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Review

A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests

by
Jianhang Chen
1,2,3,4,
Baoyang Wu
1,3,
Peng Li
1,4,
Guojun Zhang
1,3,5 and
Yong Yuan
2,*
1
State Key Laboratory of Water Resource Protection and Utilization in Coal Mining, CHN Energy Shendong Coal Group Co., Ltd., Beijing 102211, China
2
Key Laboratory of Deep Coal Resource Mining (CUMT), Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
3
National Institute of Clean-and-Low-Carbon Energy, CHN Energy, Beijing 102211, China
4
School of Energy and Mining Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
5
China Academy of Safety Science and Technology, Beijing 100012, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(24), 16543; https://doi.org/10.3390/su142416543
Submission received: 24 November 2022 / Revised: 2 December 2022 / Accepted: 8 December 2022 / Published: 9 December 2022
(This article belongs to the Special Issue Sustainable and Responsible Development of Minerals, Safety-Driven Mining Operations, Digital Mining, Critical Minerals)
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Abstract

:
Entirely grouted cable tendons are commonly used in mining engineering. They have superior working ability in reinforcing the excavated rocks and soils. During the working process of cable tendons, the force-transferring ability and the corresponding mechanism are significant in guaranteeing the safety of underground openings. To further understand the force-transferring mechanism of entirely grouted cable tendons, this paper provided a literature review on the force transfer of cable tendons. First, the force-transferring concept of entirely grouted cable tendons was summarised. The force-transferring process and failure modes of cable tendons were illustrated. Then, the experimental test program used in testing the entirely grouted cable tendons was summarised. The advantages and disadvantages of various test programs were illustrated. After that, the working ability of entirely grouted cable tendons was reviewed. The effect of various parameters on the working ability of cable tendons was summarised and compared. These parameters include the rock stiffness, embedment length, cement grout property, resin grout property, modified geometry, borehole size, rotation and pre-tensioning. Last, a discussion was provided to elaborate the working ability and force-transferring mechanism of entirely grouted cable bolts. This literature review is beneficial for researchers and engineers, furthering their understanding of the working ability of cable tendons.

1. Introduction

A cable tendon is a flexible bolt that is manufactured by twisting wires (Figure 1) [1]. Generally, there are two different cable tendons. One is the plain cable tendon, which is manufactured by twisting steel wires directly. Therefore, the cross-section area of plain cable tendons at each position is identical. The other one is the modified cable tendon. It is manufactured by adding bulbs or buttons on plain cable tendons. Therefore, the cross-section area of modified cable tendons at different positions is different. Cable tendons are normally used in civil and mining applications for rock slopes and underground excavations [2,3,4]. The historical application of cable tendons in the field was discussed by Farah and Aref [5]. Since its application in mining, cable tendons have become increasingly favoured in highly stressed ground conditions due to their much higher force-transferring ability.
In earlier years, cable tendons often behaved as temporary reinforcement elements. This is because cable tendons were sourced from discarded steel ropes. However, they were subsequently found to have poor force-transferring ability because of their smooth surface profile. These were plain cable tendons constructed by winding steel wires to form a 15.2 mm diameter tendon. The ultimate force-transferring ability of these cable tendons could reach up to 260 kN. As they were relatively inexpensive, easy to install, and could be used in boreholes with lengths measured in tens of meters, as compared with a few meters of rock bolts, cable tendons were adopted for use in several underground mining systems, such as overhand cut-and-fill methods.
Gradually, to improve the working ability of cable tendons and, in particular, the force-transferring properties, steel discs or buttons were inserted within the standard cable tendons. The working ability indicates the anchorage performance of cable tendons, especially the maximum anchorage capacity of cable tendons. These were termed buttoned or swaged tendon. The swaged cable tendons increased the effective resistance to axial loads, although it required larger boreholes to accommodate the swelling of the cable diameter. Another approach was to insert two cable tendons in the one drilled borehole together with spacers to provide some separation between the cable tendons. These are referred to as double-plain cable tendons. A common issue with these types of cable tendons was corrosion of the steel tendons, which led to the development of epoxy-coated cable tendons. Although this extended the service life, it was difficult to install with a faceplate near the outside of the borehole.
A rapid development in cable tendon design began in the 1990s. As an alternative to the traditional steel cable tendons, Mah [6] proposed a cuttable cable tendon manufactured from fiberglass. This type of cable tendon has a much larger bond strength (BS), compared with the seven-wire steel cable tendon. The birdcage cable tendon was fabricated by unwinding the cable tendon at regular intervals, forming a suite of nodes and antinodes along the tendon. This enhanced the force-transferring ability. However, it also increased the difficulty of inserting the cable tendon into a hole. The bulbed cable tendon is another popular tendon produced by compressing the cable axially to create a number of bulbs along the tendon. The advantage of this design is that the bulb diameter and spacing can be varied to suite particular rock conditions. Another variant is the nutcaged cable tendon produced by installing a hexagonal nut on the central wire of a standard cable tendon and spinning the six peripheral wires. The big advantage of this design was that it doubled the force-transferring ability, compared to plain cable tendons, making it suited for support in highly fractured ground. These products were generically termed modified cable tendons. The modified cable surface geometry improved the force-transferring ability, contributing to their increased application as permanent reinforcement.
A summary of the commonly used cable tendons is tabulated in Table 1.
To analyse force-transferring ability, the maximum force-transferring ability and system stiffness should be confirmed [21]. Specifically, most researchers are more likely to rely on the load versus displacement relationship to analyse the force-transferring characteristics of cable tendons obtained from laboratory tests [22]. Thomas [23] proposed the force-transferring index to analyse the cable force-transferring ability.
Hartman and Hebblewhite [24] summarised three parameters influenced the force-transferring ability of cable tendons: the cable tendon, the rock and the loading property. They include cable tendon diameter, borehole size, grout quality, joint roughness and confinement [25]. Although numerous methods can be used [26,27,28,29,30], this paper mainly focused on experimental tests and provided a review of the force-transferring ability of cable tendons. The force-transferring concept, the experimental program and the working ability of cable tendons were reviewed.

