non-conventional movements due to longwall mining
TRANSCRIPT
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Non-Conventional Movements due to
Longwall MiningProf. Zach AgioutantisUniversity of Kentucky
Lexington, KYAugust 26, 2016
Presented at the 2016 PEM Seminar
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Main Presentation Points• Conventional and Non-Conventional Movements• Where and under what conditions have Non-Conventional
Movements been observed?• Why do Non-Conventional Movements Occur?
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A typical Longwall Panel Cross Section
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Typical Subsidence and Horizontal Strain Curves
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Definition of the Limit Angle (complementary to the Angle of Draw)
(Brauner, 1973)
GOB
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Normal Mining Induced Movements above an Extracted Area
Whittaker et al (1985)
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Angle of Draw Limit or Limit for Conventional Movements
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Non-Conventional Movements
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Location angles for every point on the surface
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Conventional movements• Typically smooth in shape and can be explained by the expected
caving mechanisms associated• Vertical and Horizontal Movements within the angle of draw• Australians use 20mm (0.79”) as the limit subsidence value• Other approaches include 0.6% of Smax or other absolute values
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Non-Conventional Movements • Observed irregular ground movements are not easily explained –
some use the term “anomaly” when referring to such movements.• Movements typically inside the angle of draw
• Upsidence• Valley Closure• Downslope Movements
• Movements typically outside the angle of draw• Far Field horizontal movements
• A lot of relevant data have been collected from the Southern Coal Fields, Australia
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Non-Conventional Movements Explained (1/3)• Upsidence: Upward movement of
valley floor• Valley closure: Inward movement of
valley sides and shoulders, and most pronounced when valley is directly above longwall
Keilich (2006)
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Upsidence in Creek and River Beds• When mining occurs under creeks and /or river valleys, the observed
subsidence in the base of the creek or river is generally less than what would be expected in flat terrain. This is due to the floor of the valley buckling upwards.
Patton and Hendren 1972
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Closure in Steep-Sided Gorge and V-Shaped Valley
Waddington and Kay, 2003
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IESC, 2014
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16
Conceptual Model
Excavate longwall
Increased horizontal compressive stress from
valley formationRedistributed horizontal stress
Valley closure, upsidence & valley base failure
Keilich, 2006
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High Resolution Survey Lines to Measure Valley Closure
Walsh et al, 2014
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Downslope Subsidence due to Steep Topography• Non-conventional movements can also result from downslope
movements where longwalls are extracted beneath steep slopes. In these cases, elevated tensile strains develop near the tops of the steep slopes and elevated compressive strains develop near the bases of the steep slopes.
• The potential impacts resulting from down slope movements include the development of tension cracks at the tops of the steep slopes and compression ridges at the bottoms of the steep slopes.
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Far Field Displacements
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Areas of LW Extraction in the Vicinity of Appin and Wilton, Southern Coalfields, NSW.
Pells, 2011
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FEM used for Predicting Far Field Movements
Pells, 2011
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Horizontal Displacements Due to the Extraction of LW-16 and LW-17
Hebblewhite et al, 2001
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Computed Horizontal Movements
Pells, 2011
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Possible Causes for Far Field Displacements• Stress relaxation towards mining excavations (Hebblewhite 2009)• Horizontal movements aligned with the principal in-situ compressive
stress direction (Hebblewhite 2009)• Redistribution of the stresses in the strata between the seam and the
surface due to regional mining activity (MSEC 2009)• Perturbation of the natural horizontal stress field due to macro
changes to the stiffness of the strata above areas of total extraction (Pells 2011)
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High Horizontal Stresses – Are they responsible?
MSEC, 2007
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