Resin Rebar – Rebar/hole diameter and why size matters

The use of resin rebar as a support system is common in underground mines and most people involved with underground mining are familiar with it. The early support capacity, corrosive resistance, and relatively ease of installation provides certain benefits and makes it a suitable option for rapid development of longer-term excavations. However, it is not necessarily suitable for every scenario. Factors such as temperature (hot (>30°C) and cold (<10°C)) and ground conditions influence the performance. In other cases (e.g. absence of a corrosive environment) the longevity and early support associated with resin rebar may not be required. In these scenarios, alternative support products such as split sets may be a more appropriate option. Resin rebar is a great product under the appropriate conditions provided that the support system is designed and installed correctly.

Along with the benefits of resin, there are a number of potential pitfalls. The resin bond strength is sensitive to numerous factors, most of which are within the control of the geotechnical engineer and operator. The factors which could impact the success of the support system include:

• Resin temperature – slow setting of cold resin and shorter shelf life in a hot environment
• Installation rate – some resin setting before the resin at the toe of the hole is mixed
• Resin set time – fast-setting resin could form bonds before the installation is complete, resulting in broken bonds
• Spin and hold time – inadequate mixing or breaking of the bond
• Resin volume – partial bonding, poor mixing, or wastage
• Hole dimensions – challenges installing the rebar, insufficient bond strength, and inadequate bonding

I have encountered a number of these issues during previous involvement in the trialing, testing, and roll-out of resin rebar bolting systems. The human factor contributed to the majority of the issues, but challenges associated with the design were also present. In this post the focus is on the drill hole annulus and how that could impact performance.

Annulus is defined as half of the difference between hole diameter and rebar diameter. The annulus for a 20 mm rebar installed in a 32 mm hole will therefore be 6 mm. The annulus has an impact on the resin bond strength as well as the amount of resin to be used for a given hole length. In coal mining the general practice is to maintain a small annulus (6 – 7 mm). In hard rock mining an annulus of up to 13 mm is generally accepted. Resin suppliers typically provide guidance on the recommended annulus for their products, and this information can usually be found in the product’s technical documentation. The table below is such an example; it also provides guidance on the amount of resin and the resin cartridge diameter to be used for various combinations of rebar and hole diameters. The blank blocks represent combinations not recommended by the resin supplier. 

A recent project I was involved with in once again highlighted the impact that oversized holes could have on support performance. In this case a contractor was responsible to source and install a range of ground support types which included #6 (19.5 mm diameter) resin rebar of various lengths, with performance specifications defined during the support design. The contractor selected the products and equipment based on their preference and experience. Support performance was assessed through industry-accepted methods during the commissioning and thereafter at predefined intervals. Performance of the resin rebar was primarily based on short (300 mm) encapsulation pull tests and visual observations. Pull tests were required to achieve 10 tonnes (t).

The contractor elected to install the rebar (19.5 mm diameter) with mechanical bolters equipped with 33 mm drill bits. The expected hole diameter was on the upper limit of the resin supplier’s recommendation but aligned with the contractor’s experience at other operations within Canada. Although this is considered to be on the upper limit, I have since observed the same combination of 19.5 mm rebar and 33 mm drill bits at well-established Canadian operations.

Initial pull test results were highly variable and failed to consistently achieve the minimum load requirement (10 t). The trouble-shooting process investigated the sensitivity to several factors, which included the spin speed, spin time, resin temperature, and resin containment. These factors had no material impact on the bond strength. A closer inspection of the drill holes revealed variability of the drill hole break-out. The figure below shows four holes drilled in one of the testing areas. The rock type was the same but overbreak drastically different. Holes with greater overbreak were associated with resin bond failure at lower loads. This led to the conclusion that the larger annulus was the critical factor.

The obvious solution to test the hypothesis would have been to use smaller drill bits with the same installation setup. In this case the available mechanical bolters were not compatible with smaller drill bits, which left two alternatives:

1. Install the 19.5 mm diameter rebar in holes drilled with 32 mm drill bits using jacklegs, or
2. Install 22 mm diameter rebar with the mechanical bolter using the 33 mm drill bits. 

A series of tests were conducted to investigate both options. Results for the three test configurations are presented to show the influence of the hole annulus on short encapsulation tests. Pull tests were typically conducted in sets comprised of five tests. More bolts were installed with the jackleg but some of the tests were stopped when the design load (10 t) was reached.

 The issues encountered during this project emphasized the potential pitfalls associated with resin and the bond strength sensitivity to the hole annulus. Applying past practices and experience to a new project led to technical challenges and costly delays. Failure to identify this at the onset of the project would have led to the installation of questionable or substandard support and even greater cost to complete the rehabilitation. A few recommendations to manage, and prevent, similar challenges include:

1. Consider the resin supplier’s guidelines and design based on the lower limit. Do not design according to the maximum recommended diameter.
2. Test the support configuration in all the intended rock types or rock mass classes.
3. Equipment should be flexible and compatible with smaller drill bits.
4. Routine short encapsulation pull testing will help to identify potential issues and changing conditions.

More information on the ground support QA/QC provided for this project can be found in papers presented at the Ninth International Symposium on Ground Support in Mining and Underground Construction which was organized by the Australian Centre for Geomechanics.

• Viljoen, BN; Murphy, BA, 2019. Ground support installation quality controls and possible pitfalls: a case study from a critical fall of ground contract rehabilitation project
• Gibbons, O; Lee, C, 2019. How convincing is the quality of our resin rebar installation? A case study.