Case Studies in Construction Materials

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Case Studies in Construction Materials 15 (2021) e00784

Available online 15 November 2021

(http://creativecommons.org/licenses/by/4.0/).

On the design of the fiber reinforced shotcrete applied as primary rock support in the Cuiab ´ a underground mining excavations: A case study

Vitor Moreira de Alencar Monteiro, Fl ´ avio de Andrade Silva

^*

Department of Civil and Environmental Engineering, Pontifícia Universidade Cat´olica (PUC-Rio), Rio de Janeiro, RJ, Brazil

A R T I C L E I N F O Keywords:

Fiber reinforced shotcrete Rock support

Round and square panel tests Design

A B S T R A C T

The Cuiabá mine is an underground gold operation located in Sabará (Minas Gerais, Brazil), which began to use fiber reinforced shotcrete as rock support since 2012. In order to carry on a mechanical analysis and to design the steel and polypropylene fiber reinforced shotcrete linings, the work in hand presents an experimental investigation on the mechanical behavior of the applied composite material inside the Cuiabá mine. Through standardized panel tests, the paper evaluates the toughness performance levels for each studied fiber reinforced shotcrete and applies the current design guidelines for mining works. Based on the rock mass classification of the analyzed mine slopes, while the use of 4.2 kg/m³of polypropylene fiber volume fractions seems to be suitable where fair rock quality is observed, the addition of 25 kg/m³of steel fibers is more indicated where severe ground movement is expected. On the other hand, a mismatch between the standardized panel tests (ASTM C1550 and EN 14488-5) on the shotcrete design guidelines was also assessed in the present research. The analyzed fiber reinforced shotcrete received different classifications depending on the used standardized panel test, resulting on an overestimation of the PP fiber reinforced shotcrete for the square panel tests. Safety factors can be applied in order to mitigate the possible mismatch between square and round panel tests.

1. Introduction

The potential for instability in rock mine opening after excavation has become a significant threat for the safety of the miners on the working field. The support system must be strong to resist the elevated loads and appropriately flexible to enable large displacements of excavation walls [1,2]. In order to contain the great rock mass deformations, fiber reinforced shotcrete (FRS) has been applied as surface rock support due to its easy and agile spraying [3] providing the appropriate strain ability and post-crack reinforcement [4–6].

On one hand, steel fibers have been traditionally used in underground tunnel applications due to its high post-crack strength and energy absorption capacity [7–9]. On the other, the use of polypropylene fibers [10–12] has become increasingly more relevant recently due to its lower market price. In general, hooked-end steel fibers in the concrete matrix is more efficient in enabling superior post-crack strength in relation to polypropylene fibers [13]. The superior mechanical response is strongly linked to the higher modulus of elasticity and the stronger bond inside the concrete matrix of hooked-end steel fibers [14].

According to Cengiz et al. [13] research on the mechanical performance of fiber reinforced shotcrete, the addition of 50 kg/m³of

* Corresponding author.

E-mail address: [email protected] (F. de Andrade Silva).

Contents lists available at ScienceDirect

Case Studies in Construction Materials

journal homepage: www.elsevier.com/locate/cscm

https://doi.org/10.1016/j.cscm.2021.e00784

Received 13 September 2021; Received in revised form 2 November 2021; Accepted 13 November 2021

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Case Studies in Construction Materials 15 (2021) e00784

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steel fibers achieved higher values of both toughness and post-crack strength when compared to the use of 10 kg/m³of polypropylene fibers. On the other hand, Ratcliffe [15] verified that the addition of around 10 kg/m³of macro polypropylene fibers promoted higher values of the analyzed post-peak mechanical parameters in relation to 40 kg/m³of steel fibers. Therefore, the application of synthetic fiber may promote a similar post-crack performance depending of the used fiber volume fraction.

