Difference between revisions of "Ground support"

Revision as of 16:16, 11 February 2011

From Queen's University Mine Design Wiki

This article is about the application of ground support strategies and techniques to "underground mining projects". For the article on general geotechnical design see Geotechnical design.

Ground support is a set of techniques, elements and methods that enable the conservation or mobilization of a rockmass' initial strength. The rockmass is given the ability to self-support throughout the course of mining. The ultimate goal of this practice is to allow for mining to occur safely while maintaining the stability of created underground excavations.These techniques can be divided into two main categories ^[1].

Support techniques refer to the action of applying an external reactive force to a rock surface in the process of deformation, i.e. submitted to strain causing changes in the shape, size and volume of the rock ^[2]. Reinforcement techniques refer to the action of adding internal support, e.g. rockbolts, to maintain or improve the rockmass' properties prior to deformation. Of course, the selection of ground support is an integral part of the underground mine design process. See Main Article: The information required for the mine design

Significant properties of support elements

Uses of ground support

Miner in Kentucky, 1950, U.S. Steel^[3]

Active vs. passive

Unlike the distinction between temporary and permanent support, the distinction between active and passive support refers to the load existing in the chosen support prior to its installation. For example, active support forces a load onto the rock surface in order to support broken rockmass and ensure its stability. Examples of active support types are: tensioned rockbolts, cablebolts, hydraulic jacks and powered supports for longwall mining. Passive support on the other hand is reactive to the ground's movements. It develops its load as the rock deforms. Examples of passive support types are: untensioned rockbolts, meshes and screens, reinforcing bars, shotcrete, timbered sets and steel arches. A combination of active and passive supports is typically optimal for most mining situations.

Primary vs. secondary

A more modern way of relating ground support techniques and systems is to define a primary and secondary support strategy. The primary support is installed in conjunction with excavation and serves supporting and reinforcing functions. This enables simultaneous control of boundary displacements. Any support applied at a later stage will be defined as secondary support. This ground control strategy maximizes the capacities of the support selected. As the excavation size and displacements are increased, the selected support develops the required load to maintain the required internal pressure^[4]. Figure 1 below clearly demonstrates this principle.

[[Image:Support-Displacement Relationship (Daeman 1977).jpg|frame|center|Support-Displacement Relationship, (Daeman, 1977)^[4]

This design strategy is also explained through the analysis for ground support reaction curves. These analyses are necessary in the design stages of a ground support strategy in order to determine the true failure mode of an excavation.

Types of support ^[5]

Since the development of the first mechanical rockbolts in the United States, where the first rockbolting occurrence is recorded in 1936 at the St. Joseph Lead Co. in Missouri ^[6],
the types of ground support available have greatly increased and offer a range of functionalities and capacities. The types of support can be divided in the following categories:

Various Rockbolts, DeHuia (China)^[7]

• Rockbolts (mechanical, frictional, grouted)
• Cablebolts (plain, birdcage, bulbed, nutcaged)
• Other Rockbolts (cone bolts, yieldable bars)
• Shotcrete (covers, reinforced, arches, pillars)
• Mesh, straps and screens

Often times, rockbolts and cablebolts are categorized according to their anchor type^[1] mechanical, frictional, etc.), their permanency (grouted or not grouted) and their deformation reactions (pre-tensioned or nor, dynamic vs. static).

The use of timber supports is seldom used for mining applications as its use underground is related to many hazards such as fire and is easily replaced by other support types.

Rockbolts and cablebolts

Rockbolts and cablebolts  are the most common type of ground support in current use in underground mines. The great diversity of rockbolts and cablebolts allows for selection of the most geotechnically and economically appropriate ground support element for a specific situation. The sections below describe some of the characteristics ^[8] and cost^[9] of various rockbolts and cablebolts in greater detail:

Mechanical Bolts, (McKinnon, 2001)^[5]

Mechanical rockbolts

Mechanical rockbolts are the type of ground support traditionally used. These are favourable for short term support as they are only anchored in the rockmass at both ends of the bolt. Furthermore, they are subject to blast vibrations and corrosion^[5].These bolts come in two varieties: threaded or forged ends. The threaded end rockbolts which are inserted into the rock face and held in place by a face plate and a nut allow for adjustment to the rock face surface and the angle of the face plate. As for forged ends, the end of the bolt is forged to the nut allowing for less flexibility.

