1. SCT Operations Investigation into Abnormal Subsidence above a Longwall Panel in the Southern Coalfield, Australia Winton Gale Managing Director SCT Operations PTY LTD, Australia Ian Sheppard Environment and Community Manager Xstrata Coal, Tahmoor Colliery, Australia Winton Gale Managing Director SCT Operations PTY LTD, Australia Ian Sheppard Environment and Community Manager Xstrata Coal, Tahmoor Colliery, Australia

2. SCT Operations Location of the Mine Area

3. SCT Operations Picton Mittagong Thirlmere Tahmoor Tahmoor Colliery Mining Operations Tahmoor Colliery Mining Operations  25 longwall panels have been extracted at Tahmoor Colliery with subsidence occurring to predicted levels  However subsidence at the commencement of Longwalls 24 and 25 was more than double that which was predicted  25 longwall panels have been extracted at Tahmoor Colliery with subsidence occurring to predicted levels  However subsidence at the commencement of Longwalls 24 and 25 was more than double that which was predicted

4. SCT Operations Subsidence Characteristics Measured for Longwalls 24a and 25

5. SCT Operations Geotechnical Investigation of the Cause of Abnormal Subsidence Investigations were conducted in two stages Stage 1. Existing data and Regional Geotechnical Characteristics of the Strata Stage 2. Additional Geotechnical Data and Hydrological Data as required by the Investigation

6. SCT Operations  Computer Model of the Strata Section to Investigate a Range of Common Variations in the Overburden and Stress Regime Geotechnical Investigation of the Cause of Abnormal Subsidence Stage 1. Geotechnical Investigation of the Cause of Abnormal Subsidence Stage 1.   Data Review

7. SCT Operations Data Review Geotechnical Investigation of the Cause of Abnormal Subsidence

8. SCT Operations Caving and Overburden Fracture Zones relative to Empirical Data

9. SCT Operations Regional Subsidence Measured for Longwalls 24a,b and 25

10. SCT Operations Mine Plan and General Features Horizontal stress TSF 0.7-0.8 Overburden section Some variation Topographic variation

11. SCT Operations UCS Range for Boreholes over the Area

12. SCT Operations Large View of Longwalls 24A, B and Longwall 25 Relative to the Bargo Gorge Zone

13. SCT Operations Geotechnical Investigation of the Cause of Abnormal Subsidence Computer Model of the Strata Section to Investigate :-  Variation in rock strength  Range of horizontal stress magnitudes  Variation in pre-existing bedding partings and joint planes due to structural zones  Stress changes relating to the gorge  Variation in rock strength  Range of horizontal stress magnitudes  Variation in pre-existing bedding partings and joint planes due to structural zones  Stress changes relating to the gorge

14. SCT Operations Computer Modelling Approach and Background  Has been used successfully over many years by SCT  Rock mass must be well defined and rock failure criteria match the actual field mechanics  Validation with measurement  Flac code with SCT rock failure routines  Has been used successfully over many years by SCT  Rock mass must be well defined and rock failure criteria match the actual field mechanics  Validation with measurement  Flac code with SCT rock failure routines

15. SCT Operations Model Input Properties Intact and Post Failure Properties  Bulk and shear stiffness  UCS  Cohesion  Friction  Tensile strength  Dilation Bedding and Joint Properties  Normal and shear stiffness  Cohesion  Friction  No tension  Dilation Intact and Post Failure Properties  Bulk and shear stiffness  UCS  Cohesion  Friction  Tensile strength  Dilation Bedding and Joint Properties  Normal and shear stiffness  Cohesion  Friction  No tension  Dilation

16. SCT Operations Generic Strength Characteristics of the Rock Material

17. SCT Operations Model Section about the Longwall Area

18. SCT Operations Model Outputs Intact and Post Failure  Shear fracture through the material  Tensile fracture  Angle of fractures formed Bedding and Joint Post Failure  Bedding shear fracture  Bedding tension fracture  Joint and Pre existing bedding mobilisation Intact and Post Failure  Shear fracture through the material  Tensile fracture  Angle of fractures formed Bedding and Joint Post Failure  Bedding shear fracture  Bedding tension fracture  Joint and Pre existing bedding mobilisation

19. SCT Operations Rock Failure Mode for the “Normal” Overburden

20. SCT Operations Model Validation Relative to Measured Subsidence

21. SCT Operations Subsidence Monitoring Lines Most lines are suburban streets

22. SCT Operations Subsidence Lines used for Validation of Computer Results

23. SCT Operations Model Results Subsidence Profile for the Range in Strength Properties Regional Joint and Bedding Properties

24. SCT Operations Comparison of the Model with Normal Subsidence Characteristics Note the two end member cases are presented

