The popularity of small-diameter tunnelling is on the rise, driven by the increasing use of renewable energies and the demand for more and longer utility tunnels, even in very shallow or great depths. These tunnels serve various purposes, such as sewage, stormwater, freshwater, hydropower, casing tunnels and pipelines for gas or hydrogen transport. When performing pipejacking with small-diameter slurry microtunnel boring machines (MTBMs), such as AVN type slurry shields, there exists a wide range of technological adaptations like cutting wheel designs and cutter types, whose application is strongly influenced by the geotechnical conditions.
Geotechnical preliminary investigation data heavily influences the technological options available for project implementation. Tunnelling through hard, abrasive rock or soft, cohesive soil presents significant challenges due to the variability and complexity of the ground parameters and the complex interactions between geological, geotechnical, and engineering factors. Two case studies are discussed, where accurate characterisation of the subsoil was crucial for assessing the feasibility of the tunnel construction project and to select the best suited MTBM design specifications for successful project planning and execution.
INTRODUCTION
Climate change and energy transition have a direct impact on tunnel and pipeline construction, for example: existing sewage systems must be expanded and upgraded to make urban infrastructure more resilient to extreme weather events, such as heavy rainfall; and, to ensure that new hydroelectric power plants, wind farms and power grids can fulfil their long-term purpose, new infrastructure for generation, transport and storage of these renewable energies are essential.
The result is an increasing number of longer, trenchless small-diameter tunnels, with pipejacking being the tunnelling method of choice for inner diameters of up to 3.2m (see Figure 1).
The MTBMs can be adapted to a variety of geological conditions and can therefore also be used in complex subsoil. Small-diameter slurry MTBMs (such as AVNs) are the most widespread machine type used due to their versatility and wide range of geological applications. However, it is essential to carry out detailed preliminary geotechnical investigations, particularly for small diameters.
This article demonstrates how geotechnical risk factors can be identified through appropriate investigation methods, mitigated using tailored machine technology, and how demanding projects in hard rock or soft, cohesive soils can be successfully executed.
IMPORTANCE OF PRELIMINARY GEOLOGICAL INVESTIGATIONS
A detailed analysis of geological conditions is essential for the safe and successful construction of tunnel structures (see Figure 2) which is why international guidelines, such as the European Eurocode 7 (EN 1997), the DAUB (German Tunnelling Committee) recommendations or the German ATV DIN 18319 and DWA-A 125, define corresponding standards. Prescribed geotechnical investigations should not be regarded as unnecessary obstacles. On the contrary, comprehensive subsoil analysis prior to project initiation significantly reduces the likelihood of unforeseen complications during construction and enhances the probability of a successful outcome for all stakeholders.
This applies, in particular, to tunnel projects with major challenges, which may include the subsoil itself, for example due to the nature of the hard rock, the soft soil or high groundwater pressure. Typical geological risk factors include the presence of gas, clogging, high permeability of the subsoil, high strength, high abrasiveness, the occurrence of boulders, mixed ground conditions and aggressive groundwater. Further requirements may arise from the tunnel alignment itself, such as in case of long tunnel sections, extremely low or very high overburden, small curve radii or steep gradients.
In addition, small tunnel diameters with limited or no accessibility, e.g., for replacement of worn cutting tools, present specific challenges to the smooth execution of tunnelling projects. Examples of relevant MTBM design parameters directly influenced by ground conditions are face support, cutterhead design, anti-clogging measures, excavation tools, muck transport, performance and wear, appropriate ground support measures, and measures for mitigating risks associated with ground instability or dangerous gases along the tunnel alignment (see Figure 3).
APPLICATION RANGES AND GEOTECHNICAL PARAMETERS
In practice, most of the underground proves to be a mixture or, especially in the case of long sections, a sequence of different geological conditions.
The geotechnical parameters and the respective investigation methods are theoretically as diverse as the geological conditions worldwide.
Each ground naturally has its own requirements in terms of the most suitable machine specifications. Specifically for the preliminary geotechnical investigation of pipejacking projects, eight indices have emerged as recommendations over the course of many successful projects – four for soft soil and four for hard rock. These should provide a pragmatic and reasonably cost-effective geotechnical planning basis for every pipejacking project.
Classic soft soil geologies consist of gravel, sand, silt or clay in varying proportions. The following four parameters are central to the analysis of their exact geotechnical properties:
- The grain size distribution is determined by sieving (for coarser grain sizes) and by slurry sedimentation analysis (for finer grain sizes) and is represented as a distribution curve.
- The hydraulic conductivity or permeability of a soil are important hydrological parameters. The hydraulic conductivity, known as the kf value, depends on the grain size, porosity, bulk density and pore structure.
