Health and Safety Executive
Evaluation of tensioned and non-tensioned long tendon reinforcement in UK deep mining conditions
Prepared by Rock Mechanics Technology Limited for the Health and Safety Executive 2010
RR831 Research Report
Health and Safety Executive
Evaluation of tensioned and non-tensioned long tendon reinforcement in UK deep mining conditions
David Bigby, PhD, BSc (Hons) Ken Hurt, PhD, BSc (Hons) Chris Reynolds, BSc (Hons) Robert Brown, BSc (Hons)
Rock Mechanics Technology Ltd Bretby Business Park Ashby Road Burton-on-Trent Staffordshire DE15 0QD
A research programme has been carried out by RMT in support of the revision of Part 2 of the British Standard for strata reinforcement components in coal mines, covering flexible systems for roof reinforcement. This continued work commenced under a previous HSE Project, Testing and standards for reinforcement consumables.
A particular focus was to compare tensionable and non-tensionable reinforcement systems, prompted by the introduction of tensionable systems to British coal mines. A review of previous research indicated conflicting claims for tensionable systems in terms of theoretical advantages and practical experience. The research included laboratory testing, underground measurement and analysis of underground monitoring data. Advice and draft Annexes were provided to the BS Committee and a revision of the DMCIAC guidance on the use of cable bolts to support roadways in coal mines drafted. The work highlighted practical problems concerning application of the tensionable systems in use in UK coal mines but did not exclude their future applicability provided they comply with the revised Standard.
This report and the work it describes were funded by the Health and Safety Executive and co-funded by the EU Research Fund for Coal and Steel. Aspects were also co-funded by UKCoal Ltd and various manufacturers through supply of materials for testing. The reports contents, including any opinions and/ or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy nor the opinions of any of the co-funding parties.
HSE Books
Crown copyright 2010
First published 2010
You may reuse this information (not including logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit www.nationalarchives.gov.uk/doc/open-government-licence/, write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email [email protected].
Some images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to [email protected].
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CONTENTS
EXECUTIVE SUMMARY vii
1. INTRODUCTION 1
2. THEORETICAL ASPECTS OF TENSIONED TENDONS 3
2.1 Rockbolting and Tensioning Practice 3
2.2 Analysis of the Pretensioning Effect 9
2.3 Evidence for the Pretensioning Effect 12
2.4 Summary 15
2.5 Bibliography 17
3. LABORATORY AND UNDERGROUND TESTING OF LONG TENDONS 21
3.1 Grout Encapsulation Testing 21
3.2 Variation of Bond Performance with Grout Strength 24
3.3 Field Sample Grout Testing 25
4. UNDERGROUND MONITORING OF PERFORMANCE AND EFFECTS OF TENSIONED LONG TENDONS 27
4.1 Historical Data Analysis 27
4.2 Underground Monitoring of Long Tendon Tensioning at Colliery A 30
5. IMPROVED MODELLING OF FLEXIBLE LONG TENDONS 37
5.1 Representation of Tensioned Tendons and Truss Systems 37
5.2 Application to Maingate Support on Drivage at Colliery C 39
5.3 Application to Maingate Support on Retreat at Colliery C 42
5.4 Conclusions on Modelling Flexible Long Tendons 44
6. ADVICE AND DRAFTS PROVIDED TO B S REVISION COMMITTEE 47
6.1 Advice and Recommendations 47
6.2 Draft Test Procedures / Annexes 48
7. LABORATORY TESTING OF ALTERNATIVE RIB REINFORCEMENT
SYSTEMS 51
7.1 Rib Reinforcement Systems Tested 51
7.2 Test Procedures 52
7.3 Test Results and Discussion 53
7.4 Conclusions and Recommendations 60
8. SUGGESTED REVISION OF DMCIAC CABLEBOLTING GUIDANCE
DOCUMENT 63
8.1 Introduction 63
8.2 Suggested Revision of DMCIAC Cablebolting Guidance 63
8.3 Revised Cablebolting Guidance Appendix 1 70
8.4 Revised Cablebolting Guidance Appendix 2 72
8.5 Revised Cablebolting Guidance Appendix 3 74
8.6 Revised Cablebolting Guidance Appendix 4 76
9. CONCLUSIONS AND RECOMMENDATIONS 79
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v
83 10. REFERENCES
FIGURES
Figure 2.1 Comparison of the load distribution of a normal anchor and a single bore multiple anchor (after Barley and Windsor, 2000) 85
