Sample diameter effect on bonding capacity of fully grouted cable bolts
Introduction
The stability of underground excavations is of great importance in both civil and mining activities (Cai et al., 2004, Li et al., 2016). To increase the integrity of rock mass around openings, fully grouted cable bolts are commonly used. Numerous laboratory and field tests have demonstrated that cable bolts are effective in improving the inherent shear strength of rock mass (Kaiser et al., 1992, Hyett et al., 1995, Barley and Windsor, 2000, Singh et al., 2001, Moosavi and Grayeli, 2006, Ren et al., 2010, Chen et al., 2015, Chen et al., 2016, Li et al., 2017). Nevertheless, failure of cable bolting systems, especially bonding failure can present a problem caused by insufficient interfacial bonding capacity.
The bonding capacity of cable bolts is usually indicated by the maximum pull-out load (Fuller and Cox, 1975, Cox and Fuller, 1977, Hutchinson and Diederichs, 1996). To study the bonding capacity of cable bolts, a number of research has been conducted. The most commonly used method is pulling cable bolts from cylindrical samples made up of cement-based or concrete materials.
Stillborg (1984) used concrete materials to cast cylindrical samples with a diameter of 300 mm and conducted pull-out tests on plain cable bolts. His results showed that the greasy substance coated on plain cable bolts had a negative effect on the bonding capacity of cable bolts and recommended that the cable bolt surface should be kept clean before installation. Farah and Aref (1986a) also conducted pull-out tests on plain cable bolts installed in cylindrical blocks. Two different block diameters, 150 mm and 250 mm, were used. They found that cable bolts grouted with concrete materials had larger bonding capacity than those grouted with plain cement. Hassani and Rajaie (1990) used concrete blocks with a diameter of 250 mm to confine cable bolts and studied the effect of grout materials on the bonding capacity of cable bolts. The results showed that compared with the plain cement grout, the aggregate-cement grout can improve the peak and residual bonding capacity of cable bolts. Benmokrane et al. (1995) conducted pull-out tests on plain cable bolts installed in samples with a diameter of 200 mm, finding that the cable surface geometry and grout had a significant effect on the bonding capacity of cable bolts, especially when large displacement occurred. Altounyan and Clifford (2001) used a different material, namely sandstone to confine cable bolts and conducted pull-out tests. The diameter of samples is 142 mm. Testing results showed that pre-tension can effective improve the performance of cable bolts. The same test approach was also used by Bigby (2004), who found that the cable geometry had a significant effect on the stiffness of the cable bolting system. Blanco Martín (2012) developed a laboratory short encapsulation pull test for rock tendons and conducted pull-out tests on plain cable bolts. The sandstone samples with a diameter of 142 mm were used. Test results showed that plain cable bolts can maintain a high bonding capacity to a large displacement of 60 mm in pull-out tests.
According to the literature review, the effect of different parameters such as cable surface geometry, grout quality and coating materials on the bonding capacity of cable bolts has been studied. However, a common problem occurring in those tests is that the sample diameter is largely different. This is due to the fact that there is no common test standard in requiring the samples’ diameter and the variation in sample diameter made it difficult to compare previous test results.
In fact, little research has been conducted to evaluate the sample diameter effect on the bonding capacity of cable bolts. The pioneering work was conducted by Rajaie (1990), who evaluated the sample diameter effect on the pull-out performance of plain cable bolts. Specifically, he used concrete materials to cast samples with the diameter ranging from 100 mm to 300 mm. The test results showed that when the sample diameter is smaller than 200 mm, the bonding capacity of cable bolts apparently increased. However, once the sample diameter is larger than 250 mm, the bonding capacity became stable, as shown in Fig. 1. Therefore, he concluded that the sample diameter had a direct effect on pull-out performance of cable bolts and recommended that when pulling a cable bolt from the sample, the sample’s diameter should be large enough. However, his results are only applicable to plain cable bolts.
Holden and Hagan (2014) used same approach to test the sample diameter effect on a bulbed cable bolt. Four different sample diameters ranging from 150 mm to 450 mm were used. It was found that there is a linear relationship between the bonding capacity and sample diameter. However, during the test, no confinement was provided on the sample.
The sample diameter effect on the bonding capacity of cable bolts has not been fully understood. Furthermore, no research has been conducted in evaluating the sample diameter effect on bulbed cable bolts in the confined condition.
Therefore, this paper aims at studying the sample diameter effect on the bonding capacity of a bulbed cable bolt. The single embedment pull-out test method was used. Because first, it is the most commonly used method to test the performance of cable bolts (Stillborg, 1984, Farah and Aref, 1986b, Farah and Aref, 1986a, Hassani and Rajaie, 1990, Benmokrane et al., 1992, Benmokrane et al., 1995, Altounyan and Clifford, 2001, Tadolini et al., 2012, Holden and Hagan, 2014). Second, Rajaie (1990) conducted the pioneering work to study the sample diameter effect on cable bolts and he used the single embedment pull-out test. Following his work, the single embedment pull-out test method was also used in this study. This makes it possible to compare test results with Rajaie (1990)’s results.
In this paper, first, the pull-out test design was depicted. Then, a number of pull-out tests were conducted using a bulbed cable bolt. After that, the pull-out test results were compared. The effect of sample diameter and boundary condition on the bonding capacity of cable bolts was also analysed.
Section snippets
Experimental program
Two series of pull-out tests were performed using a high capacity bulbed cable bolt with different boundary conditions. In the first series of tests, samples were left in an unconfined state, while in the second series of tests, the samples were placed in a steel cylinder to be tested in confined state.
Test results of the SUMO cable bolt
The bonding capacity at different sample diameters in the unconfined condition is shown in Fig. 8 and Table 1. This shows that the sample diameter has a direct impact on the bonding capacity. Specifically, as the sample diameter rose up to 356 mm, the bonding capacity increased almost linearly to 100 kN.
A different relationship between the ing capacity and sample diameter occurred in the confined sample environment, as shown in Fig. 9 and Table 2. This also shows that bonding capacity increases
Conclusions
A number of pull-out tests were conducted, using a high capacity cable bolt to investigate the effect of sample diameter on the bonding capacity of cable bolting systems. A cement-based material was used to cast samples with diameters ranging between 150 mm and 508 mm. Two different boundary conditions, namely unconfined and confined boundary conditions, were examined.
A new approach was proposed to create the rifled borehole effect. A plastic wire was wrapped around the PVC pipe. After casting,
Acknowledgements
The authors would like to thank the Australian Coal Association Research Program (ACARP) for funding this research project. In addition, the authors would like to acknowledge the China Scholarship Council (CSC).
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