- Plasticity is a decisive factor in determining how prone a material is to clogging and lumping. It is determined in the laboratory and described using the Atterberg limits (shrinkage limit WS, plastic limit WP and liquid limit WL).
- Soil strength (of cohesive soils) and compactness (of non-cohesive soils) are determined by standard penetration testing (SPT).
Hard rock, in the sense of this classification, typically includes rock types such as sandstone, claystone, limestone, granite, basalt and gneiss. Four indices are also particularly relevant to this area in relation to tunnel construction:
- The degree of weathering describes and classifies the extent to which a rock has already been altered or decomposed by physical, chemical or biological processes.
- The strength and quality of the rock are quantified using the Rock Quality Designation (RQD) method and other parameters such as the joint set number or spacing. In the RQD, the proportion consisting of contiguous core pieces of a certain length is measured by visual inspection.
- Compressive and tensile strength – Unconfined Compressive Strength (UCS), Brazilian Tensile Strength (BTS) and Point Load Index (PLI) are determined by laboratory tests. The greater the number of rock samples used for this purpose, the more reliable the statistical statements on the strength of the material.
- And, not least, the abrasiveness of the rock must be determined. The Cerchar Abrasivity Index (CAI) 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.
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 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 southeast 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.
The geological conditions along the tunnel route posed a particular challenge in this demanding project: the soil in south-east Paris consists mainly of ‘Argiles Vertes’, a clay with swelling and clogging properties. In addition, the MTBMs were supposed to pass through layers of Cyrene marl and occasionally limestone layers at the upper edge of the tunnel cross-section.
After intensive discussions with the customer, Herrenknecht engineers and geologists recommended the use of the AVN technology. However, the machine design was specially modified and tailored to the prevailing underground conditions. In order to counteract the effects of swelling clay, for example, the overcut was increased to 45mm. This created a larger annular gap around the shield of the machine to prevent jamming of the MTBM caused by the volume increase of swelling clay.
Herrenknecht also countered the adhesive properties of the clay with special design details on the cutterheads: two of the three MTBMs used a three-spoke cutterhead (see photo p10), while the third machine has a fourspoke cutterhead. The smaller number of struts reduced the area where the clay could adhere. In addition, eight instead of six nozzles were installed in the cutting wheel to flush away the clogging clay with water pressure. The openings in the cone crusher were also designed with a larger diameter than usual. They ensure that the excavated material is sucked out in the form of a slurry.
The MTBMs, which drove the microtunnels with a limestone layer at the upper edge of the MTBM, were equipped with disc cutters in addition to the cutting bits on the outer edge of the cutting wheel. This new design not only offered an optimal solution for the complex geological conditions but also ensured consistently high performance rates during tunnelling: an average tunnelling speed of 40mm/min–50mm/min was achieved, with peak rates in certain sections reaching up to 200mm/min.
In total, the ambitious construction project required three launch shafts and three target shafts for five sections. Due to the limited space available in the densely built-up urban area, the construction site could only be organised on public ground. To save space, the work areas were installed on several levels of the launch shaft, including the AVN’s slurry pump.
Finally, the development and integration of operational technology and information technology create new opportunities to make data accessible and transparent via the cloud. Parts of the project in Paris (and that in Canada) were covered with the Herrenknecht IoT platform Herrenknecht.Connected. Predefined views enable continuous monitoring and interpretation of key MTBM parameters—such as sensor data—in near real-time, from any location. Solutions like MT.ON, part of the IoT platform, help to detect machine failures or deviations at an early stage and enable quick reaction and correction measures. In Paris, this led to improvements in terms of communication, transparency, planning, costs, and safety.
CONCLUSION
Like many other tunnel construction projects worldwide that have been realised with TBMs, such as from Herrenknecht, the construction of the new rainwater tunnel in the south-east of Paris and the hard rock tunnels in Canada are proof that the geological conditions of the construction site are a decisive factor for the success of tunnel projects and contribute significantly to the success of the project.
Therefore, a detailed and individual analysis of the specific subsoil conditions is essential. A generalised approach and standard, off-the-shelf solutions are not sufficient – what is needed are tailor-made concepts that are precisely customised to the respective conditions on site. In this context, Herrenknecht focuses on geotechnical assessment at a very early stage of project planning, enabling the technical implementation of appropriate TBM solutions.
The approach consistently takes geological conditions into account from the outset, and close cooperation with the customer and their practical experience and expertise play a central role in the outcomes. Only through continuous exchange, tailor-made technical solutions can be developed and optimally implemented in the project context. Aspects such as the quality and quantity of the bentonite, the selection of suitable concrete pipes or efficient logistics have to be integrated into the planning process at an early stage and in a practical manner – a decisive factor for the successful completion of a pipejacking project in any geological conditions.
