Underground Infrastructure for the Energy Transition
With Tunnelling and Trenchless Installation

Introduction

According to a forecast by the International Energy Agency (IEA), renewables are likely to replace coal as the world‘s leading energy source for electricity production in 2025 already. More than a third of global electricity production would then come from renewables such as solar or wind power, according to the Paris-based agency‘s 2024 annual report [1, 2]. And the energy transition is also progressing inexorably in many other areas.

However, in order for electricity to flow unhindered from the windy coasts to the energy-hungry interior of the country, for example, and for new hydropower plants, wind farms and expanded power grids to be able to do their job, new infrastructure for generating, transporting and storing these forms of energy is essential. In many places, building these by means of tunnelling or trenchless installation is proving to be the best alternative.

1 Tunnelling

Often, electricity from renewable energy sources is not generated where it is later needed. Expanding the electricity grid therefore requires new power lines over very long distances. On their way, the cables naturally have to cross thousands of obstacles, for example rivers, mountain ranges, railway lines or existing underground infrastructure such as electricity, oil, gas, water and telecommunication lines. Entire areas such as settlements or nature reserves must be avoided or bypassed at specified distances. Impacts during the construction and operation phase must be reduced as much as possible. At the same time, in cases like these too, the straight-line principle – i.e. the shortest possible connection – must always be taken into account for cost reasons.

Trenchless tunnels and pipelines are the technical answer to these challenges. Unlike power lines buried underground, lines in protective tunnels are also well protected from external influences and often remain accessible for service purposes during operation. These structures are built either by mechanized tunnelling with segmental lining or by pipe jacking.

1.1 TBM Tunnelling with Segmental Lining

Accessible tunnels lined with segments are particularly suitable for long distances: they allow drive lengths of more than 10 km, large diameters, and very flexible horizontal and vertical alignments, even with small curve radii.

Elbe Crossing for SuedLink

These advantages can also be seen, for example, in the German ultra-high-voltage direct current transmission line SuedLink currently under construction by the two transmission grid operators TenneT and TransnetBW. Once completed, it will transport electricity over 700 km from Brunsbüttel in Schleswig-Holstein to Grossgartach in Baden-Württemberg. As part of this project, the Elbe between Wewelsfleth (Schleswig-Holstein) and Wischhafen (Lower Saxony) will be crossed under by a protective tunnel around 5.2 km long.

The machine technology for the section known as ElbX was manufactured by Herrenknecht at its headquarters in Schwanau and was handed over to the contractor and network operator in July 2024 (Fig. 1). Specifically designed to meet the requirements of the project, as it passes under the Elbe the tunnel boring machine (TBM) navigates a varied geology of clay, tidal mud deposits, peat, sand, gravel, stones and boulders. The Mixshield has a diameter of 4900 mm, is 190 m long and weighs 700 t.

The highly complex Mixshield is sealed against the water pressure 20 meters below the Elbe with a multiple sealing system. The TBM not only excavates the tunnel but also lines it with concrete segments at the same time. The finished tunnel will have an inner diameter of 4000 mm and an outer diameter of 4600 mm.

National Grid’s London Power Tunnels Phase 2 (LPT Phase 2)

An example of an already successfully completed segmentally lined tunnel is National Grid’s London Power Tunnels Phase 2 (LPT Phase 2) – Package 2 Tunnels & Shafts Project (Fig. 2). It aims to reinforce and future-proof London’s electricity network as demand in the capital grows. It has involved 32.5 km of tunnelling at depths of up to 60 m under seven south London boroughs (Fig. 3).

Three Herrenknecht EPB machines with shield diameters of 3580 mm were used to drive the bulk of the five tunnel sections, 26.5 km in all. In September 2023, the last of the project’s TBMs, named Grace, broke through after tunnelling more than 11 km eastward to Eltham shaft site in Greenwich, from National Grid’s New Cross substation in Southwark. This final breakthrough occurred approximately three weeks ahead of the original programme set at the start of the project in December 2019.

All 32.5 km of the project’s underground route are now complete, and the installation of 200 km of high voltage cable is under way between substations at Wimbledon and Crayford. The project’s new transmission infrastructure is becoming operational in phases, with the first circuit between Crayford and Hurst energised in 2024.

