International Journal of Mining Science and Technology 24 (2014) 325–328

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International Journal of Mining Science and Technology

journal homepage: www.elsevier .com/locate / i jmst

Introducing aggregate into grouting material and its influence on loadtransfer of the rock bolting system

http://dx.doi.org/10.1016/j.ijmst.2014.03.0062095-2686/� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

⇑ Corresponding author. Tel.: +61 0425 334 939.E-mail address: ccao@uow.edu.au (C. Cao).

Cao Chen ⇑, Ren Ting, Chris CookFaculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 October 2013Received in revised form 15 November 2013Accepted 8 December 2013Available online 29 April 2014

Keywords:Bolting strengthResin improvementParallel shear failureDilational slip

A fully grouted bolt provides greater shear load capacity for transmitting the load from the rock to thebolt, and vice versa. When grout fills irregularities between the bolt and the rock, a keying effect is cre-ated to transfer the load to the bolt via shear resistance at the interface and within the grout. Previousresearch has revealed that the mechanical properties of the grout had a great impact on the load transfercapacity of the rock bolting system. This paper presents a method to enhance the rock bolting strength byintroducing metal granules into the grouting material. Experimental results suggest that both the averagepeak load of pullout tests and the total energy absorption of the system will increase if some metal gran-ules are mixed into the resin.

� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

Many researchers have worked theoretically and experimen-tally on the mechanism of load transfer of fully grouted rock bolts.Up to date, it is commonly accepted that fully grouted bolts aremuch more successful in supporting roof strata than other boltingsystems [1].

A fully grouted bolt provides greater shear surface for transmit-ting the load from the rock to the bolt, and vice versa. The groutsupplies a mechanism for transferring the load between the rockand the reinforcing element. This redistribution of forces alongthe bolt is the result of movement in the rock mass, which transfersthe load to the bolt via shear resistance in the grout.

This shear resistance within the resin and along the interfacescan be the result of adhesion, friction and mechanical interlocking,which is a keying effect created when grout fills the irregularitiesbetween the bolt and the rock. Therefore, the performance of thereinforcement can be directly enhanced by improvement of themechanical properties of the grouting material [2,3].

At the time of writing, approximately 80% of the over 100 mil-lion roof bolts installed in U.S. mines, tunnels, and constructionprojects employ polyester roof bolt resin. It is estimated that resinconsumed in the US each year can encircle the world at the equatorapproximately three times if in a 22.9 mm diameter cartridge [4].

Current bolting technology uses a two part polyester resin car-tridge to supply the bolt and borehole with sufficient resin to

achieve the desired encapsulated length. The successful perfor-mance of the bolt and cartridge system requires that the boltshreds the plastic cartridge and mixes the separate resin compo-nents during installation.

The effectiveness of installed roof bolts can be compromised bygloving. Gloving refers to the plastic cartridge of a resin capsuleencasing a length of bolt, typically with a combination of mixedand unmixed resin filler and catalyst remaining within thecartridge.

This paper introduces a method to enhance the rock boltingstrength by mixing metallic granules into the grouting material,as shown in Fig. 1. Experimental results show that the peak loadof pullout tests will increase if some metal granules are mixed intothe resin. The effect of the introduced metallic granules in reducinggloving is also discussed.

2. Related theories

2.1. Fully grouted bolting

Fully grouted bolting consists of the bolt, grout, and surround-ing rock. The relationships between them depend on a continuousmechanically coupled system. A fully grouted bolt is a passive roofsupport system, which is activated by movement of the surround-ing rock. The efficiency of load transfer is affected by the mechan-ical properties of the grout, surface profile of the rock bolt,thickness of the grout annulus, anchorage length, rock properties,confining pressure and installation procedure.

Resin RockBolt

Failure surfaceMetallic granule

Fig. 1. Schematic of the concept of mixing metallic granules into the resin.

Fig. 2. Parallel shear failure of the resin observed in laboratory pull out tests.