When it comes to system support design in underground excavations, flexural toughness of fiber reinforced shotcrete is one of the main determining factors [16]. Two main FRS test standards are applied worldwide. In North America and Australia, the round panel test (ASTM C1550 [17]) is more often used to quantify energy absorption capacity. On the other hand, the square panel tests (EN 14488-5 [18]) is more frequently carried on in western Europe to measure toughness of the ground support. Asides from the geometrical properties, the main difference between ASTM C1550 [17] and EN 14488-5 [18] standards are associated with the used panel support conditions. While round panel tests are supported on three symmetrically arranged pivots, square panels tests are carried on a rigid square support frame.

Both standards are included on Papworth FRS design guidelines for system support [16], which is oriented by the correlation between the overall underground rock mass quality (Q-system [19]) and the panel toughness performance levels (TPLs). Due to the differences on the support conditions, it is expected major differences on the post-crack mechanical properties when comparing round and square panel tests [20,21]. Rambo et al. [20] verified that the application of panel tests on continuous rigid supports promoted not only higher peak and post-peak strengths, but also significantly greater values of toughness when compared to the use of pivots

Fig. 1.Surface support at the Cuiab´a mine excavation: (a) Typical application of fiber reinforced shotcrete lining subjected to internal and external forces [35], (b) weld meshes arrangement on the excavation walls and (c) crack development on shotcrete surfaces.

V.M. de Alencar Monteiro and F. de Andrade Silva

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supports. The mechanical response variations between ASTM C1550 and EN 14488-5 standards on the FRS design still need to be addressed.

The work in hand presents an analysis of the influence of the applied standardized panel test on the system support design of the fiber reinforced shotcrete used as rock support at the Cuiab´a mine excavation (Minas Gerais, Brazil). One type of hooked-end steel fiber and one polypropylene fiber were tested in distinct fiber volume fractions. The design of FRS for mining applications was performed using the results of the ASTM C1550 [17] and EN 14488-5 [18] panel tests. The present paper evaluates possible design applications for each studied FRS mixture and discusses the possible conflicts on the application of both standards on the FRS linings design. The main contribution of the present research is the better comprehension of the FRS design in mining works when it comes to the application of steel and polypropylene fibers. Moreover, a mismatch on the design guidelines between square and round panel tests is assessed in this work.

2. The use of fiber reinforced shotcrete at the Cuiaba mine excavation ´

The Cuiab´a mine is an underground gold operation located in Sabar´a (Minas Gerais, Brazil). In 2019, the underground gold mine (operated by AngloGold Ashanti) was operating up to 1300 m below surface, with sublevel stopping as the main mining method [22].

The Cuiaba mine gold deposit is part of the Quadril´ ´atero Ferrífero (Iron Quadrangle), recognized for the abundant amount of mineral resources, particularly gold and iron [23]. The geology of the Iron Quadrangle comprises an Archean greenstone belt sequence, represented by the Rio das Velhas Supergroup. For more information about the geological formation of the Iron Quadrangle region, refer to Lobato et al. [23] and Baltazar et al. [24].

Weld meshes were the most commonly used surface support at the underground Cuiab´a operations providing appropriate scat control in blocky rock mass conditions (Fig. 1(b)). After reaching excavation depths higher than 1300 m, however, the incidence of high ground deformations enhances substantially, increasing the risk of loose rock downfall. Along the deeper part of the Cuiab´a mine excavation walls, it was possible to monitor large ground displacements up to 15 cm, causing major damages on the system support [22].

Therefore, fiber reinforced shotcrete began to be used as rock support at the Cuiab´a excavations since 2012 to ensure the safety of the worker under the mine depths of 1300 m. In contrast to weld meshes, shotcrete provides higher excavation development rates and immediate resistance to ground pressure, which is vital in major underground depth operations. Ground movement can be immedi- ately noticed by the crack development on the shotcrete surfaces as shown in Fig. 1(c).

In 2018, the application of FRS reached approximately 56 m³per day using the wet process. At present, 4.2 kg/m³of polypropylene fiber is added to the developed mix composition. The rise on the production of fiber reinforced shotcrete is directly associated with the implementation of an automated concrete under-ground facility (Fig. 2(a)). After the concrete mixing process, it is transported by a truck mixer to the excavation limits. The fiber reinforced shotcrete is then applied to the excavation walls by a robotic sprayer, as shown in Fig. 2(b). The reported challenges in the deep excavation levels led to the development of the present work, which brings a better understanding of the used fiber reinforced shotcrete mechanical properties and its application in underground excavation operations.

