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Introducing aggregate into grouting material and its influence on load transfer of the rock bolting system
International Journal of Mining Science and Technology 24 (2014) 325–328
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Introducing aggregate into grouting material and its influence on load transfer of the rock bolting system Cao Chen ⇑, Ren Ting, Chris Cook Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 8 December 2013 Available online 29 April 2014 Keywords: Bolting strength Resin improvement Parallel shear failure Dilational slip
a b s t r a c t A fully grouted bolt provides greater shear load capacity for transmitting the load from the rock to the bolt, and vice versa. When grout fills irregularities between the bolt and the rock, a keying effect is created to transfer the load to the bolt via shear resistance at the interface and within the grout. Previous research has revealed that the mechanical properties of the grout had a great impact on the load transfer capacity of the rock bolting system. This paper presents a method to enhance the rock bolting strength by introducing metal granules into the grouting material. Experimental results suggest that both the average peak load of pullout tests and the total energy absorption of the system will increase if some metal granules are mixed into the resin. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Many researchers have worked theoretically and experimentally on the mechanism of load transfer of fully grouted rock bolts. Up to date, it is commonly accepted that fully grouted bolts are much more successful in supporting roof strata than other bolting systems [1]. A fully grouted bolt provides greater shear surface for transmitting the load from the rock to the bolt, and vice versa. The grout supplies a mechanism for transferring the load between the rock and the reinforcing element. This redistribution of forces along the bolt is the result of movement in the rock mass, which transfers the load to the bolt via shear resistance in the grout. This shear resistance within the resin and along the interfaces can be the result of adhesion, friction and mechanical interlocking, which is a keying effect created when grout fills the irregularities between the bolt and the rock. Therefore, the performance of the reinforcement can be directly enhanced by improvement of the mechanical properties of the grouting material [2,3]. At the time of writing, approximately 80% of the over 100 million roof bolts installed in U.S. mines, tunnels, and construction projects employ polyester roof bolt resin. It is estimated that resin consumed in the US each year can encircle the world at the equator approximately three times if in a 22.9 mm diameter cartridge [4]. Current bolting technology uses a two part polyester resin cartridge to supply the bolt and borehole with sufficient resin to ⇑ Corresponding author. Tel.: +61 0425 334 939. E-mail address: [email protected] (C. Cao).
achieve the desired encapsulated length. The successful performance of the bolt and cartridge system requires that the bolt shreds the plastic cartridge and mixes the separate resin components during installation. The effectiveness of installed roof bolts can be compromised by gloving. Gloving refers to the plastic cartridge of a resin capsule encasing a length of bolt, typically with a combination of mixed and unmixed resin filler and catalyst remaining within the cartridge. This paper introduces a method to enhance the rock bolting strength by mixing metallic granules into the grouting material, as shown in Fig. 1. Experimental results show that the peak load of pullout tests will increase if some metal granules are mixed into the resin. The effect of the introduced metallic granules in reducing gloving is also discussed.
2. Related theories 2.1. Fully grouted bolting Fully grouted bolting consists of the bolt, grout, and surrounding rock. The relationships between them depend on a continuous mechanically coupled system. A fully grouted bolt is a passive roof support system, which is activated by movement of the surrounding rock. The efficiency of load transfer is affected by the mechanical properties of the grout, surface profile of the rock bolt, thickness of the grout annulus, anchorage length, rock properties, confining pressure and installation procedure.
http://dx.doi.org/10.1016/j.ijmst.2014.03.006 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
Bolt
Resin
Rock
Fig. 2. Parallel shear failure of the resin observed in laboratory pull out tests.
Failure surface Metallic granule Fig. 1. Schematic of the concept of mixing metallic granules into the resin.
