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Roof support is the set of procedures and materials used to keep the rock above and around an underground opening stable. This guide separates the two ideas the term bundles together, reinforcement and support, then maps common elements to the rock conditions they are suited to. Who uses it: mine ground-control engineers, mine safety staff and students of mining engineering.
Scope note: the construction detail below comes from a hard-rock mining paper (Hoek and Wood), the legal baseline from US underground coal regulation (30 CFR 75.202), and the design-factor study from US coal mines (NIOSH). Each claim is attributed to its source and should not be carried across to a different mining setting without checking.
What roof support is
Hoek and Wood define rock support as the term widely used for the procedures and materials that improve the stability and maintain the load-bearing capacity of rock near the boundaries of an underground excavation. They state that the primary objective is to mobilize and conserve the inherent strength of the rock mass so that it becomes self-supporting. The word “roof” simply names the upper boundary of that excavation; the same elements are used on walls (ribs).
How it works: reinforcement versus support
Hoek and Wood draw a distinction that most overviews blur:
- Rock reinforcement (active support): the elements are an integral part of the reinforced rock mass. Their example is a pattern of untensioned grouted dowels, which are loaded as the rock deforms and interact with the rock much as reinforcing steel interacts with concrete.
- Rock support (passive support): the members are external to the rock and respond only to inward movement of the surrounding rock. Their example is steel sets.
In practice a support system combines both: reinforcement by dowels, tensioned rock bolts and cables, and support by shotcrete, mesh and steel sets that carry loads from rock blocks isolated by structural discontinuities or zones of loosened rock. The authors add that, where possible, installation should be an integral part of the excavation cycle.
Types of roof support
| Element | Class (Hoek and Wood) | What the source says |
|---|---|---|
| Mechanically anchored rock bolt | Reinforcement, tensioned | Probably the oldest form of rock reinforcement; needs rock hard enough to grip the anchor. Most effective at retaining loose blocks or wedges near the surface. If the anchor slips or the bolt breaks, capacity drops to zero. |
| Grouted or friction-anchored dowel | Reinforcement, untensioned | Cannot be tensioned, so must be installed before significant rock movement. If slip occurs or the faceplate breaks off, the remaining length stays anchored and keeps supporting. |
| Grouted cable | Reinforcement | Can replace bolts and dowels in most mining applications; can be installed in openings with very low headroom; can be grouted untensioned or tensioned before grouting. |
| Mesh and straps | Support | Mesh is added where small pieces of rock may fall out between bolt heads; straps are used across bedding planes or joints for the same purpose. |
| Shotcrete | Support | Table 1 of the paper suggests a layer of approximately 50 mm for closely jointed, blocky rock under low stress; micro-silica and steel fibre reduce rebound and increase bending strength. |
| Steel sets | Support, passive | Required for long-term support where stresses are evidently high or the rock keeps moving; capacity is estimated from the amount of loose rock to be supported. |
Matching support to rock conditions
Hoek and Wood call their Table 1 a very crude guide; it ties the choice of support to the extent of the loosened or fractured zone around the opening. Selected rows, paraphrased:
- Sound rock with a few loose wedges or blocks, low stress: scale well, then install tensioned mechanically anchored bolts to tie blocks into the surrounding rock, with straps where small pieces could fall out. In permanent openings such as shaft stations or crusher chambers, the bolts should be grouted with cement to prevent corrosion.
- Stress-induced failure in jointed rock: pattern support with grouted dowels or Swellex; split sets for small amounts of failure; mechanically anchored bolts are less suitable.
- Drawpoints: grouted rebar for wear resistance, installed before rock movement from drawing of stopes; no shotcrete or mesh, with dowels at close spacing in blocky rock.
- Rockburst potential: pattern support with some flexibility; split sets are favoured because they slip under shock loading yet retain some load, and mechanically anchored bolts are described as poor in these conditions.
- Very poor rock in faults or shear zones where bolts cannot be anchored: fibre-reinforced shotcrete, which must be drained, or steel sets for long-term support.
Sizes and spacing that matter
The same paper gives rules of thumb, not design values. For stress-induced failure in jointed rock, reinforcement length of about half the span for openings under 6 m, and between one half and one third of the span for spans of 6 to 12 m, with spacing about half the dowel length. Tensioning of mechanically anchored bolts is usually to about 70% of their ultimate breaking load. These are historic hard-rock guidelines from 1987; site-specific design must follow a qualified engineer’s assessment.
What drives bolt design in coal mines
A NIOSH paper by Molinda and colleagues (20th International Conference on Ground Control in Mining, 2001) analysed case histories from 37 US mines, comparing roof falls per 10,000 ft of drivage with roof bolt systems. It reports that roof falls are rare where the roof is strong and stress is low, even with light bolting, and proposes preliminary guidelines for bolt length, capacity and pattern based on depth of cover, the Coal Mine Roof Rating (CMRR) and intersection span. The guidelines were built into the ARBS (Analysis of Roof Bolt Systems) program.
Regulatory baseline: US underground coal
Under 30 CFR 75.202(a), the roof, face and ribs of areas where persons work or travel must be supported or otherwise controlled to protect persons from falls of the roof, face or ribs and from coal or rock bursts. Paragraph (b) states that no person shall work or travel under unsupported roof except in accordance with that subpart (Subpart C, Roof Support). This applies to underground coal mines; other mining sectors and countries have their own rules.
Common mistakes the sources warn about
- Relying on old mechanical bolts: anchors tend to slip progressively with time, probably from nearby blasting vibration, so old bolts that have lost all tension are common (Hoek and Wood).
- Leaving bolts unprotected in aggressive groundwater: in massive sulphides an unprotected bolt may last less than a year; the authors say bolts should be grouted where long life is needed.
- Installing untensioned dowels late: because they cannot be tensioned, they must go in before significant movement, ideally close to the advancing face.
- Skipping scaling and careful blasting: both reduce the loose rock that must be supported and so reduce the need for bolts and mesh.
FAQ
What is the difference between roof support and roof bolting?
Roof bolting is one form of reinforcement. Roof support as a whole also includes passive members such as mesh, shotcrete and steel sets (Hoek and Wood, 1987).
Is roof support legally required?
In US underground coal mines, yes: 30 CFR 75.202 requires roof, face and ribs where people work or travel to be supported or otherwise controlled.
Which is better, a tensioned bolt or a grouted dowel?
Neither is universally better. Hoek and Wood note that tensioned bolts suit loose blocks near the surface but fail completely if the anchor slips, while dowels keep partial capacity but must be installed before the rock moves.
Sources
- Hoek, E. and Wood, D.F., “Support in Underground Hard Rock Mines”, Underground Support Systems, CIM Special Volume 35, 1987, pp. 1-6. PDF. Read 2026-11-11.
- 30 CFR 75.202, Protection from falls of roof, face and ribs and from coal or rock bursts. Cornell LII. Read 2026-11-11.
- Molinda, G.M. et al., “Analysis of Roof Bolt Systems”, Proc. 20th International Conference on Ground Control in Mining, West Virginia University, 2001, pp. 218-225. CDC Stacks. Read 2026-11-11.