2. Force-Transferring Concept

2.1. Force-Transferring Process

The working ability of cable tendon reinforcement systems is largely dependent on the ability of force transferring [31]. The force transferring was defined by Fabjanczyk and Tarrant [32]. It represents the generated force in the rock tendon as a result of rock movement [33]. Then, Windsor [34] updated the force-transferring ability and indicated that it represented the force transferring between the unstable rock and the stable rock.
In the cable tendon reinforcement system, the grout in the borehole is paramount [35]. Specifically, the stress at the bolt/grout (B/G) and grout/rock (G/R) faces is responsible for the force-transferring mechanism [36]. That is also the reason why failure of the cable tendon reinforcement system usually occurs at those two faces [37].

2.2. Failure Modes of Entirely Grouted Cable Tendons

Littlejohn and Bruce [38] indicated that in a grouted bolt system, there are five possible modes of failure: the bolt itself; grouting material; rock surrounding the borehole; B/G face; and G/R face. The failure of the cable tendon is rarely observed. Potvin et al. [39] believed that cable tendons would fail at the B/G or G/R face, especially the former.

2.3. Experimental Test Program

Although entirely grouted cable tendons are used in mining, there is no common standard for testing cable tendons’ working ability in the laboratory. Thus, researchers used various approaches to test cable tendons and the results were largely different.
The simplest test method is the single embedment pull test (SEPT). A cable tendon with a specific length is embedded in a confining medium, usually a metal tube (Figure 2). The embedment length is normally ranged between 200 mm and 500 mm. Then, the gripping system is used to grip the cable tendon at one end, and a hydraulic cylinder extrudes the cable tendon out from the confining medium.
This test method has been widely used because of easy handling. However, because of the special helical geometry, the cable tendon tends to rotate during testing. It is not a good reflection of a cable reinforcement scenario in the field. As indicated by Bawden et al. [41], rotation cannot occur along the full encapsulation length, being restricted by the surrounding confining medium.
The rotation of cable tendons is detrimental to working ability in the SEPT because the peak force is much smaller than the cable tendon’s non-rotating working ability.
A typical non-rotating test of cable tendons was conducted by Fuller and Cox [7], where a split-pipe pull test (SPPT) set up was proposed. The test rig is composed of two parts: the upper anchor section and the lower embedment section. Tubes made up of mild steel or other materials are used to simulate the rock, confining the grouted cable tendons in the upper and lower sections. The embedment length is the most important part and usually has a short length less than 450 mm. A joint occurred between those two parts. A displacement transducer is attached to measure the longitudinal displacement and a load cell is used to monitor the pulling load.
This equipment is effective in preventing cable tendons from rotating. Thus, the SPPT has been used many times by later researchers. It should be mentioned that most SPPTs were conducted under the constant normal stiffness (CNS) environment because the stiffness of the confining pipe was invariable in the whole test process. Macsporran [42] performed several SPPTs under the CNL condition by using a modified Hoek cell to restrict the grouted cable tendon.
Although the SPPT had been accepted, Reichert [43] mentioned that an extra confinement force may occur in the embedment length because of the gripping equipment at the unloaded end. This may result in larger pulling load. To overcome this issue, a modified SPPT rig was developed by Reichert [43]. Compared with the conventional SPPT, the pulling head was fixed at the joint position, and no extra confinement force was applied within the whole embedment section. Later, the modified SPPT approach was adopted by many researchers.
Another test method that can prevent cable tendons from rotating is the double embedment pull test (DEPT). It was originally developed by Hutchins et al. [44]. A cable tendon was installed in two individual steel tubes with the same length. A gap between two pipes simulated the rock joint. This test approach has the advantage of evaluating the force-transferring ability of cable tendons on either side of the joint. Consequently, this method was widely adopted in cable tendon testing. Additionally, the British Standards Institution adopted this approach to test the working ability of birdcage cable tendons [45].
An issue occurring in the SPPT and DEPT is that the measured axial working ability of the cable tendons is much stiffer than the real scenario. This is because the mechanical response of the steel material is largely different from the rock material. Furthermore, the tubes are always threaded internally. Consequently, the failure mode of shear slippage along the B/G face is artificially induced. Furthermore, this design prevents analysing failure along the G/R face.
To overcome this issue, Clifford et al. [46] proposed the laboratory short encapsulation pull test (LSEPT). Specifically, a 142 mm diameter cylindrical sandstone is used to confine the grouted cable tendon with a specific length. Around the sandstone core, a bi-axial cell is used to generate confinement of 10 MPa. The LSEPT has been adopted as the preferred testing method for cable tendons. Furthermore, the LSEPT was later incorporated in the British Standard for testing flexible bolts, in particular [45].
Nevertheless, there are still some problems involved in the current LSEPT. As indicated by Khan [47], the dimension of the bearing plate significantly affected the tensile working ability of the cable tendons. However, the standard LSEPT does not consider this bearing plate impact. Then, the designed anchorage system cannot provide reliable BS within the anchor length. In some cases, failure was found to occur in the anchor section [48]. A constant confining pressure provided by the bi-axial cell cannot reflect the field stress, which varies during the service life of cable tendons [23]. Moreover, plain tendons tested in the LSEPT were likely to unscrew from the sandstone core. This was not a typical representation of the underground situation [49]. Additionally, it is difficult to ensure uniform properties of sandstone cores extracted from the field. This may contribute to significant differences in the working ability of the cable tendons [50].
A summary of the different experimental test programs is tabulated in Table 2.