Different manufacturers offer different lengths and steel gauges (C1060 Steel, C1070, C1070M, C1055 (A29), A29 (gr75 for USA)) for mechanical rockbolts. In the United States, these manufacturing standards follow the American Standards for Material Testing ASTM F432^[10]. (Please note that these standards are accessible through Queen's Library, see External Sources) For example, Dywidag Systems International (DSI)^[10], one of the world leaders in underground support systems, offers mechanical rockbolts in diameters of 5/8" (14.2mm), 3/4" (17.3mm) and 7/8" (20.1mm) and lengths from 2.5' (0.762m) - 10' (3m).

Costs Mechanical rockbolts, due to their temporary nature, are often used in shorter lengths. A typical cost for these bolts, including the face plate, are $US 4.63 for a 3' (0.8m) bolt and $US 9.22 for a 10' (3m) bolt both of 5/8" diameter.

Installation Installing mechanical rockbolts is a straightforward process. The workers are required to drill a hole to the specified diameter, usually using a jackleg or a stoper, seeMining equipment. The bolt is then inserted in the drilled hole with the plate and nut. The bolt is then tensioned to the specified torque.

Properties The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North (MASHA) provides technical information for a typical mechanical rockbolt of 1.5m in length of comparable steel grade to the DSI products and after 85mm of displacement (C1070 grade).

Technical Information Data Sheet - Mechanical Bolts (CANMET, 2008)^[8]

Frictional rockbolts

Frictional rockbolts or friction bolts attribute their name to their support mechanism. The bolt is maintained in the rockmass when fricition is created along the bolt. Due to this characteristic, friction bolts can slip in the hole when ground movement occurs. This makes friction bolts very efficient in dynamic conditions although they are subject to corrosion and have a lower load bearing capacity compared to supported bolts, i.e. mechanical and grouted bolts. The two main varieties of friction bolts are Swellex^TM, developed by Atlas Copco^[11], and Split-sets^TM, developed by Ingersoll-Rand and manufactured by DSI^[10].

Inflated Swellex, (McKinnon, 2010)^[5]

Swellex

Swellex^TM develop the frictional contact with the rock wall by inflation. The bolts are inflated by high water pressure. Swellex^TM are manufactured from 1.2-3.6m (standard) and 3.0-6.0m (super)in length in a regular steel gauge or a manganese alloy for corrosion resistance. The Swellex^TM rockbolt diameter is specified in terms of the required hole for installation. These hole sizes vary from 32-52mm.

Costs The cost for Swellex^TM rockbolts is not specified by the Mine Cost Service Handbook (Volume 2, p.38)^[9]. The Handbook only specifies the cost for "friction bolts". For a hole diameter of 32mm and 52mm respectively, the costs of 1.2m bolts are $US 5.31, $US 6.78 and and for 6.0m bolts $US 17.10, $US 22.90. It is specified that any galvanizing on the bolts, i.e. manganese coating, increases the price of the bolts by 30%.

Installation As with mechanical rockbolts, Swellex are installed by inserting the bolt into a pre-drilled hole. Instead of adding torque to the inserted bolt, high water pressure is forced into the bolt which inflates it to the proper rock surface profile. This, in combination with the anchorage of the plate and nut, creates the appropriate resistance between the bolt and rock wall.

Properties The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North (MASHA) provides technical information for a typical Swellex of 1.8-2.1m in length of comparable steel grade (Atlas Copco) and after 150mm of displacement.

Technical Information Data Sheet - Swellex Mn12 (CANMET, 2008)^[8]

Pile of Split-sets (McKinnon, 2010)^[5]

Split-sets

As for split sets, their split ends and slightly larger diameter than the hole diameter allows them to be squeezed inside the drilled hole to create friction along the bolt. Common sizes for split-sets are refered to as SS33, SS39, SS46 which defines the bolt diameter in mm, i.e. 33mm, 39mm and 46mm^[12]. The lengths of split-sets vary according to the manufacturer but are slightly shorter than the lengths of Swellex bolts. For a 33mm bolt, the lengths vary from 2.5' (0.762m) - 8' (2.4m). For a 46mm bolt, the lengths vary from 3' (0.91m) - 12' (3.65m).

