How Rock Bolts Reinforce the Rock Mass: The Basic Mechanisms of Bolt–Rock Interaction

Rock bolting is one of the fundamental methods of securing underground excavations, tunnels, rock slopes and cuttings. Unlike conventional standing support, which carries the load of the moving rock, a correctly designed bolting system reinforces the rock mass itself. Its purpose is to limit the development of fracturing, displacement and bed separation, so that the rock retains as much of its natural load-bearing capacity as possible.

Rock bolts do not, however, act through a single, universal mechanism. Depending on the structure of the rock mass, the orientation of the joints, the length and spacing of the bolts, the way they are anchored and the stress level, they may simultaneously bind rock layers together, increase friction on discontinuity surfaces, restrain the detachment of blocks and help to create a self-supporting rock zone around the excavation. In the literature these mechanisms are most often described as suspension, composite beam action, block keying (interlocking) and the formation of a pressure arch.

The rock mass as a system of blocks, layers and joints

Intact rock may exhibit high compressive strength. In practice, however, the stability of an excavation depends not only on the properties of the rock material itself, but also on the presence of discontinuities: joints, bedding, fissures, faults, cleavage planes and contacts between layers.

Once an excavation has been made, the original state of stress changes. The rock near its boundary is de-stressed and the stresses are redistributed. As a result, the following may occur:

• opening of existing fissures,
• sliding of blocks relative to one another,
• separation of bedded strata,
• bending of thin roof layers,
• fall of rock fragments and slabs,
• growth of a fractured zone around the excavation.

Rock bolts cross these planes of weakness and tie together fragments of the rock mass which, without reinforcement, could move independently of one another.

Binding rock layers together – the composite beam effect

One of the most important mechanisms by which bolts act is the binding of several rock layers into a single, thicker package that works like a composite beam. This is particularly significant in bedded, laminated or shaly rocks, where the roof may consist of many relatively thin layers.

A single thin layer has little bending stiffness and sags easily under its own weight and under the stresses acting around the excavation. If, however, several layers are effectively stitched together by bolts, they begin to act as a single element of greater thickness.

The increase in stiffness of such a package is far greater than would follow from simply adding up the stiffnesses of the individual layers. The bending stiffness of a layer grows with the cube of its thickness – in the ideal case, a package of n effectively bonded layers is therefore roughly n² times stiffer than the same layers deflecting independently. The condition, however, is that mutual slip and separation of the layers are restrained. The bolt then carries tensile and shear forces, maintaining contact between the layers.

This mechanism is sometimes compared to a compressed stack of sheets of paper. Loose sheets slide over one another and bend easily. Once firmly bonded together, the whole stack becomes markedly stiffer.

The effectiveness of layer binding depends, among other things, on:

• the orientation and spacing of the bedding planes,
• the angle at which the bolt crosses these planes,
• the anchorage length,
• the stiffness of the rod,
• the quality of the borehole filling,
• the bond at the rod–bonding material and bonding material–rock interfaces.

The most favourable situation is one in which the bolt crosses several potential separation planes and is anchored in a more stable part of the rock mass.

Increasing friction on discontinuity surfaces

The second important mechanism is the increase in frictional resistance between fragments of rock in contact. The resistance to sliding along a fissure depends, among other things, on the force pressing its surfaces together.

A bolt, particularly a pre-tensioned bolt, can introduce a compressive force into the rock mass. This clamps blocks and layers together, which increases friction on the contact surfaces. A greater shear force is then required to initiate slip.

This relationship is described qualitatively by the Mohr–Coulomb strength criterion used in soil and rock mechanics: the greater the normal stress acting on a discontinuity surface, the greater its shear strength can be. The roughness of the fissure, its infill, the presence of water and the degree of weathering of the surfaces also play a role.

A bolt crossing a fissure can resist displacement in two ways. First, it presses the surfaces together, increasing friction. Secondly, it works in shear and bending itself once the blocks begin to move relative to one another – the so-called dowel effect.

In practice, both phenomena occur simultaneously. In the initial phase, pre-tension and friction play the major role, whereas after larger displacements the share of the bolt’s direct shear resistance increases.

Limiting bed separation and the opening of fissures

A fractured rock mass loses load-bearing capacity not only through the sliding of blocks, but also through the opening of fissures. Separation reduces the contact area between rock fragments, limits the transfer of compressive stresses and makes further detachment easier.

