Enhancing Sustainability of Underground Works

As designers in the field of infrastructure and underground works, sustainability is always part of the work, for example, the concrete codes world over are taking a design life of 50 years into account, with infrastructure works having a design life of 100 years and more. That a design life of 100 years does not necessarily mean that a structure will be standing for 100 years without due care and maintenance, is understandable. However, it means that the designer has taken all due care and provisions to ensure that once executed (according to the specifications), the structure will be able to withstand the foreseen impacts coming over 100 years. Apart from that (and the execution quality), the choice of systems and materials affecting maintainability of the structure plays a key role in ingraining sustainability into the design. In addition to sustainability, the focus on reducing the carbon footprint has increased substantially.

Introduction

Infrastructure is going underground due to various reasons and is perceived to be costly. This may be true for the investment cost, which is scrutinized anyway and in many cases a calculation of the monetary "benefit” of a measure is calculated through IRR (internal rate of return). This may define economic sustainability but there are other parameters that may define sustainability, as the IRR has to take into account cost, also of services and goods (land use!) which may, over the lifetime of a project, become scarcer and more costly than initially assumed; or certain opportunity cost is usually not considered in the analysis. The environmental impact of a construction project by measuring its carbon footprint is relatively new, and slowly gathering importance.

So, sustainability can be allocated in different places and phases of a project and should be looked into. This starts with alignment choices and their impact; then comes the construction method, the design and implementation approach together with the requirements on a certain structure, the used materials in terms of quality and quantity, and finally the maintenance requirements of a structure.

Having laid down these different aspects, the author is of the opinion that many of these issues can be addressed with common sense, proper engineering and execution, and further improved by the introduction of new materials. The identification of waste – defined here as material and manpower input used with no benefit for the structure’s performance – seems also a very important issue to tackle, as well as the identification and quantification of risks, together with their monetary consequences.

This is a big challenge for all stakeholders – Owners, Concessionaires, Contractors, Engineers, and Designers. The approach is to look at infrastructure projects as a long-term solution for improvement of a situation.

Concept Phase

The Owner and his Consultant are the major players here; the Owner may be working on political decisions and goals. As outlined earlier, sustainability is linked to long-term usability of a structure; this may create a conflict between the solution that may look appreciable and quickly implementable, and a solution that may take longer for its implementation and has other long-term benefits.

Let us take a fictious example – the design of an alignment for a highway. The boundary conditions are construction cost, time, and land use in an ecologically sensitive area. The objective is to improve the existing two-lane road system to a highway with 4 lanes to cater to future traffic flows.

The general options are as follows:

1. Going with a bridge – tunnel alignment, trying to have the minimum allowable length by not exceeding the maximum gradient; one condition is to have the tunnel length limited and therefore several tunnels.
2. Going with a long tunnel alignment, eliminating also the bridges.
3. Going with an alignment that has a minimum of tunneling and follows the terrain.

If considering construction cost and time, it looks tempting to go for option 3 – following the terrain and keeping tunneling at a minimum. What is the consequence? Following the terrain means going along the slope, and to get the required space cuts (moving towards the hill) or elevated structures (moving towards the valley) are required. The questions here are whether bridge-type structures which are founded in the slope (bedrock level) are more cost-effective than tunnels, and whether slopes are more cost-effective than a tunnel, looking at the sustainability.

The monsoon season with its significant rainfall requires provisions in terms of slope stabilization – so either there has to be an additional investment or the risk of impairment of the road usability, including major damage to the road. In short, in many cases, an unsupported cut is not a sustainable solution as it has a high probability of failure.

The next comparison is whether a long tunnel or several tunnels with bridges in between (depending on the terrain also) is the more sustainable solution. Since 4 lanes are considered, the tunnels will be a twin tube system with cross passages – regardless of length. So, the sustainability questions will come for different angles, as we assume that tunnel maintenance effort is the same per km and year. Considerations would be as follows:

