By Giuseppe Maria Gaspari
The 2020-2030: THE DECADE OF SUSTAINABILITY
The year 2021 was a pivotal one for the future of our global infrastructures. Almost every country approved special laws to release exceptional funding for the modernization and expansion of public infrastructure, but such unprecedented action is coinciding with an impressive global effort to transform the way we design and build to contribute towards national net zero emissions by 2050.
It is undeniable the impact of the ‘Conference of Parties’ (COP) summits, which have been bringing together world leaders to discuss action on climate change for almost three decades, peaking in 2021 with COP26 in Glasgow.
The direct consequence of these activities on the daily business for the tunneling industry is already clearly tangible.
Rising urbanization continues to increase land values, at the very time when more and larger public spaces are needed. If more than 50% of the planet’s population already lives in cities, an estimated 61% will be urbanized in 2030 and 70% in 2050 as recently confirmed by a Workshop of the Working-Group 20 of the International Tunnelling Association (ITA-AITES). The answer to this increase in demand for space and natural resources, as well as to reduce the growing waste, is through underground construction and underground space expansion, creating new opportunities and approaches to sustainability address those realities. Potential economic, environmental, and social impacts of tunnels and underground space are immense—with proper planning and development of underground space promising an enhanced quality of life. Nowadays, underground construction is to be considered a spatial asset that extends beyond transport tunnels and utilities to a host of facilities currently occupying surface space. To make all this possible, we must maximize the efforts during the early planning and design stages of our new tunnel projects, to reduce the cost of changes and exploit the offered value-opportunities (Fig. 1).
THE SUSTAINABILITY OF TUNNEL PROJECTS
According to a 2021 McKinsey & Co. study “Call for action: Seizing the decarbonization opportunity in construction,” Environmental, Social, and Governance (ESG) factors are the key measures of a business’ sustainability and societal impact within the construction ecosystem. However, when focusing on the underground works, it is difficult to assess metrics that can span across the entire project ecosystem. Most tunnel projects are built to relieve these issues: a subway reduces the carbon emissions of private cars—optimizing energy consumption; while a sewer separation system and the increased treatment plants capacity mitigate the combined sewer overflow (CSO) risk of stormwater mixing with untreated human and industrial waste.
However, the construction industry is directly or indirectly responsible for almost 40% of global CO emissions from fuel combustion and 25% of overall Greenhouse Gas (GHG) emissions.
Technological advancements of the tunneling sector need to focus on two specific areas to sensibly support the effort in reducing GHG emissions: raw-material processing and infrastructures operations. The contribution from raw materials comes primarily from energy-intensive cement and steel production, thus the recent development of design solutions to introduce widespread use of steel and plastic fibers as primary structural elements for the final lining of underground structures – a critical step towards “tunnels sustainability.”
Mechanized tunneling clearly represents a cutting-edge technology in our industry, with growing research toward the re-use of tunnel spoils as construction materials wherever possible. In 2021, ITA-AITES awarded China for the introduction of a sustainable, efficient, and low-carbon solution for the disposal of tunnel shield excavated spoil being developed during the construction of Shenzhen Metro Line 14, which features an overall distance of 50 km of twin bored tunnels. Through the adoption of an automated, modular, and compact kit used on-site for the detection of valuable materials to recycle (coarse aggregate, fine aggregate, residual clay and water), a cumulative volume of nearly 11,600,000 m3 of spoil has been treated across the city, sensibly contributing to the environment by: reducing half of the dumping volume of muck, reducing site operations and trucks trips to cut energy consumption equivalent to 54,520 tons of carbon emissions, saving 1.93 km2 of land, and contributing to preserve 1.160 million tons of water.
Conventional tunneling, has also seen notable progress in the development of construction means and methods, allowing an update of the Sequential Excavation Method (SEM) toolbox of support systems for both rock and soil grounds. Moreover, the availability of innovative sprayed waterproofing membranes and the introduction of advanced numerical modeling in the tunnel support design allow, in most cases, the collaboration between primary support of excavation and final liners, thus reducing cost and time of construction. This translates into immediate benefits to the environment, reducing material usage, standardizing processes, improving equipment efficiency, and increased durability of the underground structures.
The design is the most important factor in determining GHG emissions over an underground infrastructure’s lifetime. By the time the construction process begins, most decisions affecting the project’s GHG emissions are locked in.
In urban areas, it is critical to mention that moving structures and infrastructures underground frees up surface space, unlocking land to be used for public events, for green spaces or to provide residential or commercial properties; however, this opportunity requires the definition of legal and commercial frameworks which policymakers and designers must identify early on in the planning stages.
