Rock Bolting



Rock bolting is the systematic reinforcment and/or anchorage of rock slopes by the insertion and grouting of steel bars into holes  predrilled into the more or less fractured rock mass, improving its stability. The deformed steel bars are typically 25 to 50 mm in diameter and up to 12 to 15 m in length. Long bolts are typically formed by joining shorter threaded bars using special couplers, to facilitate handling. For convenience of installation, strand anchors (see fact-sheet 6.8) are normally used where longer bolts are required. Bolts are installed across the discontinuities or the potential failure surfaces at a dip angle flatter than the normal and typically work mainly in tension and only subordinately in shear and bending.

Figure 1: Schematic detail (source: SGI-MI project files)

Typically, drillholes in rock are self supporting. However, critical drilling conditions with potential loss of borehole stability may be encountered when drilling through higly fractured or milonitic zones, especially if water is also encountered in the drillhole. In this case, it may be simpler to grout and redrill the hole, rather than using a casing.

In relation to the degree of relaxation or loosening of the fractured rock to be reinforced and/or to be tied to the more competent rock below the bolts can be un-tensioned or tensioned. Relaxation and or loosening of the rock mass is a process that takes place as a results of unloading and weathering; once relaxation or loosening has been allowed to take place there is a loss of interlock between the blocks of rock and a significant decrease in the shear strength along the discontinuities and in the rock mass as a whole. Once relaxation or loosening has taken place, it is not possible to reverse the process. For this reason:

  • where the degree of relaxation or loosening is relatively modest, it is possible to use passive (untensioned) rock bolting acting as pre-reinforcement (Moore and Imrie, 1982; Spang and Egger, 1990); the deformations necessary to activate the bolts are sufficiently small not to result in a significant reduction of the shear strength characteristics of the discontinuities and of the rock mass as a whole;

  • where significant relaxation and loosening have already taken place, it may be necessary to install tensioned bolts in order to prevent further displacements and loss of interlock.

The advantages of using un-tensioned bolts are the lower costs and quicker installation compared with tensioned bolts.

From a conceptual point of view, un-tensioned (passive) rock bolts work in the same way as nails of soil nailing structures.

They are grouted for their full length in a single operation both below and above the potential failure surface. In slope applications, where the drillhole dips into the ground, there is no need for anchoring the distal end of the bolt. Even though in many situation a head plate is not strictly required, a end plate is normally fitted to the bolt at the surface and this may be usefull to anchor netting and or other facings that may be required.

From a conceptual point of view, tensioned (active) rock bolts work like anchors in tieback retaining structures.  They are characterized by a anchor head, a free-stressing length and a bond length, located beneath the discontinuity or the potential failure surface.

Tensioned (active) bolts must satisfy three basic requirements:

  1. There must be a suitable method of anchoring the distal end of the bolt in the drill hole;

  2. A known tension must be applied to the bolt without creep and loss of load over time;

  3. The complete bolt assembly must be protected from corrosion for the design life of the project.

Methods of securing the distal end of a bolt in the drill hole include mechanical devices, resin and cement grout. The selection of the appropriate method depends on several factors such as the required capacity of the bolt, speed of installation, strength of the rock in the bond zone, access to the site for drilling and tensioning equipment and the level of corrosion protection required (Wyllie and Norrish, 1996).

The most appropriate method to ensure that bolts are not susceptible to creep and loss of load over time is to set operating loads significantly lower than the pullout resistance and below the level at which significant creep or fluage is observed in load tests. Specific test procedures have been developed for example by the Post Tensioning Institute (1985) and by AICAP (1993), which can detect the essential aspects of the behaviour of the anchor and the surrounding ground, to determine also the long term pullout resistancet rather than the short term resistance only.

Methods of protecting steel against corrosion include galvanizing, applying an epoxy coating and encapsulating the steel in cement grout. Because of the brittle nature of the grout and its tendency to crack, particularly when loaded in tension and in bending, the protection system is usually composed of a combination of grout and a plastic sleeve.

Alternatively, fibre reinforced polymer (FRP) bolts may be used to overcome problems of durability. Zhang et al (2001) provide useful guidance on these products. Up to date details may be obtained from manufacturers.