Figure 2.2 The buckling beam concept for roof stability (after Strata
Engineering, 2001) 86
Figure 2.3 External load applied to a pretensioned bolted joint 87
Figure 2.4 External load applied to a pretensioned lifting bolt 88
Figure 3.1 Laboratory short encapsulation pull test results for Megastrands and CBG grout at 1 and 3 days curing 89
Figure 3.2 Laboratory short encapsulation pull test results for Megastrands
and CBG grout at 7 and 42 days curing 90
Figure 3.3 Variation of maximum load, bond strength and system stiffness
with grout unconfined compressive strength (cube samples) 91
Figure 3.4 Variation of unconfined compressive strength with density
for bottle samples of CBG grout obtained from UK coal mines 92
Figure 3.5 Variation of unconfined compressive strength with density
for bottle samples of HPRG grout obtained from UK coal mines 93
Figure 4.1 Schematic of 10s maingate, colliery C 94
Figure 4.2 Monitoring results for station 4, 10s main gate, colliery C 95
Figure 4.3 Monitoring results for station 3, 10s main gate, colliery C 95
Figure 4.4 Monitoring results for station 2, 10s main gate, colliery C 96
Figure 4.5 Monitoring results from type B telltales, 10s main gate, colliery C 97
Figure 4.6 Monitoring results from cablebolt type A telltales,
10s main gate, colliery C 98
Figure 4.7 Monitoring results from cablebolt type B telltales,
10s main gate, colliery C 99
Figure 4.8 Monitoring results from type A telltales, 22s main gate, colliery C 100
Figure 4.9 Combined displacement from type A and B telltales,
570, 590 and 610 MM, 19s tail gate, colliery C 101
Figure 4.13 Approximate position of installed instruments on 21 March 2007 105
Figure 4.14 Approximate position of installed Megastrands and tensioning on
Figure 4.10 Schematic of T18s, face line, colliery B 102
Figure 4.11 Planned support pattern in widened face line 103
Figure 4.12 Section of widened face line support and instrumentation pattern 104
27 March 2007 106 Figure 4.15 Proposed Megastrand monitoring instrumentation, 302s face line
March 2007 107
Figure 4.16 Time trend for cablebolt telltale number 13 at 196 m 108
Figure 4.17 Colliery C face line station 6 at 192 m
Figure 4.20 Sonic extensometer 196 m, displacement & strain against
109
Figure 4.18 Colliery C 302 face 197 m CL 297 RREXTO1 110
Figure 4.19 Colliery C 302 face 197 m CL 297 RREXTO1 cont. 111
distance into strata 112
Figure 4.21 Sonic extensometer 196 m, displacement against time 113
Figure 4.22 Sonic extensometer 196 m, displacement against time during
Megastrand tensioning period 114
Figure 4.23 Strain gauged rockbolt 1. Mean microstrain and microstrain
difference against distance along bolt 115
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Figure 4.24 Strain gauged rockbolt 2. Mean microstrain and microstrain difference against distance along bolt 116
Figure 4.25 Strain gauged rockbolt 3. Mean microstrain and microstrain difference against distance along bolt 117
Figure 4.26 Strain gauged rockbolt 4. Mean microstrain and microstrain difference against distance along bolt 118
Figure 4.27 Strain gauged rockbolt 5. Mean microstrain and microstrain difference against distance along bolt 119
Figure 4.28 Strain gauged rockbolt 6. Mean microstrain and microstrain
difference against distance along bolt 120
Figure 5.1 Modelled strata sequence 3.2 m roof mudstone 121
Figure 5.3 Modelled roof displacements with roof bolts failed and no
Figure 5.4 Modelled roof displacements with roof bolts failed and pretensioned
Figure 5.5 Modelled roof displacements with roof bolts failed and untensioned
Figure 5.6 Modelled roof displacements with roof bolts failed and tensioned
Figure 5.7 Shear strains and bolt loads with pretensioned flexible bolts
Figure 5.8 Shear strains and bolt loads with pretensioned truss system as
Figure 5.2a Coal strength properties 121
Figure 5.2b Siltstone strength properties 121
Figure 5.2c Mudstone strength properties 121
additional support 122
flexible bolts as additional support 122
flexible bolts as additional support 123
truss system as additional support 124
as additional support 125
additional support 125
Figure 5.9 Proposed bolt pattern at development face 126
Figure 5.10 Strata sequences 126
Figure 5.11 Roof condition with proposed support and sequence A 127
Figure 5.12 Roof condition with proposed support and sequence B 128
Figure 5.13 Alternative bolt patterns at development face 129
Figure 5.14 Ro
