influence of roof/floor interface on coal pillar performance

Influence of Roof/Floor Interface on Coal Pillar

PerformanceDr. Kyle A. Perry, P.E., Assistant Professor

Dr. Kot F. Unrug, Emeritus ProfessorKevin W. Harris, GRA

Michael J. Raffaldi, GRA

An Equal Opportunity University

Overview Background Purpose Modeling Parameters Important Results

Pillar Strength Pillar Behavior Interfaces

ARMPS SF vs. CMRR Design Recommendations


Background Empirical

ARMPS AMSS ALPS

Analytical Wilson Equation

Numerical Finite Difference (FLAC3D) Boundary Element (LaModel)


BackgroundLoad Estimation

Not InvestigatedPillar Strength

Focus Area In-situ Coal Strength Pillar Geometry

Mark-Bieniawski (Mark, 1999)


BackgroundContributing Factors

Coal Strength Pillar Geometry Roof/Floor Properties

Strength Composition Integrity

Pillar Contacts

Behavior


PurposeInvestigate

Coal Pillar Strength Resultant Behavior

Variable Interface Properties

Numerical Study - FLAC3D

W/H Ratios: 4, 7, and 10 Lithology: Massive Sandstone, Shale, Fireclay

Fireclay Interface Constant


Model GeometryIncorporate Symmetry

¼ Pillar Section Increased Zone Density Better Accuracy

Entry Width – 20’Extraction Height – 6’Model Boundaries

Bottom – Fixed BC Sides – Roller BC


Modeling ParametersInitial Conditions

2:1 Vertical:Horizontal Stress 50% Mark-Bieniawski Strength

Next Phase Applied Velocity

-5 x 10-5 ft/step FISH Function

Average Pillar Stress Average Pillar Strain


Material K (KSI) G (KSI) σt (PSI) c (PSI) φ (Deg)

Sandstone 1350 1140 208 1650 41

Sandstone (Floor) 1600 1350 208 1030 41

Shale 722 394 150 1000 30

Shale (Floor) 642 296 150 625 30

Coal 134 375 22 235 25

Fireclay 750 160 50 100 20

Material Properties


Table 1: Strata Material Properties (Lu et al, 2008, and Budhu, 2011)

Constitutive Models


Plastic Strain Cohesion (PSI)0.000 2350.100 591.000 59

Table 2: Coal Seam Cohesion vs. Plastic Strain (Lu et al, 2008)

Plastic Strain Friction Angle ()0.000 35.00.005 32.50.500 30.0

Table 3: Coal Seam Friction Angle vs. Plastic Strain (Itasca, 2009)

Rock

Linear Elastic

Mohr-Coulomb Model

Coal

Non-linear Elastic

Strain-Softening Model

Plastic Strain Dependency

Interface PropertiesNon-linear Coulomb (Iannacchione, 1990)

Reduce Strength Initiate Slip

Bi-linear Function (Peng et al., 1983) = 20 = 10 = 500 PSI

Cohesion 75 PSI 150 PSI 300 PSI


Interface Properties


0 250 500 750 10000

100

200

300

400

500

600

Weak Moderate Strong Fireclay

Interface Normal Stress (PSI)

Inte

rface

She

ar S

tress

(PS

I)

Pillar Behavior and StrengthFocus Areas

Pillar Stress-Strain Pillar Strength Plastic Behavior

Strain-Softening Perfectly Plastic Strain-Hardening

Roof/Floor Strength Influence Interface Influence


Pillar Stress-Strain


0.0000 0.0025 0.0050 0.0075 0.01000

625

1250

1875

2500

Strain (in/in)

Stre

ss (P

SI)

0.0000 0.0025 0.0050 0.0075 0.01000

625

1250

1875

2500

Strain (in/in)

Stre

ss (P

SI)

0.0000 0.0025 0.0050 0.0075 0.01000

500

1000

1500

2000

2500

Strain (in/in)

Stre

ss (P

SI)

Shale Sandstone Fireclay

W/H 4 Coal Pillar

75 PSI

150 PSIMark-Bieniawski Pillar

Strength

300 PSI

Interface

Cohesion



0.000 0.006 0.012 0.018 0.0240

1000

2000

3000

4000

Strain (in/in)

Stre

ss (P

SI)

0.000 0.006 0.012 0.018 0.0240

1000

2000

3000

4000

Strain (in/in)

Stre

ss (P

SI)

0.000 0.006 0.012 0.018 0.0240

1000

2000

3000

4000

Strain (in/in)

Stre

ss (P

SI)

Shale FireclaySandstone

W/H 7 Coal Pillar

75 PSI


Strength

300 PSI

Interface

Cohesion



0.000 0.004 0.008 0.012 0.0160

1500

3000

4500

6000

Strain (in/in)

Stre

ss (P

SI)

0.000 0.004 0.008 0.012 0.0160

1500

3000

4500

6000

Strain (in/in)

Stre

ss (P

SI)

0.000 0.004 0.008 0.012 0.0160

1500

3000

4500

6000

Strain (in/in)

Stre

ss (P

SI)