3. Working Ability of Entirely Grouted Cable Tendons

Much research has been performed on the working ability of entirely grouted cable tendons in the laboratory. They were summarised in the following sections.

3.1. Rock Stiffness

Hyett et al. [51] analysed how the rock stiffness affected the traditional seven wire cable tendons. Cable tendons were grouted with a constant embedment length of 250 mm in three tubes, steel, aluminium and PVC, simulating strong, medium and weak rock. Additionally, one test was performed in a heat shrink sleeve, which could provide little confinement to the grouted cable tendon for comparison. Three series of experiments were conducted with the water proportion ratio ranging from 0.3 to 0.5. It indicates that the force-transferring ability of cable tendons pulled from steel pipes was more than double that for PVC pipes. This indicates that the rock stiffness significantly affected the working ability of plain cable tendons (Figure 3). A similar conclusion was also confirmed by Hassani et al. [52].
Reichert et al. [53] conducted further research by conducting pull tests on same plain tendons from granite, limestone and shale in the field. The laboratory and field test results were compared. The confining medium stiffness was calculated. To calculate the pipe stiffness, the thick wall cylinder theory was used:
K r = 2 E ( 1 + v ) ( d o 2 − d i 2 d i [ ( 1 − 2 v ) d i 2 + d o 2 ] )
where Kr is radial stiffness, E and v are the modulus and Poisson’s ratio of the confining pipe, and do and di are the outside and inside diameter of the pipe. Equation (2) was adopted to calculate the hole stiffness:
K c = d d d c K d
where Kc is the cable tendon hole stiffness, Kd is the dilatometer hole stiffness and was measured by using a pressure dilatometer, dd is the dilatometer hole diameter and dc is the cable tendon hole diameter.
Based on these experiments, the relationship between the maximum force-transferring ability of plain tendons and confining medium stiffness was acquired.
Hyett, Bawden and Reichert [51] analysed laboratory and field test results, determining that the pull load versus displacement curves of cable tendons can be divided into four different stages.
Stage 1: A linear link existed between the load and displacement. Three different components, cohesion, interlocking and friction, contributed to the bond resistance along the B/G face.
Stage 2: Slippage along the B/G face probably occurred when one of the following conditions was satisfied: (1) radial splitting of the grout column; (2) shear failure along the B/G face.
Stage 3: The pull load acquired within this stage was mainly determined by two parts: the friction along the B/G face and the residual strength of the grout column. If the confinement was low, the rock would split into wedges and the failure mechanism was the radial movement of those wedges. If the confinement pressure was large, the failure would be the shear slippage along the B/G face.
Stage 4: The ultimate force-transferring ability of cable tendons was normally acquired when the pull displacement reached more than 40 mm. In this stage, the geometric mismatch between cable tendons and grout column is maximal. After the peak, the force-transferring ability might decrease quickly. However, the tests and analysis mentioned above are only based on standard plain tendons.
The corresponding failure modes were shown in Figure 4.
Hyett and Bawden [13] focused on the impact of rock stiffness on modified cable tendons. Three types of metal tubes, Sch. 40—aluminium, Sch. 80—steel and Sch. 80—aluminium, were tested. Several tests were conducted on 25 mm Garford bulb cable tendons. It indicated that the working ability of cable tendons was quite similar. Moreover, there was no apparent difference between peak loads. Thus, the rock stiffness had almost no influence on determining the force-transferring ability of bulbed cable tendons. For this reason, it was explained that the natural confinement pressure created in pulling bulbed cable tendons was less sensitive to the stiffness of the confining medium, due to the special bulb geometry.

3.2. Embedment Length

Fuller and Cox [7] conducted several pull tests on 7 mm steel wires to analyse the influence of embedment length on the peak ability of wires. The tested embedment length ranged from 100 mm to 700 mm. It was analysed that there was a linear relationship between peak ability and embedment length. However, there is a large scatter of test results, reducing the credibility of this analysis.
A vast number of experiments were performed by Stillborg [54] to analyse the influence of the embedment length on the working ability of cable tendons. Cable tendons with a diameter of 38 mm were installed in cylindrical confining medium with an embedment length less than 266 mm. The test results showed that there was a proportional relationship between the maximum force-transferring ability of the cable tendons and embedment length (Figure 5). Furthermore, it indicated that chemical adhesion along the B/G face was removed easily after only a short displacement of 0.2 mm. This demonstrated that adhesive strength marginally affected the peak ability. Nevertheless, the SEPT was used and cable tendons rotated in the pulling process, resulting in a low pulling ability.
Since then, the relationship between the peak ability and embedment length has been analysed by many researchers. Most researchers concluded that there was a linear relationship [5]. However, most researchers focused only on plain cable tendons. A typical example of the cable tendon’s performance when the embedment length was different is shown in Figure 6.
Mah [6] performed laboratory pull tests, attempting to find the critical embedment length, which is defined as the encapsulation length to induce cable tendon failure. Fibreglass cable tendons (FCB) were used and the embedment length ranged from 432 mm to 508 mm. It indicated that increasing the embedment length was beneficial to improving the force-transferring ability of cable tendons.
Martin et al. [56] performed similar research to find the critical embedment length for standard cable tendons. It indicated that cable tendons ruptured at a pull load of 258 kN when the embedment length reached 914 mm. However, the critical embedment length is related to many factors, including the grout quality and the property of surrounding rock. Thus, the critical embedment length of 914 mm is only applicable to the situation in their research.
Chen and Mitri [57] analysed the effect of embedment length on BS, which is defined as the shear resistance to induce the shear slippage at the B/G face, along a unit contact area. Equation (3) was used to calculate the BS of the B/G face:
τ = P c ( L − s )
where τ is the interfacial BS, P is the pull load, c is the perimeter of cable tendons, L is the embedment length and s is the shear slippage.
Equation (3) is valid when the embedment length is short enough. In this scenario, the BS can be assumed uniform. Test results showed that although there was a linear connection between the load and embedment length, the BS of the B/G face was not influenced by the embedment length.
Thompson and Villaescusa [58] conducted tests on plain cable tendons to analyse the critical embedment length. It demonstrated that within an embedment length of 1500 mm, failure occurred along the B/G face. However, once the embedment length was longer than 2500 mm, the cable tendons ruptured. A non-linear extrapolation approach was used to determine the critical embedment length.
A summary of the previous research, which was conducted to evaluate the influence of embedment length on the performance of cable tendons, is tabulated in Table 3.