326 C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328

In a fully grouted rock bolt, the load transfer mechanismdepends on the shear stress developed on the bolt–resin andresin–rock interfaces. Peak shear stress and shear stress modulusof the interfaces determine the reaction of the bolt to the strata.Hence, the load transfer is determined by measuring the peakshear stress and system stiffness [5]. In addition, the post-failurebehaviour of the rock bolting system is also important as it largelydetermines the total energy absorption of the system.

ConceptualField

Granite, limestone, shale Hemlo Golden Giant Mine

Drilling damageNatural fractures

Constant radial pressure Modified Hoek cell tests

Constant radial stiffnessSteel, Aluminum, PVC pipes concrete blocks

P2

Fig. 3. Schematic of a rock bolting system [8].

Table 1Radial stiffness of commonly used experimental assemblages.

Confinement Redial stiffness K (GPa/mm)

PVC* 0.0724.2 mm resin annulus only 0.200Aluminium* 0.790Steel 1* 0.770Steel 2* 0.950UCS 40 MPa concrete block 1.100Steel 3* 1.120Infinite rock mass (E = 30 GPa, and v = 0.25) 2.0008 mm steel sleeve 3.400

* Reported in Kaiser et al. [13].

Aggregate

2.2. Failure mode

Littlejohn classified various types of axial failure when usinggrouted bolts as follows: the bolt, the grout, the rock, the bolt–grout interface or grout–rock interface [6]. The type of axial failuredepended on the properties of individual elements. The shearstress at the bolt–grout interface was smaller than that at thegrout–rock interface because of the smaller effective area. If thegrout and rock were of similar strengths, failure could occur atthe bolt–grout interface. If the surrounding rock was softer thenfailure could occur at the grout–rock interface.

Based on pullout tests of cable bolts in the laboratory and in thefield, Hyett et al. identified two failure modes in cementitious gro-uted cable bolt [7–9]. One mode was radial splitting of the concretecover surrounding the cable, while the other involved shearing ofthe cable against the concrete. The former concerns the wedgemechanism but it is rarely observed in the resin grouted boltingsystem. The shearing mechanism involved crushing of the groutingmaterial ahead of the ribs on the bar, eventually making pulloutalong a cylindrical friction surface possible. It should also be notedthat as the degree of radial confinement increased, the failuremechanism changed from radial fracturing of the cementitiousannulus under low confinement, to shearing of the cement flutesand pullout along a cylindrical friction surface under highconfinement.

Recent research work of failure mode analysis suggests that acylindrical failure surface around the bolt resin interface is a pre-dominating failure mode in rock bolting [10]. It occurs for thesmooth bars and for very closely spaced rebar bolts (like a screw)along the rib tips of the bar. For rebar bolts, experimental observa-tion suggests that if the embedded length is short and the confin-ing material is stiff, parallel shear failure occurs in laboratory pullout tests. Fig. 2 shows a pull out test bolt of 75 mm embeddedlength and confined in 8 mm thick steel tubes.

Grout Steel bolt

Fig. 4. Alteration of the failure surface due to the introduced metallic granules.

2.3. Dilatancy behaviour accompanying shearing

The discontinuity behaviour is often studied under constant nor-mal load (CNL) or constant normal stiffness (CNS) condition.For the CNL condition, dilatancy accompanying shearing of the

discontinuity surfaces is permitted to occur freely; for example,sliding of an unconstrained block of rock on a slope. Under theCNS condition, however, dilation may be depressed by the surround-ing material due to the increased normal stress with sheardisplacement.

For rock bolting systems, the dilatancy behaviour can be betterconceptualised under the CNS condition, as shown in Fig. 3. That is,dilation of the failure surface may be constrained by the resinannulus and surrounding rock.

The radial stiffness of the rock bolting system can be calculatedusing the thick-welled cylinder theory, which is widely acceptedby theoretical rock bolting research studies [8,11]. Calculation ofthe results of some assemblage examples are listed in Table 1.The 4.2 mm resin annulus confinement can be used as the lowerlimit of the radial stiffness of resin grout rock bolting. If the confin-ing material is 8 mm thick steel tube, the radial stiffness will reach

(a) Pre-test of T2 bolting specimen (b) Post-test of T3 bolting specimen

Fig. 5. Post test sheared resin and the introduced metallic granules.