Bolts counteract this process by taking up the tensile forces that arise perpendicular to the fracture surfaces. When a roof layer begins to move away from the rock above it, the bolt is loaded axially. Its task is to restrain this displacement and keep the structure of the rock mass as tight as possible.

Full encapsulation of the bolt along the entire length of the hole is particularly beneficial. A continuous connection between the rod and the rock allows load to be transferred at many points, not only at the end anchorage. Local rock displacements can then be restrained along the whole length of the bolt.

Full encapsulation also reduces the risk of large stress concentrations in any one area. The load is distributed between successive sections of the rod, the bonding material and the surrounding rock.

This does not mean, however, that every bolt completely eliminates rock movement. Many systems only begin to carry significant load once a small displacement has occurred. From the point of view of stability, the key is to limit this movement to a level at which an uncontrolled failure zone does not develop.

Creating a self-supporting rock zone

Systematically arranged bolts can create a reinforced rock zone around the excavation. Fragments of rock that would otherwise behave as independent blocks begin to act together as a larger structure.

In the roof of the excavation this zone may take the form of a rock beam or an arch, known as a pressure arch. Loads are then transferred between neighbouring parts of the rock mass and directed towards the more stable areas at the sides of the excavation.

The term ‘self-supporting rock zone’ does not mean that the bolts cease to carry load. On the contrary – it is the bolts that make this structure possible. Their role is to maintain the mechanical continuity of the rock mass, limit deformation and prevent the loss of interlock between blocks.

Under favourable conditions the reinforced layer of rock can carry loads mainly in compression, which for most rocks is a far more favourable working state than tension. Bolts thus help to change the way the rock mass deforms: instead of the free detachment of individual blocks, a tighter structure of interacting elements is formed.

Suspending rock from more stable ground

Under some conditions the bolt also acts as an element suspending a weakened layer from more competent rock deeper within the mass. For this mechanism to be effective, the bolt must pass through the fractured zone and gain adequate anchorage beyond the potential detachment surface.

The suspension model is relatively easy to visualise, but it should not be treated as the only explanation of how rock bolt support works. In many excavations there is no sharp boundary between a completely loose layer and intact rock. More often there is a zone in which the degree of fracturing changes gradually.

A correct analysis should therefore consider both the suspension of potentially unstable blocks and the reinforcement of the entire zone surrounding the excavation.

The role of pre-tensioning and installation timing

Bolts can be divided, among other ways, into passive and active. Passive (untensioned) bolts acquire significant load primarily as a result of rock displacement. Active bolts are pre-tensioned during installation and clamp the rock mass from the outset.

Pre-tension can quickly restrain the opening of fissures and increase friction between layers. Its effectiveness, however, depends on the correct transfer of force through the bearing plate, the bolt head and the rock surface. If the rock beneath the bearing plate is weak, fractured or uneven, part of the tension can be lost through local crushing and deformation.

Installation timing is equally important. Bolts installed shortly after excavation restrain the development of displacements before the fissures open significantly. Delayed installation may mean reinforcing a rock mass that has already partly lost its continuity and the ability of its blocks to interlock.

Several mechanisms acting together

In a real rock mass the mechanisms described above rarely occur in isolation. The same bolt may simultaneously:

• bind several layers together,
• resist the opening of a fissure,
• increase clamping and friction,
• carry shear load,
• suspend a rock block,
• help to form a beam or a pressure arch.

Which mechanism dominates depends on the local geometry of the fracturing and the development of displacements. For this reason, the design of a bolting system cannot be limited to determining the capacity of a single bolt. The length, spacing, drilling direction, method of anchorage, compliance of the system and the expected deformation of the rock mass must also be taken into account.

A bolt of very high strength will not provide effective reinforcement if it is too short, wrongly orientated or poorly bonded to the rock. Equally, a correctly installed bolt can be overloaded if its spacing does not match the size and arrangement of the potentially unstable blocks.

Summary

Rock bolting reinforces the rock mass above all by maintaining its continuity and its ability to carry load as a whole. Bolts bind rock layers together, increase friction on discontinuity surfaces, restrain the opening of fissures and prevent blocks from moving independently.

A properly arranged bolting system can transform the fractured zone around an excavation into a tighter, self-supporting structure. The rock is then no longer merely a load acting on the support – it becomes the main load-bearing element of the whole system.

The effectiveness of rock bolting therefore stems not only from the strength of the steel or composite material. What is decisive is the quality of the mechanical interaction between the bolt, the bonding material and the rock mass – from the moment of installation until the target load state is reached.