Building Information Modelling (BIM) is becoming an increasingly important aspect of tunnel projects worldwide. Due to the rapid development of new technology, software, data management tools, and data management concepts, BIM has the capacity to fundamentally change how tunnels, or more generally, underground infrastructure, is designed, built and maintained. While the rapid development of BIM within the past decade has certainly led to improvement in tunneling projects, it has also led to a certain degree of ambiguity concerning the core concepts behind BIM and its implementation. This ambiguity can be further exacerbated by the differences in goals of BIM implementation between project partners, i.e., between owners, engineers and contractors within a tunnelling project. To address these issues, the International Tunnelling Association (ITA) Working Group (WG) 22 has developed a guideline for the implementation of BIM within a bored tunnel project, which was officially published at the World Tunnel Congress (WTC) in Copenhagen this year, and is now available on the WG 22 website for download at https:\/\/about.ita-aites.org\/publications\/wg-publications\/ content\/208-working-group-22-information-modelling-intunnelling. This guideline intends to support the tunnelling industry by presenting international \u2018best practice\u2019 solutions for owners, engineers, and contractors. Rather than competing with existing owner\u2019s BIM guidelines, the ITA guideline is intended to provide a reference framework for the implementation of BIM for tunnel projects for which there are no pre-existing standards. The guideline provides recommendations for selected important elements to be included in a project BIM execution plan (BEP) or similar contractual documents in which an owner\u2019s BIM requirements are set forth. Because BIM is such a broad topic, the ITA guideline is specifically focused on the implementation of BIM for the heavy civil works of segmentally lined bored tunnels. Additional structures, such as stations, and additional disciplines, such as systems, are not directly covered by the guideline, as these are assumed to be addressed via general civil\/MEP standards. Nonetheless, it is the intent of WG 22 to develop further specific guidelines for different tunnel methodologies (e.g., mined tunnelling) and to include non-tunnel components (e.g., cross passages) into future editions of the guideline. Building information modelling The ITA WG 22 has adopted the following definition for BIM: Building Information Modelling (BIM) is a process that involves the generation and management of project and asset information using digital representations of physical and functional characteristics of structures and facilities over their entire life cycle. This process is supported by various digital tools and software as well as by contractual information management agreements. In current practical usage, BIM is often used as an umbrella term to describe the use of any number of digital tools, such as, but not limited to, 3D modelling, computational design, visualization, clash detection, 4D\/5D modelling and information management used to improve project delivery, asset management, and collaboration. While the ITA WG 22 does not purport to have the authority to provide a definitive definition of BIM, the above definition has been developed to address two common issues. First, in describing BIM as a process, rather than as a single software, program, model, or data structure, the definition provides a technically accurate description of BIM. In contrast, the final portion of the definition addresses the reality of the usage of the term \u2018BIM\u2019 in the tunneling industry. While experienced BIM professionals may consider BIM to be primarily an information management process supported by tools such as 3D modelling, less experienced BIM users tend to refer to the 3D models or 3D modelling tools themselves as BIM. The definition above aims to reconcile this divergence in perception. To differentiate between BIM as a process and the various models used when implementing BIM for a project, the following definition will be employed in this article: Building information models (BIMs) are digital files or models that store information regarding a built asset. When fully implemented, BIM involves the creation of a central storage location for all digital information of the project\/asset during its lifecycle, from design to operation and maintenance. This information is stored within a multitude of BIMs that accurately capture the desired project\/asset information at each project phase. The BIMs together with the information management\/storage system with which they are connected make up the \u201cdigital assets\u201d of a project. Guidelines for bored tunnels The guideline addresses the following core concepts that are necessary for BIM to be successfully implemented in a project: AssetsBIM use casesThe information management process and responsibilitiesModel interoperability and data environmentLevel of definitionClassification systemsExchange data formatsGround modellingSustainability Finally, the ITA guideline provides a list of endorsed BIM documents, such as the DAUB BIM guidelines [DAUB, 2019; DAUB, 2020] or the ISO 19650 series [ISO, 2018(1); ISO 2018(2)], as well as providing a