The rate at which “consumables” are consumed during a tunnelling project can have a significant effect on total construction costs. The nature of these costs is more complex than a simple multiplication of cost and quantity. Although this may be of paramount importance in underground operations such as money, the time element is usually more significant in tunnelling. This is a matter not only of the labour costs in replacing worn parts and materials, but also of the interruption the critical activities. These is usually no alternative to standing down the whole tunnelling line from face to surface.
Although the replacement of consumables can be planned within a set maintenance period, it seems logical that time and cost savings can be made if the number of maintenance periods could be reduced. If set maintenance periods are adopted it could still be cheaper to replace consumables within the set period, whether completely worn or not, just to save time in unplanned replacements.
Under severe conditions, such as when tunnelling through particularly hard or abrasive ground, or when working in wet conditions, maintenance periods will obviously need to be more frequent. The time taken for replacement will become even more significant. In homogeneous ground this can still be well planned after a “settling in” period for trials. However, when ground conditions are variable, the consumables wear rates will be less predictable, adding to overall project uncertainties in terms of downtime, parts ordering and stock holding.
So, what are tunnelling consumables? There is a wide variety of possible candidates ranging from the picks, cutters and drill bits for excavation, to tyres, wear parts and conveyor belts for mucking out. Explosives, stemming, detonators, lubricants and filters also deserve consideration when calculating running costs, while even the costs of bearings, hydraulic fluid, fluid couplings, oil seals, anti-vibration mountings and electric motor costs may be significant under certain circumstances.
Traditionally those parts which are used to cut rock are normally considered to be prime “consumables”since they usually receive the greatest wear per metre of tunnel advance. There have been many studies on how to reduce wear rates of both drill bits and machine picks, including improved materials or geometry. In the case of picks used on roadheaders and similar plant, the pattern of pick mounting on the cutterhead, and the way in which the cutterhead is used according to the tunnel face composition, are both ways in which overall usage can be reduced.
In all these case, however, the consumables are used on an open tunnel face. This makes it a easy proposition to replace worn cutting elements once they have been identified by an experienced eye. It is important that replacement takes place before excessive wear takes place which would reduce cutting and penetration rates drastically. The use of consumables past their best can easily result in excessive pressures and wear on more expensive, and less easily obtainable, machine parts resulting in greater downtime. The saying “A stitch in time saves nine” is never so true.
TBM cutters
The reduction of TBM cutter costs has been one of the main technological advance stories over the last thirty years. Howard Handewith, for the American Underground Construction Association (AUA), has quoted cutter costs of $4.50 (historical prices) per cubic yard ($5.88/m3) for the Chicago TARP project in 1969. These days cutter costs of less than $1.00 per cubic yard (<$1.31/m3) is the expected target. Still these costs are a substantial part of the overall excavation cost.
If the face is closed, or the cutting elements otherwise hard to access, the cost of time can be much more significant in cutter replacement, even though each part has a high unit cost. Consequently, in addition to developments in materials and geometry, there has been a major emphasis in recent years on the engineering of cutter mountings. Significant developments now include rear replacement (ie from inside the TBM), more durable thrust bearings, and fewer small parts in each mounting. Improved mounting designs have recently resulted in great time savings which can prove substantial cost savers in hard or mixed rock conditions.
Robbins has been developing cutter technology continuously since introducing roller disc cutters in 1956. The current Wedge-Lock system for 17-in (432mm) and 19-in (483mm) cutters is a market leader. The system isolates saddle-mount bolts from cutter loads, so making it easier to replace cutters and reduce downtime. The design features a hardened ring seat, an assembly suitable for both front and back loading arrangements, and a large O-ring seal for improved reliability. Fit tolerances between the shaft and bearing, and between the hub and ring have been selected to increase component life.
A variety of material grades and profiles with different tip widths are available for optimum performance in different rock types. Proprietary materials and treatment processes are designed to balance the residual stress in the disc to increase active rolling life and to minimise chipping or cracking of the cutting material.
Robbins’ Cutters Product Manager, Brian Khalighi, says that research and development is continuing with modified material and heat treatment which has shown improved cutter life of R&D discs in hard rock. This work will result in the introduction of a heavy-duty range in the near future.
Impressive though recent developments in TBM cutter technology have been over the last 20 years, there are still significant developments under way and more promised.
