1. Rock support & Reinforcement
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2. Contents Terminology Support and reinforcement principles
Support and reinforcement design
Materials and Techniques
Case-study

3. Terminology Temporary support Permanent support Primary support
Secondary support
Active support
Passive support

4. Support and reinforcement principles
Install the support and reinforcement close to the face after excavation.
There should be good contact between the rock mass and the support and reinforcement system.
Repeated removal and replacement of support and reinforcing elements should be avoided.
Minimum obstruction to the excavation and the working face.
The rock mass surrounding the excavation should be disturbed as little as possible.

5. Support & Reinforcement design
Purpose :
local support and reinforcement to support individual blocks or loosened zones on an excavation boundary;
general or systematic reinforcement in which the objective is to mobilise and conserve the inherent strength of the rock mass; and
support and reinforcement system designed to resist the dynamic loading associated with rock burst conditions.

6. Local support : Design to suspend a roof beam in laminated rock
Roof bolts may be used to suspend a potentially unstable roof beam in a laminated rock.If it is assumed that the weight of the rock in the unstable zone is supported entirely by the force developed in the rock bolts then
s = T/ γD
0.5
Where,
T=working load per bolt
γ =unit weight of the rock
D=height of the unstable zone
s = rock bolt spacing

7. Reinforced roof beam
Source :

8. General or systematic reinforcement
Here the objective is to mobilise and conserve the inherent strength of the rock mass itself.
The design approaches that may be applied in this case are
rock support interaction calculations,
the application of empirical design rules,
the use of rock mass classification-based design rules and
numerical analyses.

9. Materials & Techniques : Roof bolts
Steel bolts are frequently set in holes drilled into the rock to assist in supporting the entire roof or individual rock slabs that tend to fall. It supports the strata in one of the following ways :
It binds the thin strata together and thus increases the effective thickness of the roof beam.
It suspends the strata from the strong strata above.
It inhibits rock movement and thus the rock bolt acts to enhance the inherent strength of the rock in which it is installed.

10. Principle of action of roof bolts
The basic principles of the action of roof bolts are :
to improve the strength of the rock
to consolidate blocks or layers of rock to from a reinforced zone.
The primary action of roof bolting is to hold the rock lumps that may break off.
The second action of the roof bolt is to hold a structure in position.

11. Shotcrete
Shotcrete is the generic name for cement, sand and fine aggregate concretes which are applied pneumatically and compacted dynamically under high velocity.
Dry mix shotcrete
Wet mix shotcrete
The dry mix system tends to be more widely used in mining, because of inaccessibility for large transit mix trucks and because it generally uses smaller and more compact equipment. This can be moved around relatively easily in an underground mine environment.

12. Steel fibre reinforced shotcrete mix designs
Components
Dry mix
( % dry materials)
Wet mix
(% wet materials)
Cement 19
18.1
Silica fume additive
2.2
1.7
Blended aggregate
75.5
68.9
Steel fibres
2.7
2.6
Accelerator
0.6
Superplasticizer -
0.3
Water
Controlled at nozzle
7.7
Source :

13. Source : http://shotcrete-machine
Source : (Visited : 7/9/11)

14. Source : http://shotcrete-machine
Source : (Visited : 7/9/11)

15. Truck Mounted Shotcrete Robot
Source : (Visited : 7/9/11)

16. Comparison of features of Dry mix &Wet mix Shotcrete process
Dry-Mix Process
Wet-Mix Process
Mixing water instantaneously at the nozzle to meet variable field conditions
Mixing water controlled at plant
Longer hose lengths possible
Normal Pumping distances necessary
Intermittent use easily accommodated
for continuous application of shotcrete
Exceptional Strength performance possible
Lower strengths
Lower Production rates
Higher Production rates
Equipment maintenance cost tends to be lower
Equipment maintenance cost tends to be higher
Source :

17. Case Study

18. Name of the Mine: GDK 8A incline of Ramagundam - II Area (SCCL).
Source : Google maps

19. Details of site Bord and pillar was the system of mining in this mine.
The gallery width was 3.6 m with working height of 1.8 m to 2.2 m.
The borehole section shows that the immediate stratum above the coal seam is shale (0.3 m thick).
A Shale roof is normally riskier from a support point of view. Therefore, it was necessary to leave 1.0 m coal in the roof.
The average layer thickness of coal was 6.5 cm. The coal had two sets of cleats (perpendicular to each other).

20. The field investigation showed that at some locations the roof was not stable. Roof failure occurred from the joint plane of the cleats and not because of failure of the bolts-strata interaction.
This indicated that the induced horizontal tensile stress (developed due to excavation of the gallery) played an important role in the roof failure. Its (horizontal tensile strength) value exceeded the coal's (rock mass, considering cleats) tensile strength.

21. Existing roof bolt support
The primary support system used in the mine was full column, anchored, grouted, roof-bolting.
Four roof bolts of 1.8 m in length were used in a row of gallery at 3.6 m wide.
The spacing of the bolts was 0.3 m m m m m from one side to the other side of the roadway.
The row intervals were kept at a distance of 1 m.
The design capacity of the roof bolt was up to 6 t, and the pull out anchorage test had a given strength of 14 t.
In the junctions, which are immediate out bye, a 25% additional support was provided.
A W-strap was used wherever roof condition was weak.

22. Results
The support design was done using both the conventional and numerical methods. It is concluded from the field investigation that the roof failure condition mainly depended on the structural features of the roof strata. Conventional approach for support design gave the approximate required support. The numerical modelling for the present case was developed to simulate the roof behaviour in terms of induced horizontal stress.

23. Conclusion
Rock bolt acts to enhance the inherent strength of the rock in which it is installed.
Roof bolts are used for supporting roadways underlying weak strata.
The thickness and quality og the shotcrete depends on the rock conditions.
Shotcrete support is used in supporting permanent openings such as ramps, shaft stations etc.
Support should provide stability and maintain the load carrying capability of rock of an underground excavation.
Support should be verified with the empirical formula.

24. References
Brady,B.H.G. and Brown,E.T.(2006) Rock Mechanics for underground mining.(3rd Ed.) Kluwer Academic Publisher.Boston.pp
Papworth, F Design guidelines for the use of fiber-reinforced shotcrete for ground support. American Shotcrete Assn Shotcrete Magazine, Spring.
[Accessed 6/9/2011]
support.pdf [Accessed 7/9/2011]
7.pdf [Accessed 6/9/2011]
_Shotcrete_support.pdf [Accessed 7/9/2011]
Sharma, S. K., Jaiswal,A. And Shrivastva,B.K.(2006)” Roof Support Design of an Underground Coal Mine – A case study”, Coal International. July/August pp