Mining underground in weak, highly fractured, and squeezing rock masses presents significant problems with ground stability. Convergence, shear and extensive rock damage from stress commonly limit the extent of the excavations. For the stability of the excavation to be maintained and the work to be conducted in a safe manner, there is need for effective primary and secondary support systems. The choice of support system depends significantly on ensuring compatibility between the mechanical performance of the support system and the mode of failure in the rock mass. The friction bolt and the grouted cable bolt represent two entirely different approaches to underground support.
Mechanical load transfer and anchoring capacity are two factors that differentiate the operation of friction bolts and cable bolts. Friction bolts, which include split tubes and water expanded tubes, work on the principle of the radial pressure applied on borehole walls, creating instantaneous passive resistance on the whole length of the tube. Split friction tubes exhibit low anchoring capacity ranging between 3 and 8 tonnes per meter in fractured or soft rocks; however, the size of the borehole has great effect on the anchoring capacity; larger bores lower the normal stresses whereas smaller bores hinder the placement of the tube. On the other hand, cable bolts utilize flexible high tensile steel wire strands embedded in cement or resin grout. The plain strands, cement grouted, exhibit anchoring capacity of 10 to 30 tonnes per meter in soft grounds; however, the modified strands give anchoring capacity of 60 to 80 tonnes per meter.
The behaviour of ground under large stress field deformations demonstrates important performance compromises inherent in support methods. Friction bolts adapt to early convergence of the rock mass quite well, expanding or sliding during the process of closing. High shearing in rock joint plane often results in considerable bending or failure in shear mode of the tube. Thick faceplates might bend or slide over friction bolt rings under significant convergence pressure. High axial and shear stiffness of cable bolts prevents displacement of rock layers. Full column grouting of strands avoids progressive unwinding, though too much squeeze of ground can lead to grout-cable rupture. Corrosion resistance also influences the performance. Thin-walled friction tubes corrode very quickly both from atmosphere and moisture in underground conditions, thus losing their holding strength. Therefore, non-grouted friction bolts act as temporary support.
Logistics of installation and installation times play a significant role in determining advance rate and the overall development costs. Single-stage mechanical installation makes friction bolts quick and easy to install. Face crew work makes fast advance possible without waiting for the cure of the chemical or hydraulic grout. Cable bolts are more labor intensive and require the use of grout pumps, placement of breather tube and curing time. The cost of materials for the installation of the cable bolts is higher than that of the simple friction tubes. Modern ground-control practice frequently uses the combination of two types of anchors to offset such disadvantages of operation. The main friction bolts or rebar bolts are installed immediately at the face and make immediate protection of the surface. The additional cable bolts are installed through the thick steel channels or straps and anchor deep behind the yielding roof dome.
The selection of either friction bolts or cable bolts should involve a judicious consideration of factors such as the state of ground conditions, durability of support, and speed of installation. The use of friction bolts presents significant advantages where skin retention is required, speedy development is desired, and temporary workings are planned in moderate to soft rock. On the other hand, cable bolts continue to be the conventional form of reinforcement in deep seated rock, wide spans, and structures that are exposed to great stresses or squeeze.
