Einstein Telescope: Exploring the Universe from the Underground

The Einstein Telescope (ET) is an exciting project set to become Europe's leading third-generation gravitational wave observatory. This advanced research facility, built underground, will explore the universe by detecting gravitational waves, the ripples in spacetime. ET will consist of three interconnected interferometers, capable of identifying a wide range of gravitational wave frequencies with exceptional accuracy. This endeavour not only promises to reveal new cosmic insights but also marks a significant step forward in the fields of astrophysics and cosmology. The decision to build the Einstein Telescope underground is driven by the need to achieve unparalleled precision in detecting gravitational waves. By situating the telescope deep beneath the Earth's surface, it will be shielded from a myriad of surface-level disturbances, including seismic activity, human-made noise, and atmospheric variations. However, bringing this ambitious project to fruition involves overcoming significant engineering and logistical challenges. Among the various defying tasks within the project is the excavation of extensive tunnels and large caverns at considerable depths. Furthermore, the project’s success requires ensuring both the technical feasibility of these infrastructures and an optimal interferometer’s sensitivity. Overcoming both challenges is critical to ensuring the observatory's performance and its potential to revolutionise our understanding of the universe.

An underground facility 

Interferometers have long been essential tools in the field of physics, and have traditionally been constructed on the surface. However, there is a growing interest in building interferometers underground due to the benefits this approach offers. The interest in building ET underground relies on the aim of reducing the impact of seismic noise and gravity gradient noise induced by seismic waves and compression waves of the surrounding air. Furthermore, the underground operation will allow us to extend the frequency band of the observatory down to a few Hz (ET steering committee et al, 2020). By mitigating seismic and gravity gradient noise, ensuring stability, and protecting from environmental factors, the underground location will enable the ET to perform with unprecedented precision. The move towards building interferometers underground represents a significant step forward in enhancing the accuracy and reliability of these essential scientific instruments. The several outpointed advantages collectively make a compelling case for subsurface construction. 
 

The sites 

One of the consequences of the extension of the observation band towards lower frequencies is that environmental disturbances and therefore the quality of the observatory location is ought to play an increasingly important role (ET steering committee et al, 2020).  Evaluating the site selection for the Einstein Telescope involves a comprehensive and holistic approach. The key criteria include the impact on the infrastructure's longevity, the observatory's sensitivity and operational efficiency, and the preservation of the site quality. Additionally, considerations must cover the construction costs and the socio-economic benefits to the surrounding community. Technical feasibility plays a crucial role, ensuring that the chosen site supports the advanced requirements of the observatory, consequently optimising both performance and sustainability. Two candidate sites have been identified for a detailed site characterisation: one in the north of Lula in Sardinia, and one in the Meuse-Rhine Euroregion, locations shown in Figure 1. 

The ET layout 

The layout details to be evaluated in order to assess feasibility are yet to be defined. However, two configurations are being analysed from a physics and engineering point of view. For the first scenario, the subsurface configuration of tunnels forms a triangle with an approximate side length of 10,798 meters, excluding the access options. The schematic reference layout is illustrated in Figure 2.  Access for the assessed scenario is facilitated through either (a) vertical shafts or (b) inclined access tunnels, connecting the surface to the main caverns. The surrounding area at the corners of the triangle includes several caverns with varying geometries near the intersection points. Additionally, a lined borehole, located away from the crucial and vibration-sensitive cavern structure, is proposed for water management during operation. The described layout foresees a main big cavern for each corner point, measuring 190 m long, having a 20 m span and a 30 m height. Several smaller caverns and connecting tunnels are planned surrounding this main cavern. At the end of the main tunnel, a vertical borehole is planned to dewater the Access Area separately. The length of the “Dewatering Tunnel” is contingent on the vibrational impact of the hydraulic pumping system and the tolerable water infiltration. The second scenario would foresee an L structure, with 15 km long legs, whose angle is fixed at 90°. For this detector to perform 2L detectors should work in cooperation, and therefore two detectors should be built. 

Design requirements for construction

The design requirements taken into account for this preliminary review of the concept design relate, therefore, to geotechnical, dimensional or operational requirements of the Einstein Telescope. Feasibility constraints due to surface exigencies have not been taken into account at this stage. Boundary conditions or design requirements for the feasibility stage are listed in Table 1 below:

Table 1. Project requirements and general tasks for ET project subsurface infrastructure construction  

The design requirements identified during this first review will be further developed to define design specifications. The design specifications will define the precise characteristics of the Telescope Einstein that make up the concept design and allow evaluation of the conformity of the
project with the construction requirements. The definition of these design specifications conforms to the scope of a later, more detailed mission, which will analyse the design alternatives for the Einstein Telescope project.

Because the design specifications are strongly dependent on the design, and because several design solutions can exist for a given component (reference design and alternative designs), different design scenarios shall be defined, allowing a clear identification of which scenario each
design specification applies to.

Lifetime approach

The design approach for civil engineering is to be based on the following steps: 

• Pre-feasibility studies: Aiming to define the project’s requirements. In the EMR region, 3 boreholes have been drilled, and preliminary studies have been carried out on several fields (namely noise, geology and hydrogeology) to explore the project’s potential. The first preliminary layout was defined in 2020.
• Feasibility studies: Starting in Q1 2024, they aim to validate the feasibility of a specific chosen emplacement. A concept design, a preliminary cost estimate and a construction schedule are expected as an output. To enhance the geological and hydrogeological knowledge of the region, a borehole campaign is currently ongoing.
• Technical design: Further studies will develop one and only design scenario up to the level of detailed design. Construction schedule and budgeting are to be conducted.
• Tendering: Environmental assessment and validation, tender documents preparation, and call for tenders. Depending on the tendering scheme selected, part of the technical design might be developed during the contractor’s mandate.
• Construction: A contractor is awarded with the project’s construction mission and is therefore responsible for the construction of ET.