In total the project will have shifted 900 000 t of earth, with 99.98 % of waste material diverted from landfill. Phase 1 of the London Power Tunnels was already completed from 2011 to 2018 and carried cable circuits north of the River Thames.

Hydropower Plants With Steep Inclined Bores

TBM tunnelling is also used for the supply lines to hydroelectric power plants. Here, the flexibility of the machines in the routing of the alignment proves particularly useful. A characteristic challenge are the sometimes steep inclined bores with gradients of up to 42 degrees. As a comparison: the Bergisel ski jump in Innsbruck has a gradient of just 35 degrees.

Herrenknecht solves this special challenge with Gripper TBMs (Fig. 4). Machines of this type are designed in such a way that they cannot slip back and down through the steep tunnel they have built. For this reason, the machine has several bracing levels, so-called anti-slip systems or gripper units. Two of these gripper units press against the tunnel wall like barbs, securing the TBM in every phase of the drive.

In 2022, for example, a Herrenknecht TBM of this type with a diameter of 5800 mm bored a 770 m long pressure shaft with a gradient of 42 degrees for the “Limberg 3” pumped storage power plant in the High Tauern mountain range south of Salzburg (Fig. 5). Construction of a 1600 m headrace tunnel for the “Ritom” project in the Swiss canton of Ticino (Fig. 6), which began in 2021, has also been completed. Of particular note here: after approx. 900 m, the inclination of the tunnel changes from 42.5 % to 90 % (23 degrees to 42 degrees), which was an additional challenge for the overall machine conception. The excavation process itself was not affected by the increase of the inclination, whereas parts of the machine back-up system and components such as walkways and the control cabin had to be repositioned to function in both working angles.

1.2 Pipe Jacking

An alternative to segmental lining is pipe jacking. Here, too, the soil is excavated at the face by a tunnel boring machine. A hydraulic jacking station pushes product pipes made from various materials, mainly reinforced concrete, from a launch shaft through the ground to a target shaft. In this way, tunnel diameters (ID) of up to 4000 mm can be achieved. With microtunnelling, however, significantly smaller diameters from DN 250 mm are also possible using pipe jacking. Microtunnelling with slurry discharge and a slurry-supported tunnel face offers a high level of safety and precision. As a result, for example, railway lines can also be crossed under in compliance with guidelines and without settlement.

Thanks to the compact design, the method considerably simplifies and accelerates the commissioning and construction process compared to segmental lining. Above all, however, pipe jacking can be carried out remotely from the surface. In addition to the lower personnel requirements, the method is also gaining in importance from an occupational safety perspective. In particular for small tunnel diameters under DN 4000 mm namely, following its revision the DIN EN 16191 standard will define new safety requirements in the future.

Where the inner diameter is less than 3000 mm, these expected requirements can no longer be reasonably met in segmental lining, but they can in pipe jacking. Thanks to continuous further development of the method, corresponding machines from Herrenknecht can also handle pipe jacking drive lengths of over 2000 m.

Dresden Elbe Culvert

In 2020, an accessible protective tunnel for district heating pipes, a so-called culvert, was built under the Elbe River in Dresden using the pipe jacking method (Fig. 7). The use of district heating saves more than 3200 t of carbon dioxide each year. To build the protective tunnel, a Herrenknecht AVN machine (Fig. 8; German abbreviation for “automatic tunnelling machine, slurry”, for an inner diameter of 2500 mm) worked its way through the subsoil up to 7 m below the river bed using pipe jacking. The plan also involved the TBM boring through the target shaft wall, a 1.5 m thick reinforced concrete wall with fiberglass reinforced plastic. On August 28, 2020, after 30 working days and 245 m of tunnelling at a depth of up to 22 m, the machine achieved breakthrough in the target shaft.

2 Pipeline Construction

Pipelines (e.g. for hydrogen) and cable protection pipes (e.g. for offshore landfall cables and other power lines) are other key pieces of infrastructure in the energy transition. Trenchless underground installation is often preferable here.

Safety is a paramount consideration in the construction and operation of utility networks, particularly in the context of occupational safety and environmental protection. This becomes even more crucial in sensitive areas such as nature reserves and coastal areas, but also when water bodies and transportation routes have to be crossed. Beneath roadways and railway lines, settlement issues must be strictly avoided to ensure the integrity of the infrastructure above.

