An examination of the basic rationale for the use of rock-reinforcement methods in tunnels subject to high stress or rockburst damage, emphasizes the importance of yielding of the support elements. The concept of 'critical bond length' is proposed as a determinant of whether fully-grouted tendons will rupture or yield. Experimentally-determined characteristics of support elements are, related to actual observations of tunnel damage to confirm the relevance and importance of these ideas.
Un examen des raisons fondamentales de l'utilisation de soutènements dans le tunnels soumis à de grandes pressions ou à des coups de terrain a montre que les proprietes elastiques de ces soutènements sont primordiales. Le concept de "longueur critique de scellement" est propose pour la determination des critères de rupture ou de deformation des boulons d'ancrage. La relation trouvee entre les caracteristiques des elements de soutènement determinees experimentalement et les observations in situ des dommages dans les tunnels a confirme l'interêt de ce concept.
Eine Untersuchung primarer Stuetzmaßnahmen fuer Tunnel mit hohen Gebirgsdruecken und großer Gebirgsschlaghaufigkeit betont die Wichtigkeit yon nachgiebigen Ausbaumethoden. Das Konzept der "kritischen Haftlange" wird als Entscheidungsfaktor ob vollverklebte Anker als steifer oder nachgiebiger Ausbau wirken, vorgeschlagen. Versuchsergebnisse werden mit in situ Beobachtungen in verbrochenen Tunnelstrecken verglichen, um die Bedeutung und Wichtigkeit dieses Konzepts zu bestatigen.
Rockbolting has been used for a long time as temporary or supplementary support in rock tunnels. About fifteen years ago the South African gold-mining industry first considered using steel tendons systematically as a complete method of tunnel support to supplant the traditional systems based on steel arches Or timber sets. Recently, end-anchored rock studs have almost, entirely given way to fullygrouted reinforcing bar or looped wire rope as the preferred form of tendon. Usually the tunnel walls are fractured to a greater or lesser degree, because of depth or induced stresses. Often this fractured rock must be contained by means of a fabric of wire mesh backed-up by,wire rope laced between the tendons. On one large mine a total length of over 2000m of tunnel is supported in this manner each month at a cost of R110/linear metre. Some 98 000m of wire rope strand is required for tendons and lacing and 20 000m2 of wire mesh for the fabric. Variations in detail that have evolved to meet differing, local conditions have been empirically developed. While the empirical approach has mostly worked well it has become evident that a more careful consideration of system requirements is necessary for the extreme conditions at very great depths or more particularly, where severe rockbursts occur. This paper examines some of the premises implicit in the development of this type of support and presents some quantitative performance characteristics of the main components of the system.
In the South African gold-mining industry the rock-reinforcing system of tunnel support is usually, somewhat loosely, referred to as "wire-mesh support". Although there are some variations in detail the essential components of the system are, in order of importance, the tendons, the lacing and the wire-mesh. A brief description of the most common types of these elements is necessary before their functional requirements are considered in detail.
The length of tendons and their spacing patterns tend to be based on broad, empirical guidelines¹. Since most main tunnels have typical cross-sectional dimensions of 3 to 3,5m, the most common length for tendons is 2, 5 to 3m and the spacing between tendons is typically 1 to 2m. The tendons generally range from l2mm to 20mm in diameter. 'Full-column' grouting is almost invariably used as the means of providing anchorage. A thick cement slurry is used with wire rope tendons and, usually, resin capsules or cementitious cartridges with reinforcing bar. Where very severe increases in field stress are anticipated and the tunnel is particularly important, pre-stressed rock anchors up to 7,5m long are used to supplement the normal 2,5m grouted tendons -- Figure 1. These may be fully-grouted after tensioning or have a "free" length able to extend elastically.
The type and size of wire rope that is employed for lacing through the looped ends of the tendons is determined largely by practical considerations and availability, rather than - by design. Most mines find that single strands from large discarded hoist ropes can be used satisfactorily. While these are naturally far cheaper than new rope there is not always a sufficient quantity available. Where new rope has to be obtained scraper rope of 6 × 7 construction is usually chosen. The diameter of rope or strand used for lacing can thus vary from 9mm to l6mm.
Although it represents the largest materials cost item in the system, there has been less enquiry into the effectiveness of wire mesh than into any of the other components.
