The Main Types of Loads Acting on a Rock Bolt

A rock bolt is often pictured as an element working primarily in tension. This simplification is convenient for basic calculations, but it does not reflect the real conditions in the rock mass. In practice, the rod, cable or other load-bearing element may be stretched, sheared and bent at the same time, and additionally subjected to variable or sudden loads. How the bolt works depends not only on its type, but also on the geological structure, the way it is installed, the deformation of the excavation and the interaction with the other elements of the support system.

A correct assessment of bolt loading therefore requires looking at the whole system: the rock mass, the bonding material, the rod, the bearing plate, the nut and the surface support of the excavation. Failure can occur at any point in this chain, not necessarily in the rod itself.

Tension – the basic but not the only mode of working

Tensile loading appears when a portion of the rock mass moves towards the excavation surface, or when the bolt ties together strata with a tendency towards bed separation. Examples include roof sag, the detachment of a rock slab, or growth of the fractured zone around the excavation.

In grouted bolts, the tensile force is transferred from the rod to the bonding material and then to the walls of the hole. This does not happen at a single point but over a certain bond length. Shear stresses develop along the rod at the steel–resin and resin–rock interfaces. If the bond length is adequate, the load is spread over a larger area. If it is too short, or the bonding material does not properly fill the annulus, the result may be pull-out of the rod, slip within the bonding material, or detachment of the rock surrounding the hole.

In pre-tensioned bolts, part of the tensile force is introduced during installation. The pre-tension clamps the rock strata together, limits the development of fissures and mobilises the interaction between bolt and rock mass earlier. Passive bolts, by contrast, only pick up load once relative displacement of the rock has occurred.

Tensile capacity is not governed by the strength of the rod alone. The hole diameter, the rib geometry of the rod, the mixing quality of the resin capsule, the bond length, the strength of the rock, and the condition of the bearing plate and nut all matter. A bolt with a very strong rod may have a low system capacity if its connection with the rock mass is weak.

Shear on planes of discontinuity

Shear loading occurs when two rock blocks or strata move relative to each other across the axis of the bolt. This happens above all on joints, faults, bedding planes and other discontinuities intersecting the bolt hole.

In such a configuration the bolt acts like a dowel passing through elements sliding past one another (the so-called dowel effect). As the rock moves, it bears against the rod on both sides of the discontinuity, and shear stresses develop in the bolt material. Local bending usually occurs at the same time, because the load is not distributed perfectly symmetrically.

The shear resistance of a bolt depends on the cross-sectional area of the rod, its yield strength, its ductility and its ability to deform without fracturing. The aperture of the discontinuity and the properties of its infill material are also significant. A narrow joint can concentrate shear almost on a single plane. In a wider fractured zone, the deformation is distributed over a longer section, but it can lead to considerable bending of the rod.

The bonding material also takes part in transferring the load. A stiff material limits local displacements but can crack or spall. A more compliant system allows greater deformation, though at the cost of increased rock mass displacement. For this reason, the maximum shear force alone does not fully describe the suitability of a bolt. Equally important is the energy the system can absorb before losing its capacity.

Bending of the bolt

Bending appears when the axis of the hole no longer coincides with the direction of the force acting on the bolt. It may result from the sliding of rock blocks, the opening of joints, uneven roof sag or deformation of the excavation profile.

In practice, a bolt rarely works in pure bending. It usually occurs together with tension and shear. A rod crossing a discontinuity is first pressed against its edge, then bends and, as displacement increases, is stretched. Tensile stresses develop on one side of the cross-section and compressive stresses on the other. The maximum stresses occur at the surface of the rod, which is why even minor damage, corrosion or material flaws can significantly reduce the life of the element.

Locations of abrupt stiffness change are particularly unfavourable, e.g. at the boundary between the bonded and unbonded lengths. A stress concentration develops there, which can lead to local yielding and, under variable loads, to fatigue cracking.

The bending capacity is influenced not only by the rod diameter but also by the steel grade and the profile geometry. High-strength steel can reach a high limit force, but if its ductility is low, it copes poorly with large deformations. Where the rock mass keeps moving, an element that continues to carry load after yielding is often preferable to one that fails suddenly.

Torsion during installation

A separate case is the torsional moment, which appears mainly during bolt installation. In grouted bolts, the rotating rod mixes the resin capsule and has to overcome the resistance of the resin; in pre-tensioned bolts, the tightening torque on the nut is used to introduce the pre-tension. The rod then briefly carries torsion combined with an axial force.

Once installation is complete, the torsion generally disappears and is usually neglected in the analysis of bolt behaviour. It does, however, matter in practice: excessive mixing resistance or an excessive tightening torque can damage the rod, the thread or the fresh bond that has not yet set.

Dynamic loading

Dynamic loads differ from static ones above all in how quickly the force is applied. They may result from a rockburst, a rock mass tremor, the detachment of a rock block, blasting or the sudden failure of a load-bearing stratum.

In such situations the bolt must not only withstand a given force, but above all absorb the energy of the moving rock mass. Two bolts with similar static capacity can behave completely differently under impact. A stiff element may carry a high load but fracture after little elongation. A yielding bolt will sometimes reach a lower maximum force, but thanks to its greater deformation it will absorb far more energy.

The capacity for dynamic work is determined by the area under the load–displacement curve. The greater the force sustained over a longer deformation path, the more energy the bolt absorbs. This is why yielding bolts, controlled-slip elements, special deformable profiles or designs equipped with energy-dissipating mechanisms are used in rockburst-prone areas.

It should be remembered that dynamic loading acts on the whole system. The rod, the bond, the bearing plate, the nut or the surface support of the roof may fail. Even a bolt with a high energy-absorbing capacity will not be effective if the bearing plate punches through the mesh or the bolt loses its bearing in a heavily damaged near-surface layer.

Combined loading

A real rock bolt almost always works under combined loading. Bed separation produces tension, sliding on a joint produces shear and bending, and the sudden detachment of a block can additionally give the whole process a dynamic character.

Combined actions are particularly dangerous because the individual mechanisms reinforce one another. A rod already in tension has a smaller bending reserve. Bending raises the local stresses, so additional shear can lead to earlier yielding. Dynamic loading imposed on an already deformed bolt can, in turn, break it at an energy lower than that determined in a test on a new, straight element.

The results of simple tensile tests should therefore not be the sole basis for bolt selection. In difficult conditions, tests are needed that account for shear on joints, large displacements, load cycling and the interaction with the bonding material and the surface support.

The system matters, not just the rod

Assessing how a bolt works cannot be reduced to the question of how much force the steel element will carry. It is necessary to establish how the load arises, where it is transferred and which failure mechanism is most likely.

In a massive, relatively stable rock mass, tension may dominate. In bedded and faulted rock, shear and bending gain importance. In rockburst-prone areas, the ability to absorb energy is key. Most often, however, several actions combine, changing as deformation progresses.

An effective rock bolt support system should therefore offer not only adequate maximum capacity, but also ductility, a deformation reserve and the ability to sustain load beyond the elastic limit. Only when these characteristics are taken into account can it be judged whether a bolt will actually stabilise the rock mass or merely satisfy a formal requirement for the nominal strength of the rod.