3. Experimental program

3.1. Materials

The applied fiber reinforced shotcrete inside the Cuiab´a mine excavation employs the high initial strength Brazilian cement CP-V (equivalent to ASTM cement type III) with a water/cementitious material ratio of 0.45. The used applied particle size aggregate was designed according to EFNARC [25] recommendations (Fig. 3). After 28 days, 50 MPa of compressive strength was reached. The

Fig. 2.Application routine of fiber reinforced shotcrete at the Cuiab´a mine excavation: (a) automated concrete facility and (b) robotic shotcrete sprayer.

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4 complete mix composition can be analyzed in Table 1.

One hooked-end steel fiber (SF) and one polypropylene fiber (PPF) were added to the shotcrete matrix composition. Three distinct fiber fractions were applied for PP fiber: 4.2, 6 and 8 kg/m³. In the case of the steel fibers, 25 kg/m³was employed. While the hooked end steel fiber presents an aspect ratio of 65 and 35 mm length, the polypropylene fiber reports 62 of aspect ratio and 54 mm length.

The values of 210 GPa and 7 GPa of elastic modulus were reported for SF and PPF, respectively. According to EFNARC [25], steel fiber length in shotcrete applications should range between 13 mm and 32 mm in order to not exceed 70% of the internal diameter of the pipes or hoses and prevent shotcrete blockage during spraying. The applied shotcrete projector reports an internal diameter of around 65 mm, meeting EFNARC requirements for fiber blockage. When it comes for the addition of PP fibers, no similar standards are established.

This study was designed to evaluate not only to the FRS application in the Cuiab´a mine excavations, but also possible mismatches between square and round panel tests to mining applications. The studied mixes were selected according to the Cuiab´a mine needs in terms of ground support. The data was analyzed according to the rock class conditions provided by the engineering mine staff.

3.2. Sample preparation

The FRS compositions were named according to its fiber volume fractions and the fiber types. the addition of 4.2, 6 and 8 kg/m³of polypropylene fibers was named as C4.2PPF-S, C6.0PPF-S and C8.0PPF-S, respectively. In the case of steel fibers, the mixture with 25 kg/m³was named as C25SF-S. All mixtures were sprayed in the Cuiab´a mine and sent for the laboratory analysis. The materials were first mixed with the help of the automated concrete facility. After the concrete was sent to the excavation limits, the specimens were produced with the robotic sprayer shown in Fig. 2(b). The samples were, then, sent to the Structural and Materials Laboratory (LEM-DEC PUC-Rio) for the panel test execution and analysis. Fig. 4 brings the complete flowchart for the present research.

3.3. Test program

3.3.1. Round panel tests (ASTM C1550)

Three specimens with nominal diameter and thickness of 800 mm and 80 mm, respectively, were produced in accordance with ASTM C1550 standard [17]. A symmetric arrangement of three pivot points was used as support for the tests. The round panel specimens were centrally loaded at a displacement rate of the actuator of 2.0 mm/min. A LVDT was positioned at the underside center of the specimen in order to measure its central deflection. The tests were carried on using a MTS servo controlled hydraulic testing machine with closed loop type of control and a load cell of 500 kN. The overall test setup is shown in Fig. 5.

3.3.2. Square panel tests (EN 14488-5)

Three specimens with 600×600 x 100 mm were produced in accordance with EN 14488-5 standard [18], as shown in Fig. 6. A square metallic frame with 500×500 x 20 mm was used as support for the tests. The square panel specimens were centrally loaded at a displacement rate of the actuator of 1.0 mm/min. A LVDT was positioned at the bottom center of the specimen to measure its central deflection. A rigid steel plate with 100×100 x 26 mm was positioned on the center of the upper surface of the square panel as recommended by EN 14488-5 [17]. The tests were carried on using a MTS servo controlled hydraulic testing machine with closed loop type of control and a load cell of 500 kN.