In a fully grouted rock bolt, the load transfer mechanism depends on the shear stress developed on the bolt–resin and resin–rock interfaces. Peak shear stress and shear stress modulus of the interfaces determine the reaction of the bolt to the strata. Hence, the load transfer is determined by measuring the peak shear stress and system stiffness [5]. In addition, the post-failure behaviour of the rock bolting system is also important as it largely determines the total energy absorption of the system.
discontinuity surfaces is permitted to occur freely; for example, sliding of an unconstrained block of rock on a slope. Under the CNS condition, however, dilation may be depressed by the surrounding material due to the increased normal stress with shear displacement. For rock bolting systems, the dilatancy behaviour can be better conceptualised under the CNS condition, as shown in Fig. 3. That is, dilation of the failure surface may be constrained by the resin annulus and surrounding rock. The radial stiffness of the rock bolting system can be calculated using the thick-welled cylinder theory, which is widely accepted by theoretical rock bolting research studies [8,11]. Calculation of the results of some assemblage examples are listed in Table 1. The 4.2 mm resin annulus confinement can be used as the lower limit of the radial stiffness of resin grout rock bolting. If the confining material is 8 mm thick steel tube, the radial stiffness will reach
2.2. Failure mode Littlejohn classified various types of axial failure when using grouted bolts as follows: the bolt, the grout, the rock, the bolt– grout interface or grout–rock interface [6]. The type of axial failure depended on the properties of individual elements. The shear stress at the bolt–grout interface was smaller than that at the grout–rock interface because of the smaller effective area. If the grout and rock were of similar strengths, failure could occur at the bolt–grout interface. If the surrounding rock was softer then failure could occur at the grout–rock interface. Based on pullout tests of cable bolts in the laboratory and in the field, Hyett et al. identified two failure modes in cementitious grouted cable bolt [7–9]. One mode was radial splitting of the concrete cover surrounding the cable, while the other involved shearing of the cable against the concrete. The former concerns the wedge mechanism but it is rarely observed in the resin grouted bolting system. The shearing mechanism involved crushing of the grouting material ahead of the ribs on the bar, eventually making pullout along a cylindrical friction surface possible. It should also be noted that as the degree of radial confinement increased, the failure mechanism changed from radial fracturing of the cementitious annulus under low confinement, to shearing of the cement flutes and pullout along a cylindrical friction surface under high confinement. Recent research work of failure mode analysis suggests that a cylindrical failure surface around the bolt resin interface is a predominating failure mode in rock bolting [10]. It occurs for the smooth bars and for very closely spaced rebar bolts (like a screw) along the rib tips of the bar. For rebar bolts, experimental observation suggests that if the embedded length is short and the confining material is stiff, parallel shear failure occurs in laboratory pull out tests. Fig. 2 shows a pull out test bolt of 75 mm embedded length and confined in 8 mm thick steel tubes.
Field Granite, limestone, shale Hemlo Golden Giant Mine Drilling damage Natural fractures
Conceptual
Constant radial stiffness Steel, Aluminum, PVC pipes concrete blocks
Constant radial pressure Modified Hoek cell tests P2
Fig. 3. Schematic of a rock bolting system [8].
Table 1 Radial stiffness of commonly used experimental assemblages.
*
Confinement
Redial stiffness K (GPa/mm)
PVC* 4.2 mm resin annulus only Aluminium* Steel 1* Steel 2* UCS 40 MPa concrete block Steel 3* Infinite rock mass (E = 30 GPa, and v = 0.25) 8 mm steel sleeve
0.072 0.200 0.790 0.770 0.950 1.100 1.120 2.000 3.400
Reported in Kaiser et al. [13].
Aggregate
2.3. Dilatancy behaviour accompanying shearing The discontinuity behaviour is often studied under constant normal load (CNL) or constant normal stiffness (CNS) condition. For the CNL condition, dilatancy accompanying shearing of the
Grout
Steel bolt
Fig. 4. Alteration of the failure surface due to the introduced metallic granules.
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C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
(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.
