MECHANISED TUNNELLING | TECHNICAL


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


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


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


January 2026 | 11


Left: Three-spoke


anti-clogging cutterhead on AVN1800 PHOTO CREDIT:


VALENTIN ENVIRONNEMENT (VIA HERRENKNECHT)


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