Reinflation testing of inflatable friction rock bolts

Inflatable friction rock bolts (IFRB) are often considered temporary ground support due to their susceptibility to weakening from corrosion. The effective service life of an IFRB can be difficult to estimate due to the multitude of factors that affect the rate of corrosion. Some mine sites have experienced severalyear- long service lives of IFRBs without issue, while others experience corrosion-related falls of ground only weeks after installation. The most common industry method of assessing bolt performance is by pull testing the fixture. However, due to the IFRB’s ability to mobilize its full tensile capacity with less than 0.6 m (2 ft) of anchored length, corrosion along the bolt at depths greater than 0.6 m (2 ft) from the collar cannot be identified using this approach. Alternatively, reinflation testing is a simple and nondestructive method of evaluating the condition of an entire bolt length for breaches in the bolt wall. This can be particularly useful when assessing the extent of corrosive conditions within a particular mining area where instabilities have already occurred and the condition of nearby bolts is unknown, or when re-entering historic mine workings that have not been monitored. IFRBs are commonly used in underground mines throughout the world and have become increasingly popular due to their rapid and simple installation. To install an IFRB, a borehole is drilled out and once the IFRB is fully inserted within the hole, it is inflated to 300 bar pressure so that the bolt expands and its exterior walls are in contact with the surrounding rock. Once the bolt has sustained this inflation pressure for a minimum of six seconds, the installation is considered complete. As with all friction-type rock bolts, IFRBs are generally considered a temporary support type with a finite service life due to susceptibility to corrosion and the associated reduction in capacity. Even relatively minor levels of corrosion can significantly reduce the ultimate tensile strength of friction rock bolts, with a reduction in the crosssectional area of only 10 percent resulting in a 30 to 40 percent reduction in strength (Ranasooriya et al., 1994; Aziz et al., 2013.) Rates of corrosion and the effective service life of these bolt types vary greatly depending on conditions and bolt specifications or coatings. Many mines determine a site-specific effective service life of these bolt types through trial and error or case histories of falls of ground where the bolts were observed to have been corroded. While split-set friction bolts can be inspected with a small borescope camera to observe the interior bolt wall for corrosion, this method is not possible with inflatable bolts. Reinflation testing is a quick and simple method that evaluates the condition of the entire bolt length and can give a reasonable indication if the bolt wall has been compromised by corrosion, shearing or other factors. Corrosive conditions Corrosive conditions are difficult to predict due to many contributing factors such as the presence of water, pH of the water, dissolved oxygen in the water, the presence of soluble chlorides and sulfides, presence of corrosion inducing bacteria, humidity levels, rock and air temperatures and changes to ventilation. The geochemistry of the rock in contact with the bolt is one of the greatest contributing factors to a corrosive environment. Unlike fully encapsulated support types, friction support such as IFRBs are in direct contact with the surrounding rock. When that surrounding rock contains corrosive elements, the bolts can corrode quickly. As most rock masses are not homogenous, the corrosion from contact with corrosive minerals in the rock can be highly localized. Changes in ventilation can cause condensation to form on the bolts, particularly when the virgin rock temperature and air temperature are significantly different. This can supply the moisture required for the process of corrosion in an area that is seemingly dry. Open fractures or structures such as faults are often conduits for the water required for corrosion. They may also have a different mineral composition than the host rock. Anecdotal evidence suggests that these factors may lead to localized corrosion where a bolt passes through such structures. Since these structures often also act as release planes in fall of ground (FOGs), they represent the worst possible location for localized corrosion of a friction bolt. To further complicate the diagnosis of corrosive conditions, the mineralogy and structures in the bolt hole beyond the collar can sometimes vastly differ from the observed conditions along the drift exposure. Even if sulfide minerals or other corrosive elements are not observed in the drift back or ribs, bolts may still be in contact with them within the rock mass. Many IFRB manufacturers offer anticorrosion coatings as an option. These typically include cathodic sacrificial coatings such as zinc galvanization, epoxy coatings or various plastic-polymer coatings. These systems have proven vulnerable to scratch damage during installation or cracking and debonding of the coating due to tangential stresses induced during inflation. In practice, most of these coatings provide minimal protection against corrosion of the bolts when subjected to installation and inflation stresses in highly corrosive environments. (Ma et al., 2017). Figure 1 Failed bolt with localized corrosion and dampness. Figure 2 Failed bolt with localized corrosion. Case study An underground mine in the western United States experienced multiple FOG incidents where corrosion of the IFRBs was indicated as a primary contributing factor during post-FOG investigations. The bolting standard at this mine specified a maximum service life of 12 months for IFRBs, and the FOGs occurred where bolts had been in service for seven to nine months, indicating that the specified maximum service life was overestimated. Factors thought to potentially contribute to the corrosive environment and FOG included presence of sulfide minerals, high virgin-rock temperatures, very dense rock, highly continuous steeply dipping rock fabric, high humidity and frequent ventilation changes. In one of the FOGs, a bolt was observed to have corroded completely through the wall of the bolt in multiple places along a short length near the point of failure but had minimal corrosion on the 762-mm (30-in.) length closest to the hole collar. Corrosion on the wire mesh and bolt heads and plates was also observed to be minimal, and the heading was observed to be generally dry. The rock exposed by the FOG and the bolts where they passed through the failure plane were both observed to be damp. A photo of a recovered portion of this bolt immediately following the FOG is presented in Fig. 1, showing the dampness and corrosion present at the failure plane. A photo of the same bolt with additional details on the levels of corrosion and their corresponding lengths is presented in Fig. 2. Another failed bolt in the same FOG was observed to have mild levels of corrosion along most of the section that was