T3 bolt EL=75 mm
Load (kN)
Without impurity With impurity
80 60 40 20 0
5
10
15
Load (kN)
T2 bolt EL=75 mm 120 100
140 120 100 80 60 40 20
20
0
Displacment (mm)
5
10 15 20 25 30 Displacment (mm)
35 40
Fig. 6. Test results.
3.4 GPa/mm [12]. For an infinite medium, the radial stiffness can be found via Eq. (1):
K¼
Er að1 þ mr Þ
ð1Þ
where a is the hole radius; Er the Young’s modulus; and vr the Poisson’s ratio of the rock. 2.4. Conceptualisation of introduced aggregate Fig. 4 conceptualises introducing aggregate into the resin matrix. If failure of the rock bolting system subjected to axial load occurs along the rib tips of a rebar bolt, then the aggregate intercepting the failure surface resists the relative slipping of the interface. Due to the interruption, an irregular failure surface is formed with increasing axial loads. The irregularity of the failure surface will cause extra dilation of the interface, increasing the effectiveness of the load transfer of the system. 3. Experimental study Laboratory pull out tests were carried out using two kinds of rebar bolt, namely the T2 and T3 bolts, which are popular in the Australian mining industry. The bolt was encapsulated in a steel tube 75 mm long and 8 mm thick using mix and pour resin. The specifications of the bolts’ surface profile configurations can be found in study investigated by Cao et al. [10]. Steel wire 2 mm diameter was cut to 2–3 mm long segments and mixed into the resin, with approximately 10 per bolt profile. Fig. 5 shows the post test sheared resin and the introduced metallic granules. Fig. 6 shows the test results. As expected, the load transfer capacity and the total absorbed energy of the bolt are both increased by up to 20% by introducing metallic granules into the resin. 4. Discussion Gloving is currently seen as an industrial problem because the gloved and unmixed portions reduce the effective anchor length
and adversely affect the reinforcement for the roof strata. Research shows 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 installed using either hand held pneumatic or continuous miner-mounted hydraulic bolting rigs, under face conditions by operators, and under 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 can significantly reduce gloving, and concluded that gloving could be significantly reduced by a bolt end that nearly contacted the side of the bolt hole [15]. However, due to installation difficulties, the patent pending bolt cannot be applied using standard Australian bolting rigs. It is obvious that the plastic film will be ground into pieces if the bolt diameter equaled the bore hole diameter; however, it remains a question whether this can be achieved via other means. Introducing metallic granules into the resin will lead to extra slipping of the plastic film against the granules while the bolt is being installed. This may greatly reduce the extent of the gloving problem because the effect of the introduced metallic granules can be thought of as a way to increment bolt diameter without leading to installation difficulties.
5. Conclusions This study introduces a new method to increase the load transfer capacity of a fully grouted rock bolting system by introducing metallic granules into the resin. When the granules are mixed into the resin, the failure surface around the interfaces will become irregular for the parallel shear failure mode of the system. This will lead to extra dilation of the failure surface when and after failure occurs. Laboratory pullout tests were conducted for two kinds of rebar bolts which are commonly used in the Australian mining industry. Results show that both the peak load and total energy absorption can be substantially increased by about 20%. This innovation is also proposed as a possible solution to reduce the gloving of the system; however, more research is needed to further examine and quantify this hypothesis.
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C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
References [1] Jalalifar H. A new approach in determining the load transfer mechanism in fully grouted bolts. Wollongong: University of Wollongong; 2006. [2] Stillborg B. Professional users handbook for rock bolting, series on rock and soil mechanics. Atalas Copco 1994:1–85. [3] Cao C, Nemcik Jan, Ting Ren, Aziz Naj. A study of rock bolting failure modes. Int J Min Sci Technol 2013;23(1):79–88. [4] Blevins TC, Campoli AA. The Origin and History of U.S. Mine Resin. In: Proceeding of 25th International Conference on Ground Control. Morgantown; 2006. p. 409–11. [5] Stille H. Rock support in theory and practice. Int Symp Rock support Min Underground Construct 1992:421–37. [6] Littlejohn S. Rock reinforcement-technology, testing, design and evaluation. Oxford: Pergamon Press; 1993. p. 413–451. [7] Hyett AJ, Bawden WF, Reichert RD. The effect of rock mass confinement on the bond strength of fully grouted cable bolts. Int J Rock Mech Min Sci Geomech Abstr 1992;29(5):503–24. [8] Hyett AJ, Bawden WF, Macsporran GR, Moosavi M. A constitutive law for bond failure of fully-grouted cable bolts using a modified Hoek cell. Int J Rock Mech Min Sci Geomech Abstr 1995;32(1):11–36.
