Jun Hn, Zuoqing Bi, Bing Ling, Chen Co,b,*, Shungwen M

a College of Mining, Liaoning Technical University, Fuxin 123000, China

b ME, EIS, University of Wollongong, Wollongong, NSW 2526, Australia

Keywords:FRP bolt Laboratory SEPT Tensile strength Double shear testing Mined rib support Large deformation roadway

A B S T R A C T In underground coal mines,fibre reinforced polymer(FRP)bolt is ideal for mined rib reinforcements as it can prevent gas explosions caused by shearer frictional spark.With increasing mining depth,small diameter FRP bolts used in shallow underground mining cannot fulfil the rib support requirements.Under the engineering background of deep underground shortwall mining in Wudong coal mine,this paper systematically studies Φ27 mm FRP bolt support for large deformation coal rib.Specimens with a fan-shaped cross-section were used to enable the tensile testing of the bolt rod, the measured average tensile strength of the studied FRP bolt was (486.1 ± 9.6) MPa with a maximum elongation of 5.7%±0.6%.The shear strength of the bolt was measured as approximately 258 MPa using a self-made double shear testing apparatus.Based on the equivalent radial stiffness principle, a laboratory short encapsulation pullout test (SEPT) method for rib bolting has been developed undertaken consideration of the mechanical properties of the coal seam.Results showed that the average peak anchorage forces of the Φ27 mm FRP bolt and Φ20 mm steel rebar bolt were 108.4 and 66.4 kN,respectively,which were agreed with the theoretical calculations and field measurements.Based on theoretical analysis of the loading states of the bolt under site conditions, bolting method of full-length resin grouting was adopted to offset the weak

1.Introduction

Fiber reinforced polymer (FRP) tendons are widely used as structural and ground reinforcing materials in the civil, mining,environmental and energy engineering fields due to their high tensile strength,good resistance to degradation and creep,low weight and easy being cut compared to conventional steel rebar bolts[1,2].Although FRP materials may be produced with a range of different strength and stiffness properties, some FRP mechanical properties are considerably weaker than those of steel tendons,such as the axial ductility, shear strength and end fixture [2].For a successful FRP reinforcement system, the challenge lies within the anchorage properties itself, along with the displacement field of the applied application.

In underground coal mines with high gas contents, the steel tendons anchored in the mined rib must be removed in advance to prevent frictional spark from shearer cutting process;therefore,FRP bolt is an ideal material for rib support as it can be cut directly by the shearer without spark generation.In previous studies, partially grouted Φ16–22 mm FRP bolts were reported for rib support whereas the integrity of the surrounding rock was relatively good[3–8].Field observations suggested that the external members,including the end plate and nut, were the most vulnerable unit of the FRP anchor [7–12].Several technical measures have been developed to enhance the end fixture of the bolt,including supplementary support[8],wedged plates[11],thick plates[12],wooden plates [7] and metallic plates [9].However, a practical method to avoid the shortage of the end fixture is remaining to be found.

With the exploitation of deep resources,the ground stress of the surrounding rock increases, which causes a large convergence of coal pillars, or even rib spalling [13,14].Although Φ16–22 mm FRP bolts can be successfully used for rib support under partial grouting method in small deformation roadways,FRP bolt support for large deformation roadway is a challenge.From 2015, high strength FRP bolts were introduced to the underground coal mining industry [15].In a case study, He and Wu [16] reported that high strength Φ22 mm × 2400 mm FRP bolts were used in a –530 m roadway, and the measured maximum axial load was 113.2 kN, which accounted for 86% of the rod bearing capacity.In a large deformation roadway(750 m buried depth),it was found that previous support method using Φ22 mm FRP bolt could not be successful,Yao[17]used a kind of high strength Φ27 mm FRP bolt in a testing tunnel, but no further result was reported.It can be concluded that the Φ16–22 mm FRP bolts used in shallow seam mining can hardly fulfil the support requirements in deep underground.Therefore, research on large diameter FRP rib bolting is of great significance.