Figure 1 shows a typical example of a three-layer corrosion protection system, where the bolt is encapsulated in a grout-filled HDPE sheat, and the outer annular space between the sheat and the rock is filled with a second grout layer, with centering sleeves to ensure complete encapsulation of the steel.

Grout mix can be readily pumped down a small-diameter grout tube, so that grouting proceeds from the distal end of the drill hole towards the surface, displacing any water or debris and producing a continuous grout column. Grouting is continued until clean grout flows out of the hole at the surface. Hollow bars can be used in lieu of soild bars, in which case the grout is injected through the bar itself, avoiding the need for the grout tube.

When bolting is carried out in a unweathered rock mass with relatively widely spaced discontinuities, the spacing between bolts may be commensurably wide and there is no need for any facing. In this case the end of the bolt is fitted with a small steel plate, typically embedded in a small concrete slab for corrosion protection (See Pictures 2 and 3 for an example).

Where the rock mass is highly fractured and/or the fractured rock may degrade and ravel from under and in between the reaction plates of the bolts, a structural facing must form an integral part of the rock bolting scheme. Different solutions may be foreseen for the structural facing, including for example:

  • Reinforced concrete walls: the wall acts both as a protection against raveling of the rock and as a large reaction plate for the rock bolts; the rock bolt will be drilled through sleeves in the concrete; it is also important that there be drain holes through the concrete to prevent buildup of water behind the wall.

  • Shotcrete, reinforced with reinforcing mesh (typically steel, but other materials may be equally suitable).

  • Reinforced wire mesh, with a network of steel cables.

  • Reinforced wire mesh associated with reinstatement of vegetation.

    Picture 1: Examples of bolts fitted with mechanical devices for securing the distal end
    Picture 2: Drilling for installation of rockbolts on Polk County US-64 Rockslide
Picture 3: Rockbolts on Polk County US-64 Rockslide after completion


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Design methods

Except for the specific differences that derive from the different nature of the material, the design of un-tensioned (passive) rock bolts is governed by much the same principles and rules as described in fact-sheet 6.5 on “Soil Nailing”, while the design of tensioned (active) rock bolts is governed by much the same principles and rules as described in fact-sheet 6.8 on “Strand anchors”. In particular, the following differences are noteworthy:

  • from the point of view of the stability analyses used to determine the design load capacity and length of the inclusions, rock bolting deals with a discontinuous rock mass whose stability is typically governed by the discontinuities, as opposed to the pseudo-continuous nature of the ground involved in soil nailing schemes; appropriate methods of analyses, such as deterministic or probabilistic wedge analysis need to be applied;

  • the grout-ground bond that can be developed in rock bolting (other than in argillaceous rocks) is typically much higher than is available in soil nailing, with an impact both on the minimum length of embedment beyond potential failure surfaces and on the lower demand on the facing; in limit cases, no facing at all will be required;

  • the much greater stiffness of rock compared to soil allows the component of resistance associated with bending and shear to develop at much smaller displacements.

For tensioned cement-grout bolts the stress distribution along the bond length is higly non-uniform; the highest stresses are concentrated in the proximal end of the bolt immediately below the discontinuity or the failure surface, while ideally the distal end is unstressed (Farmer, 1975; Aydan, 1989). In practice the required length of the bond zone can be calculated with the simplifying assumption that the shear stresses at the rock-grout interface is uniformly distributed along the bond length. Limit values of the shear stresses can be estimated as a fraction of the uniaxial compressive strength of the rock in the bonded zone (Littlejhon and Bruce, 1975); allowable bond stresses related to rock strength and rock type are found in Wyllie (1991).

The diameter of the drillhole is determined by the available drilling equipment but must also meet certains design requirements. The hole diameter should be large enough to allow the bolt to be inserted in the hole without driving or hammering and be fully embedded in a continuous column of grout; a hole diameter significantly larger than the bolt will not improve the design and will result in unnecessary drilling costs and excessive grout shrinkage. A suitable ratio between the diameter of the bolt and the diameter of the hole is in the range of 0.4 to 0.6.

The working shear strength of the steel-grout interface of a deformed bar is usually greater than the working strength of the rock-grout interface; hence the length of the bond zone is typically determined from the stress level of the rock-grout interface.

Littlejohn and Mothersille (2008a; 2008b) provide guidance on issues related to maintenance and monitoring.