[9] Hyett AJ, Mossavi M, Bawden WF. Load distribution along fully grouted bolts, with emphasis on cable bolt reinforcement. Int J Numer Analyt Methods Geomech 1996;20:517–44. [10] Cao C, Nemcik J, Aziz N, Ren T. Analytical study of steel bolt profile and its influence on bolt load transfer. Int J Rock Mech Min Sci 2013;60:188–95. [11] Yazici S, Kaiser PK. Bond strength of grouted cable bolts. Int J Rock Mech Min Sci Geomech Abstr 1995;29(3):279–92. [12] Cao C, Ren T, Chris Cook. Calculation of the effect of Poisson’s ratio in laboratory push and pull testing of resin-encapsulated bolts. Int J Rock Mech Min Sci 2013;64:175–80. [13] Kaiser PK, Yazici S, Nose J. Effect of stress change on the bond strength of fully grouted cables. Int J Rock Mech Min Sci Geomech Abstr 1992;29(3): 293–306. [14] Campbell R, Mould RJ. Impacts of gloving and un-mixed resin in fully encapsulated roof bolts on geotechnical design assumptions and strata control in coal mines. Int J Coal Geol 2005;64(1–2):116–25. [15] Craig P. Addressing resin loss and gloving issues at a mine with coal roof. In: Proceedings of 12th Coal Operators’ Conference. Wollongong; 2012, p. 120– 128.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Introducing aggregate into grouting material and its influence on load transfer of the rock bolting system Cao Chen ⇑, Ren Ting, Chris Cook Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 8 December 2013 Available online 29 April 2014 Keywords: Bolting strength Resin improvement Parallel shear failure Dilational slip
a b s t r a c t A fully grouted bolt provides greater shear load capacity for transmitting the load from the rock to the bolt, and vice versa. When grout fills irregularities between the bolt and the rock, a keying effect is created to transfer the load to the bolt via shear resistance at the interface and within the grout. Previous research has revealed that the mechanical properties of the grout had a great impact on the load transfer capacity of the rock bolting system. This paper presents a method to enhance the rock bolting strength by introducing metal granules into the grouting material. Experimental results suggest that both the average peak load of pullout tests and the total energy absorption of the system will increase if some metal granules are mixed into the resin. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Many researchers have worked theoretically and experimentally on the mechanism of load transfer of fully grouted rock bolts. Up to date, it is commonly accepted that fully grouted bolts are much more successful in supporting roof strata than other bolting systems [1]. A fully grouted bolt provides greater shear surface for transmitting the load from the rock to the bolt, and vice versa. The grout supplies a mechanism for transferring the load between the rock and the reinforcing element. This redistribution of forces along the bolt is the result of movement in the rock mass, which transfers the load to the bolt via shear resistance in the grout. This shear resistance within the resin and along the interfaces can be the result of adhesion, friction and mechanical interlocking, which is a keying effect created when grout fills the irregularities between the bolt and the rock. Therefore, the performance of the reinforcement can be directly enhanced by improvement of the mechanical properties of the grouting material [2,3]. At the time of writing, approximately 80% of the over 100 million roof bolts installed in U.S. mines, tunnels, and construction projects employ polyester roof bolt resin. It is estimated that resin consumed in the US each year can encircle the world at the equator approximately three times if in a 22.9 mm diameter cartridge [4]. Current bolting technology uses a two part polyester resin cartridge to supply the bolt and borehole with sufficient resin to ⇑ Corresponding author. Tel.: +61 0425 334 939. E-mail address: [email protected] (C. Cao).