3.3. Cement Grout Property

3.3.1. Water Proportion

Cox and Fuller [59] analysed the effect of water proportion on pull working ability of single wires. Three different water proportions were used. Additionally, the corresponding UCS ranged from 18.4 MPa to 38.9 MPa. It demonstrated that the lower the water proportion, the higher the grout strength, and consequently, the larger the peak ability. However, this research only focused on steel wires and no test was attempted to analyse the water proportion impact on cable tendons.
Similar research was performed by Stheeman [60] who concentrated on cable tendons. Three different water proportions, 0.3, 0.375 and 0.45, were used. It showed that decreasing the water proportion was beneficial to improving the peak ability of cable tendons. This conclusion was confirmed by many researchers [57]. However, all these tests were conducted under CNS environments.
Hyett et al. [61] analysed the effect of water proportion on the working ability of plain cable tendons under CNL conditions. The water proportion varied from 0.3 to 0.5 (Figure 7). It shows that lower water proportion and, consequently, stronger mortar were effective in increasing the force-transferring ability of cable tendons.
To analyse the influence of water proportion on working ability of modified cable tendons, Hyett, Bawden, Powers and Rocque [12] tested nutcaged cable tendons with three different water proportions: 0.3, 0.4 and 0.5. It showed that nutcaged cable tendons could still sustain a large pulling load even when a high water proportion was used. Later, Hyett et al. [62] continued this research by testing the water proportion effect on 25 mm Garfold bulb cable tendons. The results indicated that the water proportion marginally affected the working ability of bulbed cable tendons. Based on these two series of tests, it was recommended to use modified cable tendons in the field.
Mosse-Robinson and Sharrock [63] also analysed the influence of water proportion on bulbed cable tendons installed in large boreholes, ranging from 42 mm to 106 mm. It indicated that increasing the water proportion resulted in a marginal decrease in the peak ability of the bulbed cable tendons.

3.3.2. Grout Additives

Hassani, Mitri, Khan and Rajaie [52] tested high strength cement, which was fabricated by adding silica powder into fresh cement on standard cable tendons. It was reported that the silica powder was beneficial to improving the force-transferring ability of cable tendons not only in the peak stage but also in the residual range.
Similar research was performed by Benmokrane, Chennouf and Mitri [55] to analyse the influence of grout additives on the working ability of cable tendons. Two types of grouts were made. One was cement paste with aluminium powder and the other one was cement paste mixed with silica powder and superplasticizer. Test results indicated that the aluminium additive increased the force-transferring ability of cable tendons. On the other hand, silica powder and superplasticizer had minimal influence on the working ability of the cable tendons. It also demonstrated that the friction between the cable tendon and grout was the most important part in determining the peak ability of the cable reinforcement system, compared with interlocking. However, one problem is that the mass of fresh cement was not identical, making the result comparison difficult.

3.3.3. Grout Aggregates

Stheeman [60] added sands to cement paste, aiming to analyse the influence of aggregate on force-transferring ability of cable tendons. It showed that the sands reduced the peak ability of cable tendons. However, the water proportion also decreased when adding sands into the grout. Thus, it may not be reasonable to conclude that adding sands by itself adversely affected the working ability of cable tendons.
Farah and Aref [64] compared the working ability of seven wire cable tendons pulled from fresh cement paste and aggregate–cement-based grout, which was made by mixing cement, sand and coarse aggregates. It showed that the force-transferring ability of cable tendons pulled from aggregate–cement-based grout decreased slowly after the peak ability. Furthermore, adding aggregates in cement paste was beneficial to improving the maximum force-transferring ability. However, this research was only limited to standard plain cable tendons.
Hassani and Rajaie [65] used shotcrete as aggregate and mixed it with cement paste, analysing the effect of aggregate-to-cement ratio on the force-transferring ability of cable tendons. It indicated that the shotcrete aggregates significantly affected the residual working ability of cable tendons. Specifically, cable tendons installed in aggregate–cement grout retained high residual load. Moreover, increasing the aggregate-to-cement ratio led to the decreasing of the force-transferring ability. Finally, an optimum aggregate-to-cement ratio of 2 was recommended for field practice.
Benmokrane et al. [66] used two grouts to bond cable tendons. One was fresh cement paste and the other one was sand–cement-based grout. Test results showed that adding an amount of sand increased the initial stiffness and peak ability of the cable reinforcement system.

3.3.4. Curing Time

Mah [6] analysed the influence of grout curing time on the working ability of FCB. Portland cement type III was tested, and the curing time varied from 2 to 10 days. It indicated that increasing the curing time did not apparently improve the force-transferring ability of cable tendons. However, this conclusion is only applicable to a limited curing time up to 10 days.
A different conclusion was derived by Hassani, Mitri, Khan and Rajaie [52]. They found that the curing time significantly affected the force transferring of cable tendons. Increasing the curing time improved not only the initial stiffness of the cable reinforcement system but also the peak ability of cable tendons. This conclusion was, furthermore, confirmed by later researchers [67].