list of reference projects, and a list of otherwise relevant standards to provide a set of reference documents for further education. The following sections provide a more detailed description of the contents of the sections discussed above: Assets. The ITA guideline is focused primarily on the project delivery phase of a tunnel project. Owners, however, often desire that the digital assets developed during a project be used for asset management purposes after handover. The ITA guideline therefore provides a short introduction to BIM for asset management. This introduction covers the differing terminology involved when discussing asset management, i.e., project information models (PIMs) and asset information models (AIMs) and describes important aspects to consider when transferring information between a PIM and an AIM. In addition, the guideline provides a reference to ISO 55000, which specifically covers the primary aspects of asset management. BIM use Cases. Before BIM can be used on a project, the goal of its application (e.g., BIM for spaceproofing, for cost calculation, for construction scheduling, etc.) should be clearly defined and outlined. These goals are referred to as BIM Use Cases in the ITA Guideline. It should be noted, however, that other terminology, e.g., Use Cases, as employed by buildingSMART [buildingSMART, 2020], is often used to refer to the same concept. BIM Use Cases are the tasks or processes for which BIMs are used. In order to give each project participant the information they need, it is vital to know in which way various BIMs are engaged and how they are interrelated. A BIM Use will determine the necessary software or information storage environment required to develop a BIM and at which project stage the BIM information must be provided. Within the ITA BIM guideline, the determination of BIM Use Cases before design is strongly encouraged. To aid the determination of BIM Use Cases, the WG22 has developed a summary of common examples. The BIM Use Cases provided by the WG22 are largely based on the existing literature, with several cases being adapted from the DAUB [DAUB, 2019] and buildingSMART [buildingSMART, 2020]. To provide more transparency for the project participants, the examples provided by the WG 22 have been sorted by applicability to different project stages. An excerpt of the BIM Use Cases is provided in Fig. 1. It should be noted that the ITA BIM Use table is necessarily non-exhaustive. BIM Use Cases vary with the project needs. In addition, the continuous development of BIM software leads to the continuous expansion of potential BIM Use Cases within a project. Information management process and responsibilities. A clear information management framework is required to successfully adopt, integrate, and apply BIM processes within a project. Such a framework must regulate and define the workflow which governs the process of creation, modification and verification of digital project information within a project. In doing so, it should be determined which project participant (i.e. client, engineer, contractor, etc.) is responsible for which task (e.g., creation, modification or verification of information) at each stage of a project or asset\u2019s life cycle. Once such a framework is developed, it is further recommended to adopt a contractual agreement between participants that codifies the information management process. This agreement can, for example, be made in the form of a BEP. The ISO 19650 series [ISO, 2018(1); ISO, 2018(2)] provides a standard framework for information management of built assets using information modelling processes applicable throughout the asset life cycle. As the ISO 19650 series is already frequently adopted by the tunneling industry, the ITA WG 22 has chosen to endorse the adoption of the ISO 19650 series, rather than developing an independent guideline. To support this process, the ITA WG 22 has developed a companion document to the ITA BIM guideline regarding the adoption of the ISO 19650 series. This companion document is titled \u201cITA-AITES Recommendations for the Application of ISO 19650 Series during the Delivery of Underground Projects and Assets \u2013 Information Management Process and Responsibility Matrix.\u201d These ITA recommendations are intended to provide a guideline for the adoption of the ISO principles in the underground construction industry. The ITA recommendations for the application of ISO 19650 Series will be officially published at the WTC in Copenhagen in 2022. FIG. 1 Excerpt of BIM use list as provided in the ITA WG 22 document \u201cBIM in Tunneling \u2014 Guideline for Bored Tunnels.