Large-diameter FRP bolting practice is closely relevant to several important issues.Literature review shows that, most of the field application of the FRP bolts is heavily relied on previous engineering experiences, with a lack of theoretical analysis and experimental verification.Reliable and practical laboratory testing method should also be developed to effectively determine the mechanical properties of the bolt,nominated mechanical specifications only indicate a manufacturing standard and may vary greatly from actual parameters.Moreover, the anchorage performance of the FRP bolts should be tested under consideration of the field geo-conditions.Another point is that, both of the stress environment and deformation requirement of the FRP bolts used in civil or structural engineering are quite different to those used in coal mining roadways, in which ground stress exists in surrounding rock and plastic deformation or even partial failure of the bolt are allowed due to short service period.So, the theories and conclusions drawn from FRP bolting research underground engineering condition should be critically evaluated before underground application.With this in mind, the development of a laboratory short encapsulation pullout testing (SEPT) method to investigate the rib bolting performance of FRP anchor is essential.

With the engineering background of large deformation roadway reinforcement in Wudong coal mine, China, this work systematically studies a kind of high strength large-diameter FRP bolt for rib support, including the related theories, methodology of mechanical properties testing, SEPT design, end member protection technology and reinforcing effect prediction.In laboratory SEPT, PVC tube was employed as confining material to simulate the deformational behaviour of the coal mass based on thickwalled cylinder theory, which provides a new approach to effectively determine the bolting effect and greatly reduce testing costs.In addition, Φ20 mm steel rebar bolt was also tested as a comparison.

2.Mechanisms

A rockbolting system consists of 3 media and 2 interfaces:bolt,grout, rock, bolt-resin and resin-rock interfaces.The anchorage performance of the bolt is the result of the interaction between the elements in the system, or the so-called load transfer mechanism.When a bolt is subjected to axial load, shear stress is generated within the resin and along 2 interfaces (Fig.1).Research shows that interface shear failure is the dominated failure mode of the anchorage[18–20].The relationship between the axial force of the bolt and the shear stress distribution along the bolt-resin interface can be expressed as:

where F is the axial force of the bolt;r the bolt radius;τ(x)the shear stress of the interface;x the displacement from the borehole collar;and L the length of the anchorage section.

In rockbolting mechanics,the shear lag model(SLM)is the most popular mechanical model for rock bolts subjected to axial load.In the model, the shearing stress-strain relationship of the interface can be assumed to be linear [18], bi-linear [19] or tri-linear [20],and their correlated shear stress distributions along the interface are shown in Fig.1.However, the shear stress distribution developed by SLM involves unrealistic assumptions, and its expression is also complex.In practices, the average shear strength of the anchorage is often employed to indicate the bonding strength,which can be simply calculated from the field SEPT data.Then,the deformation of the roadway can be predicted using the numerical methods.In any case, the relationship between the axial force of the bolt and the average shear stress at the interface can be expressed as:

where τaveis the average shear stress at the interface; and L the bolting length, it is about 100 mm in the laboratory SEPT, and is around 300 mm in field SEPT.

Eq.(2) shows that the anchorage bearing capacity is proportional to the diameter of the anchored bolt, indicating that the maximum anchorage force of a Φ27 mm bolt increases about 24% compared to that of a Φ22 mm bolt.In addition, the weight of FRP bolt is lighter than steel bolt and drilling of large borehole is easier in coal mass than in the roof, both are advantages of largediameter FRP bolting engineering.It suggests that the largediameter FRP bolt support is a proper material and technology upgrade in deep underground coal mining.

It should be noted that low shear strength and vulnerable external fixture are weaknesses of FRP bolts.A successful FRP bolting application relies on the compensation of the material and structure shortages.In addition, the tensile performance of a largediameter FRP bolt can hardly be measured using conventional laboratory machinery.Especially,in terms of anchorage performance,the on-site SEPT is time-consuming and laborious, nor there is no test standard for laboratory SEPT, resulting in the testing result is seriously divorced from reality.In the following sections, a comprehensive testing of Φ27 mm FRP bolt will be carried out to provide new experimental methodology and reliable parameters for the reinforcing design using large diameter FRP bolts.

3.Experiments and results

3.1.Tensile test

The mechanical properties of the FRP bolt rod should be measured as their composition or manufacturing method may be quite different.For a high strength large diameter FRP rod,the laboratory tensile test is difficult as the clamping force of the testing machine is often low for a such material.Tensile test using both ends anchored sample can be conducted for small diameter FRP rods[21].However, the maximum tensile force of a Φ27 mm FRP bolt is estimated as over 250 kN, so, the sample length would be large due to long anchored ends.To shorten the specimen,several methods have been developed to improve the anchorage force capacity of a FRP tendon in structure engineering research; for example,using contoured sleeve [2] or curved angle wedge anchor [22],which may be adopted for tensile testing of the large-diameter FRP rods.