140120

100806040

20

50 1510 2520 3530 40

Displacment (mm)

Loa

d (k

N)

120

100

80

60

40

20

0

Loa

d (k

N)

5 10 15 20

Displacment (mm)

Without impurity

With impurity

T2 bolt EL=75 mm T3 bolt EL=75 mm

Fig. 6. Test results.

C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328 327

3.4 GPa/mm [12]. For an infinite medium, the radial stiffness canbe found via Eq. (1):

K ¼ Er

að1þ mrÞð1Þ

where a is the hole radius; Er the Young’s modulus; and vr the Pois-son’s ratio of the rock.

2.4. Conceptualisation of introduced aggregate

Fig. 4 conceptualises introducing aggregate into the resinmatrix. If failure of the rock bolting system subjected to axial loadoccurs along the rib tips of a rebar bolt, then the aggregate inter-cepting the failure surface resists the relative slipping of the inter-face. Due to the interruption, an irregular failure surface is formedwith increasing axial loads. The irregularity of the failure surfacewill cause extra dilation of the interface, increasing the effective-ness of the load transfer of the system.

3. Experimental study

Laboratory pull out tests were carried out using two kinds ofrebar bolt, namely the T2 and T3 bolts, which are popular in theAustralian mining industry. The bolt was encapsulated in a steeltube 75 mm long and 8 mm thick using mix and pour resin.

The specifications of the bolts’ surface profile configurations canbe found in study investigated by Cao et al. [10]. Steel wire 2 mmdiameter was cut to 2–3 mm long segments and mixed into theresin, with approximately 10 per bolt profile. Fig. 5 shows the posttest sheared resin and the introduced metallic granules.

Fig. 6 shows the test results. As expected, the load transfercapacity and the total absorbed energy of the bolt are bothincreased by up to 20% by introducing metallic granules into theresin.

4. Discussion

Gloving is currently seen as an industrial problem because thegloved and unmixed portions reduce the effective anchor length

and adversely affect the reinforcement for the roof strata. Researchshows that gloving is a systematic and widespread phenomenon,occurring across the range of resin and/or bolt manufacturers,and in a variety of roof types. It has been found in bolts installedusing either hand held pneumatic or continuous miner-mountedhydraulic bolting rigs, under face conditions by operators, andunder controlled manufacturers best practice conditions [14].

Recent research reported that testing of specific bolt ends of 26–28 mm widths installed into a hole drilled with a 27 mm bit cansignificantly reduce gloving, and concluded that gloving could besignificantly reduced by a bolt end that nearly contacted the sideof the bolt hole [15]. However, due to installation difficulties, thepatent pending bolt cannot be applied using standard Australianbolting rigs.

It is obvious that the plastic film will be ground into pieces if thebolt diameter equaled the bore hole diameter; however, it remainsa question whether this can be achieved via other means. Introduc-ing metallic granules into the resin will lead to extra slipping of theplastic film against the granules while the bolt is being installed.This may greatly reduce the extent of the gloving problem becausethe effect of the introduced metallic granules can be thought of as away to increment bolt diameter without leading to installationdifficulties.

5. Conclusions

This study introduces a new method to increase the load trans-fer capacity of a fully grouted rock bolting system by introducingmetallic granules into the resin. When the granules are mixed intothe resin, the failure surface around the interfaces will becomeirregular for the parallel shear failure mode of the system. This willlead to extra dilation of the failure surface when and after failureoccurs. Laboratory pullout tests were conducted for two kinds ofrebar bolts which are commonly used in the Australian miningindustry. Results show that both the peak load and total energyabsorption can be substantially increased by about 20%. This inno-vation is also proposed as a possible solution to reduce the glovingof the system; however, more research is needed to further exam-ine and quantify this hypothesis.

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