\u201d Model interoperability and data environment. It is often falsely believed by non-BIM experts that all digital project information can be stored within a single BIM. This is largely impossible, as computing power and software capabilities are not yet sufficient to do so. Rather, several BIMs (e.g., separate geotechnical, structural, and systems models) according to the selected BIM Use Cases are typically developed for a tunneling project. Although not all BIMs interact with one another, all BIMs should be stored in a centralized location, referred to as the common data environment (CDE), which is subject to the information exchange requirements set forth in the BEP or by ISO 19650. A CDE can be, for example, a ProjectWise environment (or similar in another platform, e.g., Autodesk BIM360) in which the file structure as well as the uploading, editing and approval process is strictly controlled. FIG. 2 Schematic of the relationship between tunnel and ring model in a BIM for a bored tunnel CDEs are well defined in ISO 19650 and may be directly adopted in the field of tunneling. The WG 22 therefore recommends that the ISO 19650 standard be followed for the creation of a CDE in tunneling projects. Level of definition. Within the context of the ITA BIM Guideline, the level of definition describes the level of complexity to which a BIM model is developed. This is further divided into the level of detail (LOD), which defines the level of geometrical detail to which a BIM object is developed, and the level of information (LOI), which is used to refer to non-geometrical information (i.e., material type, volume price, equivalent CO2 output per kg, etc.). For example, a tunnel segment can be modelled to a LOD incorporating only its inner radius, outer radius, and faces, or a tunnel segment can be modelled such that it accounts for all the geometrical details such as the gasket groove, contact area for the longitudinal joint, etc. The LOD and LOI of each object within a BIM develop throughout the life of a tunneling project. This terminology is borrowed from the PAS 1192-2:2013 [BSI, 2013], within which it was introduced. It should, however, be noted that this terminology is no longer used by the BSI as they have moved to use the term \u201clevel of information need.\u201d Nevertheless, the terms LOD\/LOI have proved to be helpful in the context of the tunnel and therefore have been continued to be used. To simplify this concept for easier inclusion into a tunnelling BIM environment, the ITA guideline provides a simplified table that accounts for most of the objects found in a tunnel and provides recommendations at which stage which object or detail should be included. An excerpt is provided in Fig. 4. To account for the complexities in the delivery process, the WG 22 guideline proposes to split the bored tunnel BIM into two models: a ring model and a tunnel\/alignment model. The ring model is included as a reference within the tunnel model through the tunnel model\u2019s level of information (LOI). The LOI describes semantic, i.e. nongeometrical, information associated with objects in a BIM. A schematic of the interaction between the tunnel and ring model is provided in Fig. 2. The tunnel model is a tube model that defines the location of the tunnel in the three-dimensional space. The tunnel model also includes all information generalizable to the tunnel as a whole (clearance envelopes, linear internal structures, etc.). It does not contain the location of the ring segments, as the achieved construction tolerances, and corresponding segment location, are unknown during the design process. The segmentation information is contained in the ring model. During design, only a single ring of each ring type generally needs to be modelled. In addition to the segmentation, the ring model should contain all relevant information needed to define the segmental lining, i.e, exact geometry, number and location of embedded items, reinforcement content, etc. The ring model is intended to form the basis of the segmental lining drawings and can be used at a later date by the contractor to generate the as-built tunnel models with the exact known ring orientation and locations. The as built models, in contrast to the design models, should contain the asbuilt location of the individual ring segments. An image of an exemplary tunnel model is shown in Fig. 3 (a), whereas a schematic of a ring model is shown in Fig. 3 (b). FIG. 3 (a) Tunnel model including interior systems, (b) ring model including segments Classification systems. Objects within BIMs (i.e. the tunnel lining or tunnel segments) should be properly named or labelled so that a model may be properly queried. Classification systems are used within a BIM context to achieve this purpose. These systems provide a naming hierarchy which allows all objects within a BIM to be named in a consistent but unique manner. Classification systems may be project specific or may be dictated by pre-existing owner\u2019s requirements. In the absence of owner\u2019s requirements, the ITA recommends the adoption of existing classification systems. Examples are the Uniclass [NBS, 2021] or DAUB [DAUB, 2020] classification systems. The DAUB standard is tunnelling focused and provides an extensive naming convention for BIM objects within both TBM and conventional tunnelling frameworks. The DAUB standard is, however, complex and results in long object names that adopt local national conventions. The NBS Uniclass 2015 system has been more broadly developed for the entire construction industry. In being broader, the Uniclass system provides less direct guidance on naming conventions for specific tunnel-based objects, but is therefore also easier to manipulate. A schematic of the Uniclass structure is provided in Fig. 5. An example of named objects using the Uniclass convention is provided in Table 1. FIG. 4 Excerpt of the LOD\/LOI table developed by the ITA WG 22. NR signifies \u201cNot Required.