25. SCT Operations Comparison of Subsidence Profiles for the Normal and Abnormal Subsidence

26. SCT Operations Effect of Modifying the Horizontal Stress through the Overburden

27. SCT Operations Effect of Variation in the Density of Bedding Partings and Joints

28. SCT Operations Conclusions from Initial Models  Model simulated typical subsidence of the overburden for the normal and anticipated range of overburden properties  Variation of stress, rock properties from surrounding area and the density of jointing and bedding partings did not cause the abnormal subsidence  Review of data indicated a lack of bridging in the Hawkesbury sandstone  Must be a variation in defect properties in that section of overburden ( not rock material itself)  Model simulated typical subsidence of the overburden for the normal and anticipated range of overburden properties  Variation of stress, rock properties from surrounding area and the density of jointing and bedding partings did not cause the abnormal subsidence  Review of data indicated a lack of bridging in the Hawkesbury sandstone  Must be a variation in defect properties in that section of overburden ( not rock material itself)

29. SCT Operations Stage 2 Additional Data  Definition of water table anomalies and weathered zones  Inclined and vertical boreholes for testing of bedding and joint planes in weathered and non-weathered zones  Packer testing of weathered and non-weathered zones  Estimation of joint spacing from inclined boreholes  Definition of water table anomalies and weathered zones  Inclined and vertical boreholes for testing of bedding and joint planes in weathered and non-weathered zones  Packer testing of weathered and non-weathered zones  Estimation of joint spacing from inclined boreholes

30. SCT Operations Cross Section Indicating Weathered Zone and Water Table Thanks to Ian and Xstrata for their co operation and patience during the Investigation Thanks to Ian and Xstrata for their co operation and patience during the Investigation

31. SCT Operations displacement Shear stress High stiffness ; high friction Low stiffness, low friction Shear Stiffness Characteristics

32. SCT Operations Shear Stiffness and Friction Angle Characteristics for the Weathered and Non-weathered Zones

33. SCT Operations Review of Additional Exploration Information  Water table much lower towards gorge zone  Hydraulic conductivity of the joints and bedding 100-1000 times higher about the gorge zone. Joints “open” in weathered zone  Shear stiffness and friction angle of partings in the weathered zone much lower than in non weathered zone  Major joint spacing approximately 5m  Weathered zone estimated as 100m deep  Water table much lower towards gorge zone  Hydraulic conductivity of the joints and bedding 100-1000 times higher about the gorge zone. Joints “open” in weathered zone  Shear stiffness and friction angle of partings in the weathered zone much lower than in non weathered zone  Major joint spacing approximately 5m  Weathered zone estimated as 100m deep

34. SCT Operations Review of Exploration Information  Water table much lower toward the gorge  Inference that joints and bedding had been “leached” by water flow and had reduced shear stiffness and frictional properties  Water table much lower toward the gorge  Inference that joints and bedding had been “leached” by water flow and had reduced shear stiffness and frictional properties

35. SCT Operations Strata Properties and Model Updated with New Information  Jointing 5m spacing  Normal stress regime with lower horizontal stress in weathered zone  Weathered zone 100m  Shear stiffness in weathered zone reduced to reflect measured data  Joint and bedding friction reduced in weathered zone  Jointing 5m spacing  Normal stress regime with lower horizontal stress in weathered zone  Weathered zone 100m  Shear stiffness in weathered zone reduced to reflect measured data  Joint and bedding friction reduced in weathered zone

36. SCT Operations Rock Failure Mode in the Weathered Zone

37. SCT Operations Rock Failure Mode for the “Normal” and “Weathered” Overburden

38. SCT Operations Subsidence Characteristics with Properties of the Weathered Zone

39. SCT Operations Comparison of Equivalent Panel Dimension with Subsidence Measured

40. SCT Operations Mechanics of the Process  Modified shear stiffness reduces bridging  Causes additional “weight” on the caving zone and subsidence  Caving zone modified  Occurrence at Tahmoor due to the fact that the height of the subsidence caving zone is within or close to the “weathered” bridging zone. If this geometry was different the outcome may be different  Modified shear stiffness reduces bridging  Causes additional “weight” on the caving zone and subsidence  Caving zone modified  Occurrence at Tahmoor due to the fact that the height of the subsidence caving zone is within or close to the “weathered” bridging zone. If this geometry was different the outcome may be different

41. SCT Operations Conclusions  Modified shear stiffness and frictional properties caused subsidence consistent with that measured  Most likely cause of the abnormal subsidence  This phenomenon is likely to be more widespread in dissected topography with lowered water table  Modified shear stiffness and frictional properties caused subsidence consistent with that measured  Most likely cause of the abnormal subsidence  This phenomenon is likely to be more widespread in dissected topography with lowered water table

42. SCT Operations Forward Application  Definition of water table and weathered zones is an indicator of the potential for abnormal subsidence  Measurement of hydraulic conductivity, shear stiffness and frictional properties in the weathered zone provides confirmation  Computer simulation is a reliable tool to assess the overburden subsidence characteristics  Definition of water table and weathered zones is an indicator of the potential for abnormal subsidence  Measurement of hydraulic conductivity, shear stiffness and frictional properties in the weathered zone provides confirmation  Computer simulation is a reliable tool to assess the overburden subsidence characteristics

43. SCT Operations displacement Shear stress High stiffness ; high friction Low stiffness, low friction Shear Stiffness Characteristics