Fig. 3. Particle size distribution according to EFNARC recommendations [25].

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4. Discussion and analyses

4.1. Fiber reinforced shotcrete design for the Cuiab´a mine excavation support system

The support system of the Cuiab´a mine excavation was designed based on the ASTM C1550 [17] round panel tests. Fig. 7 presents the results obtained from round panel tests carried on steel and polypropylene fiber reinforced shotcrete specimens. Toughness is calculated as the area below the Force-Deflection curves. Table 2 displays the mean peak strength (Pu), the calculated toughness from the analyzed panel tests curves for different deflection levels (10, 20, 25, 30 and 40 mm) and its classification according to Papworth design guidelines [16].

Papworth [16] design guidelines links the FRS panel test results with the Q rock class system proposed by Barton et al. [26], as shown in Table 3. Each rock class is associated by a Toughness Performance Level (TPL), obtained through the panel tests, and classified according to Morgan’s [27] performance recommendations as follows:

•TPL IV – Recommended for cases involving severe ground movement with the multiple FRS cracks. Rock bolts and/or cable bolts may be necessary;

•TPL III – Appropriate for steady rock in hard rock mines or tunnels where relatively low rock displacement and stresses are expected. Presence of minor FRS cracks;

•TPL I/II – Low possibility of rock mass movement. Outcome due to minor cracking is not severe. Fibers can be added to provide shrinkage and thermal crack control.

Table 1

Applied mix compositions.

Constituent Mix (kg/m³)

EFNARC Aggregate 1530

Cement CPV 480

Water 216

Superplasticizer MasterGlenium 3.60

Accelerator MasterRoc 190 38.40

Stabilizer Delvo Crete 1.20

Fig. 4. Research work flowchart.

Fig. 5. Round panel test in accordance with ASTM C1550 [17]: (a) test setup and (b) setup details with specimen geometry. Dimensions in mm.

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CaseStudiesinConstructionMaterials15(2021)e00784

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Fig. 6.Square panel test in accordance with EN 14488-5 [18]: (a) test setup, (b) setup details and (c) specimen geometry. Dimensions in mm.

V.M. de Alencar Monteiro and F. de Andrade Silva

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Applying the discussed guidelines for the studied fiber reinforced shotcrete at the Cuiab´a mine excavation, while the addition of 6.0 kg/m³PPF is only indicated in hard rock mines where relatively stress and movement is expected (III, D), the use of 25 kg/m³of steel fibers is recommended in areas with severe ground movement with expectation for cracking of the fiber reinforced shotcrete linings (IV, E). As already indicated by the material characterization, high levels of PP fiber volume fractions, around 8 kg/m³, is required to achieve the toughness performance levels when using 25 kg/m³of steel fibers.

Barbosa [28] classified the rock mass located in the depths of 1300 m of Fonte Grande Sul and Serrotinho slopes inside the Cuiab´a mine according to the Q system [26], reporting areas of poor and good rock mass qualities. The Q values ranged between 1 and 22 in the analyzed location. With a maximum excavation span of 10 m, it is possible to design the system support based on the modified Fig. 7.Round panel tests, based on ASTM C1550, for steel and polypropylene fiber reinforced shotcrete sprayed at the Cuiab´a mine excavation facilities: (a) force per deflection curves and (b) toughness per deflection.

Table 2

Peak strength (Pu) and toughness results from round (ASTM C1550 [17]) and square panel (EN 14488-5 [18]) tests. Standard deviation presented in parentheses.