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Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 9 Typically most suitable to prevent widespread sliding and toppling on competent rock masses whose behaviour is governed by discontinuities.
Topple 9
Slide 0
Spread 0
Flow 0
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Material type

Descriptor Rating Notes
Earth 0 Requires both potentially unstable mass and underlying stable material to be competent rock.
Debris 1
Rock 9
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Depth of movement

Descriptor Rating Notes
Surficial ( 5 Typically suitable to stabilize multiple slabs or wedges up to 8 m depth; requires additional facing where superficial instability occurs.
Shallow (0.5 to 3 m) 8
Medium (3 to 8 m) 6
Deep (8 to 15 m) 0
Very deep (> 15 m) 1
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Rate of movement

Descriptor Rating Notes
Moderate to fast 0 Rock face must be stable at time of bolting.
Slow 0
Very slow 0
Extremely slow 9
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Ground water conditions

Descriptor Rating Notes
Artesian 1 Suitable for all groundwater conditions but groundwater in the drillholes may affect the grout-ground bond in some rock types, especially shales and mudrocks; ”artesian” not applicable to the type of situation treated by rock bolting.
High 7
Low 9
Absent 9
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Surface water

Descriptor Rating Notes
Rain 9 Not practical within or close to water courses.
Snowmelt 9
Localized 6
Stream 0
Torrent 0
River 0
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Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Relatively simple schematization and analysis. Possible pitfalls in the systematic identification of wedges or slabs to be treated.
Feasibility and Manageability 8 Well established technique, widely used where applicable. Often insufficent attention paid to durability.
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Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 6 Requires access on steep slopes, implying specialist equipment. Works must be planned carefully to avoid exposing workers to rockfall from above.
Environmental suitability 4 will be updated
Economic suitability (cost) 6 Typically moderate; access conditions may have a strong impact on cost.
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  • AICAP (1993). ”Ancoraggi nei terreni e nelle rocce – Raccomandazioni”. In Italian, 43pp..

  • Aydan O. (1989). ”The stabilization of rock engineering structures by rock bolts”. PhD thesis, Department of Geotechnical Engineering, Nagoya University, Japan, 204  pp..

  • BS 8081:1989 British Standard code of practice for Ground Anchorages. BSI, London.

  • EN 1537:2000 European Standard for Execution of special geotechnical work . Ground Anchors.

  • Farmer I.W. (1975). ”Stress distribution along a resin grouted anchor”. Int. Journal of Rock Mechanics and Geomechanics Abstracts, vol. 12, 347-351.

  • Littlejohn G.S., Bruce D.A. (1975). ”Rock anchors – State of the Art. Part I: Design”. Ground Engineering, vol. 8, n° 4, 41-48.

  • Littlejohn S., Mothersille D.M. (2008a). ”Maintenance and monitoring of anchorages: guidelines”. Geotechnical Engineering 161, issue GE2, 93-106.

  • Littlejohn S., Mothersille D.M. (2008b). ”Maintenance and monitoring of anchorages: case studies”. Geotechnical Engineering 161, issue GE2, 107-114.

  • Moore D.P., Imrie A.S. (1982). ”Rock slope stabilization at Revelstoke Damsite”. In Transactions, 14th Int. Cong. On Large Dams, ICOLD, Paris, vo. 2, 365-385.

  • Post Tensioning Institute (1985) ”Recommendation for Prestressed Rock and Soil Anchors”. 2nd edition, Phoenix, Arizona, 57 pp..

  • Spang K., Egger P. (1990). ”Action of fully grouted bolts in jointed rock and factor of influence”. Rock Mechanics and Rock Engineering, vol. 23, 201-229.

  • Wyllie D.C. (1991). ”Rock Slope Stabilization and Protection Measures”. In Proc. Of  National Symposium on Highwaay and Railway Slope Stability, Association of Engineering Geologists, Chicago, 41-63.

  • Wyllie D.C., Norrish N.I. (1996). ”Stabilization of rock slopes”. In Landslides: Investigation and Mitigation, Special Report 247, Transportation Research Board, National Research Council, A.K. Turner and Schuster R.L. editors, 474-504.

  • Zhang B., Benmokrane B., Chennouf A., Mukhopadhyaya P., El-Safty A. (2001). ”Tensile behaviour of FRP tendons for prestresses ground anchors”. Journal of composite construction, ASCE, 5,  85-93.

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