achieve the desired encapsulated length. The successful performance of the bolt and cartridge system requires that the bolt shreds the plastic cartridge and mixes the separate resin components during installation. The effectiveness of installed roof bolts can be compromised by gloving. Gloving refers to the plastic cartridge of a resin capsule encasing a length of bolt, typically with a combination of mixed and unmixed resin filler and catalyst remaining within the cartridge. This paper introduces a method to enhance the rock bolting strength by mixing metallic granules into the grouting material, as shown in Fig. 1. Experimental results show that the peak load of pullout tests will increase if some metal granules are mixed into the resin. The effect of the introduced metallic granules in reducing gloving is also discussed.
2. Related theories 2.1. Fully grouted bolting Fully grouted bolting consists of the bolt, grout, and surrounding rock. The relationships between them depend on a continuous mechanically coupled system. A fully grouted bolt is a passive roof support system, which is activated by movement of the surrounding rock. The efficiency of load transfer is affected by the mechanical properties of the grout, surface profile of the rock bolt, thickness of the grout annulus, anchorage length, rock properties, confining pressure and installation procedure.
http://dx.doi.org/10.1016/j.ijmst.2014.03.006 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
326
C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
Bolt
Resin
Rock
Fig. 2. Parallel shear failure of the resin observed in laboratory pull out tests.
Failure surface Metallic granule Fig. 1. Schematic of the concept of mixing metallic granules into the resin.
In a fully grouted rock bolt, the load transfer mechanism depends on the shear stress developed on the bolt–resin and resin–rock interfaces. Peak shear stress and shear stress modulus of the interfaces determine the reaction of the bolt to the strata. Hence, the load transfer is determined by measuring the peak shear stress and system stiffness [5]. In addition, the post-failure behaviour of the rock bolting system is also important as it largely determines the total energy absorption of the system.
discontinuity surfaces is permitted to occur freely; for example, sliding of an unconstrained block of rock on a slope. Under the CNS condition, however, dilation may be depressed by the surrounding material due to the increased normal stress with shear displacement. For rock bolting systems, the dilatancy behaviour can be better conceptualised under the CNS condition, as shown in Fig. 3. That is, dilation of the failure surface may be constrained by the resin annulus and surrounding rock. The radial stiffness of the rock bolting system can be calculated using the thick-welled cylinder theory, which is widely accepted by theoretical rock bolting research studies [8,11]. Calculation of the results of some assemblage examples are listed in Table 1. The 4.2 mm resin annulus confinement can be used as the lower limit of the radial stiffness of resin grout rock bolting. If the confining material is 8 mm thick steel tube, the radial stiffness will reach
2.2. Failure mode Littlejohn classified various types of axial failure when using grouted bolts as follows: the bolt, the grout, the rock, the bolt– grout interface or grout–rock interface [6]. The type of axial failure depended on the properties of individual elements. The shear stress at the bolt–grout interface was smaller than that at the grout–rock interface because of the smaller effective area. If the grout and rock were of similar strengths, failure could occur at the bolt–grout interface. If the surrounding rock was softer then failure could occur at the grout–rock interface. Based on pullout tests of cable bolts in the laboratory and in the field, Hyett et al. identified two failure modes in cementitious grouted cable bolt [7–9]. One mode was radial splitting of the concrete cover surrounding the cable, while the other involved shearing of the cable against the concrete. The former concerns the wedge mechanism but it is rarely observed in the resin grouted bolting system. The shearing mechanism involved crushing of the grouting material ahead of the ribs on the bar, eventually making pullout along a cylindrical friction surface possible. It should also be noted that as the degree of radial confinement increased, the failure mechanism changed from radial fracturing of the cementitious annulus under low confinement, to shearing of the cement flutes and pullout along a cylindrical friction surface under high confinement. Recent research work of failure mode analysis suggests that a cylindrical failure surface around the bolt resin interface is a predominating failure mode in rock bolting [10]. It occurs for the smooth bars and for very closely spaced rebar bolts (like a screw) along the rib tips of the bar. For rebar bolts, experimental observation suggests that if the embedded length is short and the confining material is stiff, parallel shear failure occurs in laboratory pull out tests. Fig. 2 shows a pull out test bolt of 75 mm embedded length and confined in 8 mm thick steel tubes.
