Abstract
Mechanized excavation of tunnels with Earth Pressure Balance-Tunnel Boring Machines requires the use of foaming agents. The latter contain the anionic surfactant sodium lauryl ether sulphate (SLES) as the main compound. The re-use as a by-product of excavated soil containing foaming agents (spoil material) can pose a risk for soil and particularly for aquatic ecosystems if they are close to the spoil material final destination site. This work reports the chemical results (SLES residual concentrations) and ecotoxicological effects (battery of five tests) of 28 day-mesocosm studies performed at a tunnelling construction site. The soil mesocosms were set up with two different lithologies, which contained four different foaming agent products at the highest amounts used for excavation. The decrease in SLES concentrations and the ecotoxicological tests were performed in soil and its water extract (elutriate) at different times (0, 7, 14, 28 d). Elutriates were prepared in order to simulate a possible SLES leaching from soil to water. The results showed a decrease in SLES over time and different ecotoxicological responses depending not only on the initial amount of each product, but also on the soil lithology and organism tested (aquatic or terrestrial). This study showed how only site-specific ecotoxicological evaluations can ensure a safe management of the spoil material, making possible the re-use of soil and avoiding production of waste.
Highlights
The use of millions of tons of excavated soil as a by-product can be a unique opportunity for recycling a useful resource for various purposes, in line with the circular economy [1,2,3]
The data made it possible to calculate the effective concentrations of the foaming agent products
The highest EC20 (5.75 mg/L) and EC50 (17.86 g/L) values were found for P3, demonstrating it was less toxic than the others
Summary
The use of millions of tons of excavated soil (spoil material) as a by-product can be a unique opportunity for recycling a useful resource for various purposes, in line with the circular economy [1,2,3] It is handled in different ways worldwide. Some recent publications have posed the question of the re-use of spoil material from mechanized tunnelling [6,7,8,9,10,11,12], since high amounts are produced at each excavation site They highlighted that the final destination of this material needs to be considered with care, in particular to protect the water compartment. The storage of spoil material in temporary deposit areas for several days before its final use can be useful for ensuring soil drying and degradation of any chemicals (e.g., those contained in foaming agents) used for soil conditioning during tunnelling [16,17]
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