• Construction time: for a long twin tube tunnel without intermediate adit there is always 4 faces, whereas (say) 3-4 short tunnels offer more faces. Consequently, the construction time and cost can be reduced.
• Downtime during maintenance or at incidents: with the same maintenance requirement, the interventions when done in shorter tunnels may have lesser impact than interventions in the long single tunnel.
• Operational cost: the installed power required for ventilation will (most probably) be more in the long tunnel, as the losses are not in a linear, but exponential relationship based on the length.
• Cost of the bridges: eliminated for the long tunnel (so some cost shifting). Looking at the above points, finding the more sustainable solution will require an in-depth study which must also look into the priorities of the Owner and the criticality of the availability of the structures. However, the following principles have to be pointed out for tunnelling and slope modification works, which are nearly always applicable:
• Go for the fresh: the closer to the surface, the higher the weathering would be (exception – laterite!). It also means that searching for the minimum overburden is not always beneficial.
• Less disturbance means less support: a just stable slope which is cut along the alignment will require support and attention to the full extent. Slope bridges will require attention at their foundations and a tunnel will require attention at its portals and initial drive.

The concept phase is the foundation stone to a sustainable structure, and, in infrastructure, the considerations need to be from start to end of the alignment with the whole set of proposed measures along the alignment. Isolated analysis for single structures may lead to neglecting other parts of the alignment, which may create weak spots.

Investigation and Design phase

Once the concept is approved by the Owner and the basic parameters and boundary conditions are agreed to, the design phase starts. In this phase the structures are defined in detail, the proposed construction methodology is refined and reviewed, and the ground investigations (GI) are done. Proper ground and ground water investigations are part and parcel of a sustainable design. The earlier all parties are prepared (for e.g. aggressive environmental conditions), the more options can be explored on time. A lot has been deliberated already on the necessity of proper ground investigation programs, so it will not be touched in this paper - the successful implementation of many underground metro projects is already proof.

In the design phase, the general design and implementation approach is defined, and material choices are made, both of which have an impact on the sustainability of the works. These choices not only depend on the designer, in many cases the readiness of the Owner to implement new technologies and approaches is a key driver.

Steel Fibre Reinforced Concrete

One recent example is the driving force of Delhi Metro Corporation behind the publishing of a guideline for the use of Steel Fibre Reinforced Concrete (SFRC). The replacement of rebar cages with steel fibers has several benefits in terms of durability, reduction of steel use, and simplification of the production process. Since the Client is now ready to employ this technology, the Designers are nudged to use it; earlier, the Designers had to justify their material choice and, at times, the Client accepted such proposals with a transfer of risk to the Contractor and his Designer. Needless to say, such risk transfer was not accepted by the Contractors and therefore such proposals were hardly implemented.

Un-reinforced Inner Linings

A perfect example for a design opportunity towards increased sustainability is the inner lining design approach. A modern tunnel should have a rounded cross section, at least in the overt. The obvious fact that it is most efficient to load a material that has high compressive strength (essentially in compression) shall then be the guiding principle for the design.

The ongoing trend that inner linings have to be designed with systematic reinforcement as bending is becoming the governing load case, or some Clients have chosen this due to whatever reason that a cast in-place inner lining is to be treated like a “short column” with corresponding minimum reinforcement, which is not a progress but a setback in terms of sustainability and cost efficiency in design.

It is proposed to have the simple check from Strength of Materials, which basically says that in case the resulting force is within the core of the cross section, the material is under compression only; means for plain cement concrete that no reinforcement is required. This in turn reduces the risk of puncturing of the waterproofing membrane and enhances the durability, as the absence of steel bars means no corrosion issue. It needs to be mentioned here that the readiness of some Clients to go for Steel Fibre Reinforced Concrete for cast-in-place inner linings is already taking the development to the right direction.

Single Shell and Composite Linings

The use of a single shell precast element lining is state of the art for mechanized tunnelling and is in itself a solution that is definitely making the best use of the materials employed. However, not every tunnel can be done with a circular cross section and therefore sequentially driven tunnels (NATM) will still be constructed.

The conventional approach is to have a double shell system, consisting of a primary support, a waterproofing layer and the inner lining. This leads (in many cases) to a combined support thickness which can reach more than 50 centimeters, with each layer being designed to take the ground load and over and above – depending on the applicable Employers Requirements – each layer being designed and checked as per the rules for structural concrete.

In the first design phase, assessment of the requirements, and going one step further, the in-depth check whether the support quantities can be reduced without losing the durability and serviceability of the structure, is necessary. The most radical approach would be to design the lining as one layer; in challenging geological conditions this will not be applicable, however, in good geological boundary conditions the integration of the functionality of the final lining into the primary support is doable.

