MATERIAL FOR INSTALLING BOLTS
Material for Installation
Windsor (1997) introduced the bolt load transfer concept that has four principal elements as shown in Figure 17: rock, the internal fixture, the external fixture and the reinforcing element. The reinforcing element and external fixture refer to the bolt, the face plate and the threaded nut, respectively. Coupling between the rock and the reinforcing element is via the internal fixture, which could be either cementitious or polymeric resin for grouted bolts, or a mechanical friction anchorage along the bolt-rock surface for frictionally coupled bolts. Accordingly, Windsor (1997) classified the current reinforcement devices into three fundamental groups:
- Continuous Mechanically Coupled (CMC),
- Continuous Frictionally Coupled (CFC) and
- Discretely Mechanically or Frictionally Coupled (DMFC).
The fully encapsulated rockbolt system belongs to CMC, while expansion shell bolts belong to DMFC system and the Swellex and Split set rockbolts to CFC system.
In comparison to the expansion shell or wedge type bolt anchors, fully grouted rockbolts are usually much stiffer than the point anchor bolts and can hold greater load transfer capacity because of the apparent fact that the fully grouted bolting system provides greater shear surface in the load transmission from the bolt to the rock and vice versa. The resistance of the bolt to axial movement in this type of bolt occurs at any point along the bolt and thus has a cumulative reinforcement effect. This characteristic leads to the application of fully grouted bolts in weaker or fractured rock strata. The mechanism of fully grouted bolt system seems similar to continuous mechanically coupled bolt system however, the fully grouted bolts have larger cross-section and perform better when loaded in lateral shear than the bolts such as swellex bolt. Hence the grouted rock bolts are increasingly being used in many geotechnical and mining applications. The fully grouted bolting system has three components: the grout, the bolt, and the rock surrounding the hole. Grouting materials in common use are usually cementitious grouts and chemical resin grouts.
Pumped cementitious grouts
These grouts tend to be relatively low cost but have various performance mainly due to poor discipline in the mixing operation. The uncontrolled addition of excess water to make pumping easier is a common issue, which adversely influences the performance of the grout and eventually the performance of the anchor. The main use of these grouts is to install secondary support in conjunction with long tendon bolts and birdcage style bolts.
Resin capsules, supplied in a two-component capsule, are widely used in the installation of rock bolts and cables. Installation of resin grouted rockbolts involves spinning the bar or cable through the capsule, the action of which tears the capsule and mixes the mastic and catalyst together. The resultant mix then hardens, anchoring the bolt into the rock borehole.