Ground support

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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

Historically, the installation of ground support was confined to temporary or permanent excavations.
Timber Supports.jpg
Temporary excavations such as small and narrow shrinkage mining stopes, see article on The selection of mining methods, would have been supported with long timber members to prevent ground failures in these workings. These workings would typically have remained opened for short periods of time (1-2 weeks). As for permanent excavations such as shaft stations which can remain opened for years, permanent support and reinforcement was installed. Often times, the temporary support previously put in place was removed and replaced with more permanent forms. This is contrary to modern practice where the rock-support interactions are an important design consideration.
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[3]. Figure 1 below clearly demonstrates this principle.

Support-Displacement Relationship (Daeman 1977).jpg


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 [4]

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 [5],
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: 

Rock bolts.jpg
  • Rockbolts (mechanical, frictional, grouted)
  • Cablebolts (plain, birdcage, bulbed, nutcaged)
  • Shotcrete (covers, reinforced, arches, pillars)
  • Mesh, Straps and Screens

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 [6] and cost[7] of various rockbolts and cablebolts in greater detail:


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[8]

These bolts come in two varieties: threaded or forged ends. The threaded end rockbolts which are inserted into the rock face IMAGE OF THREAD VS. FORGED 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[9]. 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).

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.

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).TABLE OF TECHNICAL

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 SwellexTM, developed by Atlas Copco, and Split-setsTM, developed by Ingersoll-Rand and manufactured by DSI.


  • Swellex

Swellex develop the frictional contact with the rock wall by inflation. The bolts are inflated by high water pressure IMAGE OF SWELLEX.   

Swellex 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 rockbolt diameter is specified in terms of the required hole for installation. These hole sizes vary from 32-52mm.

The cost for Swellex rockbolts is not specifically specified by the Mine Cost Service Handbook (Volume 2, p.38). 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%.

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. TABLE OF TECHNICAL


  • 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. IMAGE OF SPLIT-SET

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[11]. 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). 

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

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. TABLE OF TECHNICAL


Grouted Rockbolts

Grouted rockbolts refer to the use of reinforcement bars (rebar)[12] or threadbars[13] in combination with a grouting agent for creation of a bond along the rebar's entire length. IMAGE 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. Since the development of resins, the use of cement is less common in mining applications. Just as in mechanical rockbolts, rebars and threadbars come with forged or threaded ends. Furthermore, their chamfered end allows the rebar to break through the resin or cement cartridges[14]


  • Resin Grouted Rebar


  • Cement Ground Rebar
  • Threadbar




Cablebolts


Shotcrete
Mesh and Screens

[15]

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

[16] [17]


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. 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. Daeman. "Support-Displacement Relationship (Fig. 11.1b)." Brady. Rock Mechanics. 2005.
  4. McKinnon, Steve. "Moodle - Ground Support Slides." 2010. Moodle - Queen's University - Mine 469
  5. Bieniawsky. "Ground Control, Section 10.5" Cummins, Hartman, Given, Howard. SME Engineering Handbook. SME, 1992
  6. 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>
  7. USA, InfoMine. Mine Cost Service - Volume 2. Jennifer B. Leinhart, 2009
  8. McKinnon, Steve. "Moodle - Ground Support Slides." 2010. Moodle - Queen's University - Mine 469
  9. International, Dywigad Systems. Products. January 2009. <http://www.dsigroundsupport.com/products/mechanical-rockboltsbrextension-boltsbrstelpipe-bolts/mechanical-rock-bolts.html>
  10. International, Dywigad Systems. Products. January 2009. <http://www.dsigroundsupport.com/products/mechanical-rockboltsbrextension-boltsbrstelpipe-bolts/mechanical-rock-bolts.html>
  11. International, Dydiwag Systems. Friction Stabilizers. January 2009. <http://www.dsigroundsupport.com/products/friction-stabilizers-expandable-bolts/friction-stabilizers.html>
  12. International, Dywidags Systems. Rebar Rock Bolts. January 2009. <http://www.dsigroundsupport.com/products/rebar-rock-bolts/rebar-rockbolts.html>
  13. International, Dywidag Systems. DYWIDAG Threadbar. January 2009. <http://www.dsigroundsupport.com/products/dywidag-threadbar/threadbar-properties.html>
  14. —. Resins . January 2009. <http://www.dsigroundsupport.com/en/products/resins-and-cement-cartridges/ground-lok-h2o-resin-cartridges.html>
  15. International, Diwydags Systems. Plates and Mesh. 2010. &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;http://www.dsigroundsupport.com/products/plates-and-mesh/mesh.html&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;
  16. Bieniawsky. "Ground Control." Cummins, Hartman, Given, Howard. SME Engineering Handbook. SME, 1992
  17. Hoek, Evert. Practical Rock Engineering. 2009

External Sources

http://en.wikipedia.org/wiki/Wikipedia:Citing_sources