Decisions can have a huge impact on cost, performance, and sustainability early in the project life; therefore, it is important to base all choices on a pre-defined series of principles (an example in Fig. 2) and tools, such as the following:
- Building Information Modelling (BIM) for Sustainability;
- Enhanced structural flexibility for future different uses;
- Parametric design to easily implement multiple options;
- Multi-criteria evaluations and hybrids between disciplines;
- Innovative materials information = easy carbon costing;
- Smart planning and digital tools to minimize waste;
- Carbon costing as a quantitative method to estimate the emissions related to an activity or material.
The Institution of Civil Engineers (ICE) Civil Engineering Standard Method of Measurement, 4th edition (CESMM4) includes CO2 emissions calculations for tunneling and related activities. Trenchless market sector associations, such as the Pipejacking Association, have developed their own specialized tools.
THE RESILIENCE OF UNDERGRUND SPACES
Underground structures have intrinsic resilience thanks to the isolation provided by the soil or rock overburden, protecting from the disastrous events that occur above ground. Take hurricanes, earthquakes, tornados and ballistic threats for example, underground structures typically provide a great resistance to these, but they are not completely protected, even when the entrances are fully sealed. In fact, underground structures do have vulnerabilities for example, it has fatal consequences when an internal fire or explosion occurs, more so than aboveground structures. A fracture of water service pipes during an earthquake (like in Kobe) can also lead to fires that cannot be easily controlled.
Therefore, tunnels resilience should not be taken for granted and it is necessary to understand risks, benefits, and capital / operational impact of the required mitigation measures. Rail and metro systems have excellent resilience in dealing with earthquakes, but with floods or tsunamis the station entrances and tunnel portals make them extremely vulnerable. In the Netherlands, the NoordZuidlijn is built with pen stocks at each end of the submerged tunnel sections. This could be a great solution for the New York metro system, where the vulnerability is very tangible, seen particularly after Hurricane Sandy. In Bangkok the issue with regards to flooding is tackled by bringing the entrance up two to three meters above ground to prevent the metro tunnel from flooding during the Monsoon Season. The SMART tunnel in Kuala Lumpur is designed in such a way that it can interchange between a traffic tunnel and a flood relief tunnel several times a year. As long as the issues are acknowledged, underground project flexibility allows the risks to be tackled and improve the resilience of the system itself and the overall city.
Back in the 1960s, Montreal in Canada embarked on a visionary project from a small initial development, the RÉSO, commonly known as “The Underground City,” has since developed into a network of pedestrian tunnels spanning 33 km in length, connecting hotels, museums, universities, offices, banks, homes, a bus terminal, metro stations, train stations, an amphitheater amongst other buildings allowing 500,000 people to traverse the city underground safe from the inclement weather daily. Other cities worldwide are increasing the use of underground space including Toronto, Houston, Singapore and many others including Hong Kong’s development of the first underground WWTP (waste water treatment plant) freeing up many acres of surface land.
Based on the literature, underground structures resilience can be confirmed by identifying an appropriate list of characteristics which shall make the systems flexible enough to ensure a redundant responsiveness to the threats identified. For example, in Rotterdam the multipurpose parking facility Museumpark in case of heavy rainfall has a water storage basin that helps relieve the city from flooding, thanks to its spatial and functional diversity, thus increasing the flexibility of the whole city. The MAUDC (Metropolitan Area Underground Discharge Channel) in Tokyo was designed for a similar scope but the resulting damages from a critical event would still risk flooding surrounding areas; thus, the City is looking at improving its resilience against major earthquakes. Introducing a Resilience Matrix is a powerful tool as much as it is a Risk Matrix
A FRAMEWORK FOR THE SUSTAINABILITY OF TUNNELING PROJECTS
When transferring the general concept of “sustainability” to underground infrastructures, it is important to make sure a practical approach is offered to all players in the industry.
As decarbonization is rising to the top of the business agenda in this decade, players in the construction industry can have a significant impact. In fact, climate change risks and sustainability are now factored into traditional business approaches by investors and asset managers, most contractors and designers are setting ambitious carbon-reduction targets to modify the way they operate, and several organizations are following guidelines to enhance disclosure and mitigation of climate-related financial risks.
An optimized design process is critical to reduce embodied and operational carbon, as 25% of it is embedded in the construction value chain and can be mitigated through development of new materials (in the last decade over 80 patents have been registered for low carbon cement) and implementation of digital technologies. A successful example is offered by a cloud based intelligent system for fully automated real-time design of tunnel supporting system developed in 2021 by Tongji University. This is one of the first application of artificial intelligence to a tunnel system consisting of:
- Twin model digitalization via multi-view photogrammetry;
- Automatic identification of the geological structure;
- Rock mass quality classification based on cloud algorithms;
- Automatic tunnel support structure design and verification.