In such scenarios, slurry microtunnelling methods can assure this high level of safety as excavation of the borehole and installation of the jacking or product pipes takes place at the same time. Settlement risk thus is avoided by continuous mechanical borehole support. Successfully executed projects, with low risk and the least possible impact on environment and surroundings, help the public to gain trust and acceptance of necessary installations. Likewise, they contribute to convince planning authorities of the opportunities and the benefits of trenchless technologies for installation lengths of over 2000 m.

Horizontal Directional Drilling (HDD) is still a preferred option in the drilling industry, as it is an economical and industry-proven technology. Nevertheless, HDD can be risky or even not applicable in highly permeable or non-stable ground. Today, slurry microtunnelling methods like Direct Pipe and E-Power Pipe (and also pipe jacking) cover all application fields, ground conditions, respective pipe materials and diameters. They use a slurry microtunnel boring machine for excavation. A thrust unit on the launch side pushes the product pipes or temporary jacking pipes forward. Continuous face and borehole support in slurry microtunnelling methods emerge as the key advantage over HDD.

2.1 Direct Pipe

For nearly 20 years, the Direct Pipe method developed by Herrenknecht has been used globally for the trenchless installation of prefabricated steel pipelines, in more than 250 pipeline crossings and landfall projects. It combines the advantages of slurry microtunnelling with the Pipe Thruster technology to enable trenchless installation of pipelines or casings in difficult ground conditions while reducing the risks typically associated with HDD.

Direct Pipe allows excavating the borehole while simultaneously installing a prefabricated and already tested pipeline in one single continuous step. Typically, Direct Pipe is used to safely cross rivers or traffic ways, infrastructure objects and other man-made or natural obstacles, but also to cross shorelines and beaches on landfalls and outfalls.

Protective Pipes for the Biggest Wind Farm in the USA

The Direct Pipe method recently demonstrated its strengths during construction of what is currently the largest offshore wind farm in the USA, the Virginia Coastal Offshore Wind Project (Fig. 9): around 40 km off the coast of Virginia Beach, 176 turbines are to produce electricity for up to 660 000 households. To this end, from the shore, using the Direct Pipe method three AVN machines from Herrenknecht with a diameter of 1110 mm installed a total of nine steel pipelines, each around 550 m in length. The plastic pipes for the cable protection tunnels were then placed within the protection of the installed steel pipes. If all goes to plan, electricity will begin to flow in 2026.

2.2 E-Power Pipe

In addition to proven trenchless installation technologies from the tunnelling and the pipeline industry, Herrenknecht has further developed slurry microtunnelling to the two-phase E-Power Pipe technology (Fig. 10). It is used to install non-pressure-resistant HDPE pipes as cable protection pipes, or small-diameter pipelines of 10” to 28” diameter underground, wherever conventional methods, like HDD, reach their economic and technical limits. It was originally designed for crossings in difficult, non-stable ground conditions or drilling with low ground cover, over long distances of up to 2000 m.

Installing Protective Pipes With E-Power Pipe

From April 2025, Heitkamp Construction Swiss GmbH will be installing protective pipes for power lines using the E-Power Pipe method as part of the construction of the SuedLink alignment mentioned previously. Herrenknecht has already handed over the machine for the project. At the heart of the solution is the fully remote-controlled AVNS350XB tunnelling machine with a boring diameter of 505 mm, which is designed for drive lengths of over 1000 m. The machine can keep to the planned alignment with high precision and thus cross safely under existing infrastructure such as pipelines, roads, railways or smaller bodies of water. Individual boreholes can be drilled just one to two meters apart, allowing several pipelines to be laid in parallel.

2.3 Outlook: Underground District Heating Pipelines

The energy transition is not only taking place in the electricity sector, but also in heating. In Germany alone, 100 000 new buildings are to be connected to district heating networks every year. [3] According to an estimate by Herrenknecht (based on the Prognos study from 2024), this means more than doubling the existing or recorded district heating lines from currently around 35 000 to 74 000 km.

The construction and expansion of district heating networks is carried out by conventionally installing the pipelines in open trenches or using trenchless laying methods, or alternatively, if there are several pipes – as in Dresden – in protective tunnels. However, trenchless installation methods are also being used in more and more projects – and for good reason: among other things, district heating pipelines are naturally located in densely populated areas and usually only require smaller diameters. Microtunnelling and HDD methods often offer the best and most cost-effective solution for this.