Costs As with the Swellex bolt, the only specified cost is for a general "friction" bolt, refer to the costs of a Swellex bolt.

Installation The installation procedure for split-sets is simple. The bolt is inserted in a pre-drilled hole of a specified diameter. The bolt is then hydraulically placed into the hole. This creates resistance along the walls of the bolt. The bolt is then anchored with a plate and nut.

Properties The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North (MASHA) provides technical information for a typical split-set (39mm) of comparable steel grade to the DSI standards and after 150mm of displacement.

Technical Information Data Sheet - Split-Set (CANMET,2008)^[8]

Resin Grouted Rebar with Chamfered End (Hoek, 2009)^[13]

Grouted rockbolts

Grouted rockbolts refer to the use of reinforcement bars (rebar)^[14] or threadbars^[15] in combination with a grouting agent for creation of a bond along the rebar's entire length or a selected length.

The bonds are most often created using plain cement or a set of resins; fast acting at the toe, and slow acting along the bar. The required length of the bond is easily determined from strength factors of the bolt and diameter of the bolt.

Equation 1: Bond Length Calculation (McKinnon, 2010)^[5]

Since the development of resins, the use of cement is less common in mining applications. Just as in mechanical rockbolts, rebars come with forged or threaded ends. Furthermore, their chamfered end allows the rebar to break through the resin or cement cartridges ^[16].

Fasloc Resin Cartridge (DSI)^[17]

Resin grouted rebar is the most commly used grouted support. When installed appropriately, see Installation methods, its full length bond allows for efficient long-term support in static conditions. Furthermore, the resin used allows for corrosion protection as only the tip of the rockbolt is exposed.

Rebar is typically manufactured in four sizes #5 (15M - 16mm), #6 (20M), #7 (22M), and #8 (25M) which refer to the bar's diameter in mm or the thread size. Thread sizes are also refered to in imperial measurements with the following naming convention 5/8”- 11 UNC, 3/4”- 10 UNC, 7/8”- 9 UNC, 1”- 8 UNC. Rebars can be cut to specific lengths for the application up to 10' (3m). Of course, as with other ground support elements, their design follows manufacturing standards. In Canada they are manufactured to ASTM F432, Canadian Standards Association's CSA M430 and CSA G30.18M specifications. (Please note that these standards are accessible through Queen's Library, see External sources).

As for the manufactured resins, they come in various set times, from 15 seconds to 7 minutes, at a specified temperature. In order to create a quick bond at the end of the rockbolt a combination of fast set and the appropriate spin rate is used. These resins come in two compartment cartridges when the resin and the activator are kept separate until installation. The cartridges come in various diameters to accomodate the selected rebar/hole size (23 - 40mm) and various lengths, 12"- 60" (30.4 - 152.4cm). Most commonly, because various set times are desired at the bottom and top of hole, 12" cartridges are used. The following figures refer to appropraite spin times and temperatures for the different products. 

Costs

Required RPM and set-time for resins (DSI)^[17]

Set time for various temperatures (DSI)^[17]

The costs for this type of support depends on the selected bolt and resin. The Mine Service Cost Handbook assigns costs of $US 4.63 for a 3' rebard and $US 9.22 for an 8' rebar. As for the resin cartridges, the price for one cartridge, regardless of its set-time, is estimated at $US 0.55 for a 17" cartridge and $US 3.07 for a 49" cartridge.

Installation The installation of resin grouted rebars require more fine tuning that other bolts. First, a hole needs to be drilled to the specified diameter and flushed with air or water to ensure a clean surface contact. Then the first resin cartridge is inserted. This cartridge usually has a fast setting time

Properties The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North (MASHA) provides technical information for a typical resin grouted rebar (1.5m long) of comparable steel grade to the DSI standards and after 25mm of displacement.

