TECHNICAL | MECHANISED TUNNELLING
The expected ground conditions in the Kamloops
area, as inferred from numerous boreholes, were described as Iron Mask batholith, an early Jurassic, northwest-trending alkalic intrusive complex. Rocks of the Triassic Nicola Group, the principal volcano-sedimentary component of the Mesozoic arc complex that characterises the Quesnel terrane, were also supposed to be faced. A total of four Herrenknecht AVN 2000 pipejacking
machines with a shield diameter of approximately 2.5m, and special hard rock cutting wheel designs and tooling, were delivered to execute these challenging drives. Special emphasis was given on the high rock strength and high abrasivity, leading to the application of anti-roll solutions and special wear protection. The anti-roll unit consists of grippers just behind the telescopic station. In combination with the jacking stations, this measure makes sure that full thrust can be applied to the cutterhead at all times in order to achieve maximum performance rates. A further measure is the incorporation of a strong
main bearing and main drive, which is the critical link between the cutterhead and the jacking system of the MTBM, including the steering cylinders. It has not only to transfer the jacking loads via the cutterhead to the tunnel face but, at the same time, it has to deal with the torque generated by the main drive and the rotation speed of the cutterhead to chip the rock. The main bearing also seals off the excavation chamber against the atmospheric pressure in the tunnel. Finally, steering cylinders robust enough to transfer
the required thrust force to overcome groundwater pressure and the contact force of the cutters to break the anticipated hard rock were deployed. Additionally, the stroke of the steering cylinders was sufficient to allow the machine to navigate curved drives and follow the designated tunnel route. The rocks encountered close to Kamloops were gabbro,
diorite, basalt, sandstone, siltstone, chert and variations of these rocks. The geotechnical conditions were very heterogeneous with high-strength rocks, fault zones and soft rock conditions and deviated from the expected conditions. The southernmost section over approximately 980m was excavated exclusively in soft rock conditions, whereas the rock strength on other sections was given as up to 350MPa. The steepest gradient was predicted to be at approximately 20%. The latest section, completed in November 2023 by
Below: Anti-roll solutions IMAGE COURTESY OF HERRENKNECHT
The Tunneling Company, is considered as the longest microtunnel of its kind in Canada, with a total length of 1,173m. The experienced jobsite team managed to overcome rocky conditions, hard rock strata of basalt with
UCS values of up to 250MPa and a CAI of up to 3, while safely navigating transition zones of clay, sand and gravel as well as weathered rock, with multiple vertical curves on a continuous horizontal curve radius.
SOFT SOIL Soft soil, like hard rock, involves many challenges for mechanical tunnelling. When adapting the machine design to the geological conditions, the main focus is on the cutterhead design, tooling, material removal, separation and tunnel face support. Here, the AVN technology becomes relevant: the water-saturated, coarse-grained tunnel face can be stabilised effectively by forming a bentonite filter cake. This is particularly important in urban areas, which are sensitive to settlement.
Cutterhead design for soft soil conditions As with hard rock, the design of the cutting wheel is crucial to the success of the project in soft ground, as it must be adapted to the specific subsoil conditions. It can be adapted to the respective soil, by selecting the opening ratio: the more open the design of the cutting wheel, the less potentially sticky clay can clog to it, whereby sufficient space is required for the installation of excavation tools and grain size limiters to fit the openings in the cone crusher; the more closed the design, the better the additional mechanical support for a more coarse-grained soil. Furthermore, depending on the nature of the soft
subsoil, a wide variety of tools can be used on the cutting wheel – such as ripper tools (for weathered material or low-strength rock layers) and cutting bits in a wide variety of designs, for example with special wear protection, with straight or conical geometry, and in different sizes.
Cohesive soil reference project: Rainwater tunnel in south-east Paris Another current project demonstrates in practice how preliminary geotechnical investigations and MTBM design can ideally be combined: For the 2024 Olympic Games, the host city of Paris undertook considerable efforts to optimise its water disposal. The objective was to ensure that the swimming competitions could be held in a sufficiently clean section of the River Seine. For this, the Val-de-Marne department in south- east Paris commissioned a new rainwater tunnel with an internal diameter of 1.80m and a length of just under 1,500m. The construction companies Eiffage, Valentin and Bessac decided to use a total of three AVNs designed and built by Herrenknecht.
14 | January 2026
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