3.4. Resin Grout Property

Kent and Bigby [14] installed two series of Megabolt cable tendons in two different grout-anchored systems, in which one was resin and the other was cement grout. After the tests, the working ability of the Megabolt tendons was compared, showing that the BS and initial stiffness of the cement-grouted system was better than the resin-anchored system.
Meikle et al. [68] found that increasing the curing time from 1 day to 28 days would only improve the peak ability of cable tendons by a small extent.
Faulkner et al. [69] installed cable tendons in resin anchored systems with different installation methods. In one test, the cable tendon was rotated clockwise when installing and the other test used a counter-clockwise rotation method. It indicated that the counter-clockwise rotation of the cable tendons in resin reduced the force-transferring ability of the cable reinforcement system.

3.5. Modified Geometry

3.5.1. Buttons

Goris and Conway [10] conducted pull tests on buttoned and plain cable tendons, finding that adding a button apparently improved the working ability of the cable tendons. Furthermore, the button position relative to the joint affected the maximum force-transferring ability of cable tendons. Specifically, the greater the distance between the button and the joint in the embedment section, the greater the force-transferring ability of the cable tendons.
Later, more pull tests were performed on buttoned cable tendons by Goris [70] to find that if the distance between the button and joint was larger than 152.4 mm, the force-transferring ability of buttoned cable tendons became stable. Thus, a minimum distance of 111 mm was recommended. However, when installing buttoned cable tendons in field practices, it is rather difficult to determine the distance between the button and the known joint.
Martin, Girard and Curtin [56] found that the stiffness of cable reinforcement systems could also be improved by adding buttons along the cable tendon.

3.5.2. Bulbs

Numerous pull tests were conducted by Strata Control [71] to analyse the influence of bulbs on the force-transferring ability of cable tendons. It showed that increasing the bulb frequency along the cable length significantly improved the maximum force-transferring ability of cable tendons. This finding was confirmed by other researchers [72].
Hyett and Bawden [13] analysed the influence of bulb size on the working ability of bulbed cable tendons. The tested bulbs were from 25 mm to 40 mm in diameter. It indicated that increasing the bulb size resulted in the twisting of the steel wires around the bulb structure, reducing the force-transferring ability of cable tendons. Finally, a standard bulb size of 25 mm was recommended.
Pull tests on bulbed cable tendons under CNL conditions were performed by Moosavi et al. [73]. It indicated that the occurrence of bulb structure improved the working ability of cable tendons to a great extent. Other researchers used different confining materials to test bulbed cable tendons and reached the same conclusion [74].

3.5.3. Birdcages

Hutchins, Bywater, Thompson and Windsor [44] analysed the working ability of birdcage cable tendons, finding that the birdcage geometry improved the force-transferring ability of the cable tendons. Renwick [11], Clifford, Kent, Altounyan and Bigby [46] and Thomas [23] had the same result. It also revealed that the position of the nodes and antinodes relative to the joint made no apparent difference on the force-transferring ability of cable tendons when both were embedded in the grout column.
However, Goris [70] derived a different conclusion based on his experimental test results. Specifically, several birdcage cable tendons were tested. It showed that the position of nodes and anti-nodes significantly affected the working ability of cable tendons. For this reason, it was assumed that antinodes acted like anchors, increasing the force-transferring ability of cable tendons. Finally, it was recommended to use grout to fully submerge the antinodes in engineering applications.

3.5.4. Multi-Strands

Goris [75] conducted pull tests on twin plain cable tendons, finding that the force-transferring ability of twin plain tendons was more than double that of a single plain cable tendon. Similar research was later performed on bulbed or birdcage cable tendons by others [14,23,56,70,71]. The results showed that the existence of the second tendon could increase the force-transferring ability to a high value.

3.5.5. Fiberglass Material

Mah [6] conducted pull tests on cable tendons made up of fiberglass materials. It was reported that the FCB was far superior to standard plain tendons. Further tests showed that the water proportion, the encapsulation length and the reinforcement geometry were significant in deciding the working ability of FCB [76].

3.5.6. Coating Materials

Fuller and Cox [7] analysed the influence of rust on the working ability of steel wires. It indicated that compared with smooth wires, the rusted steel wires had a higher force-transferring ability. It was determined that rust improved the interlocking and friction between the wire and surrounding grout, which is beneficial to increasing the force-transferring ability. This was further confirmed by Cox and Fuller [59] with more experiments.
The influence of epoxy coating material on the working ability of cable tendons was analysed by Dorsten, Hunt and Kent [8], showing that the epoxy coating material is beneficial to improving the working ability. This was later confirmed by Goris and Conway [10], Goris [77], Nosé [78], Satola [79], Satola and Hakala [80], Satola and Aromaa [81], and Satola and Aromaa [82]. Furthermore, Satola [16] found that galvanized material was also beneficial to increasing the working ability of cable tendons.
Stillborg [54] used a greasy substance as a coating material and painted it along the surface of plain cable tendons. A comparison was performed on pull tests on coated cable tendons and plain tendons. It was found that the greasy substance reduced the force-transferring ability of the cable tendons. Thus, it was recommended to ensure that the cable tendon surface was clean before installation. This conclusion was also confirmed by others [44]. The only difference was that after adding painting materials, modified cable tendons still attained a high load but with a much lower stiffness.

3.5.7. Nutcases and Ferrules

Renwick [11] conducted pull tests on the Ultrastrand cable tendons to analyse the influence of the ferrule on the working ability of cable tendons. It indicated that the ferrule significantly affected the force-transferring ability of the cable tendons. Furthermore, the Ultrastrand with a 5 mm ferrule had greater BS, compared to a 2.5 mm ferrule configuration. However, the ferrule size was only varied in a small range in this research.
Hyett, Bawden, Powers and Rocque [12] analysed the working ability of nutcaged cable tendons, finding that the nutcaged cable tendon had a much better working ability than plain cable tendons. Moreover, the optimal nutcase size should be limited to 12.7 mm. The superior working ability of nutcaged cable tendons was also confirmed by Moosavi, Bawden and Hyett [73] and Moosavi [83].
A number of axial pull tests were performed by Kent and Bigby [14] to analyse the influence of ferrules on the working ability of cable tendons. It indicated that adding a ferrule along the middle grout tube evidently improved the force-transferring ability of the cable tendons, which was also agreed by Bigby [15].