Technical Information Data Sheet - Resin grouted Rebar (CANMET, 2008)^[8]

Cement grouted rebar^[17]

Cement grouted rebars follow the same principles as resin grouted rebars. As the name implies, the only difference is in the bonding agent; cement cartridges instead of resin cartridges. For lengths and dimensions of typical rebars see the section on resin grounted rebars.Contrarily to resin cartridges, cement cartridges have a fixed set time. The cartridges, which contain dry cement material, need to be submerged in water for approximately 1.5 minutes (no longer than 5) prior to installation. The set-time is fixed all along the selected bond length of the bar at approximately 20 minutes and full yield strength is typically obtained after 24 hours of curing.Diameters for these cartriges vary from 25-38mm in lengths of 305mm. Due to the less common usage of cement cartridges, the cost of these cartridges can be inferred from the typical cost of Portland cement($CAN 273.00/ton).

The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North(MASHA) provides technical information for a typical cement grouted rebar.

Technical Information Data Sheet - Cement Grouted Rebar (CANMET, 2008)^[8]

Threadbar

Threadbars are simple dywidags very similar in design to rebars. Their entire length is threaded with a simple nut that ties on to one end. The end is also chamfered for breakage of cartridges. The use of simple threadbars, not to confuse with threaded rebars, see Threaded-end rebars, is the least common in the mining industry due to their lack of adjustability to variable rock surface, i.e. no head. Their length and diameter are similar to those of manufactured rebar: #6 (19mm), #7 (22mm), #8 (25mm), #10 (32mm), #11 (35mm), #14 (44mm) at lengths up to 10' (3m). Costs for threadbars are not readily available. The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North(MASHA) provides technical information for a typical cement or resin grouted threadbar of 2.2m in lenght and 0.5m bond at 65mm displacement.

Technical Information Data Sheet - Resin or Cement Grouted Threadbar (CANMET,2008)^[8]

Types of Cablebolt Lays (McKinnon, 2010)^[5]

Cablebolts are also commonly used in underground mining applications. Their development, which occured in South African and Australian cut-and-fill(Clifford, 1974, Brown, 1999b)^[1], seeMining method, led to the modern cablebolt configuration. Due to their greater capacities, in comparison with standard bolt-type support, cablebolts are most often used in large span applications such as in hanging wall support and open stoping support designs. The bolts can be used as pre or post reinforcement with pre or post tensioning. Cablebolts are manufactured as 7-wire cables with various strand diameters: 0.5", 0.6", 0.7" ^[10] and, as they are purchased in rolls, can be cut to any specified lengths, typically 5' (1.52m) - 50' (15.2m).

Bircaged vs. Plain Lay (McKinnon, 2010)^[5]

The wires are layed using one of the defined types: plain, birdcaged, bulbed and nutcaged. All of these bolt types are grouted in cement along their entire length. The selected lay impacts the cement grout/rockmass/cable contact. The bulbed or birdcaged lays offer the best frictional resistance. Furthermore, the wires of the cable can be galvanized for corrosion protection. Also, in dynamic conditions cablebolts can be partially covered in order to prevent bonding of the cable with the grout. This allows the bolt to extend with rock deformation. The cost for a birdcaged cablebolt of 0.6" diameter, excluding the cement, anchor grip and bearing plate, is estimated at $US 1.29 per foot^[9] plus $US 5.28 for the anchor and $US 7.55 for the plate. The cost can also said to increase by 30% for galvanizing. The 2008 report on dynamic behaviour of rock support tensors provided by CANMET and Workplace Safety North(MASHA) provides technical information for cemented plain and birdcaged cablebolts.

Technical Information Data Sheet - Plain Cemented Cablebolt (CANMET, 2008)^[8]

Technical Information Data Sheet - Birdcaged Cemented Cablebolt (CANMET, 2008)^[8]

Shepperd's Hook Cone Bolts (Steeldale SCS Pty. Ltd.)^[5]

As the mining industry is evolving, new technologies in ground support are allowing for the testing and implementation of new ground support elements. In combination with a deep underground mining trend (in Canada), the new elements presented by manufacturers are often dynamic elements that perform well in rockbursting conditions, see Rockbursting. Similarly to friction bolts, these new bolts use created friction and yield strength to accomodate for greater ground movemements. Cone bolts (threaded, shepperd's hook) and yieldable bars and tendons have been recently tested for rockbursting conditions^[8]. For example, for a cone bolt of comparable dimensions to a typical friction bolt, the obtained yield energies (kJ) are twice as large in dynamic loading (40 kJ).

