Environmentally friendly soil conditioning for EPB TBMs — The state of environmental considerations around the world

Environmental considerations are widely present in the modern world, and there is a global push in most industries for sustainability and environmental consciousness. In the tunneling market, many projects being built are intended to benefit the environment. One critical aspect of tunnel projects that must be considered is muck handling and disposal. Tunnel boring machines (TBMs) generate a lot of muck. Earth pressure balance (EPB) machines are one of the most common TBMs, especially in urban areas. Chemical products such as foaming agents and polymers are added to the soil to allow for the safe and effective operation of EPB machines. When it exits the TBM the muck produced is laden with these soil-conditioning chemicals. These residual chemicals may pose an environmental impact if improperly managed. Special disposal restrictions on EPB muck can add logistical challenges and increase the overall cost of a project, as acceptable disposal sites may be farther away with the possibility of more expensive disposal fees. Several countries in Europe have developed programs intended to minimize the environmental impact of tunnel muck. As a direct response, MAPEI has developed innovative new foaming agents with significantly lower ecotoxicity and higher biodegradability, and their use has been widely adopted in these countries. Many localities around the world have limits for substances such as heavy metals and hydrocarbons but these are not used in the formulation of foaming agents, while other potentially harmful ingredients common in foaming agent formulations like alcohol and glycol remain unregulated. Soil-conditioning overview With the addition of water, air and special soil conditioning agents, including foams and/or polymers, soil conditioning is used to modify the characteristics of the ground, making it more suitable for tunneling with EPB TBMs. Well-conditioned soil improves face stability, reduces wear on machines, improves the flow of material out of the chamber, increases productivity and reduces downtime. Proper soil conditioning is critical to the success of any EPB TBM project. Foaming agents. The most common type of soil conditioners are foaming agents. They are concentrated solutions that are first mixed with water and then passed through a foam generator with the addition of air to create a foam. The foam is injected into the cutting head, mixing chamber and/or screw conveyor. There are several parameters of the soil conditioning that may be adjusted including: Concentration (cf) — the volumetric ratio of foaming agent to water. Foam expansion ratio (FER) — the volumetric ratio of air to foam solution. Foam injection ratio (FIR) — the volumetric ratio of foam to soil. On an EPB TBM, the operator of the soil-conditioning system will adjust these parameters as needed for the specific ground conditions. Composition of foaming agents. Foaming agents are typically formulated with the following ingredients: Surfactants. Polymers. Solvents. Surfactants are the main component of most soil conditioners. They lower the surface tension of water to aid in the generation of foam. Different foaming agents may contain different types of surfactants, such as cationic, anionic and others. Polymers are included in foaming agents to enhance the technical properties of the foam, such as half-life, lubrication and dispersion characteristics. Polymers may be of either synthetic or natural origin. Solvents are used in some foaming agents and include chemicals like alcohols, glycols, dichloromethane and more. Although solvents, if present, are typically present in low quantities, they are not rapidly degradable and are toxic to the environment. Technical aspects. With the proper balance of ingredients, foaming agents are designed to meet technical requirements for excavating with EPB TBMs. In choosing which soil conditioners to use, preliminary analysis and testing is typically done before the start of the project. First, relevant project information is analyzed, including review of TBM characteristics, tunnel alignment, geotechnical baseline report (GBR), hydrological aspects of the tunnel route and past projects in the area. Next, preliminary laboratory testing is performed with soil samples acquired along the alignment, typically from investigative borings. Testing done on the natural soil include moisture content, density, grain size distribution and Atterberg limits. Next, soil conditioning agents are mixed with the soil, which is then tested for slump, flow, visual consistency, stickiness and impermeability. Soil-conditioning parameters and doses are adjusted until the soil exhibits behavior ideal for EPB tunneling. These tests are useful to find the most suitable products for conditioning, and to determine the initial soil conditioning parameters. Once tunneling begins, the soil conditioning parameters are optimized on the TBM. As the geology changes along the alignment, the parameters are further adjusted and refined. Technical performance is a critical aspect of soil conditioners, but as the construction industry moves to more environmentally conscious practices, the impact of soil conditioners must be considered. TABLE 1 Muck utilization suitability chart. Environmental impact of soil conditioning A tunnel project can create an enormous amount of spoil. As an