Composition Test Standard Pu T25.0 T30.0 T40.0 TPL^a AuSS Classification [37]

(kN) (J) (J) (J) Rock Class

C4.2PPF-S EN 14488-5 54.94 730.16 800.97 – III Non-structural Support

(5.14) (95.55) (116.06) – D

C4.2PPF-S ASTM C1550 31.48 149.75 164.94 188.51 0 –

(0.69) (5.42) (6.47) (9.22) A

C6.0PPF-S ASTM C1550 32.78 244.80 269.00 304.46 III Non-structural Support

(4.63) (42.08) (46.72) (53.85) D

C8.0PPF-S ASTM C1550 30.51 338.23 381.29 448.91 IV High–level Support

(2.84) (32.93) (38.14) (46.06) E

C25SF-S ASTM C1550 41.216 332.43 362.92 406.75 IV Moderate Support

(4.84) (32.67) (34.03) (34.03) E

aToughness Performance Level.

Table 3

Papworth [16] design guidelines for fiber reinforced shotcrete based on Q-system rock classes, EN 14488-5 [18] and ASTM C1550 [17] standards.

Ground Condition TPL

TPL^a Class EN 14488-5 ASTM C1550 (RDP40 mm)

– – (J) (J)

IV F >1400 >560

IV E >1000 >400

III D >700 >280

II C >500 >200

I B >500 >200

0 A 0 0

aToughness Performance Level.

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Grimstad & Barton chart [19] proposed by Papworth [16], which includes the link between the Q system classification with Toughness Performance Levels obtained in panel tests. Fig. 8 indicates the support system design area recommended for the analyzed Cuiab´a mine location.

Therefore, according to the verified toughness performance levels through round panel tests (Table 2), higher polypropylene fiber volume fractions around 6 kg/m³are indicated for poor and fair rock quality areas with 5–9 cm of shotcrete thickness, as indicated in Fig. 8. When rock mass quality is classified as good, bolt spacing with unreinforced shotcrete is recommended. In this cases, the application of a minimum of 4.2 kg/m³of polypropylene fiber with 4–10 cm of shotcrete thickness is adequate to guarantee scat control of the rock surface. According to the information provided by the mine operators, the addition of only 4.2 kg/m³of PP fibers is not appropriate regarding the poor rock quality present in the Fonte Grande Sul and Serrotinho slopes. The addition of at least 6 kg/m³ of polypropylene is more adequate, since this fiber volume fraction can reach the minimum toughness performance level of 280 J, recommended by Papworth [16].

Beyond the fiber material properties, the fiber efficiency as a concrete reinforcement is also strongly related to the fiber bond with the concrete matrix [29]. The bond properties of fibers are divided into physicochemical and mechanical properties [29]. The physicochemical bond is influenced by adhesion and friction at the interface, while the mechanical bond is associated with the anchor effect at the fiber end or along the fiber [30]. Synthetic macro fibers are known for their poor bond when used in a concrete matrix due to their hydrophobic nature. According to past research on polypropylene fiber pullout [31,32], a significant range of pullout strength is presented depending on the studied fiber geometrical and surface properties, matrix resistance and embedment lengths. Nogueira

Fig. 8.Cuiab´a mine system support design based on the modified Grimstad & Barton chart [19] by Papworth [16].

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[33], Castoldi [31] and Di Maida et al. [32], for instance, reported peak pullout strengths below 1 MPa. Babafemi et al. [14] clarify that the higher adherence of some PP fibers is usually associated with the surface deformation (embossment) created during their production process. Modifications to the surface geometry of synthetic fibers from straight and smooth to a deformed configuration aim to increase the fiber-matrix interface shear resistance, enhancing, as a consequence, the pullout strength.

The stronger steel fiber bond with the concrete matrix supports the verified higher flexural residual strength when compared to the application of PP fibers. This significant difference on the pullout behavior between both fibers is strongly associated with the fiber anchorage of the steel fiber inside the concrete matrix [11]. While the pullout results of the PP in the concrete matrix is associated only with shear stress, the steel fiber presents another component related to the mechanical anchorage of the hook.

Due to the stronger bond and the higher value of modulus of elasticity, hooked-end steel fibers can present a better potential to reach much higher values of both post-crack residual strength and toughness when compared to the PP fibers. When more severe cases with very poor rock conditions are concerned, higher volume fractions of steel fibers may be the best option to guarantee the safety of the workers underground. The addition of higher fiber fractions of around 60 kg/m³, for instance, of steel fibers shall contain the rock mass deformations in more extreme conditions.