Comparison of Bolt Strengths (Pakalnis and Vongpaisal,1993)^[5]

The comparison for the strengths (yield and break) of various ground support elements is useful for the design of a support system.

Shotcrete

Shotcrete, and its historical predecessor gunite^[1], is applied in order to passively support rockmass. For the last 20 years, shotcrete has proven to be one of the most useful ground support elements in underground mining. The combination of cement, various additives (retarders, activators, reinforcing fibres etc.) and a mix of fine and coarse particles create a powerful tool for the mining industry. Shotcrete can be used as standard ground support in combination with mesh or in light rockbursting conditions as strain bursting protection, see Rockbursting. It can also be used to create shotcrete arches and pillars.

Shotcrete and mesh Although shotcrete is used in light rockbursting conditions, its nature makes it poorly resistant to rock deformation. In order to increase its "flexibility" it can be sprayed over steel mesh and mixed with steel / polymer reinforcing fibres. Of course, these fibres increase the cost and wear and tear applied on the applicators.

Shotcrete arches The design of shotcrete arches is still experimental. Created arches around drifts to create rib-like structures reduces the risk of fracturing during rockbursting.

Shotcrete Pillar at Final Stage (McKinnon, 2010)^[5]

Shotcrete pillars Shotcrete pillars are used in the same manner as rock pillars, i.e. in areas where the span exceeds same mining limits. The main difference with shotcrete pillars is that they are engineered for the specific situation. The bases can be pre-fabricated on surface and the shotcrete is sprayed on a mesh frame for reinforcement. These pillars have very high-load carrying capacities. The cost of shotcrete without any additives is approximately $US 300 per cubic yard. Added steel fibres are estimated to cost $US 0.80 per pound^[9]. This cost does not include the cost of application, the capital costs of the required equipment, the sprayers and the required labour. According to an article on shotcrete standards^[18] shotcrete, after a curing time of 28 days, develops strengths of 65MPa.

Laced Wall (McKinnon, 2010)^[5]

Lacing Another technique that can be used in combination with shotcrete, especially in high stress excavations is lacing. Cables are basically laced in a spiderweb pattern along the wall with anchorage at intersections to diffuse stresses along the cables. This is mostly used in deep South African mines.

Mesh and screens

Mesh and screens, also straps, have two main functions. The first one being the support of tumbling and bulging pieces of rock and the second one being in support to shotcreting applications. Mesh, which is the most common ground support element in underground mines, can be build from welded wires or chain-link. Typically welded-mesh is used due to its higher strength, i.e. bigger gauge, and more adequate function for shotcreting. Chain-link mesh can also be used in highly variable rock surface because its smaller gauge allows for better contouring of the asperities^[5].

Strapping over Mesh (McKinnon, 2010)^[5]

It is evident that mesh is always used in combination with rock bolts to ensure attachment of the mesh to the rock wall. Furthermore, attachment and reinforcement of the screens/mesh layers of mesh are overlapped and sometimes double with higher gauge steel straps. The main downfall with mesh is its poor resistance to equipment damage.

Welded Mesh is manufactured in sheets of 48" to 96" in width and 48" high (or rolls of 100'). The steel gauges manufactured also vary from size 4 (0.232"), 6 (0.192"), 8 (0.160"), 9 (0.144") and 10 (.0128"). The spacing of the mesh squares is typically 4" x 4".^[19]

Chain-link mesh, similarly to welded mesh, is manufcatured in rolls of 4'-10' in width and 25' - 100' in length. The steel gauges are slightly smaller than the welded mesh; 6 (0.192"), 9 (0.144"), 11 (0.120") and 12 (0.106"). As for the spacing, diagonally across the mesh, it is offered in 2" - 3" openings.^[20]

The general cost for support mesh is estimated at $US 84.00 for a gauge 9 mesh and $70.00 for a gauge 11 mesh in rolls of 6' x 50'^[9].