Field Granite, limestone, shale Hemlo Golden Giant Mine Drilling damage Natural fractures
Conceptual
Constant radial stiffness Steel, Aluminum, PVC pipes concrete blocks
Constant radial pressure Modified Hoek cell tests P2
Fig. 3. Schematic of a rock bolting system [8].
Table 1 Radial stiffness of commonly used experimental assemblages.
*
Confinement
Redial stiffness K (GPa/mm)
PVC* 4.2 mm resin annulus only Aluminium* Steel 1* Steel 2* UCS 40 MPa concrete block Steel 3* Infinite rock mass (E = 30 GPa, and v = 0.25) 8 mm steel sleeve
0.072 0.200 0.790 0.770 0.950 1.100 1.120 2.000 3.400
Reported in Kaiser et al. [13].
Aggregate
2.3. Dilatancy behaviour accompanying shearing The discontinuity behaviour is often studied under constant normal load (CNL) or constant normal stiffness (CNS) condition. For the CNL condition, dilatancy accompanying shearing of the
Grout
Steel bolt
Fig. 4. Alteration of the failure surface due to the introduced metallic granules.
327
C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
(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.
T3 bolt EL=75 mm
Load (kN)
Without impurity With impurity
80 60 40 20 0
5
10
15
Load (kN)
T2 bolt EL=75 mm 120 100
140 120 100 80 60 40 20
20
0
Displacment (mm)
5
10 15 20 25 30 Displacment (mm)
35 40
Fig. 6. Test results.
3.4 GPa/mm [12]. For an infinite medium, the radial stiffness can be found via Eq. (1):
K¼
Er að1 þ mr Þ
ð1Þ
where a is the hole radius; Er the Young’s modulus; and vr the Poisson’s ratio of the rock. 2.4. Conceptualisation of introduced aggregate Fig. 4 conceptualises introducing aggregate into the resin matrix. If failure of the rock bolting system subjected to axial load occurs along the rib tips of a rebar bolt, then the aggregate intercepting the failure surface resists the relative slipping of the interface. Due to the interruption, an irregular failure surface is formed with increasing axial loads. The irregularity of the failure surface will cause extra dilation of the interface, increasing the effectiveness of the load transfer of the system. 3. Experimental study Laboratory pull out tests were carried out using two kinds of rebar bolt, namely the T2 and T3 bolts, which are popular in the Australian mining industry. The bolt was encapsulated in a steel tube 75 mm long and 8 mm thick using mix and pour resin. The specifications of the bolts’ surface profile configurations can be found in study investigated by Cao et al. [10]. Steel wire 2 mm diameter was cut to 2–3 mm long segments and mixed into the resin, with approximately 10 per bolt profile. Fig. 5 shows the post test sheared resin and the introduced metallic granules. Fig. 6 shows the test results. As expected, the load transfer capacity and the total absorbed energy of the bolt are both increased by up to 20% by introducing metallic granules into the resin. 4. Discussion Gloving is currently seen as an industrial problem because the gloved and unmixed portions reduce the effective anchor length
and adversely affect the reinforcement for the roof strata. Research shows 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 installed using either hand held pneumatic or continuous miner-mounted hydraulic bolting rigs, under face conditions by operators, and under 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 can significantly reduce gloving, and concluded that gloving could be significantly reduced by a bolt end that nearly contacted the side of the bolt hole [15]. However, due to installation difficulties, the patent pending bolt cannot be applied using standard Australian bolting rigs. It is obvious that the plastic film will be ground into pieces if the bolt diameter equaled the bore hole diameter; however, it remains a question whether this can be achieved via other means. Introducing metallic granules into the resin will lead to extra slipping of the plastic film against the granules while the bolt is being installed. This may greatly reduce the extent of the gloving problem because the effect of the introduced metallic granules can be thought of as a way to increment bolt diameter without leading to installation difficulties.