An approach which has seen some support from the industry is the composite lining approach – in short to “upgrade” the primary lining (first layer of Sprayed Concrete) with a second layer to achieve the factors of safety required in permanent use, together with the durability. Between these layers, the waterproofing in the form of a membrane (sprayed and with full bonding) may be placed. The primary lining is then not a “temporary” support but a part of the permanent works. In such cases also the “composition” of the primary support needs to be looked into. The standard solution of lattice girder and wiremesh might need to be replaced with solutions that reduce any spraying defects to a minimum, to ensure the durability of the first layer of support.

At this point also the application of the inner lining comes into play. A cast in situ lining requires formwork which is not very flexible in terms of changing geometry. On the other hand, the inner lining design assumes a minimum thickness. So, any larger excavation profile in a tunnel that is being blasted, will naturally be filled with concrete. It has been experienced in many cases that the inner lining with the theoretical / minimum thickness of 30cm exhibited a thickness up to 50cm, which is 67% more (!) concrete. If such cast-in-place linings are replaced by Sprayed Permanent Linings (with prior survey of the surface) there is more flexibility to apply the thickness required and keeping the additional material use in check. The efforts to achieve an adequate finish (surface smoothness) need to be included in the overall assessment.

Reducing Cement for Reducing the CO2 Footprint

Apart from the reduction of steel reinforcement where possible – and the possibilities increase when the design approaches allow for it – the concrete mix design and the cement as one main ingredient can be tackled. Especially so-called “non-cementious binders” that replace the Portland cement promise to reduce the CO2 footprint substantially. Compared to solutions outlined above, these materials are yet to be developed for general consumption and represent the mid-term look ahead.

Tendering Phase

The tender conditions in many projects do not consider sustainability in the sense that the overall environmental impact is assessed and weighed. Since the overall CO2 footprint is becoming a commercial topic, an introduction into the tender conditions is debatable and may provide financial incentives towards emission avoidance and what is called carbon neutrality.

Tender Conditions and Flexibility

One thing that is to be reviewed by the Client and the Consultant who does the tender design and is involved in the tender documents are possible and unintended incentives for waste; such incentives are to be eliminated.

Secondly, the local conditions, especially in terms of availability of bulk materials, shall be assessed (if not yet done in a prior phase) and alternative sourcing or materials shall be defined. This also implies that the tender shall give the Contractor the opportunity to go for alternatives and shall also clearly define the requirements to make such alternatives acceptable. Nothing is more discouraging for a Contractor than to develop new solutions and being sent into approval loops as the approval requirements were not clearly defined in the Contract itself, and one risk-averse stakeholder increasing the approval requirements with every round in absentia of clear rules. The same is applicable to a Client who wants to implement new technologies but faces resistance.

Execution Plans

In large infrastructure projects – also underground projects – the existing infrastructure is on the one hand used for logistics and on the other hand also disrupted by the construction activities and their space requirements. The assessment of different options for the execution and their impact on the public life (e.g. traffic disruptions) and a financial recognition of the same in the tender may, on the one hand, put emphasis on the planning and reward the same, and, on the other hand, may be an incentive to keep disruptions at a minimum. The logic behind the argument is that a disruption is increasing costs for all involved and additional CO2 load – so it compromises the objective of sustainability. At the same time, the unavoidable disruptions need to be assessed and put in as a benchmark.

Financial Recognition

Finally, the matter comes down to financial incentives, which can be taken or be lost, and it is the Client who can exert his discretion at the time of formulation of the conditions of contract. The monetary recognition of fulfilling sustainability goals is important, especially when the business environment is competitive and the L1 principle is prevailing.

A transparent mechanism that has clear goals and limits inside the contract means also that the application of outdated and non-sustainable practices has a (hefty) price tag and will be adjusted from the remuneration. Like the development of the integration of the Geological Baseline Report (GBR) as part of the risk sharing mechanism between Client and Contractor in underground works, a monetary mechanism for sustainability based on proper objectives has to be developed; this mechanism shall know reward and penalty.

