1. Clarifying Cable Bolting Practices and Principles Mine Managers Association of Australia 2012 Annual Seminar 7 – 8 June 2012 Jim Galvin

2. Characterised currently by: Controversy Contradictions Conflicts Competition = CONFUSION Amongst practitioners Amongst those statutorily accountable = RISK  2012 JM Galvin - 2 Support Mechanics and Design

3. Objective Take one of these issues – cable bolting - and provide a basic understanding of cable bolting principles and practices to assist mine managers in evaluating advice relating to cable bolting: Pattern Anchorage Length  2012 JM Galvin - 3

4. Some Basic Principles  2012 JM Galvin - 4

5. Tendon Support Systems Tendon All forms of bar, tube, cable (wire stand) support systems anchored to the rock mass in boreholes Fully Encapsulated End Anchored Friction Anchored  2011 JM Galvin - 5

6. Stiffness  2012 JM Galvin - 6 Yield Ultimate Stiff Soft Unencapsulated Cable Length Extension at Yield (1%) Extension at Ultimate (6%) 110 mm60 mm 440 mm240 mm 880 mm480 mm

7. Fully Encapsulated Tendons  2011 JM Galvin - 7 Load Transfer For parting to open, rock mass on free side of parting has to slide past the tendon This generates shear stress in the encapsulation medium, or grout, which is then transmitted to the tendon where it generates axial load The converse then occurs on the opposite side of the parting Tension created in tendon acts to clamp parting and resist further opening Clamping force increases with displacement across parting, until peak load capacity of a support system component is exceeded

8. Fully Encapsulated Tendons Considerations Load Transfer Exponential axial load transfer  2011 JM Galvin - 8 Signer, 1990 Bar Cable Moosavi et al, 2002

9. Fully Encapsulated Tendons Q. What does face plate display?  2011 JM Galvin - 9

10. Fully Encapsulated Tendons Location of Parting Multiple parting planes  2011 JM Galvin - 10

11. End Anchored Tendons  2011 JM Galvin - 11 Load Transfer

12. Pre-tensioning  2012 JM Galvin - 12

13. Learnings A given amount of displacement, or opening of a parting, generates a much higher load, or resistance against further displacement, in a fully encapsulated tendon than in an end anchored tendon. That is, fully encapsulated support systems are stiffer. A broken fully encapsulated cable can still provide considerable confinement across partings. An end anchored cable is susceptible to unloading if the rock around the collar crushes, or the face assembly is damaged. Pre-tensioning significantly increases the stiffness of the loading system, thereby generating higher reaction load for a given amount of displacement  2012 JM Galvin - 13

14. Tendons Support deadweight load - Suspension Increase self supporting capacity of rock mass – Reinforcement  2011 JM Galvin - 14 What do tendons do?

15. Suspension  2011 JM Galvin - 15

16. Suspension

18. Pressure Arch Conceptual Model Pressure Arch: Transfers weight of overlying strata above arch to the abutments Immediate roof relies on its own capacity to span the excavation Immediate roof strata bends, or sags, due to: Its own weight (transverse load) Axial (horizontal) stress Expansion of failing strata in the roof (transverse load)  2011 JM Galvin - 18

19. Beam Theory  2011 JM Galvin - 19 Beam with Clamped Edges Sag = B 1/ t 2 Tensile stress = B 2/ t Where B 1, B 2, are proportional factors

20.  2012 JM Galvin - 20 Reinforcement – Laminated Strata In order to sag, layers have to slide past each other No sliding at centre of each beam

21. Learnings Principle of beam building is to prevent sliding between layers so as to increase the effective thickness of beam. If four beams of thickness t/4 are formed into one composite beam of thickness t: Maximum deflection (sag) is reduced 16 fold Maximum horizontal stress is reduced 4 fold  2011 JM Galvin - 21

22.  2012 JM Galvin - 22 Tendon Reinforcement Pattern No Change in Length and so no change in clamping force Compressed- Tendon shortened and so unloaded Stretched a lot, to generate very high clamping force

23.  2011 JM Galvin - 23 Impact of Bolt Location and Patterns (Spann et al, 1983) Number of Bolts Tendon Reinforcement Pattern

24.  2011 JM Galvin - 24 Tendon Angle Effect of Tendon Inclination on Shear Resistance Generated by the Tendon (After Windsor et al, 1993)

25.  2011 JM Galvin - 25 Example of Effectiveness of Angled Bolts in Controlling Failure of a Laminated Coal Roof Tendon Angle

26.  2012 JM Galvin - 26 Tendon Reinforcement Pattern Eastern Distributer Motorway, Sydney

27. Angled Tendons  2011 JM Galvin - 27

28. Angled Tendons  2011 JM Galvin - 28

29. Learnings In a laminated roof environment: Shear stresses increase towards flanks of excavations. Therefore, tendons provide most resistance to shear when concentrated towards flanks of excavation. Tendons angled outwards (towards flanks) load up more for a given amount of shear displacement Tendons angled inwards (towards centre of excavation) initially unloaded by shear movement.  2012 JM Galvin - 29