3.5.8. Indentations

Fuller and Cox [7] analysed the influence of indentation position on the working ability of steel wires. It was revealed that although the indentation position had little influence on the peak ability of steel wires, it greatly influenced the residual working ability.
Tadolini, Tinsley and McDonnell [17] compared the working ability of smooth and indented PC tendons, finding that the existence of indentations along the wire improved the working ability of the cable tendons, which was also determined by Thomas [23].

3.6. Borehole Size

The borehole size effect on the working ability of cable tendons has been analysed by several researchers. For example, Rajaie [84] pulled plain cable tendons installed in concrete cylinders with different borehole diameters. He found that the borehole size had no influence on the working ability of cable tendons. Mah [6] used steel tubes to represent the rock and pulled FCB from tubes with different diameters, showing that the size of the borehole had minimal influence on the peak ability of cable tendons. The same conclusion was later confirmed by many researchers [63].
Thomas [23] used LSEPT to analyse the borehole size effect on the BS of cable tendons, finding that increasing the borehole diameter reduced the force-transferring ability of plain cable tendons. On the other hand, for bulbed and nutcaged cable tendons, larger borehole size was beneficial to improving the working ability of the cable tendons. However, no explanation was given regarding this different influence.
A summary of the previous research regarding the influence of borehole size on the working ability of cable tendons is tabulated in Table 4.

3.7. Rotation

Rotation of cable tendons was monitored by Stillborg [54] when he pulled plain cable tendons from concrete cylinders. In those tests, many cable tendons unscrewed from the confining material. Furthermore, rotation partly reduced the force-transferring ability of the cable tendons (Figure 8), which has been discussed and confirmed by later research [41,85,86,87].
Moosavi [83] conducted pull tests on bulbed cable tendons, finding that the modified surface geometry was effective in preventing cable tendons from rotating and improving the capacity of cable tendons. This phenomenon was also observed by Ito et al. [88] (Figure 9).
Zhang et al. [89] compared the working ability of plain tendons and modified cable tendons, finding that plain tendons clearly rotated in axial tests. By contrast, modified cable tendons only rotated a little bit in axial tests.

3.8. Pre-Tensioning

Thompson and Windsor [90] analysed the influence of pre-tension on the force transferring of cable tendons. It was found that the pre-tensioning was effective in improving the shear strength of the rock. However, it did not make a difference in determining the internal stiffness of cable tendons, which was also confirmed by Mirabile et al. [91].
Kent and Bigby [14] conducted pull tests on pre-tensioned Megabolt cable tendons. It indicated that pre-tension was beneficial to enhancing the working ability of the cable reinforcement system, especially under a low stiffness environment. However, pre-tension will also undoubtedly increase costs.

3.9. Stress Change

Macsporran [42] analysed the influence of confining stress change on the BS of cable tendons. A CNL environment was applied to the confining medium and the confining pressure varied from 2 MPa to 15 MPa. It indicated that the working ability of cable tendons was critically determined by the confining pressure. However, this conclusion was limited to plain cable tendons.
Nosé [78] used the same approach to test coated cable tendons. He found that modified cable tendons could still retain high force-transferring ability when the confining stress changed. This implies that the modified cable tendons were less sensitive to stress change. Later, Prasad [74] tested bulbed cable tendons and deduced the same conclusion.

3.10. The Breather Tube

Goris [75] conducted pulling tests on both single and twin cable tendons to analyse the influence of the breather tube on the working ability of cable tendons. It indicated that the existence of the breather tube did not influence the BS of the cable tendon system if the breather tube was filled with grout. This was supported with later research [48]. Furthermore, the diameter of the breather tube had no influence on the working ability of the cable tendons.

3.11. Loading Rate

The influence of loading rate was carefully analysed by Farah and Aref [64]. Three different loading rates were used, simulating rock movement from fast to slow. Two types of cement-based materials were used to bond the cable tendon with the confining medium. One was concrete while the other one was cement paste. Testing results showed that cable tendons grouted with concrete had more ductility and larger peak ability. It indicated that using concrete rather than cement paste was more effective under a dynamic loading environment.
However, pull tests performed by Hassani, Mitri, Khan and Rajaie [52] showed that for cable tendons installed with shotcrete bonding material, the loading rate had no influence on the working ability of cable tendons.

3.12. Confining Medium Size

Rajaie [84] analysed the influence of cylindrical confining medium size on the working ability of standard cable tendons. It indicated that if the confining medium diameter was larger than 200 mm, the peak ability of the cable tendons levelled off. However, this result is only applicable to plain cable tendons.
Holden and Hagan [92] used the same approach to analyse the size effect of test specimens on the working ability of bulbed cable tendons. It indicated that the force-transferring ability of the cable tendons increased with confining medium diameter.

3.13. Ambient Temperature

Goris [75] found that increasing the curing temperature is beneficial to improving the working ability of cable tendons. It was explained that higher temperature increased the hydration of the cement grout, reaching a larger UCS strength sooner.