In this study, the middle section of a Φ27 mm × 500 mm FRP rod was processed to 80° in a fan-shape cross-section around the rod axis; then, it was clamped and pulled by the testing machine.In the testing procedure, it was observed that the matrix material of the bolt peeled off in early stage of the testing.When the tensile load reached 60%–70%of the ultimate load,the specimen began to sound.Finally,the specimen broke with a loud sound.Post-testing sample showed that the glass fibres were cross-fractured and the bundling dispersed, as shown in Fig.2.The failure mode of the testing sample was identical to those of tensile tests of small diameter FRP rods using fully cross-sectional sample [23,24], it can be concluded that the testing result is reliable.The measured tensile strength of the rod was(486.1±9.6)MPa,with a maximum elongation of 5.7%±0.6%.

Fig.2.Post-testing sample of the FRP rod in the tensile test.

3.2.Shearing test

Although the tensile behaviour of the FRP bolt is a major concern for rib bolting,the shearing strength should also be evaluated to ensure the lateral deformation capacity of the bolt.Referring to several testing methods [20,24,25], a self-made double shear testing device(Fig.3)was developed to test the shearing performance of the FRP bolt.The results showed that the shear strength of the rod was (258.4 ± 10.5) MPa, which was approximately 50 % of its tensile strength.

Fig.3.Shear testing of the FRP bolt.

Table 1 summarizes the mechanical properties of FRP rods used for underground coal mines in the literature and in this study.It shows that the nominated mechanical properties were employed in most of the research.For high strength FRP bolts,the nominated tensile strengths were 750 MPa in Ref.[6]and 650 MPa in Ref.[26],but the measurement was 579 MPa in Ref.[5]and 486 MPa in this study.Therefore, the tensile strength nominated by the manufacturers may be overestimated.

Only nominated shear strengths were provided in the related literatures.In this study,the measured shear strength was approximately 258 MPa, which can be used as a preliminary estimation for a high strength Φ27 mm FRP bolt.It is strongly recommended to measure the shear strength of the utilized FRP bolt in each application, as shear failure is a dominant failure mode for FRP rib bolting.

3.3.Laboratory and field SEPT

Table 1 Mechanical properties of the FRP rods used in underground coal mines.

Anchorage parameters are essential for reinforcing system design and ground deformation prediction; it should be evaluated accurately, precisely and comprehensively [27–29].Field tests are time-consuming and laborious, and the data consistency is low.In laboratory testing, variables can easily be controlled to obtain a consistent result, which is favourable for scientific research.Therefore,the laboratory SETP is the most commonly used method to evaluate bolting performance.

It is worth noting that the conditions of the FRP bolts anchored in underground roadway are quite different to those in ground engineering, such as civil, soil, dam, and foundation application.In underground excavation, not only the environment harsh, but the loading state of the bolt are also quite different.The ground stress in a civil structure is often negligible;however,high ground stress is a typical environment for a bolt anchored in deep underground roadway.Moreover, the reinforcing requirements are normally different between civil structures and mining roadways.The anchors in civil engineering are often required undergoing elastic deformation; however, large deformation or even partial failure of underground support members is allowed due to short service period.Therefore,the theories and conclusions drawn from ground engineering need to be re-evaluated and justified before being applied to underground engineering.

The bolting performance of a rib bolt is related to the ground stress and the mechanical properties of the coal mass.In laboratory SEPT, concrete blocks or steel sleeves with different wall thicknesses are often used as confining materials to prepare bolting samples,but there is no testing standard at present.Based on thick welled cylinder theory, Zhang et al.[30] developed a laboratory bolting specimen design method using the equivalent radial stiffness of the surrounding rock to improve the accuracy of the laboratory SEPT result:where n is the inner diameter of the sleeve,mm;w the outer diameter of the sleeve,mm;E the Young’s modulus of the sleeve,GPa;v the Poisson’s ratio of the sleeve;ERthe Young’s modulus of the rock in GPa; and vRthe Poisson’s ratio of the rock.