SandstoneShale Fireclay

W/H 10 Coal Pillar

75 PSI


Strength

300 PSI

Interface

Cohesion

Pillar Strength Results


W/H 7 Pillar 75 PSI Interface

Shale Sandstone Fireclay0

500

1000

1500

2000

2500

3000

3500

4000

75 150 300

Pilla

r Str

engt

h (P

SI)

Summary of ResultsW/H 4 Pillar

Strain-Softening Mark-Bieniawski

W/H 7 Pillar Perfectly Plastic (Transition) Mark-Bieniawski

W/H 10 Pillar Strain-Hardening Functional Strain


Important FindingsRoof/Floor Composition

Mechanical Properties - Secondary Roof/Floor Pillar Contact

Interface Properties - DominantExplanation

Strong Interface - High Confinement Weak Interface - Low Confinement


Fireclay Floor ResultsStrength Reduction

Average 22%Higher W/H, More Effect

W/H 10 Strength Reduced 26%Su & Hasenfus, 1997

Weak Roof/Floor Peak 30% Reduction

Reason Poor Interface Strength


ARMPS SF vs. CMRRDatabase

645 Total Cases Limited Statistics (Mark, 2010)

CMRR Classification 330 Definitive CMRR Ratings Weighted – High SF Diversity

Logistic Regression


ARMPS SF vs. CMRRResults

Minimal Correlation Mark & Barton, 1997

Supports Model ResultsCMRR

Roof Quality – Excellent Support Requirements

Pillar Strength – Poor Correlation – Interface Quality


0 10 20 30 40 50 60 70 80 90 1000

1

2

3

4

5

6

7

8

SuccessFailure

CMRR

Stab

ility

Fac

tor

Design RecommendationsPurpose

Site-Specific Parameters Importance Advantageous

Increase Accuracy Pillar Strength Pillar Behavior

Result In-situ Coal Strength Modification

900 PSI Constant


Design RecommendationsQuantify Roof/Floor Material Properties

Triaxial TestingInterface Properties

Direct Shear Testing Case Studies

Reasonable Thresholds

Boundary Conditions Stress Field

Horizontal Stress


Design RecommendationsSolution

Calculate Average Pillar Strength Back-Calculate In-Situ Coal Strength Representative of Conditions

Correlation Mark-Bieniawski Equation Software

ARMPS, ALPS, AMSS LaModel


ConclusionsRoof/Floor Interface

Significant Higher W/H

Pillar Strength Pillar Mechanical Behavior Confinement Partings

Roof/Floor Strength Secondary Supported by ARMPS SF vs. CMRR


ConclusionsDesign Recommendations

In-situ Coal Strength Modification Implementation

ARMPS, AMSS, ALPS, LaModel

Limitations Single Pillar vs. Panel Span Mechanical Properties

Roof/Floor Deformation Stress Distribution

Interface Classification System


References• Budhu, M. (2011). Soil Mechanics and Foundations. 3rd Ed. Hoboken, NJ: University of Arizona, p. 724.• Heasley, K. (2007).“LaModel Calibration.” In: WVU College of Engineering and Mineral Resources Faculty,

Staff, and Course Web Server. [http://web.cemr.wvu.edu/~kheasley/LaModelDownloads/Documents/Details/Calibration.pdf].

• Iannacchione, A.T. (1990). “The effects of roof and floor interface slip on coal pillar behavior.” In: Proceedings of 31st U.S. Symposium on Rock Mechanics. Golden, CO: Colorado School of Mines, pp. 153–160.

• Itasca Consulting Group, Inc. (2009). FLAC3D Version 4.0 Example Applications. Minneapolis, MN, pp. 2-1–2-15.

• Lu, J., Ray, A., Morsy, K., Peng, S. (2008). “Effects of rock/coal interface property on coal pillar strength.” In: Proceedings of the 27th International Conference on Ground Control in Mining, pp. 262–267.

• Mark, C. (1999). “Empirical methods for coal pillar design.” In: Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design, NIOSH IC 9448, pp. 145–154.

• Mark, C. (2010). “Pillar design for deep cover retreat mining: ARMPS Version 6 (2010).” In: 3rd International Workshop on Coal Pillar Mechanics and Design. ICGCM Pillar Design Workshop. Morgantown, WV, pp. 106–121.

• Mark, C., Barton, T.M. (1997). “Pillar design and coal strength.” In: Proceedings of the New Technology for Ground Control in Retreat Mining, NIOSH IC 9446, pp. 49–59.

• Peng, S.S. (2008). Coal Mine Ground Control. 3rd Ed. Morgantown, WV: West Virginia University, p. 78.• Peng, S.S., Patrick, C.W., Khair, A.W. (1983). “Direct shear strength of Appalachian coals.” Geotechnical

Testing Journal. 6(3): 144–150. • Su, D.W.H., Hasenfus, G.J. (1996). “Coal pillar strength and practical coal pillar design considerations.” In:

15th Conference on Ground Control in Mining. Golden, CO: Colorado School of Mines, pp. 155–162. • Su, D.W.H., Hasenfus, G.J. (1997). “Effects of in-seam and near-seam conditions and asymmetric pillar

loading on coal pillar strength.” In: 16th Conference on Ground Control in Mining. Morgantown, WV: West Virginia University, pp. 329 –344.


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