ICGCM Papers:
Fundamental Research Studies
 
 
Evaluation of Drill Hole Geometry on Bolted Roof Beam Performance
35th International Conference on Ground Control in Mining
Evaluation of Drill Hole Geometry on Bolted Roof Beam Performance
by
David R Burkhard, San Juan Coal Company, Waterflow, United States
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Fundamental Research Studies
[Author] David R Burkhard, San Juan Coal Company, Waterflow, United States
[Abstract] 
Key Conclusions:
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In seeming contradiction to published pull test results, when conducted in an active development setting, bolted roof beam delamination was consistently less in the smooth walled study area then in the grooved walled study area. Factors such as bolt type, bolted lithology, and grouted length must be accounted for, however enlargement of the effective drill hole diameter as a result of incising a groove in the drill hole may be of greatest importance. This increase in effective diameter may contribute to a decrease in resin mixing during bolt installation. The use of resin dams for short encapsulation pull tests may mask this fact, becoming evident in an actual development setting.
Key Findings:
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Analysis of bolted roof beam delamination data yielded no statistical difference in roof beam performance between areas utilizing grooved drill hole geometry verse areas with smooth walled drill hole geometry. Analysis of delamination measurements taken a 2 ft., 5 ft., and 8 ft. heights within the roof consistently yielded less delamination in the area supported with smooth walled drill holes compared to the area utilizing grooved drill holes. Mean total roof delamination from zero to 8 feet was 11.1 mm greater for grooved drill holes than for smooth walled drill holes. Condensing of data for statistical analysis resulted in low confidence levels and contributed to a finding of no statistical difference. Short encapsulation pull tests did indicate grooved drill holes have greater pull strength then smooth walled, though the difference was significantly less then previously reported. As grouted bolt length is increased, pull strength differences are decreased. At 2.5 ft. of grouted length, bolts in both grooved and smooth walled drill holes pulled to the region of plastic deformation with no slippage.
Objective of the Paper:
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To assess the potential strengthening of the bolted roof beam by utilizing a modified drill hole geometry during roof bolt installation in a development setting. Comparative practical and statistical analysis of bolted roof beam performance, as measured by roof beam delamination. Delamination of the bolted roof beam was measured as a function of longwall face position and grooved verse smooth walled drill hole geometry. Verification of prior short encapsulation pull test results and influences of roof bolt type and grouted bolt length on these results
Problem Statement:
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Prior work has indicated modifying roof bolt drill hole geometry through the introduction of a spiral groove in the drill hole wall results in an increase in roof bolt pull out strength based on short encapsulation pull tests. Analysis of bolted roof beam performance when modified drill hole geometry is utilized in a development setting is requisite for further assessment of this technique.