Execution Phase

After having set goals, having gone through an exhaustive design process and having awarded a contract with clear cut objectives and their incentive tags, the stakeholders are set for execution. In many cases, a Detailed Design Consultant is part of the Contractor’s team to work out the details and develop the tender design further, so this work will get some attention and the ideas coming below will be based on the assumption that the Detailed Design is with the (EPC) Contractor. Then the Contractor’s implementation plans are important as they may have an impact on the goals formulated in the contract and the execution itself carries the potential to meet or miss the sustainability goals.

Detailed Design and its Approval

The common setup in infrastructure projects is that the Contractor engages a Detailed Design Consultant (DDC) who takes up the task to deliver the documents that allow for the construction of the project. This means observance of all codal provisions and other requirements given by the Client while optimizing the design for the execution. In many cases, the Contractor has to engage a Lead Design Checker (LDC) or “Independent” Third Party Checker, who checks the designs coming from the DDC and gives his concurrence. This four-eyes principle is useful, however, both the DDC and the LDC need to have a common ground and the roles must be clear, or dissent may hamper the process.

The Client usually also has his representative as a General Consultant (GC) or a Project Management Consultant (PMC), an Authority Engineer (AE) or any other setup. It may be noted here that the entity taking the role of the “Engineer” as defined in FIDIC is appointed by the Client and takes over defined responsibilities from the Client. Ideally the Client then shall not interfere with the Engineer’s decisions for the delegated responsibilities.

Whichever setup is chosen (one of) the Client’s Representatives or the Engineer will also review the Design submitted by the Contractor and will comment on the same so that finally the design can be implemented without any objection (Notice of No Objection). Here again, the alignment of the objectives shall happen under strict observation of the objectives given in the tender, as these were the ones the Contractor was bidding for. Also, the approach to non-codified standard practices and codal interpretations need to be addressed here, so that objectives will be met without major quantity changes, as any increase building material quantities means increase in CO2 footprint.

Implementation Planning

The implementation plan for a large infrastructure project usually affects the public at large, and it may be argued that not addressing, for example, traffic diversions, jams, or traffic disruptions is a matter of sustainable project implementation. If not already formulated in the Contract, it is the Contractor who needs to present his planning to the Client and the Client’s representatives. At times, a short but drastic solution (e.g. road closure) may have benefits over a long dragging compromise, and its proper planning can benefit the overall sustainability goal. Here comes in the designer also as the construction sequence may be affected and so also the (temporary) works design. In the positive sense, the Designer and the Contractor can contribute with innovative solutions to reduce the impact of construction.

The idea to have a benchmark in terms of impact on the public, will require manpower to be implemented and there might be costs associated with that. However, once the opportunity cost of avoidable disruptions is assessed, it is a balancing calculation.

Quality Control

Several points mentioned above as objectives in the design improvement, like the use of composite liners, require that whatever is on paper is also executed. In other words, without achieving the specified strength and durability, the design is compromised, and instead of having optimized, the contrary may happen, and we end up with a structure that does not fulfill even the basic requirements.

In underground works, we work with the material “ground”, and we realistically cannot excavate the theoretical excavation line as we need allowances and tolerances. On the one hand, this topic can and should be addressed by the blasting and excavation technology employed, and on the other hand its impact should be quantified by measurements, which allow to define mitigation measures to avoid additional material to be required.

In underground projects, Sprayed Concrete is widely employed and therefore the quality of the same needs to be ensured. This starts with the logistics (transportation time and also admixtures to control the setting process) and ends with the application, which requires proper equipment and well-trained people. Sprayed Concrete rebound is there, the amount defines the sustainability of the work; and again the cost to use admixtures together with the cost of training for the operators needs to be set off against the overall cost of rebound.

Conclusion

This paper is not meant as a report but a stimulus for the stakeholders in a project to gather and exchange ideas where improvements can be made in the short-term, mid-term and long-term. The scientific research in the field of construction materials will be a key driver of sustainability, together with the efficient design by the Designers and the Contractors’ constant improvement of execution quality. The planning and design have a major role in the implementation process, which attracts costs, however, if the opportunity cost is monetized, a comparison is possible, and will give the stakeholders a useful tool to assess the overall impact and cost.

References

Golser, J.; Friess, J.; Luniaczek, T. (2022) CO2 Reduktion im Tunnelbau aus der Sicht der Planung und Umsetzung beim Bau. Geomechanik und Tunnelbau 15, H. 6, S. 792–798. https://doi.org/10.1002/geot.202200045