MECHANISED TUNNELLING | TECHNICAL
describes the degree of wear on the tool caused by hard rock. For this purpose, a standardised test is used to examine the extent to which a rock causes abrasive wear on steel.
For all ground conditions, whether soft soil or hard
rock, the basis for these parameters should always be a sound geological profile, where the geological and geotechnical information can be correlated with project-specific information like the depth of the tunnel alignment. On the basis of these parameters, a basic performance estimation for the MTBM project is possible. Other properties, like slaking and dispersion, affect the design and configuration of the slurry circuit and the slurry treatment plant. These parameters must be obtained from representative samples near the planned tunnel route, ensuring high data quality.
CHALLENGES AND SUCCESS STORIES IN EXTREME GROUND Tunnelling through hard rock or soft soil presents significant challenges due to the variability and complexity of the ground parameters and the complex interactions between geological, geotechnical, and engineering factors. The following two sections present the geotechnical
challenges for pipejacking and technical solutions in hard rock and cohesive soil, respectively.
HARD ROCK Knowing the harsh conditions in hard rock, more attention is paid to the MTBM performance factors, and the design of the critical components described later in this section, such as cutterhead, tooling, capacity of main bearing or anti-roll measures. It is important to note that all components and their capacity must be adjusted to each other. Thus, successful pipejacking is determined by the successful incorporation of all components into one functioning system. To increase performance, it is important to find the system’s bottleneck and increase its capacity. The whole system can be only as effective as its weakest component allows. The following equipment parts are key to further increasing the performance of a MTBM:
Cutterhead design for hard rock conditions The cutterhead’s tooling composition is probably the most critical part in rock applications. Tool size and arrangement are determined based on the rock properties. This arrangement leads to reasonable rock chip sizes that can be handled by the discharge system. Wear protection plates and hard facing play a key role in protecting the cutting wheel steel structure from excessive wear. New cutterheads have been developed for
microtunnelling in hard rock to 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 stable solid structure to take the higher loads on the cutters. This
development has further increased the feasible drive lengths in hard rock, especially in the non-accessible diameter range of ID 800-1000 (AVN800-1000 HR). Another aspect increasing the efficiency of the
cuttings transport, and therefore of the overall cutting mechanism, is the application of a flushing ring right behind the cutterhead, preventing the accumulation of fines below the machine can and therefore facilitating the steering of the MTBM. For MTBMs designed for hard rock, three main
cutting tool types can be considered: disc cutter; TCI cutter; and, milled tooth cutter. The most common cutting tool remains the disc cutter, which is well known from large diameter hard rock TBMs. However, due to space constraints, the disc cutter is normally modified into a double- or triple-disc cutter, which decreases the spacing and therefore leads to faster chipping of the rock. TCI or button cutters exert a point load force on
the rock, resulting in numerous small chips which are further downsized in the cone crusher and beneficial to the small-diameter slurry circuit. Due to the tungsten carbide insert, the TCI cutter is considered especially wear-resistant, which is beneficial for long drives in small diameters, where cutterhead interventions are not possible. Milled tooth cutters can fill a niche in small-
diameter tunnelling based on the observation that normal disc and TCI cutters have comparably low performances in low strength, ductile-behaving rocks. Therefore, a tooth cutter can be an option in low- strength rocks to ensure high penetration rates.
Hard rock reference project: Trans Mountain Expansion Project (TMEP) In the following, the Jacko Lake microtunnel project in Canada is presented as an outstanding example of pipejacked tunnel construction in hard rock, built to serve as a casing tunnel to host an oil pipeline. Within Canada’s Trans Mountain Expansion Project (TMEP) a total of approximately 980km of new pipeline must be built between Strathcona County (near Edmonton), Alberta and Burnaby, British Columbia (BC). This involves the construction of a second pipeline. In Spread 5A of the TMEP with a total length of
4.25km, southwest of Kamloops, BC, four sections were planned as pipejacking drives to avoid and preserve nature conservation areas.
January 2026 | 13
Above: Figure 3. Risk factors in pipejacking IMAGE COURTESY OF HERRENKNECHT
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