\u201d Exchange data formats. BIMs within a project often need to exchange and share information. Importing, exporting, creating, or editing data, may, however, require software-specific exchange formats. These formats may have limited interoperability with other software used in the BIM environment. Consequently, data requirements and file formats for data interactions between BIMs must be pre-selected and codified in a contract document (e.g. BEP or similar) before use. If data between BIMs cannot be directly transferred through native file formats, interfaces modifying the export or import information must be manually created using specialized coding tools. FIG. 5 Uniclass object hierarchy. File formats for BIM programs are typically proprietary and often unique to a specific program or software family. To increase transparency and compatibility between BIM programs, the ITA WG 22 guideline supports the adoption of the Industry Foundation Class (IFC) format. The IFC format presents a vendorindependent format for the exchange of information between BIMs. Tunneling-specific object classes (titled IFCTunnel), have been in development by Building Smart International [buildingSMART, 2020] since 2019. Although significant progress has been made towards the adoption of IFC in commercial BIM software, the IFC format may not be available in all commercial programs. In lieu of the IFC format, it is generally advantageous to combine software packages from one developer to improve interoperability between disciplines and tasks. In doing so, the ITA guideline provides the following additional recommendations: Tunnels are linear structures, and not all software are capable of handling chainages. Care should be taken in determining the right software to provide the ideal working environment for tunnels.In contrast to the above, local structures (e.g. shafts or stations) may require different modelling software than the primary tunnel alignment.Generally, the adoption of fewer software platforms leads to better integration between BIMs as the number of interfaces is minimized.All tunnel and other project models should share the same co-ordinate system from commencement of modelling.A federation strategy to transmit information containers or models should consider the maximum file size that is practical for upload and download with the specified IT infrastructure (e.g. 250MB, 1 GB, 10 GB, etc.). The information model should be subdivided such that no single information container exceeds these limits. These limits are typically set forth in a project Master Information Delivery Plan (MIDP) and Task Information Delivery Plans (TIDP). Ground modelling. The inclusion of ground information (e.g., borehole data, geophysical data, geological models) in a BIM environment is often hindered due to a variety of reasons, with some being: The development of a full geological database of all available ground information is often difficult due to the large volume of geological information available.In contrast to a civil design, ground information cannot be largely determined a priori.Ground information changes during tunneling, and previous assumptions concerning geological layering are updated or replaced as the project progresses (e.g., borehole vs face map records, latest readings from I&M, etc.).Much available geological information is not factual, and is a result of specialist interpretation. TABLE 1 Tunnel-specific examples of Uniclass object naming structure. Despite the complexities surrounding the subject, ground information is a vital component of underground construction, as many of the risks and successes of a project hinge on the correct interpretation of its geology. The ITA guideline strongly encourages the inclusion of ground information within a BIM environment. FIG. 6 Geotechnical BIM showing positions of boreholes along a tunnel alignment. Integration of geological and geotechnical data within a BIM context. Ground information in a geotechnical \u2018BIM\u2019 environment often follows a different data structure than structural or architectural data included within BIM models. For this reason, the ITA guideline recommends that the geotechnical \/ geological model be kept separate from the main tunnel model. This also supports the practicality of reducing model sizes in line with software \/ hardware limitations. Furthermore, the inclusion of different types of information will be dependent on the stage of a project. Some examples of information to include in geotechnical BIMs at different project stages are: Conceptual and preliminary design model \u2013 Historical borehole data.Baseline reference design model \u2013 borehole data (with links to relevant reports), initial geotechnical\/geological models and sections.Detailed\/contractor design model \u2013 borehole data, geotechnical \/ geological models and sections, baseline I&M readings.Construction model \u2013 Borehole data, I&M (realtime or not), updated models and sections.Handover\/operational model \u2013 The asset management model is assumed to be the construction model as often no further information is created after completion of construction. The asset stage is, however, outside of the scope of this work as it needs to be defined by the asset owner suitable to their systems. An example of a BIM showing borehole data is provided in Fig. 6. Factual vs. non-factual (or contractual vs. noncontractual). Ground