4.2. The influence of panel test type on the FRS design

Fig. 9 presents the results of square panel tests based on EN 14488-5 standard [18] and its comparison with ASTM C1550 [17]

round panel curves. This confrontation between test methods was carried on for polypropylene fiber reinforced shotcrete with 4.2 kg/m³of volume fraction. Table 2 shows the mean toughness and peak force (Pu) values for both studied guidelines.

Based on the comparison between ASTM C1550 [17] and EN 14488-5 [18], it is possible to verify higher values of residual strength, peak strength and toughness when FRS post-crack behavior is evaluated through the square panel test. While the overall toughness for square panel tests reported around 800 J, the final energy absorption capacity reached 188 J when the round panel tests were evaluated.

Similar results were reported by Rambo et al. [20], which evaluated the round panel tests in different support configurations. As far as the support conditions reached a circular continuous support, higher results of post-peak strength and toughness were verified for the studied fiber reinforced composite, similar to what is observed when analyzing both EN 14488-5 [18] and ASTM C1550 [17]

standards. Moreover, the degree of hyperstaticity in structural tests potentiate the mechanism of multiple cracking in concrete, while the support configuration with three symmetrically arranged pivots causes the formation of three major cracks [20].

In terms of the Papworth [16] design guidelines, higher values of toughness are indicated when the square panel tests are used for the system support analysis (Table 3). On the other hand, an inconsistency is observed in relation to the toughness performance level (TPL) depending on the used panel standard. While C4.2PPF-S is recommended for very good ground conditions (0, A) when the ASTM C1550 [17] results are used, the same mix composition is classified for poor ground conditions (III, D) for EN 14488-5 [18] tests.

Therefore, there was an overestimation of the toughness performance level when the square panel was applied. Contemplating both the support design guidelines and the workforce safety after excavation, a conservative choice should be considered by classifying the application of 4.2 kg/m³of PP fiber for good ground conditions (0, A).

Papworth [16] classification (Table 3) is based on the Bernard correlation between square and round panel tests. Bernard [34]

estimated that 1000 Joules of energy absorption capacity at 25 mm deflection in an EN 14488-5 panel was equivalent to 400 Joules of energy absorption at 40 mm deflection of the ASMT C1550 panel test. This correlation between the two types of panel showed to be strongly linear with a coefficient of determination (r²) of 0.90. Eq. (1) presents the correlation developed by Bernard [34].

TEN14488−5,25mm=2.5xTASTMC1550,40mm (1)

where TEN14488-5,25 mm and TASTMC1550,40 mm are, respectively, the overall toughness achieved for both EN 14488-5 and ASTM C1550 panel tests.

Fig. 10 compares the Bernard [34] correlation between panel tests with Manfredi et al. [21], Gallo et al. [35] and the current results. Both Manfredi et al. [21] and Gallo et al. [35] carried on both square and round panel tests for both steel and PP fiber reinforced concrete in range of volume fractions and compared the achieved toughness levels for all verified data. Fig. 10 is also divided according to the Papworth [16] design guidelines for fiber reinforced shotcrete in excavation applications. It is possible to observe that the majority of the achieved results were closely related to the developed Bernard [34] correlation for both panels and can be properly classified by Papworth [16] guidelines independently of the chosen panel standard. On the other hand, three results showed in- consistencies depending on the used panel test. For both the addition of 6 kg/m³of polypropylene fiber in Manfredi et al. [21] paper and the C4.2PPF-S of the present research, the use of the square panel test overestimates the toughness performance level. In terms of the Gallo et al. [35] study, the application 10 kg/m³of PP fiber had its TPL overestimated by the round panel test.

As fiber reinforced concrete does present an intrinsic scatter [36] for its post-crack mechanical parameters, the estimated toughness values may range for the same applied concrete matrix and fiber parameters depending on the curing conditions and mixing pro- cedures. Therefore, applying only an overall linear tendency between toughness values for both panel tests may not be enough to adjust all possible deviations from fiber reinforced composites, as seen in Fig. 10.