For rib bolting, the surrounding rock was coal mass (Young’s modulus 1.0–3.5 GPa), whose radial rigidity was equivalent to the selected PVC sleeve of thickness 3.6–62.3 mm.In laboratory testing, PVC sleeves with an inner diameter of 37 mm, wall thickness of 35 mm, and length of 100 mm were used as the confining material to prepare the bolting specimens, as shown in Fig.4.In contrast,Φ20 mm steel rebar bolt used in the coal mine was also tested using PVC sleeves with an inner diameter of 30 mm and wall thickness of 31 mm as the confining material.

The laboratory SEPT results are shown in Fig.5a,which showed that the average anchorage capacities of the Φ27 mm FRP and Φ20 mm steel rebar bolting samples were 108.4 and 66.4 kN, respectively.According to Eq.(2), the average bonding strengths of the two kinds of tested anchor were 12.8 MPa and 10.6 MPa, respectively.That is, the bonding strength of the FRP anchor was 21%higher than that of the steel rebar anchor, and its axial loading capacity was 63%higher than that of the steel bolt,which indicates that increasing the bolt diameter is an efficient manner to increase the anchorage strength.

Fig.5.Laboratory and field SEPT results.

Field SEPT of the Φ27 mm FRP anchor and Φ20 mm steel rebar anchors were also carried out at the mined rib of +425 level B3 roadway of the Wudong coal mine.The anchorage length of both kinds of bolt was 300 mm,which is specified in the industrial testing standard [21].The average diameters of the drilling holes for the FRP bolt and steel bolt were measured as 37 and 30 mm,respectively.The average maximum pullout force of the FRP bolt was 101.0 kN, which was approximately 31% higher than that of the steel rebar bolt, as shown in Fig.5b.

Although PVC tubes were carefully selected as the confinement material to simulate the deformation of the coal mass in the field,however,the differences in the results between the laboratory and field SEPTs were still large.This may be related to the fracture development around the drilling hole, remaining coal powder in the borehole, and resin lost in the field.Nevertheless, the equivalent radial stiffness principle in the laboratory SEPT design should be followed as, within authors’ knowledge, the bonding strength measured in the laboratory is the closest to reality in the literature.

4.Application case

4.1.Background and field conditions

Wudong coal mine is located 34 km northeast of Urumqi,China.The annual production is around 6.0 Mt.The seam thickness is(37.5 ± 5.6) m, and the buried depth is around 450 m.The seam is totally located in the south wing of the Badaowan syncline,whose dipping angle is 87°, i.e., it is a nearly vertical coal seam(Fig.6).The in-situ stress was measured as 14.3, 8.1 and 7.2 MPa in the horizontal towards rib, horizontal along roadway and vertical directions, respectively.

As the coal seam is nearly vertical,shortwall with top coal caving mining method was employed, as shown in Fig.7.The panel width and excavation height were approximately 25 m.The mining roadway was fully developed within the seam, with a crosssectional area of about 17 m2, one hard rock rib and one coal rib.

Fig.7.Physical model of the roadway in the coal seam.

In the original support scheme (Fig.8), Φ20 mm × 2500 mm steel rebar bolts were used as the primary support in an 800 mm × 800 mm arrangement; and Φ18.9 mm × 10000 mm cable bolt was used as the secondary support.The grouting length of the rebar bolt was 700 mm, with pre-tensioning torque around 60 N∙m.

Fig.8.Original support scheme in Wudong coal mine.

With increasing mining depth, the deformation of the roadway increases due to high ground stress of the surrounding rock, manifesting as a large roof subsidence, floor heave, and spalling of the mined rib.The maximum convergence of the mined rib is up to 500 mm.

Another problem in the mining process is spark generated by the shearer cutting.As the working face is only 25 m width, its advancing speed is quite high.If the steel bolt was anchored in the mined rib, it must be removed from the coal mass before the shearer cutter reaches it.Bolt removal is a time-consuming,laborious, and dangerous operation as it is done manually.As a result,the cutter has to be frequently stopped waiting for steel bolt removal.Operation safety and mining productivity can be greatly improved if steel bolt can be replaced by FRP bolt.

Partially grouted Φ18 mm FRP rib bolts were used in Wudong coal mine several years ago;however,with the increase of the mining depth, FRP rod breaking, end shearing, plate damage and nut retreat were observed.To fulfil the support requirements in deep underground environments, high strength Φ27 mm FRP bolts was introduced to Wudong coal mine.Consequently, theoretical analysis relevant to support design should be conducted to optimise the reinforcing effect.