The Tiger Shark tunnelling ring from Barker Hughes was developed to provide better performance and durability than conventional steel cutter rings, but continuous development will result in further improvements for better penetration rates. This Mark III version was due for production in early May. When used for rock tunnelling in a Montana, US, mine the 17.5in. (445mm) Tiger Shark ring cutter produced an average life in the gauge position of 1054ft (321m) compared to 318 ft (97m) for a conventional steel ring.
Stein Narvested says that his Norwegian development and manufacturing company has produced a new cutter ring design which is planned for site testing in Italy and the US during the second half of this year. Compared to an earlier Narvested design, the rear-loading cutter has only two main parts. It is this design which, it is claimed, has reduce the replacement time for each cutter from around 45 min to under 15 min. Although the saving may seem small, multiplied over the number of discs on a TBM face it is certainly significant. Initial reactions from contractor customers have been encouraging but a major problem has often been the source of the TBM.
Narvested complains that contractors often feel that they cannot order independent wear products like his because the contractor is already tied to a project-long supply contract with the TBM manufacturer. Alternatively it is implied that use of cutters other than original manufacture forms will infringe warrantee or service agreements. Thus the easier target market appears to be refurbished or owner-designed plant.
Drill bits
In parallel with the drill tool developments arising from more traditional rock excavation tasks in underground construction and mining, there have been increasing amounts of technology transfer from the oil and gas market. Companies such as Baker Hughes and Wirth, which serve both markets, are in a good position to exploit applicable technologies arising from the high cost, high value market. A prime example is the tools used for horizontal directional drilling, a technique now widely used in underground utilities and pipeline construction with increasingly larger maximum diameter capabilities.
Robit of Finland emphasises that, as a small, specialist company, it puts all its efforts into button bits as its only product range. Continuous monitoring of manufacture, in-house heat treatment, and the use of microscopically homogenous tungsten carbide are all aimed at ensuring quality in the delivered product for use in modern top-hammer drilling. Steel for the bit body is selected for both its wear resistance and its capacity for resisting the high frequency (around 3600 impacts/ min), high energy action of modern rock drills. The bit body is finished on CNC machining centres for consistency. A special machining centre drills holes for the tungsten carbide buttons to an accuracy of 0.02mm to ensure optimum button retention and stability in use, discouraging early failure. Robit produces different shapes of tungsten carbide buttons to suit the rock type, from standard round to ‘ballistic’.
Tony Torquato of Torquato Drilling Accessories advocates a grinding programme to extend the life of new bits and reduce the chances of drill-machine damage for a grinder investment of under $700 excluding grinding cups. Grinding can be carried out in a surface workshop or on the drilling rig. As flat spots develop on the tungsten carbide inserts of bits, increased torque will be required, resulting in more vibration and ultimate damage to the drill string and drifter. Advantages claimed for a bit grinding programme are increased drilling rates, less rig maintenance and repair, and even less power costs.
Faults with the drill bit will exhibit themselves as increased wear at drilling string joints and adapters. Although these have been combated by manufacturers with improved materials technology and connector thread design, the best answer is good drilling practice. This has traditionally depended on experienced operators but modern computerised drill control systems form an automatic means of reducing the risk of exceeding optimum drilling parameters, especially in mixed ground conditions. Atlas Copco claims that in difficult rock conditions, the life of shank adapters has been increased to more than 10,000 drilled metres when using a computerised drilling control system.
Explosives
The other main category of drill-and-blast consumables is explosives. A major advance has been the availability of safe bulk explosives for underground use. Originally developed for surface use from the 1960s, emulsion bulk explosives have increased density, energy and water resistance compared to ANFO slurry mixtures. As a consumable the main advantage of bulk explosives is reduced cost compared to cartridged explosives, although there will need to be an investment in bulk handling and charging equipment which may only be justified on large projects.
ANFO explosives may be mixed on the surface or is supplied ready mixed by the explosives manufacturer. The mix can be poured into holes (depending on orientation) or pneumatically charged. According to Orica, one advantage of bulk emulsion explosives over traditional bulk ANFO is easier control of loading which gives more accurate and precise blast ratios, thereby reducing costs either directly in saved explosives economy or indirectly through reduced overbreak or improved advance rates. Emulsion is a higher energy product which can also allow the number of blast-holes to be reduced or fragmentation to be improved.