The tunnelling industry is not new to the introduction of automation in both design and construction: not only the whole process of excavating and supporting the ground has been industrialized, but at the design stage, top engineering firms offer tools that can create a digital twin of the underground system to verify impacts on operations and optimize geometries and solutions not only to meet a more efficient construction schedule but also to minimize maintenance costs and maximise durability. This is related to both transportation tunnels, where simulation of entire fire events in a subway line can help identifying the most vulnerable areas, and water tunnels, thanks to hydraulic modelling which can quickly inform changes of shafts and tunnels via a parametric design tool.
If design is important to offer sustainable underground spaces, the application of principles of Lean Construction to the tunneling industry is uttermost important to incrementally improve performance and possibly achieve zero waste for:
Studies of the University of Toronto and of the University of British Columbia are currently identifying areas were cost of underground projects can be optimized through data analytics and appropriate models for demand forecast. Tunneling, more so than other types of construction, benefits from repeatable tasks and therefore lends itself to benefit from LEAN philosophies:
- Materials are delivered when and where they are needed;
- The length of the critical path can be minimized;
- Zero defects can be achieved through pre-casting;
- Flexibility in the processes and in the design is allowed;
- Components and methods are standardized (toolboxes);
- Procurement from nearby sources is a state of the practice;
- Selection of the right methods minimizes the re-work;
- Reducing equipment is required due to reduced spaces;
- The ability to control emissions underground is critical;
- Top-down methods allow to reduce surface land needs;
- Shoring systems can approach close to existing structures;
Nowadays, most tunnels across the globe are TBM excavated with other underground excavation methods limited. With TBM technology significantly improved, larger diameter TBM tunnels can also include station platforms and now non-circular shaped TBMs (elliptic, rectangular) are already in use for the construction of shorter tunnels. According to United States Environmental Protection Agency (USEPA) with application of more TBMs, emissions are declining. This reduction proves the fact that the underground systems’ ecological footprint declines and the hope for the future is that Carbon Credits and reduced taxation can be accordingly introduced for the underground space preferred utilization.
In summary, a specific framework needs to be considered to identify key action areas in order to improve the sustainability of underground projects:
- Design for flexibility of the underground spaces;
- Foster materials innovation and re-utilization;
- Standardization and industrialization of processes;
- Equipment optimization and re-cycling;
- Improve resilience towards climate change and future pandemics, thus reducing cost / carbon for reconstruction.
As we navigate through very challenging times where news on climate change and the pandemic is balanced out by the hope for the future. Thanks to new policies and investments, introducing sustainability parameters while evaluating alternative design options for new infrastructures is essential, particularly in our rapidly growing cities. An intrinsic component of this parameter is urban resilience, which should be managed properly and take advantage of underground spaces.
Hurricanes, earthquakes, typhoons, heavy rainfall, heat waves, rising sea levels are only some of the examples of new scenarios to be included in the resiliency verification as part of a new infrastructure planning. Not only underground structures offer constructible solutions to all such threats, but they can achieve it, limiting the environmental footprint, thanks to available technologies across the entire value chain, from planning and design, to construction, to operation and maintenance.
As a state of the practice, nowadays the tunnelling industry supports better project planning, design, and delivery by addressing a broad range of social, economic, and environmental indicators. However, several steps must be followed in the near future if we want to achieve the challenging targets recently confirmed by the COP26:
- technical codes and regulations need to be updated to specify and adopt technological innovations and to create legal frameworks for competitive procurement methods;
- customer demand circularity needs to be implemented by lowering demand for primary resources through design and process optimization, including re-use of materials;
- construction and materials decarbonization need to be supported by fiscal policies and incentives to shift to alternatives that are more energy efficient.
- raw materials emissions must be reduced introducing lean manufacturing, electrification of processes, and increasing closed-loop circularity for materials and components;
- access to capital for technological innovation is key to achieve the ultimate goal of net-zero emissions at an ecosystem level, but abatement costs are not currently standardized globally on a net-present-value (NPV) basis.
The pathway towards a sustainable resilient future of our infrastructures has been laid out. However, no single player can capture the large opportunities alone, thus the most important mission for all players in the tunnelling industry is now to improve communication and collaboration across the value chain, keeping in mind the vision of a greater good for all humankind.
Giuseppe Maria Gaspari, PEng, MBA, MSc, MSE, is Associate Vice President and Tunnel Practice Lead at AECOM, Toronto, Canada.