recovered, and an elevated (moderate) level of corrosion near the point of failure. In both cases, corrosion was significantly greater where the bolts intersected the structures that ultimately acted as release planes than on the remainder of the recovered bolt lengths. A schematic showing an interpreted cross section of the wedge failure is presented in Fig. 3. This clearly demonstrates how the bolts installed could easily pass a pull test despite extreme corrosion near the failure planes rendering the bolts completely ineffective. Reinflation test procedures The IFRBs were tested in the same way that they were installed, which is by attaching a wand to the bolt head, pressurizing the bolt with water to 300 bar, and sustaining that pressure for six seconds. Any bolt that held pressure for six seconds was recorded as a passing test, while any bolt that did not pressure up to the full 300 bar or hold pressure for the full six seconds was recorded as a failure. The heads and plates of all tested bolts were spray painted with contrasting colors to enable easy visual identification and tracking of which bolts had passed and which had failed. While a bolter can be used for this testing, it can be difficult to align the boom accurately, which can lead to premature damage and failure of the rubber boot. Using a hand wand from a scissor deck proved to be the quickest and easiest method of testing and had the added benefits of reducing wear and tear on the rubber boot and enabling easy marking of the bolt heads and plates with spray paint. The reinflation testing plan used in the case study followed the logic flowchart presented in Figs. 4 and 5. The premise is to commence testing in an area where the ground support is known to be in good operating order and then advance toward the area of concern to minimize exposure for personnel performing the tests. One bolt was tested in the back or shoulder of each ring and tests were advanced in a zig-zag pattern. Bolts were targeted where indicators of corrosive elements were observed, such as water, higher corrosion levels on mesh or plates, and sulfide minerals. Each bolt that failed to inflate triggered an additional two bolts to be tested in that ring and two additional tests in each of the two adjacent rings. When areas with many failed inflation tests were encountered, the area was to be resupported with new ground support prior to resuming the testing campaign. The results of the reinflation test program are in Table 1. Reinflation test versus pull test A pull test for installed rock bolts typically involves attaching a hydraulic ram to the head of the bolt and pulling to between 80 and 100 percent of the rated ultimate capacity of the bolt. A standard-capacity inflatable rock bolt typically has an ultimate capacity of 10.8 t (12 st) and a typical bond strength of 5.4 t/ft (6 stpf), though the bond strength can vary significantly based on installation parameters, borehole diameter and rock conditions. Therefore, a bolt with less than 0.6 m (2 ft) of bonded length can successfully pass a pull test, which can be verified through limited embedment pull tests. Since bond strength and corrosion-related reduction in strength are not distributed evenly along the length of a bolt, a pull test is only able to verify the integrity of the bolt and the bond strength in the short length nearest the collar of the bolt hole. A reinflation test does not verify bond strength to any degree but can give indication that the integrity of the bolt has been compromised at some point along its full length. Knowing that the wall of an inflatable bolt has been compromised is useful information regardless of whether shearing, corrosion or some other factor was the cause. Applications and limitations A primary application for the use of reinflation testing is re-entry of mine workings supported by inflatable bolts that have not been accessed recently or where there are concerns about the age or conditions of the installed support. The criteria for an acceptable failure rate will vary and are dependent on the level of risk tolerable to the mine operator. In this case study, a failure rate of up to 10 percent was considered acceptable, but areas with failure rates greater than this were resupported. In this case, the ground support was designed to a factor of safety equal to 1.3, and 10 percent of the bolts completely failing would still provide a safety factor greater than 1 for the ground support system, assuming the failures were evenly distributed. Reinflation tests should only be used as a supplement to other quality assurances/quality controls for bolt integrity, not as a replacement. While a bolt passing a reinflation test does indicate that the bolt integrity is not entirely compromised, it does not give any indication of bond strength, or degree of corrosion that may have occurred. It should be noted that the reinflation test only gives an indication of bolt integrity at a specific moment in time and does not give any indication of when a bolt may be nearly compromised. The results of the case study indicate that the service life of IFRBs can vary significantly and is difficult to predict based on observed conditions. However, the testing program did successfully quantify the extent of exposed ground where IFRBs had exceeded their service life and aided the prescription of a targeted rehabilitation program. The data collected also provided sufficient evidence to amend the ground control standards to a more accurate service life for IFRBs in that project site’s unique environment. Figure 3 Interpreted schematic of wedge failure. Figure 4 Schematic of testing strategy. Figure 5 Example procedure for failed inflation tests. Table 1 Reinflation test data. Conclusions Reinflation tests of inflatable bolts offer an additional tool for geotechnical engineers and mine operators to verify the integrity of inflatable friction bolts after they have been put into service. The approach offers a rapid solution when performing forensics on a FOG occurrence, or when re-entering mine workings where the age or condition of the IFRBs is unknown. Furthermore, it is recommended that this approach be used to aid in the accurate prediction of IFRB service life for a mine’s ground control management program. References Ranasooriya, G., Richardson, G.W. and Yap, L.C. (1994), “Investigation of Corrosive Behaviors of Friction Rock Stabilizers Used in Underground Mines in Western Australia,” Geological Survey of Western Australia, 1993-94 Annual Review. Aziz, Naj, Craig, Peter, Nemcik, Jan and Hai, Faisal I. (2013), “Rock Bolt Corrosion – An Experimental Study,” Proceedings of the 2013 Coal Operators’ Conference, Mining Engineering, University of Wollongong, Feb. 18-20, 2019. Ma, Kevin J., Stankus, John and Faulkner, Dakota (2017), “Development and Evaluation of Corrosion Resistant Coating for Expandable Rock Bolt Against Highly Corrosive Ground Conditions,” International Journal of Mining Science and Technology.

Reinflation testing of inflatable friction rock bolts

Authored by:
M. Conley, R. Cook, J. Hallowell

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