4. Discussion

Based on earlier research, there is common agreement that certain parameters have an influence on the force transferring of cable tendons. For example, high confinement increases the force-transferring ability of cable tendons. Furthermore, it appears that much research focused on axial force transferring of cable tendons and LSEPT approach has become more widely accepted. The work reported by Thomas [23] found several deficiencies, especially in testing high-strength modified cable tendons. One is related to the specifications of the test sample, where the cable is embedded. A series of tests using a high-strength cable tendon showed that the maximum pull load varied with the sample diameter up to around 300 mm but remained constant thereafter. It is recommended that the sample size effect tests should be performed beforehand on cable tendons to determine the appropriate diameter for specimens.
Compared with axial testing, there is less work reported on the shearing ability of cable tendons. An even lesser amount has focused on combined actions of axial and shear loading conditions. To better understand the force-transferring mechanism of entirely grouted cable tendons, the following work is recommended.
It is valuable to assess the influence of borehole diameter on the force transferring of cable tendons along the axial direction. Although some work on this topic has been reported, there are some inconsistences. Martin, Girard and Curtin [56] indicated that borehole diameter had no impact on the force transferring of cable tendons, which was also supported by Chen and Mitri [57]. However, Hutchinson and Diederichs [93] stated that the force-transferring ability varied inversely with the borehole diameter, which was also reported by Mosse-Robinson and Sharrock [63]. It should be noted that the same impact was initially found by Yazici and Kaiser [94] with the development of a conceptual model. Later, Thomas [23] demonstrated that the impact of borehole diameter differed depending on the design of the cable tendon. When conventional cable tendons are used, the working ability varies inversely with the borehole diameter.
Moreover, there should be a relationship between the parameters used in the cable tendon reinforcement. For example, the grout strength used in the cable tendon reinforcement may be dependent on the surrounding rock mass strength. However, the working ability of cable tendons is influenced by numerous parameters. Until now, the specific relationship between those parameters in the cable tendon reinforcement is still not clear. Therefore, further research is recommended to reveal the specific relationship between those parameters used in the cable tendon reinforcement.

5. Conclusions

Entirely grouted cable tendons are used in the underground mining industry, becoming more popular in rock reinforcement design. The main purpose of cable tendon reinforcement is to increase the stability of the rock surrounding excavations and provide a safe working environment.
Generally, two cable tendons, plain and modified cable tendons, are commonly used. Standard plain tendons have a smooth surface and a small axial force-transferring ability. Consequently, they are usually used for temporary reinforcement. By contrast, modified cable tendons have more superior working ability and are commonly installed for permanent reinforcement.
Both cement and resin grouts are used as bonding materials. Nevertheless, cement grout is more widely used in cable reinforcement due to its low cost and convenient installation. Furthermore, cable tendons grouted with cement-based grout are used in most underground excavations, such as cut-and-fill stopes and open stopes.
In the field, there are basically five different failure modes for cable tendons. Among them, failure along the B/G face is more frequent. The force transferring of cable tendons mainly relies on the resistance at the B/G face, which is composed of three parts: adhesion, interlocking and friction. However, adhesion marginally affected the interfacial shear strength. Interlocking and friction are more important, especially the friction, significantly affecting the working ability of cable tendons.
Much research has been undertaken on the axial working ability of cable tendons. Since there is no common test standard, various approaches have been used. Compared with the SEPT, SPPT and DEPT, the LSEPT is a better procedure to analyse the working ability of cable tendons subjected to tensile load. This is because it can overcome the problems occurring in previous test methods. Nevertheless, there are still some problems with the current LSEPT, especially in testing high-strength modified cable tendons. Therefore, it is valuable to propose a new test approach to analyse the working ability of cable tendons.
The influence of many diverse parameters on the working ability of cable tendons was analysed. It indicated that the working ability of cable tendons can be improved by various methods, such as the installation of buttons along the tendon and the adding of epoxy coated materials. These experiments were beneficial to understanding the cable reinforcement mechanism. However, most focused on the working ability of cable tendons under strong rock environments by using steel, aluminium or concrete blocks with high strength to simulate the rock. Moreover, little research has been performed on the axial working ability of cable tendons in weak rock conditions.

Author Contributions

Writing: J.C., B.W., P.L. and G.Z.; Conceptualisation: Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [National Natural Science Foundation of China] grant number [51904302, 52174093], [Open Fund of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining] grant number [WPUKFJJ2019-08], [Key Laboratory of Deep Coal Resource Mining (CUMT), Ministry of Education] grant number [KLDCRM202203].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are included in this article.

