Rock Bolt

Hollow rock bolts were designed to make the grouting process more reliable (Figure 33.11). In hollow rock bolts, there is a hole in the center of the rock bolt. This hole is used for the grouting process and entangling tubes are eliminated. Hollow rock bolts may have the same cone and wedge anchor mechanism.

Rock bolts are a type of drilled soil nail or anchor used when the ground to be stabilized consists mostly of rock materials. These types of inclusions are typically grouted in place and posttensioned, similar to soil anchors (Figure 15.8). The bolts hold potentially unstable jointed or fractured rock masses together in compression with bearing faceplates providing passive resistance, forming a more stable structural entity. Rock bolts have been used for many years as temporary roof support in the mining industry and for tunneling, and are now used routinely for stabilizing roadcuts, rock cliffs, steep slopes, bridge abutments, and reinforced concrete dams. Two other versions of rock bolts sometimes used are those that have expansive shells (like a drywall anchor) rather than grout for resistance, and untensioned steel rods grouted into boreholes referred to as dowels (Hausmann, 1990).

Fig. 3.82 illustrates the excavation process. Rock bolts and prestressed anchors were used as reinforcements; the rock bolts were installed in a pattern of a 2 × 2 m grid over the entire arch and a 3 × 2 m grid on the two sidewalls. Reinforcement in each stage was conducted in the following sequence: (1) install rock bolts, (2) embed prestressed anchors, and (3) place shotcrete. The rock bolt and prestressed anchor were both treated as cable elements with different properties in the numerical model; the shotcrete was modeled as a structural element with a thickness of 0.32 m. A FISH function was written to perform automatic excavation and reinforcement installation.

Table 3.38 lists the input parameters for the structural elements and cable elements in the simulation (Li, 2009). Blasting dramatically affects the rock mass stability. In the simulation, the rock masses within 2 m from the wall of the excavation have been treated as a blast-induced damaged zone with a decreased deformation modulus that is 36% of the original value (Li, 2009).

In addition to friction at the bolt-rock interface, the bolt also interacts with the rock through mechanical interlock owing to the roughness of the borehole surface. The bolt tube is plastically deformed under the high water pressure during installation to match the rough profile of the borehole surface. A secondary contact stress q₂ will be induced at the bolt-rock interface when the bolt tube tends to longitudinally slip along the borehole. Letting i represent the average roughness angle of the borehole surface, the radial contraction of the bolt steel tube, u_i, is related to the axial slippage x as follows:

The secondary contact stress is expressed, according to Eq. (4.3.9), as

The products under consideration by Huade include FRP rock bolt, vessels, pipelines, duct valves and fittings, cooling tower, spiral, safety helmet, electric fan, cable pipe, railing, grating, crane span, and others. Rock bolt (Fig. 16.18) is widely used in underground mining to provide support to the roof or sides of the cavity. It can be used in any excavation geometry and it is simple and quick to apply, and is relatively inexpensive. The installation can be fully mechanized. The length of the bolts and their spacing can be varied, depending on the reinforcement requirements. However, in aggressive environmental conditions such as in coal mines, steel bolts deteriorate in a matter of days rather than years. FRP rock bolt is particularly suitable in harsh chemical and alkaline environments because of its corrosion resistance. It is durable, lightweight, easy to install, non-conductive, dimension-ally stable under thermal loading, anti-static rating, and can be cut without the danger of sparks. Currently, four production lines are being installed at the Beijing Huade facilities. Furthermore, spiral (Fig. 16.19) is widely used in coal preparation equipment and fine coal washing systems, and is being manufactured using a hand layup method. It is made of FRP composites with a wear-resistant coating.

1.

For D-shaped tunnels, θ = 90°.

2.

The directional rock bolts should be designed for tackling loads due to the wheels of the crane on the haunches.

3.

Support must be installed within the stand-up time (Figure 6.1).

Stresses in the shotcrete lining and rock anchors may be reduced significantly by delaying subsequent layers of shotcrete (except initial layers), but no later than the stand-up time. Instrumentation for the measurements of stress and deformation in the roof and the walls of a cavern or in tunnels is a must to ensure a safe support system. Instrumentation would also provide feedback for improvements in the designs of such future projects. Location of instrumentation should be judiciously selected depending upon the weak zones, rock mass quality, and intersection of openings.

The life of SFRS is the same as concrete in a polluted environment of approximately 50 years. Life may be increased to 60 years by providing an extra cover of 5 cm of SFRS. If SFRS is damaged later, the corroded part should be scratched and a new layer of shotcrete should be sprayed that will last for 100 years. For this the recommended thickness of SFRS is t_fsc = 13 cm = 21 cm (near portals).

Recent research work of failure mode analysis suggests that a cylindrical failure surface around the bolt–resin interface is a predominating failure mode in rockbolting (Cao et al., 2013). It occurs for smooth bars and for very closely spaced rebar bolts (like a screw) along the rib tips of the bar. For rebar bolts, experimental observation suggests that if the embedded length is short and the confining material is stiff, parallel shear failure occurs in laboratory pull-out tests. Fig. 2.27 shows a pull-out test bolt of 75-mm embedded length and confined in 8-mm thick-steel tubes.