Rock support & Reinforcement MINE PORTAL TEAM www.MINEPORTAL.in CALL/WHATSAPP-8804777500 www.fb.com/mineportal.in

Contents Terminology Support and reinforcement principles Support and reinforcement design Materials and Techniques Case-study

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

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.

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.

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

Reinforced roof beam Source :http://140.194.76.129/publications/eng-manuals/em1110-2-2901/c-7.pdf

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.

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.

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.

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.

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 : http://ezinearticles.com/?Shotcrete-Components-in-Mix-Design&id=3251343

Source : http://shotcrete-machine Source : http://shotcrete-machine.com/Shotcrete-Machine/Dry-mix/BBDS-3-Dry-mix-shotcrete-machine.html (Visited : 7/9/11)

Source : http://shotcrete-machine Source : http://shotcrete-machine.com/Shotcrete-Machine/Wet-mix/BBDS-4-Wet-mix-shotcrete-machine.html (Visited : 7/9/11)

Truck Mounted Shotcrete Robot Source : http://www.gunitesupply.com/Gunite-C-10.html (Visited : 7/9/11)

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 : http://constructionz.com/articles/introduction-to-shotcrete-applications.html

Case Study

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

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).

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.

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 - 1.0 m - 1.0 m - 1.0 m - 0.3 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.

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.

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.

References Brady,B.H.G. and Brown,E.T.(2006) Rock Mechanics for underground mining.(3rd Ed.) Kluwer Academic Publisher.Boston.pp.312-346. Papworth, F. 2002. Design guidelines for the use of fiber-reinforced shotcrete for ground support. American Shotcrete Assn Shotcrete Magazine, Spring. http://www.cdc.gov/niosh/mining/pubs/pdfs/aorbs.pdf [Accessed 6/9/2011] http://www.rockmass.net/ap/45_Barton-et-al_on_Design_tunnel_ support.pdf [Accessed 7/9/2011] http://140.194.76.129/publications/eng-manuals/em1110-2-2901/c- 7.pdf [Accessed 6/9/2011] http://www.rocscience.com/hoek/corner/16 _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 14-18.