Ongoing feasibility studies

The design approach at the feasibility stage has been based on an up-to-down methodology. The main steps of the design are summarised below:

• The preliminary layout and the geometric definition of the caverns and shafts, especially the configuration of adjoining caverns
• Definition of the geological and geotechnical context of the area where the project will be built. This study is based on the available information (borehole information at our disposal as reference, laboratory or in-situ tests, geological plans, etc…) and on the judgement of expert geologists. It is to be noted that an underground context is a highly heterogeneous environment. This heterogeneity is usually encountered even within the same geological formation or within the same geotechnical unit. In consequence, a certain degree of uncertainty will always exist, and it means that the characterisation of the mechanical parameters of the geotechnical units should follow a probabilistic approach.
• Full preliminary design of the underground caverns and tunnels considering the geological and geotechnical context, the design loads, and the preliminary definition of the project.
• Optimization of the project design (tunnels and caverns support, geometric definition, etc…) based on the results of the preliminary studies.

Some of the general design requirements for deep underground works that need to be considered are the following: 

• Ensuring an adequate lifetime for the constructions.
• Considering the construction design codes in effect.
• Following the advice of trustworthy recommendations, as underground works are not fully described in the construction design codes.
• Identifying any missing geological and geotechnical information deemed essential for the design.
• Developing a design adapted to an observational excavation approach in the case of huge geological and geotechnical uncertainty. It means that several excavation and support methods need to be defined to be prepared for all the potential scenarios.
• Ensuring the feasibility from a logistical point of view. It means that the shaft and the ET dimensions need to be large enough to allow all equipment (e.g. ventilation ducts, laser pipes) to be installed and operational facilities to be emplaced.
• Ensuring that the crossings between galleries and caverns or galleries and shafts are large enough to allow the rotation of the longest operational facilities.
• Ensuring enough escape routes for workers during construction and limiting the length of dead-end galleries, if any.
• Ensuring geometrical layout to meet ventilation and fire safety requirements. Safety at this stage refers only to the structural resistance of the infrastructure to the design loads. No further measures regarding safety will be assessed at this stage of the concept design analysis

Technical hurdles

One of the principal technical challenges for ET construction is related to the complex nature of the ET layout. Corner points hosting multiple caverns have been identified as a highly critical aspect regarding feasibility.

Multiple recesses along the tunnel alignment pose extra technical challenges since they are likely to be excavated by traditional methods. Impacts on costs shall be taken into account. It is important to note that considering insufficient spacing between caverns may lead to pillar instability issues.

Restrictions regarding water flow within the infrastructure to ensure proper operation might well determine water tightness criteria of caverns and tunnels, which will determine inner lining requirements as well as the expected effort for pre-excavation grouting works. In case the tunnels must remain flat, and thus no slope is allowed, dewatering management will pose a significant challenge. Defining these outstanding issues will help reduce the level of uncertainty the project is currently facing.

International context and stakeholder engagement

The EMR site is strategically positioned at the intersection of three European countries: Belgium, Germany, and the Netherlands. This prime location offers significant opportunities for funding and technological collaboration. Ensuring the success of the project is therefore set as the top
priority for everyone at the ET EMR Project's office.

However, the alignment of the scientific community's objectives with the engineering feasibility of the project presents a significant challenge despite collaborative efforts. Overcoming this challenge will require a unified approach and for both engineers and scientists to push the limits of their respective state-of-the-art.

Environmental aspects

The construction of the Einstein Telescope must ensure minimal disturbance to the protected areas of the EMR region. This involves adhering to stringent guidelines on noise, vibration, and air quality. Furthermore, there must be a focus on maintaining the hydrological balance of the area. Alterations to the water table caused by the excavation and construction processes could adversely affect the delicate ecosystems within the Limburg’s Natura 2000 sites. Therefore, hydrological studies and sustainable water management practices will play a determining role regarding feasibility. 

International collaboration and engagement with stakeholders, including environmental organisations and local communities, are imperative for navigating the legal and ecological complexities of this project. Only through continuous dialogue and compromise on environmental regulations can the Einstein Telescope project proceed responsibly, ensuring the preservation of the natural heritage while advancing scientific knowledge.

Located at the strategic intersection of Belgium, Germany, and the Netherlands, this project aims to achieve groundbreaking advancements in gravitational wave detection, all while addressing the complex combination of technical challenges, environmental compliance, and cost efficiency. However, this project also presents an opportunity to innovate and find cost-efficient solutions that do not compromise quality or sustainability. Collaborative efforts between engineers, scientists, and financial experts will be vital in optimising resources and achieving the desired outcomes. The challenge of aligning scientific requirements with tunnelling feasibility is an opportunity to push the boundaries of technology. A holistic and collaborative approach will help to create a truly unique and groundbreaking project that will inspire both the tunnelling industry and the scientific community, redefining the possibilities of underground use and setting a milestone on how large-scale scientific endeavours can be realised responsibly and sustainably.

• ET steering committee et al.2020. Einstein Telescope: Science Case, Design Study and Feasibility Report
• ET steering committee.2020. Design Report Update 2020 for the Einstein Telescope, ET Steering Committee Editorial Team
• ITA Report N°17. 2016. Recommendations on the development process for Mined Tunnels
• Amberg Engineering .2023. Minimum subsurface requirements from tunnels, shafts and caverns. Brussels
• Amberg Engineering & Tractebel.2024. Preliminary feasibility assessment based on Aubel’s borehole results. Brussels