information can be factual or non-factual (i.e., interpreted data). It is recommended to include factual data (examples outlined above) in projectwide BIM models. Non-factual data include interpolations for geological models and sections, recommended baseline parameters or interpretations from geophysics. The inclusion of non-factual data should be carefully considered since this information may impact risk-sharing arrangements within a project. The inclusion of non-factual data within a project-wide geological BIM model does, however, carry significant benefits. Interpretive data, such as the in situ stratigraphy, and other actual ground conditions can be very useful to make informed engineering decisions, and provide direct comparisons to the baseline or reference model, especially in projects with complex geology. In addition, such data included within a BIM model can significantly streamline future engineering decisions, as future engineers may use past interpretations as a basis for their own assumptions or interpretations. This is especially true with regard to BIM models intended to be used as asset management aids during the use\/operation phase. If non-factual data is to be included in the BIM, it should be explicitly evident that this information is an interpretation from factual data. Uncertainties in this interpretation should be quantified and reported. Methods for clear classification of factual vs. non-factual data vary based on projects and are owner dependent. One example of how to distinguish geotechnical data is that provided by Building Smart International [buildingSMART, 2020] in which geotechnical data is stored as \u201cfactual data,\u201d \u201cinterpreted data,\u201d and \u201cconception (design) data.\u201d In addition to correctly identifying non-factual geotechnical data, the source of interpretive information should be traceable. Traceability within a BIM model can be achieved by, e.g. consistently tracking author information within a BIM object. Sustainability. BIM can facilitate the early tracking of sustainability parameters and quantify the emissions associated with geometrical objects. The ITA WG 22 guideline recommends tracking equivalent carbon emissions as a primary sustainability marker of a project. This approach supports informed decision making by clients and consultants. Other life-cycle analysis design tools specific to tunnelling projects that include geology, structural design options and alignment can additionally provide early embodied carbon calculations for bestpractice results. Conclusion The field of BIM is continuously changing due to the ever-increasing number of tools available to architects and engineers. Nevertheless, some core concepts, such as organized data management and workflows or centralized data structures, have established themselves as necessary for the successful integration of BIM into a tunneling project. The ITA \u201cguideline for the implementation of Building Information Modeling concepts for Bored Tunneling Projects\u201d aims to clearly depict these core concepts to both owners and engineers and therewith support the continued adoption of BIM within the tunneling industry. Acknowledgements The authors would like to thank all the members of the ITA WG 22 for their assistance in developing the guideline. In addition, we would like to thank the UCA of SME Working group on interaction modelling in tunneling for their review on the applicability of the guideline for the U.S. market. The current members of the UCA of SME WG are Jon Berkoe, Jeff Fontana, Jacob Grasmick, Rajat Gangrade, Ivan Hee, Mark Johnson, Jay Mezher, and Eric Westergren. Former members involved in the review are Foteini Vasilikou, and Anthony Bauer. The Group is chaired by Vojtech Ernst Gall. References Building Smart International (buildingSMART). 2020. IFC-Tunnel Project Report WP2: Requirements analysis report (RAR). Retrieved from: https:\/\/ www.buildingsmart.org\/the-final-draft-of-the-ifc-tunnel-requirements-analysisreport- is-now-available. British Standards Institute (BSI). 2013. PAS 1192-2:2013 - Specification for information management for the capital\/delivery phase of construction projects using building information modelling (withdrawn). BSI Standards Limited, London, United Kingdom. German Tunneling Committee (DAUB). 2019. Digital Design, Building and Operation of Underground Structures BIM in Tunnelling. DAUB e.V.: Cologne, Germany. German Tunneling Committee (DAUB). 2020. Digital Design, Building and Operation of Underground Structure Model requirements \u2013 Part 1: Object definition, coding and properties. DAUB e.V.: Cologne, Germany. International Organization for Standardization (ISO). 2018. ISO 19650-1:2018 \u2013 Organization and digitization of information about buildings and civil engineering works, including building information modelling (BIM) \u2013 Information management using building information modelling Part I: Concepts and Principles. ISO: Geneva, Switzerland. International Organization for Standardization (ISO). 2018. ISO 19650-2:2018 \u2013 Organization and digitization of information about buildings and civil engineering works, including building information modelling (BIM) \u2013 Information management using building information modelling Part 2: Delivery phase of the assets. ISO: Geneva, Switzerland .