example, a tunnel 10 km (6 miles) long with a diameter of 10 m (33 ft) would require the removal of approximately 1,000,000 cu yd of soil. Conditioning this soil for excavation would require 757,000 to 908,000 L (200,000 to 240,000 gal) of foaming agent, given typical dosing. Management of muck. Muck is often thought of as waste that needs disposal, but in the right conditions, muck can be a valuable resource. While excavated material from hard-rock tunnels is widely considered the most valuable and easiest to process type of spoil, soft-ground muck can be valuable too, and engineers and contractors are continually looking for ways to better utilize softground spoils. In a 1977 handbook sponsored by the U.S. Department of Transportation titled “Muck Utilization Planning,” many of the possible uses for tunnel muck were considered (see Table 1). Table 1 summarizes information from the publication and demonstrates some of the best uses for different types of soft-ground tunnel muck. Today, these various uses remain relevant to softground muck; however, it is important to note that the handbook was published before EPB TBMs became popular or their use in tunneling became widespread. The presence of soil conditioners and how they would affect the muck was not considered. Today, research programs like the DRAGON Project (a collaboration between the governments of Austria, France, Germany, Switzerland and the UK) and the “Re-Muck” program in Italy continue to push for efficient reuse of muck as a resource (International Tunneling and Underground Space Association ITA report No. 21, April 2019). These programs envision a more sustainable future of tunneling. Rather than disposing of tunnel muck in a landfill, these programs advocate for repurposing and reuse. Efficient use and recycling of tunnel muck can reduce the need for mining of mineral resources while sending less material to landfills. Often, carbon emissions can be reduced by minimizing material handling and transportation. Environmentally friendly soil conditioners play a critical role in this process, allowing for more options and flexibility in the reuse of EPB TBM muck. Development of environmentally friendly soil conditioners Many manufacturers are working to develop soil conditioners that are less harmful to the environment. With large investments in research and development, MAPEI has developed one such new product line of foaming agents called Polyfoamer ECO. FIG. 1 EC50 to Daphnia magna, comparison of Polyfoamer ECO line to traditional foaming agents. FIG. 2 LC50 to Danio rerio, comparison of Polyfoamer ECO line to traditional foaming agents. FIG. 3 IC50 to green algae, comparison of Polyfoamer ECO line to traditional foaming agents. These new foaming agents are based on biodegradable anionic surfactants in combination with a natural biodegradable polymer. They contain no glycols or other solvents with low biodegradability (common to many traditional foaming agents). While significantly less harmful to the environment, with minimum ecotoxicity and high biodegradability, the foaming agent maintains a high technical performance standard, comparable to that of traditional foaming agents. Environmental impact. When considering the environmental impact of soil conditioners, criteria for comparison testing must be defined. Two important environmental benchmarks for soil conditioners are ecotoxicity and biodegradability. Ecotoxicity. Ecotoxicity is defined as the amount of a chemical product required to have an adverse effect on populations of living organisms exposed to it. Exposure testing is done on a variety of aquatic and terrestrial organisms. Adverse effects are recorded including lethality, inhibition of growth, movements or reproduction. The ecotoxicity of the newly developed foaming agents is far lower than those of traditional foaming agents. Figures 1, 2 and 3 show comparisons with selected soil conditioners’ toxicities to the planktonic crustacean Daphnia magna, fish Dario rerio, and green algae, respectively. The toxicity toward Daphnia magna (Fig. 1) is measured according to the Organization for Economic Cooperation and Development (OECD) Test No. 202, and the values registered are EC50, the concentration of each product at which 50 percent of the initial population of Daphnia magna is immobilized in 48 hours. The toxicity toward Danio rerio (Fig. 2) is measured according to OECD Test No. 203, and the values registered are LC50, the lethal concentration of each product at which 50 percent of the population is killed in 96 hours. The toxicity toward green algae (Fig. 3) is measured according to OECD Test No. 201, and the values registered are IC50, the inhibitory concentration of the test substance that results in 50 percent inhibition of growth rate of the green algae in 72 hours. These figures demonstrate that the new foaming agent is 10 to 15 times less toxic toward these living organisms than traditional foaming agents. Biodegradability. Biodegradability is the ability of a material to break down over time. Biodegradability of chemicals is typically evaluated considering the chemical oxygen demand (COD) and the biochemical oxygen demand (BOD), which measures how much oxygen is consumed to degrade the organic content. The lower the organic content of the foaming agent introduced to the soil, the lower the oxygen demand will be. The state of environmental considerations. Around the world, no one standard exists