Active and Passive Rock Bolting: How the Two Approaches Differ

Mining and tunnelling bolts do not always begin to reinforce the rock mass at the same moment. Some act on the rock almost immediately after installation, introducing a defined pre-tension into the system. Others reach their full effectiveness only once the rock mass starts to move, separate along bedding or break away. This is the basis for distinguishing between active and passive bolting.

The distinction does not refer merely to the type of rod or bonding material. What matters most is the way in which capacity is mobilised – in other words, the answer to the question: is the force in the bolt introduced during installation, or does it only develop as a result of rock mass deformation?

Active bolting – acting from the moment of installation

Active bolting consists of introducing a controlled initial force into the bolt – a pre-tension. Once the far end of the rod is anchored, the bolt is stretched and the bearing plate presses against the excavation surface. As a result, a state of compression is created in the rock before any larger displacements occur.

A typical example of an active system is a mechanical bolt – with an expansion shell or a wedge-type head – tightened with a nut after the head has been set in the hole. Grouted bolts can also be active if part of the bolt is first fixed in a fast-setting material and the rod is then tensioned before the remainder of the bonding material sets, or before full grouting is carried out.

Pre-tension produces several favourable effects:

• it presses rock slabs and blocks against the more stable rock mass,
• it restrains the opening of fissures and discontinuity surfaces,
• it increases friction between layers,
• it reduces the possibility of block slip,
• it limits initial bed separation in the roof,
• it improves the interaction of mesh and other support elements with the excavation surface.

An active bolt therefore does not have to wait until the rock visibly begins to move. Its task is to counteract the development of deformation from the moment of installation.

The role of installation torque

In many active systems the initial force is obtained by tightening a nut. The torque then generates an axial force in the rod. The relationship can be described approximately by the formula:

F ≈ T / (K · d)

where:

• F is the axial force in the bolt,
• T – the tightening torque,
• d – the thread diameter,
• K – a coefficient that depends above all on friction; for dry, clean threads a value of K ≈ 0.15–0.25 is usually assumed, often around 0.2.

The formula is, however, only indicative. The same tightening torque does not always produce the same initial force. The result is influenced by, among other things, the condition of the thread, the presence of lubricant, corrosion, dirt, the type of nut, the thread geometry, the stiffness of the bearing plate and the friction between the nut and the plate.

Installation torque should therefore not be automatically equated with the actual tension in the bolt. Calibration of the specific assembly is required: rod, nut and bearing plate. In more demanding applications the initial force is controlled directly, for example by hydraulic tensioning of the bolts, load-indicating washers or systems monitoring rod elongation.

Too low a tightening torque means insufficient pre-tension. Too high a torque, on the other hand, can lead to overloading of the thread, rupture of the bolt, destruction of the expansion shell, deformation of the bearing plate or local crushing of the rock beneath the plate.

Passive bolting – capacity mobilised by deformation

A passive bolt is not deliberately loaded with any significant axial force during installation. Its capacity is mobilised only once the rock begins to move relative to the bolt.

This group includes, above all, many bolts grouted over their full length. The steel rod is surrounded by hardened resin or mineral grout, which bonds it to the walls of the hole. Immediately after the bonding material has set, the stress in the rod may be small. When, however, the rock layers begin to separate or slide, the deformation is transferred through the bonding material to the bolt.

The load then develops through:

• shear stresses at the rock–bonding material interface,
• shear stresses at the bonding material–rod interface,
• tension in the rod,
• local bending of the bolt at discontinuity surfaces,
• the interaction of the bearing plate with the excavation surface.

Friction bolts also work passively. They are driven into a hole of slightly smaller diameter than the bolt tube: frictional contact with the rock is established immediately after installation, but the bolt only takes up significant axial force as rock mass displacements develop.

A passive bolt is therefore not an idle or less effective one. The term ‘passive’ describes only the way in which its capacity is mobilised. The system responds to the deformation of the rock mass, taking up load as the deformation develops.

Capacity mobilisation in the two systems

The most important difference between active and passive bolting concerns the relationship between the force in the bolt and the displacement of the rock mass.

In an active system a large part of the axial force is present as soon as installation is complete. The initial deformation of the rock can thereby be limited. If displacements continue, the force in the bolt rises above the value of the pre-tension.

In a passive system the initial force is small, and its growth requires relative displacement between the rock and the bolt. The properties of the system therefore depend not only on the strength of the rod, but also on the stiffness of the bonding material, the anchorage length, the quality of resin mixing, the filling of the borehole annulus and the bond to the rock.

In practice the boundary between the two groups is not always entirely clear-cut. A bolt may be partially tensioned during installation and then develop additional capacity passively. Full-length grouted bolts with a tightened nut and bearing plate are an example. The initial tightening provides a certain clamping force, while further load is transferred through the layer of bonding material.

Applications of active bolting

Active bolting is particularly advantageous where rock movement needs to be restrained quickly. This applies, among others, to:

• bedded roofs prone to separation,
• rock blocks separated by distinct fissures,
• excavations in which the permissible displacements are small,
• support systems requiring immediate clamping of the mesh,
• temporary installations in which rapid capacity is needed,
• tensioned bolts and tendons used for large excavation spans.

The main advantage is the immediate stabilising effect. A limitation, on the other hand, can be the loss of tension due to rock creep, deformation of the bearing plate, loosening of threaded components or displacement of the expansion shell.

Applications of passive bolting

Passive bolts are widely used as permanent rock mass reinforcement, especially where the load needs to be distributed over a considerable length of the hole. Full encapsulation of the rod provides corrosion protection, limits stress concentrations and allows the bolt to interact with the rock along many sections.

Passive systems perform particularly well in:

• excavations with moderate, gradually developing deformations,
• fractured rock masses in which many blocks need to be tied together,
• permanent support of tunnels and chambers,
• systems requiring high stiffness and good bonding,
• conditions in which point anchoring would be unreliable.

The condition for effectiveness, however, is correct drilling of the hole and proper bonding of the bolt to the rock. Voids in the bonding material, inadequate mixing of the components, too large a hole diameter or the presence of water and cuttings can significantly reduce the capacity.

System selection should follow the failure mechanism

It cannot be said that active bolting is always better than passive bolting, or the reverse. The two approaches answer different geotechnical needs.

If the principal hazard is rapid separation of layers or slip of blocks, the immediate introduction of pre-tension may be advantageous. If, on the other hand, the aim is the permanent bonding of a fractured rock mass and the transfer of loads along the entire length of the hole, full encapsulation of the bolt may be the better solution.

In practice, hybrid systems are often used, combining pre-tension with full bonding. This makes it possible to exploit the immediate action of active bolting and the long-term ability of passive bolting to carry loads that develop together with rock deformation.

The final selection should take into account the geological structure, the orientation of discontinuities, the expected magnitude of displacements, the service life of the excavation, water conditions and the required compliance of the support. What matters most is not the bolt type itself, but whether its working mechanism matches the actual mechanism of deformation and failure of the rock mass.