3 Other Areas of Application

The energy transition also requires new infrastructure for the generation, transport and storage of environmentally friendly forms of energy in other areas. Two examples show that drilling technology concepts transformed from horizontal to vertical applications also offer the best or only solution: creating the foundations of offshore wind turbines and in geothermal drilling.

3.1 Foundations for Offshore Wind Turbines

The success of offshore wind power generation means that many areas with favorable construction conditions have already been developed. In turn, this means that new offshore wind farms are increasingly being built in more challenging locations, for example in greater water depths or on geologically more problematic ground. This has also increased the demands on offshore foundation building. One effective response to this is Offshore Foundation Drilling (OFD), a new technology developed by Herrenknecht since 2007.

Today, the OFD technology uses a full-face drilling concept to overcome the limits of conventional methods, especially in challenging non-drivable ground conditions (boulders, rock layer) and where established pile driving methods would cause critical noise emissions. The main focus for OFD application are wind farm foundation piles, including anchoring of floating wind turbines, with drilling diameters today from 3000 to 12 000 mm.

Reference Projects in France

For the drilled subsea installation of a total of 73 monopile foundations for the St. Nazaire wind farm in France, Herrenknecht delivered an Offshore Foundation Drilling machine in September 2020. The Belgium marine and offshore engineering specialist DEME Offshore started operation of the OFD equipment on the project in spring 2021 within an innovative engineering framework.

The employed equipment is currently the biggest offshore drilling rig with a drilling diameter of 7700 mm, the project being a premier for the drilled and grouted installation of offshore monopiles. The St. Nazaire offshore wind farm is the first offshore wind farm in France. By May 2022, within less than a year of construction time, all 73 monopiles had been successfully installed by drilling and grouting, in water depths of 15 to 25 m in rocky seabed and with an average installation time of three days per pile, including drilling and grouting.

After this work was completed, the Offshore Foundation Drilling machine (Fig. 11) was thoroughly refurbished by Herrenknecht for use in the next project. The Noirmoutier wind farm off the French Atlantic coast is due to be completed in summer 2025. Boreholes for 61 monopiles are being executed for the new wind farm. Each monopile has a diameter of 7000 mm and will serve as the foundation for one of the more than 200 m high wind turbines. The weather, sea and ground conditions off Noirmoutier are just as challenging as those off St. Nazaire. The monopiles are embedded in the rocky seabed at water depths of up to 36 m.

3.2 Geothermal Energy

Geothermal energy is climate-friendly, reliable and practically inexhaustible. However, as heat can only be transported to a limited extent, it must be tapped directly where it is needed. This in turn poses new challenges for the drilling industry, particularly in urban areas, where demand for climate-friendly energy is increasing. Application-specific drilling rig technology for geothermal projects must therefore combine two key features:

high power density to reduce project duration

reduced noise emissions to increase local acceptance.

This represents an extreme challenge, as noise sources can arise in many different areas of a drilling rig.

The automated pipe handling system of Herrenknecht‘s drilling rigs (Fig. 12) enables safe, hands-free and low-noise handling of the horizontally stored drill rods and casing pipes. The pipes are positioned on the drilling site in hydraulic, tilt-adjustable pipe storage facilities and stored in layers in an open steel frame.

Another aspect is important for increasing the overall energy efficiency of the equipment: it is now standard practice here to operate it via the local power grid. This reduces the logistical effort for diesel and increases the necessary acceptance among the population because there is no noise or odor nuisance. Energy recovery in the electro-hydraulic systems during the drilling process is monitored using artificial intelligence and ensures that the drill rigs have a low overall energy requirement.

4 Conclusion

The realization of underground infrastructure for the generation and transport of alternative forms of energy is essential for the success of the energy transition. In Germany alone, the upcoming grid expansion projects involve around 4500 infrastructure crossings – the most complex sections of underground high-voltage cable alignments. Both tunnel structures and trenchless pipelines and protective pipes are of crucial importance for this. Geographical obstacles such as rivers, swamps and wetlands must be overcome. Nature reserves and recreational areas, groundwater zones, the landscape and valuable agricultural land must be protected. Furthermore, transport infrastructure, supply and disposal lines and buildings must be crossed under. To realize these projects, broadly positioned manufacturers like Herrenknecht offer a range of different tunnelling, drilling and installation technologies to meet all project requirements.