Acknowledgments

All authors thank the reviewers for their valuable comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Cable tendons used in engineering: (a) plain cable; (b) modified cable [1].
Figure 1. Cable tendons used in engineering: (a) plain cable; (b) modified cable [1].
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Figure 2. SEPT for cable tendons [40].
Figure 2. SEPT for cable tendons [40].
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Figure 3. Working ability of plain cable tendons under different confinement conditions [51].
Figure 3. Working ability of plain cable tendons under different confinement conditions [51].
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Figure 4. Failure modes of cable tendons in those four stages: (a) Stage 1; (b) Stage 2; (c) Stage 3; (d) Stage 4 [51].
Figure 4. Failure modes of cable tendons in those four stages: (a) Stage 1; (b) Stage 2; (c) Stage 3; (d) Stage 4 [51].
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Figure 5. Peak ability of cable tendons with different embedment length.
Figure 5. Peak ability of cable tendons with different embedment length.
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Figure 6. Performance of cable tendons when the embedment length was different [55].
Figure 6. Performance of cable tendons when the embedment length was different [55].
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Figure 7. Working ability of cable tendons with different water proportions under CNL condition. (a) 0.4; (b) 0.5 [61].
Figure 7. Working ability of cable tendons with different water proportions under CNL condition. (a) 0.4; (b) 0.5 [61].
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Figure 8. Influence of rotation on the cable tendon’s performance [41].
Figure 8. Influence of rotation on the cable tendon’s performance [41].
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Figure 9. Comparison between a plain cable tendon and a modified cable tendon [88].
Figure 9. Comparison between a plain cable tendon and a modified cable tendon [88].
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Table
Table 1. Development on the design of cable tendons.
Table 1. Development on the design of cable tendons.
Cable Tendon NameYearReporterDescription of the Cable Tendon
Plain cable tendon1975Fuller and Cox [7]7-wire cable tendon with a diameter of 15.2 mm
Epoxy coated cable tendon1984Dorsten et al. [8]7-wire cable tendon with the epoxy material coated on the cable tendon surface
Birdcaged cable tendon1986Nguyen et al. [9]10-wire birdcaged cable tendon with a maximum diameter of 45 mm
Buttoned cable tendon1987Goris and Conway [10]Buttoned cable tendon with the button diameter ranging between 24.4 mm and 31.8 mm
Ultrastrand cable tendon1992Renwick [11]Ultrastrand cable tendon with the spacer wall thickness ranging between 2.5 mm and 5.0 mm
Nutcaged cable tendon1993Hyett et al. [12]7-wire cable tendon with the hexagonal nut diameter ranging between 12.7 mm and 19.05 mm
Bulbed cable tendon1994Hyett and Bawden [13]7-wire cable tendon with the bulb diameter ranging between 25 mm and 40 mm
Fibreglass cable tendon1994Mah [6]Cable tendon with the tendon diameter ranging from 1 mm to 15 mm
Megabolt cable tendon2001Kent and Bigby [14]9-wire cable tendon with a central grouting tube whose diameter is 12.7 mm
Double minicage cable tendon2004Bigby [15]7-wire cable tendon with the bulb profile having a maximum diameter of 25 mm
Galvanised steel cable tendon2007Satola [16]7-wire cable tendon with a diameter of 15.7 mm and coating material of zinc
Indented PC strand2012Tadolini et al. [17]7-wire cable tendon with a diameter of 15 mm and indentation along steel wires
Sumo cable tendon2015Ur-Rahman et al. [18]9-wire cable tendon with the tendon diameter of 28.5 mm
Constant-resistance large deformation cable tendon2017He et al. [19]Cable tendon with the tendon diameter of 21.8 mm and a constant resistance device
Goliath cable tendon2022Rastegarmanesh et al. [20]19-wire cable tendon with a tendon diameter of 28.6 mm and bulb diameter of 28.6 mm
Table
Table 2. Comparison between different experimental test programs.
Table 2. Comparison between different experimental test programs.
Test MethodFull NameObjectiveAdvantagesDisadvantages
SEPTSingle embedment pull testThe objective is to provide a simple test method to evaluate the working ability of cable tendons.The test method is simpler and it is relatively easy to be conducted.During the test process, the cable tendon has the tendency to rotate. This decreases the working ability of cable tendons.
DEPTDouble embedment pull testThe objective is to provide a test method to prevent the cable tendon from rotating during the axial test process.Two embedment lengths are used. Therefore, the force and displacement can be measured on either side of the joint between those two embedment lengths. Additionally, rotation of the cable tendon can be restricted.Metal tubes are used to simulate the rock mass. It is not a true flection of the confinement that the rock masses apply on the cable tendon.
LSEPTLaboratory short encapsulation pull testThe objective is to provide a more realistic test method to reveal the working ability of cable tendons in the axial test.The full length of the cable tendon was divided into two parts: the embedment length and the anchor length. In the embedment length, realistic or artificial rock can be used to confine the cable tendon. It can better simulate the interaction between the cable tendon and the surrounding rock. In the embedment length section, the size of the realistic or artificial rock sample is usually large. Therefore, it is more complicated to conduct this test.
Table
Table 3. Previous research summary regarding the influence of embedment length on the performance of cable tendons.
Table 3. Previous research summary regarding the influence of embedment length on the performance of cable tendons.
ReporterYearTested ElementElement DiameterEmbedment Length
Fuller and Cox [7]1975Steel wire7 mm100 mm to 700 mm
Stillborg [54]1984Cable tendon38 mm76 mm to 266 mm
Farah and Aref [5]1986Cable tendon15.2 mm178 mm to 710 mm
Mah [6]1991Cable tendon15 mm432 mm to 508 mm
Benmokrane, Chennouf and Mitri [55]1995Cable tendon15.8 mm63.2 mm to 316 mm
Martin, Girard and Curtin [56]1996Cable tendon15.2 mm457.2 mm to 914.4 mm
Chen and Mitri [57]2005Cable tendon15.2 mm152.4 mm to 304.8 mm
Thompson and Villaescusa [58]2013Cable tendon15.2 mm500 mm to 2500 mm
Table
Table 4. Previous research regarding the influence of the borehole size on the working ability of cable tendons.
Table 4. Previous research regarding the influence of the borehole size on the working ability of cable tendons.
ReporterYearTested ElementElement DiameterBorehole Diameter
Rajaie [84]1990Cable tendon15.2 mm20 mm–60 mm
Mah [6]1994Cable tendon15 mm48 mm–77 mm
Mosse-Robinson and Sharrock [63]2010Cable tendon15.2 mm42 mm–106 mm
Thomas [23]2012Cable tendon21.8 mm–31 mm28 mm–55 mm
Thomas [23]2012Cable tendon21.8 mm–46 mm42 mm–64 mm
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Chen, J.; Wu, B.; Li, P.; Zhang, G.; Yuan, Y. A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests. Sustainability 2022, 14, 16543. https://doi.org/10.3390/su142416543

AMA Style

Chen J, Wu B, Li P, Zhang G, Yuan Y. A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests. Sustainability. 2022; 14(24):16543. https://doi.org/10.3390/su142416543

Chicago/Turabian Style

Chen, Jianhang, Baoyang Wu, Peng Li, Guojun Zhang, and Yong Yuan. 2022. "A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests" Sustainability 14, no. 24: 16543. https://doi.org/10.3390/su142416543

APA Style

Chen, J., Wu, B., Li, P., Zhang, G., & Yuan, Y. (2022). A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests. Sustainability, 14(24), 16543. https://doi.org/10.3390/su142416543

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