5. Conclusions This study introduces a new method to increase the load transfer capacity of a fully grouted rock bolting system by introducing metallic granules into the resin. When the granules are mixed into the resin, the failure surface around the interfaces will become irregular for the parallel shear failure mode of the system. This will lead to extra dilation of the failure surface when and after failure occurs. Laboratory pullout tests were conducted for two kinds of rebar bolts which are commonly used in the Australian mining industry. Results show that both the peak load and total energy absorption can be substantially increased by about 20%. This innovation is also proposed as a possible solution to reduce the gloving of the system; however, more research is needed to further examine and quantify this hypothesis.
328
C. Cao et al. / International Journal of Mining Science and Technology 24 (2014) 325–328
References [1] Jalalifar H. A new approach in determining the load transfer mechanism in fully grouted bolts. Wollongong: University of Wollongong; 2006. [2] Stillborg B. Professional users handbook for rock bolting, series on rock and soil mechanics. Atalas Copco 1994:1–85. [3] Cao C, Nemcik Jan, Ting Ren, Aziz Naj. A study of rock bolting failure modes. Int J Min Sci Technol 2013;23(1):79–88. [4] Blevins TC, Campoli AA. The Origin and History of U.S. Mine Resin. In: Proceeding of 25th International Conference on Ground Control. Morgantown; 2006. p. 409–11. [5] Stille H. Rock support in theory and practice. Int Symp Rock support Min Underground Construct 1992:421–37. [6] Littlejohn S. Rock reinforcement-technology, testing, design and evaluation. Oxford: Pergamon Press; 1993. p. 413–451. [7] Hyett AJ, Bawden WF, Reichert RD. The effect of rock mass confinement on the bond strength of fully grouted cable bolts. Int J Rock Mech Min Sci Geomech Abstr 1992;29(5):503–24. [8] Hyett AJ, Bawden WF, Macsporran GR, Moosavi M. A constitutive law for bond failure of fully-grouted cable bolts using a modified Hoek cell. Int J Rock Mech Min Sci Geomech Abstr 1995;32(1):11–36.
[9] Hyett AJ, Mossavi M, Bawden WF. Load distribution along fully grouted bolts, with emphasis on cable bolt reinforcement. Int J Numer Analyt Methods Geomech 1996;20:517–44. [10] Cao C, Nemcik J, Aziz N, Ren T. Analytical study of steel bolt profile and its influence on bolt load transfer. Int J Rock Mech Min Sci 2013;60:188–95. [11] Yazici S, Kaiser PK. Bond strength of grouted cable bolts. Int J Rock Mech Min Sci Geomech Abstr 1995;29(3):279–92. [12] Cao C, Ren T, Chris Cook. Calculation of the effect of Poisson’s ratio in laboratory push and pull testing of resin-encapsulated bolts. Int J Rock Mech Min Sci 2013;64:175–80. [13] Kaiser PK, Yazici S, Nose J. Effect of stress change on the bond strength of fully grouted cables. Int J Rock Mech Min Sci Geomech Abstr 1992;29(3): 293–306. [14] Campbell R, Mould RJ. Impacts of gloving and un-mixed resin in fully encapsulated roof bolts on geotechnical design assumptions and strata control in coal mines. Int J Coal Geol 2005;64(1–2):116–25. [15] Craig P. Addressing resin loss and gloving issues at a mine with coal roof. In: Proceedings of 12th Coal Operators’ Conference. Wollongong; 2012, p. 120– 128.