for quantifying the environmental impact of tunnel spoils or the soil conditioners contained in them. When considering the environmental impact of soil conditioners, much of the world has some catching up to do. The most common testing done on tunnel muck does not consider soil-conditioning agents. For example, European guidelines 2008/98/CE establish a test known as lixiviation, which determines the concentration of several harmful substances that may present in soil. Most are naturally occurring in trace quantities, and include heavy metals like lead, cadmium and mercury, as well as hydrocarbons. However, since soil conditioners do not contain these materials, the environmental impact of soil conditioners themselves is not considered or evaluated. Projects in the United States, like many other countries around the world, generally do not consider biodegradability or ecotoxicity of soil conditioners present in muck. Limitations are typically related to hydrocarbons and heavy metals only. Because regulations do not exist, traditional soil-conditioning agents are widely used, even those with a high dose of solvents that may be particularly harmful to the environment. On the other hand, countries such as Italy, Denmark, the UK and others use a holistic, case-by-case process. In practice, testing of soil conditioners begins in the design phase of a project. First, the biodegradability and ecotoxicity of potential soil conditioning agents are evaluated alone in their pure state at an independent laboratory. This establishes baseline values for the soilconditioning products that are being considered. In the next phase, laboratory work begins with soil samples from the tunnel, where soil conditioning parameters and dosages are determined and the conditioned soil is evaluated for both technical attributes and environmental aspects (ecotoxicity and biodegradability). Finally, as the excavation begins, and throughout the course of the job, regular tests are conducted on the muck to ensure compliance with biodegradability and ecotoxicity guidelines established by respective environmental regulatory agencies. Once the muck leaves the tunnel, it must be stockpiled until it has been deemed safe for transport for final disposal or reuse. Tests on the conditioned soil are carried out at different times (after one day, three days, seven days, and so on) to measure biodegradation. Curves can then be plotted to determine when the muck has reached an acceptable threshold for biodegradation and ecotoxicity. This way, muck conditioned with rapidly biodegradable, low-toxicity foaming agent is able to pass the tests for removal from the site in shorter time. Reducing this waiting time through the use of environmentally friendly soil conditioners is of particular benefit to jobs in urban areas with small site footprints. Conclusion Excavated material disposal is a critical part of any tunneling job and is especially important for EPB TBMs. In general, soft-ground spoils are less desirable for reuse than rock. In addition, the chemicals present in the conventional EPB muck make disposal even more difficult, and it is not uncommon for projects to be negatively impacted due to the limited muck disposal options and cost. Global regulations, protocols or standards should be studied and instituted for TBM muck management. The use of environmentally friendly soil conditioners is not only about respect for the environment and sustainability, but may also reduce project costs. In some cases, the use of environmentally friendly soil conditioners may allow reuse of the excavated soil, while also avoiding longer trucking distances needed for special muck disposal. Several miles of EPB TBM tunnels have already been completed using new environmentally friendly soil conditioners. This demonstrates that ecofriendly soil conditioners bring an exceptional opportunity because they are formulated with natural ingredients that provide the high technical performance of traditional foaming agents, but greatly decrease the impact of EPB TBM muck on the environment. References Dal Negro, E., Boscaro, A., & Barbero, E. (2019). Management of the muck produced by EPB TBM tunnel excavation: Environmental aspects and consistency control for efficient handling. In Tunnels and underground cities, Engineering and Innovation Meet Archaeology, Architecture and art: Proceedings of the WTC 2019 ITA- AITES World Tunnel Congress (WTC 2019), May 3–9, 2019, Naples, Italy. essay, CRC Press. Dal Negro, E., Boscaro, A., Barbero, E., Stefanoni, M., & Darras, J., (2020). Environmental aspects of mechanized tunnelling with EPB-TBM: chemical products for soil conditioning and management of the muck produced during the excavation. AFTES International Congress 2021, Paris (FR), September 6–8, 2021. International Tunnelling Association Working Groups 14 and 15 Underground Construction and the Environment and Mechanized Tunnelling. (2019). Handling, Treatment and Disposal of Tunnel Spoil Materials. ITA report N°21 April 2019. Liu, T.K., Gifford, D.G., Dugan, J.P. (1977). Muck Utilization Planning Urban Transportation Tunneling: A handbook of rational practices for planners and designers. Haley & Aldrich, Inc. OECD Guideline for testing of chemicals. ENV/JM/ TG(2005)5/REV1. (2005)

Environmentally friendly soil conditioning for EPB TBMs — The state of environmental considerations around the world

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