practice. Apparently it may be cost-effective. There are always going to be zones of poor strata conditions that can be sumped, but with increased potential of inbye roof falls.
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

24. FLAC3D model
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

25. First workings in Argo Seam – 6 m roadway
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

26. Secondary extraction – 4 m sump and 7 m fender
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

27. Secondary extraction – 16 m sump/strip with 8 m fender
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

28. First workings in Argo Seam beneath Castor roadways
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment

29. Secondary extraction – 16 m sump/strip with 8 m fender
BBUGS Blackwater February Recent secondary extraction experience at Cook Colliery in a multi-seam environment