Mesh strengths can be estimated according to the table below:

Tested Mesh Strengths (Pakalnis and Vongpaisal, 1993)^[5]

Empirical support design

The appropriate ground support strategies are selected through various engineering design techniques. In Ontario, this design selection is governed by legal requirements defined in the Ontario Occupational Health and Safety Act. and Masha design guidelines (GCMP)

Historical methods of support

^{[21] [22]}

1879 development of rock mass classfication for ground support requirements in tunelling applications reference

Design parameters

Select for the right mode of failure

Installation methods

Equipment

References

1. ↑ ^{1.0 1.1 1.2 1.3} Brady, B. H.G. and E. T. Brown. "Rock Support and Reinforcement." Brady, B. H.G. and E. T. Brown. Rock Mechanics. Springer Science, 2005. p. 312 - 346.
2. ↑ Nelson, Stephen A. "Physical Geology - EENS 111." 2007. Tulane University. http://www.tulane.edu/~sanelson/geol111/deform.htm
3. ↑ Mining USA. (n.d.). Old Photo - Lynch Kentucky. Retrieved from http://www.miningusa.com/old_photo/2002.htm
4. ↑ ^{4.0 4.1} Daeman. "Support-Displacement Relationship (Fig. 11.1b)." Brady. Rock Mechanics. 2005.
5. ↑ ^{5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14} McKinnon, Steve. "Moodle - Ground Support Slides." 2010. Moodle - Queen's University - Mine 469
6. ↑ Bieniawsky. "Ground Control, Section 10.5" Cummins, Hartman, Given, Howard. SME Engineering Handbook. SME, 1992
7. ↑ insert ref
8. ↑ ^{8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9} Beauchamp, Luc. "Technical Information Data Sheets - Design Guidelines for the Dynamic Behaviour of Ground Support Tendons." 2009. Workplace Safety North - Technical Projects. http://www.masha.on.ca/ground_support_tendons.aspx
9. ↑ ^{9.0 9.1 9.2 9.3 9.4} USA, InfoMine. Mine Cost Service - Volume 2. Jennifer B. Leinhart, 2009
10. ↑ ^{10.0 10.1 10.2 10.3} International, Dywigad Systems. Products. January 2009. http://www.dsigroundsupport.com/products/mechanical-rockboltsbrextension-boltsbrstelpipe-bolts/mechanical-rock-bolts.html
11. ↑ Atlas Copco. (2008). Swellex Bolts. Retrieved from Atlas Copco - Mining Products: http://pol.atlascopco.com/SGSite/default_prod.asp?redirpage=products/area.asp&redirid=Rock Bolting and Miscellaneous&view=&plid=EN&slid=ES&GetonBoard=Yes&LanguageID=Yes
12. ↑ International, Dydiwag Systems. Friction Stabilizers. January 2009. http://www.dsigroundsupport.com/products/friction-stabilizers-expandable-bolts/friction-stabilizers.html
13. ↑ Hoek, Evert. Practical Rock Engineering. 2009
14. ↑ International, Dywidags Systems. Rebar Rock Bolts. January 2009. http://www.dsigroundsupport.com/products/rebar-rock-bolts/rebar-rockbolts.html;
15. ↑ International, Dywidag Systems. DYWIDAG Threadbar. January 2009. http://www.dsigroundsupport.com/products/dywidag-threadbar/threadbar-properties.html
16. ↑ Dywidag Systems International. Resins . January 2009.;http://www.dsigroundsupport.com/en/products/resins-and-cement-cartridges/ground-lok-h2o-resin-cartridges.html
17. ↑ ^{17.0 17.1 17.2 17.3 17.4} Resins. January 2009. http://www.dsigroundsupport.com/products/resins-and-cement-cartridges/fasloc-resin-cart.html
18. ↑ Jones, W. (n.d.). Meeting Shotcrete Standards. Retrieved from Tunnels Industry: http://www.tunnelsonline.info/story.asp?sectioncode=8&storycode=62301&c=3
19. ↑ International, Diwydags Systems. Plates and Mesh. 2010. http://www.dsigroundsupport.com/products/plates-and-mesh/mesh.html
20. ↑ International, Diwydags Systems. Plates and Mesh. 2010. http://www.dsigroundsupport.com/products/plates-and-mesh/mesh.html
21. ↑ Bieniawsky. "Ground Control." Cummins, Hartman, Given, Howard. SME Engineering Handbook. SME, 1992
22. ↑ Hoek, Evert. Practical Rock Engineering. 2009

External sources

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