MECHANISED TUNNELLING | TECHNICAL


affect the stability of structures and cavities to be excavated in the rock. The most important rock mass classification systems are: ● Rock Quality Designation (RQD) ● Rock Mass Rating (RMR, BIENIAWSKI1974 (Council for Scientific and Industrial Research (CSIR) Geomechanic Classification)


● Q-system, BARTON et al. 1974 (NGI-index) ● Geological Strength Index (GSI), HOEK 1995


MTBM DESIGN FOR HARD ROCK All rock and rock mass properties have to be taken into account when choosing the correct MTBM type, including designing the best-suited cutting wheel, tools, main bearing, anti-roll measures, and to predict the tunnelling performance. Components and their capacities must be adjusted in relation to each other and optimised for the optimal benefit of the whole functioning system. For increased performance and capacity, bottlenecks need to be found and addressed. It is well known that for small-scale, small-diameter


projects it is not possible to determine all geotechnical parameters. The most important and minimum parameters should be: ● Strength: UCS; TS, PLI ● Abrasivity: CAI ● Rock mass: RQD, (Q) ● Information about groundwater (pressure, chemical composition, etc.,)


According to the most important single parameter, the UCS, an application range of slurry MTBMs is recommended with diameters (see Figure 2).


Cutting wheel design for hard rock The cutting wheel’s tooling composition is the most critical part in rock applications, especially selecting tool sizes and arrangements based on rock properties. The tool arrangement is also related to cutting wheel diameter for generation of reasonable rock chip sizes to be handled by the discharge system (see Figure 3). Wear protection plates and hard facing play a key role


in cutting wheel design. For hard rock microtunnelling, new cutting wheels have been developed that provide: extra wear protection, like TCI cutters with hard facing


and sandwich wear plates on the rim; high performance bearing of the cutters for higher loads; and, a stable solid structure to take the higher loads on the cutters. A project example of a dedicated hard rock cutting


wheel is on the machine design – for a larger AVN, the AVN 2500 – used on a water pipeline project in New Zealand (see box panel-2).


Hard rock tools and selection There are three main types of cutting tool for MTBMs (see Figure 4): ● Disc cutters – for microtunnelling these are normally double- or triple- disc cutters with decreased spacing (compared to large TBMs) enabling faster rock chipping. But more rings on cutters reduces the available thrust per ring, which decreases the probability of creating tensile cracks for chipping. So, small-diameter hard rock AVN machines need high jacking force plus a strong main bearing and cutter bearing.


● TCI or button cutters – these are recommended if the problems noted above cannot be solved with a technical solution, or if there is high requirement on the lifetime of the cutters exists. These exert a point load force on the rock, resulting in many small chips. The TCI cutter is considered especially wear-resistant, which is of benefit for long drives in small diameters where cutterhead interventions are not possible.


● Milled tooth cutters – these fill a niche for performances in low strength, ductile-behaving rocks which are difficult to chip. Conventional cutters tend to mark or create grooves into such rock faces but do not create tensile fractures to result in chips, whereas tooth cutters penetrate deeply and can lever out loose pieces. Also, soft to mixed ground tools (chisels, rippers, knives, scrapers) can work in low-strength rocks.


Calculating wear While wear prediction for small-diameter hard rock MTBMs is a largely undeveloped field with very little academic research, and the scarce information available is kept within the companies involved in the industry, it is noted that this is based on empirical and semi- theoretical approaches. Several lab tests help to


Disc cutter RING VERSION (1/2-ring)


Disc cutter MONOBLOCK (1/2/3-ring)


Disc cutter HARD FACING (1/2/3-ring;


opt. TCI inserts) Left, figure 4:


Overview of hard rock cutting tool types for microtunnelling


TCI CUTTER (opt. hard facing)


MILLED TOOTH CUTTER


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