This inconsistency may be associated to the different crack formation depending on the used support conditions. Bernard [34] work does not take into account the crack opening displacement into its correlations, which vary significantly between both panel tests. As explained by Rambo [20], the formation of multiple cracks, when a continuous support is used, promote a distinct post-crack behavior in relation to the traditional symmetrically arranged pivots of the ASTM C1550 standard. The degree of hyperstaticity in structural

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tests, such as the panel tests, may potentiate the mechanism of multiple cracking. Rambo [20] shows that the application of continuous supports was responsible for not only modifying the specimen failure mode, but also on enhancing the values of energy absorption capacity. As a similar mechanical response can be observed when comparing square and round panel tests, the further development of new equations that consider the crack formation under distinct supports may eventually entail more accurate panel test correlations.

A third reference of toughness performance level based on round panel tests can been assessed at the Australian shotcrete guidelines (AuSS) [37]. Recent Australian mining projects divided the ground support in three levels: non-structural, moderate and high-level supports. The values of toughness for square panel tests (EN 14488-5 [18]) were calculated with the Bernard [34] correlation (equation 2) as AuSS is based on ASTM C1550 [17] standard. The classification of the fiber reinforced shotcrete according to AuSS [37]

can be analyzed in Table 2.

The Australian shotcrete guidelines seemed to be more conservative, requiring higher values of toughness in relation to Papworth [16] orientations. While C25SF-S is recommended for situations involving severe ground movement with the presence of cracking of FRS lining based on Papworth [16], AuSS classifies the same mix for moderate ground support. Moreover, the same mismatch was observed on the application of ASTM C1550 [17] and EN 14488-5 [18]. C4.2PPF-S did not reach a minimum toughness level for mining application and was recommended for non-structural supports simultaneously depending on the used standardized panel test.

Although AuSS [37] guidelines are more conservative, the Australian recommendations do not present any correlation with the rock quality for shotcrete design.

Fig. 9. Round (ASTM C1550 [17]) and square (EN 14488-5 [18]) panel test results for 4.2 kg/m³polypropylene fiber reinforced shotcrete.

Fig. 10.Comparison between the Bernard [34] correlation with Manfredi et al. [21], Gallo et al. [35] and the current results for round and square panel tests. Papworth [26] toughness performance level classification according the rock condition classes is also displayed.

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5. Conclusions

The current work presents the results of an experimental study on the mechanical behavior of the fiber reinforced shotcrete applied as rock support at the Cuiaba mine excavation (Minas Gerais, Brazil) and its design for underground mining applications. The main ´ contribution of the present paper is the better understanding of the conflicts between the main shotcrete design standards. The following conclusions can be drawn from the present work:

•While the addition of low fiber volume fractions of polypropylene fibers is indicated for hard rock mines where relatively stress and movement is expected, the application of steel fiber reinforced shotcrete is recommended in areas with severe ground movement with expectation for cracking of the shotcrete linings. Based on the rock mass classification of the analyzed mine slopes, the application of 6.0 kg/m³of polypropylene fibers seem to be suitable to prevent the downfall of rock sets inside the Cuiab´a mine.

•There is an overall mismatch between round and square panel standards when it comes to the FRS classification for underground applications. Although the design guidelines consider the support conditions peculiarities, the analyzed fiber reinforced shotcrete received different classifications depending on the used standard. Square panel tests seem to overestimate the FRS support in relation to round panel tests. The use safety factors for FRS design with square panel tests in mining works can be recommended for this application.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This study was financed by Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Coordenação de Aper- feiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance code 001. This research was carried on in cooperation with AngloGold Ashanti and the engineering staff at the Cuiabá mine operations. The authors would like to thank Belgo Bekaert and ArcelorMittal for the endorsement, in particular, to Gelmo Chiari Costa, Belgo Bekaert manager, for the continued encouragement to our work.

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