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SCT Operations
Investigation into Abnormal Subsidence above a Longwall Panel in the Southern
Coalfield, Australia
Investigation into Abnormal Subsidence above a Longwall Panel in the Southern
Coalfield, Australia
Winton GaleManaging Director
SCT Operations PTY LTD, Australia
Ian SheppardEnvironment and Community Manager
Xstrata Coal, Tahmoor Colliery, Australia
Winton GaleManaging Director
SCT Operations PTY LTD, Australia
Ian SheppardEnvironment and Community Manager
Xstrata Coal, Tahmoor Colliery, Australia
SCT Operations
Location of the Mine AreaLocation of the Mine Area
SCT Operations
Picton
Mittagong
Thirlmere
Tahmoor
Tahmoor CollieryMining OperationsTahmoor 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
SCT Operations
Subsidence Characteristics Measured for Longwalls 24a and 25
Subsidence Characteristics Measured for Longwalls 24a and 25
SCT Operations
Geotechnical Investigation of the Cause of Abnormal Subsidence
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
SCT Operations
Computer Model of the Strata Section to Investigate a Range of Common Variations in the Overburden and Stress Regime
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 Data Review
SCT Operations
Data ReviewData Review
Geotechnical Investigation of the Cause of Abnormal Subsidence
Geotechnical Investigation of the Cause of Abnormal Subsidence
SCT Operations
Caving and Overburden Fracture Zones relative to Empirical Data
Caving and Overburden Fracture Zones relative to Empirical Data
SCT Operations
Regional Subsidence Measured for Longwalls 24a,b and 25Regional Subsidence Measured for Longwalls 24a,b and 25
SCT Operations
Mine Plan and General Features
Mine Plan and General Features
Horizontal stress
TSF 0.7-0.8
Overburden section
Some variation
Topographic variation
SCT Operations
UCS Range for Boreholes over the AreaUCS Range for Boreholes over the Area
SCT Operations
Large View of Longwalls 24A, B and Longwall 25 Relative to the Bargo Gorge Zone
Large View of Longwalls 24A, B and Longwall 25 Relative to the Bargo Gorge Zone
SCT Operations
Geotechnical Investigation of the Cause of Abnormal Subsidence
Geotechnical Investigation of the Cause of Abnormal Subsidence
Computer Model of the Strata Section to Investigate :- 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
SCT Operations
Computer Modelling Approach and Background
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
SCT Operations
Model Input PropertiesModel 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
SCT Operations
Generic Strength Characteristics of the Rock MaterialGeneric Strength Characteristics of the Rock Material
SCT Operations
Model Section about the Longwall AreaModel Section about the Longwall Area
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Model OutputsModel 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
SCT Operations
Rock Failure Mode for the “Normal” OverburdenRock Failure Mode for the “Normal” Overburden
SCT Operations
Model Validation Relative to Measured Subsidence
SCT Operations
Subsidence Monitoring Lines
Subsidence Monitoring Lines
Most lines are suburban streets
Most lines are suburban streets
SCT Operations
Subsidence Lines used for Validation of Computer Results
Subsidence Lines used for Validation of Computer Results
SCT Operations
Model Results
Subsidence Profile for the Range in Strength Properties
Regional Joint and Bedding Properties
SCT Operations
Comparison of the Model with Normal Subsidence Characteristics Note the two end member cases are presented
Comparison of the Model with Normal Subsidence Characteristics Note the two end member cases are presented
SCT Operations
Comparison of Subsidence Profiles for the Normal and Abnormal Subsidence
Comparison of Subsidence Profiles for the Normal and Abnormal Subsidence
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Effect of Modifying the Horizontal Stress through the Overburden
Effect of Modifying the Horizontal Stress through the Overburden
SCT Operations
Effect of Variation in the Density of Bedding Partings and Joints
Effect of Variation in the Density of Bedding Partings and Joints
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Conclusions from Initial ModelsConclusions 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)
SCT Operations
Stage 2 Additional Data 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
SCT Operations
Cross Section Indicating Weathered Zone and Water Table
Cross Section Indicating Weathered Zone and Water Table
Thanks to Ian and Xstrata for their co operation and
patience during the Investigation
SCT Operations
displacement
Sh
ea
r s
tre
ss
High stiffness ; high friction
Low stiffness, low friction
Shear Stiffness Characteristics
SCT Operations
Shear Stiffness and Friction Angle Characteristics for the Weathered and Non-weathered Zones
Shear Stiffness and Friction Angle Characteristics for the Weathered and Non-weathered Zones
SCT Operations
Review of Additional Exploration Information
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
SCT Operations
Review of Exploration InformationReview 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
SCT Operations
Strata Properties and Model Updated with New Information
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
SCT Operations
Rock Failure Mode in the Weathered ZoneRock Failure Mode in the Weathered Zone
SCT Operations
Rock Failure Mode for the “Normal” and “Weathered” Overburden
Rock Failure Mode for the “Normal” and “Weathered” Overburden
SCT Operations
Subsidence Characteristics with Properties of the Weathered Zone
Subsidence Characteristics with Properties of the Weathered Zone
SCT Operations
Comparison of Equivalent Panel Dimension with Subsidence Measured
Comparison of Equivalent Panel Dimension with Subsidence Measured
SCT Operations
Mechanics of the ProcessMechanics 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
SCT Operations
ConclusionsConclusions
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
SCT Operations
Forward ApplicationForward 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
SCT Operations
displacement
Sh
ea
r s
tre
ss
High stiffness ; high friction
Low stiffness, low friction
Shear Stiffness Characteristics