30. Reinforcement Mechanics What naturally determines resistance to sliding?  2012 JM Galvin - 30 (Galvin, 1990)

31. Bolting Mechanics How do tendons modify resistance to sliding?  2012 JM Galvin - 31 1. Increase  n by prestressing at time of installation Applies to both fully encapsulated tendons and end anchored tendons

32. Bolting Mechanics How do tendons modify resistance to sliding?  2012 JM Galvin - 32 2. Increase  n due to axial force developed by sliding along parting Develops earlier in fully encapsulated tendons – less sag Develops faster in tendon angled towards rib – less sag Benefits lost in bolts angled towards roadway centre

33. Bolting Mechanics How do tendons modify resistance to sliding?  2012 JM Galvin - 33 3. Increase  n due to axial force developed by opening of parting Applies to both fully encapsulated tendons and end anchored tendons

34. Bolting Mechanics How do tendons modify resistance to sliding? 4. Increase C o due to lateral component of axial force Develops earlier in fully encapsulated tendons Develops faster in bolts angled towards rib Lost in bolts angled towards roadway centre

35. Bolting Mechanics How do tendons modify resistance to sliding?  2012 JM Galvin - 35 5. Increase C o due to lateral resistance to shear provided by dowel Develops earlier in fully encapsulated tendons

36. Reinforcement Mechanics How do tendons modify resistance to sliding?  2012 JM Galvin - 36 What gives the most benefit? Say- 30 tonne (300 kN) Ultimate Tensile Strength tendon Shear strength = 50% UTS = 150 kN 5 tonne (50 kN) pretension =  n1 Bolts 1m apart, 2m row spacing = 2 m 2 per bolt Therefore C 5 = 150/2 = 75 kPa  n1 = 50/2 = 25 kPa

37. 30 t UTS bolt 5 t pretension  n1 (kPa) ()()  = Tan(  )  f =  n Tan (  ) (kPa)  c = C 5 Slickensided25170.37.575 Shale2526.60.512.575 Sandstone25451.02575  2012 JM Galvin - 37 In a laminated (sliding) setting: Dowelling benefit far out-weighs pretension benefit Provided tendon is fully encapsulated Reinforcement Mechanics

38. What about C 4,  n2 and  n3 ? Can get a variety of outcomes depending on properties of rock mass, angle of bolt, bolt diameter, whether bolt is fully encapsulated in hole.  2012 JM Galvin - 38 Reinforcement Mechanics

39. Why Install Centre Tendons?  2012 JM Galvin - 39 Most obviously, to reduce effective span – sag is proportional to “span x span x span x span” Limit span between bolts of immediate roof plies so they do not fall under their own weight Suspend delaminated immediate roof plies to higher stable strata – using cables

40. Why Fully Encapsulate Centre Cables if They are not Subjected to Shear?  2012 JM Galvin - 40 They may be subjected to shear, as loading regime is not always symmetrical. Varies in mining cycle. They have superior load/displacement characteristics to end anchored tendons (any claims to the contrary should be tested to confirm that tendons, rock properties, anchorage characteristics and installation mediums are comparable). They are not susceptible to unloading as a result of damage and rock crushing at the collar. They can still perform a support function in a broken state.

41. When might a Centre Cable not be Fully Encapsulated?  2012 JM Galvin - 41 To avoid breaking cable bolt in a high displacement environment. Requires strata to reach a quasi state of equilibrium with sag An option is to allow a certain amount of “stretch” (sag) before then encapsulating cable to make it stiffer and give it a higher resistence to further displacement

42. When might a Centre Cables not be Fully Encapsulated?  2012 JM Galvin - 42 8m long cable Yield load = 60 t Elongation at yield = 1% Elongation at ultimate load = 6% 2m encapsulated, 6m free Elongation at yield = 0.01 x 6 m = 60 mm Elongation at failure = 360 mm Fully encapsulated Load transfer rate either side of parting = 60 t/m Therefore, at yield, load is distributed over a length of 2 m, so elongation at yield = 20 mm. The same displacement in the 6 m free end of an end anchored cable would give a strain of 20/6000 = 0.33%, at which point the load in the cable would be only 20 t.

43. When might a Centre Cables not be Fully Encapsulated?  2012 JM Galvin - 43 The end anchored cable can sustain 60 mm of displacement before reaching yield as compared to 20 mm for the fully encapsulated cable. Some mines do not immediately fully encapsulate in order to allow rock to relax and to reach a quasi state of equilibrium without cable going into yield, before then fully encapsulating cable. In this example, if the cable were not to be fully encapsulated until after 20mm of convergence, it could still sustain another 14mm of convergence before going into yield.

44. Conclusions When it comes to support design, there is no “one size fits all”. There are many and varied factors that impact on the design and performance of cable bolting systems. These should be assessed through the application of applied mechanics supported, preferably, by numerical modelling. Some designs need to accepted with caution. Whilst they may have application in some circumstances, both theoretical considerations and operational experience suggest that, in general, the benefits associated with them are not commensurate with the risk presented by pushing the limits to such an extent. Subject all ‘novel’ advice to robust independent risk assessment Third party review Premised on data Supported, if necessary, by numerical modelling Subject to cost/benefit analysis in respect of material and personal consequences if design fails – Is it worth the risk?  2012 JM Galvin - 44