Rockbolts and cables(2)

Rockbolts

Mechanically anchored rockbolts

Expansion shell rockbolt anchors come in a wide variety of styles but the basic principle of operation is the same in all of these anchors. As shown in Figure 2, the components of a typical expansion shell anchor are a tapered cone with an internal thread and a pair of wedges held in place by a bail. The cone is screwed onto the threaded end of the bolt and the entire assembly is inserted into the hole that has been drilled to receive the rockbolt. The length of the hole should be at least 100 mm longer than the bolt otherwise the bail will be dislodged by being forced against the end of the hole. Once the assembly is in place, a sharp pull on the end of the bolt will seat the anchor. Tightening the bolt will force the cone further into the wedge thereby increasing the anchor force.

These expansion shell anchors work well in hard rock but they are not very effective in closely jointed rocks and in soft rocks, because of deformation and failure of the rock in contact with the wedge grips. In such rocks, the use of resin cartridge anchors, described later in this chapter, is recommended.

At the other end of the rockbolt from the anchor, a fixed head or threaded end and nut system can be used. In either case, some form of faceplate is required to distribute the load from the bolt onto the rock face. In addition, a tapered washer or conical seat is needed to compensate for the fact that the rock face is very seldom at right angles to the bolt. A wide variety of faceplates and tapered or domed washers are available from rockbolt suppliers.

In general, threads on rockbolts should be as coarse as possible and should be rolled rather than cut. A fine thread is easily damaged and will cause installation problems in a typical underground environment. A cut thread weakens the bolt and it is not unusual to see bolts with cut threads that have failed at the first thread at the back of the nut.Unfortunately, rolled thread bolts are more expensive to manufacture and the added cost tends to limit their application to situations where high strength bolts are required.

Tensioning of rockbolts is important to ensure that all of the components are in contact and that a positive force is applied to the rock. In the case of light 'safety' bolts, the amount of tension applied is not critical and tightening the nut with a conventional wrench or with a pneumatic torque wrench is adequate. Where the bolts are required to carry a significant load, it is generally recommended that a tension of approximately 70% of the capacity of the bolt be installed initially. This provides a known load with a reserve in case of additional load being induced by displacements in the rock mass.

One of the primary causes of rockbolt failure is rusting or corrosion and this can be counteracted by filling the gap between the bolt and the drillhole wall with grout. While this is not required in temporary support applications, grouting should be considered where the ground-water is likely to induce corrosion or where the bolts are required to perform a 'permanent' support function.

The traditional method of grouting uphole rockbolts is to use a short grout tube to feed the grout into the hole and a smaller diameter breather tube, extending to the end of the hole, to bleed the air from the hole. The breather tube is generally taped to the bolt shank and this tends to cause problems because this tube and its attachments can be damaged during transportation or insertion into the hole. In addition, the faceplate has to be drilled to accommodate the two tubes. Sealing the system for grout injection can be a problem.

Many of these difficulties are overcome by using a hollow core bolt. While more expensive than conventional bolts, these hollow bolts make the grouting process much more reliable and should be considered wherever permanent rockbolt installations are required. The grout should be injected through a short grout tube inserted into the collar of the hole and the central hole in the bolt should be used as a breather tube. When installing these bolts in downholes, the grout should be fed through the bolt to the end of the hole and the short tube used as a breather tube.

Since the primary purpose of grouting mechanically anchored bolts is to prevent corrosion and to lock the mechanical anchor in place, the strength requirement for the grout is not as important as it is in the case of grouted dowels or cables (to be discussed later). The grout should be readily pumpable without being too fluid and a typical water/cement ratio of 0.4 to 0.5 is a good starting point for a grout mix for this application. It is most important to ensure that the annular space between the bolt and the drillhole wall is completely filled with grout. Pumping should be continued until there is a clear indication that the air has stopped bleeding through the breather tube or that grout is seen to return through this tube.