4.2.FRP reinforcement analysis

The axial loading capacity of Φ27 mm FRP bolt is high (31%increment in field tests),which can effectively reduce the rib deformation.For shear deformation of the coal mass (including vertical and tunnel axial displacement), considering that the surrounding rock is isotropic (full coal roadway) and the maximum principal stress is nearly normal to the rib surface, it is supposed that the shear displacement of the rib coal would be small,which can avoid one weakness of the FRP material.In addition,the shear strength of the tested FRP bolt(258 MPa)was 3.4 times the industry standard[21], showing good shearing resistance.

The elongation (5.7%) of the FRP bolt was lower than that of steel bolt (13.3%), although the end connection was also a notable weakness.To offset these defects, the anchorage method was designed as full-length resin grouting.According to the rockbolting mechanism[31,32],the axial stress concentration can be generated at each location of the thick-plated coal body(similar to the strata separation) under full-length grouting, which is conducive to maintaining the integrity of the surrounding rock.Large axial displacement may cause bolt breakage; however, the remaining bolt segments can have certain ground control capability as they are still grouted.Moreover, the magnitude of pre-tensioning for fulllength anchorage is low, which avoids high bearing load of the end members and protects the integrity of the support structure.

Based on the rockbolting mechanism and the experimental results, a fully grouted large diameter FRP rib bolting scheme was proposed.To further examine the proposed support design,the numerical method was employed to predict the support effect and to obtain insights of the deformational behaviour of reinforced surrounding rock.

4.3.Numerical study

FLAC 3D was used to optimize the reinforcing design.The model was built based on the geological conditions of Wudong coal mine(Fig.9).In the model,the FRP bolt was simulated as a pile structure whose interface parameters were drawn from testing result and similar research [33].

Fig.9.Numerical model of the site geo-conditions.

The size of the numerical model of the mining roadway in Wudong coal mine was 35 m × 5 m × 45 m.A total of 1956500 units and 2026944 nodes were established.The loading conditions were applied according to the measured field stress.The initial stress distribution is shown in Fig.10.

Fig.10.Stress distribution cloud of the numerical model after mining.

The calculation results show that, under the condition of fully grouted rib bolting, the maximum shear displacement of the FRP bolt was 25 mm,and the maximum axial load of the pile elements was 117 kN.The calculated maximum horizontal displacement of the rib coal was 198 and 124 mm using the rebar bolt and Φ27 mm FRP bolt,respectively,as shown in Fig.11.It can be concluded that the Φ27 mm FRP rib bolting is feasible for Wudong coal mine +425 level roadway support.

Fig.11.Displacement contour of the mining roadway.

5.Conclusions

In this study, the mechanical properties and bolting effect of Φ27 mm FRP bolts was tested based on laboratory and field experiments.The application of large diameter FRP bolts for mined rib support was studied using the numerical method.The following conclusions can be drawn.

(1) For laboratory SEPT, based on the principle of equivalent radial stiffness, PVC sleeves should be selected as confining material to simulate the coal mass, so that the test results could be as close as possible to reality.The results showed that the peak anchorage capacities of the Φ27 mm FRP bolt and Φ20 mm steel rebar bolt were 108.4 and 66.4 kN,respectively.Correspondingly, the average bonding strengths of the two kinds of testing anchors were 12.8 and 10.6 MPa.

(2) Tensile and shearing tests of the large-diameter FRP bolts are strongly recommended in each application.The measured tensile strength of the rod used in this study was(486.1±9.6) MPa, with a maximum elongation of 5.7%±0.6%, and the measured shear strength of the bolt was approximately 258 MPa.

(3) The bolting effect of large-diameter FRP bolts in Wudong coal mine was predicted by the numerical method.The results showed that, under the condition of full-length grouting, the maximum shear displacement of the FPR bolt was 25 mm, and the maximum axial load was 117 kN.The calculated maximum horizontal displacement of the panel coal was 198 and 124 mm using the rebar bolt and large diameter FRP bolt, respectively.It can be concluded that the FRP bolt can be used in Wudong coal mine rib support.

This study provides new laboratory testing methods and technical support for large-diameter FRP bolts used in deep underground roadway reinforcement.

Acknowledgements

The work presented in this paper was financially jointly supported by General Project of the National Natural Science Foundation of China (No.52074145) and Liaoning Revitalization Talents Program (No.XLYC2002110).