ABSTRACT
The necessary work that the civil engineering sector must undertake to improve the sustainability of infrastructures cannot overlook a more conscientious and attentive design with regard to the various impacts of the project. In mechanized tunnelling, the annular gap formed outside the tunnel lining is filled with mortar. Even though the mortar contains much less cement than concrete, it is usually injected in very large quantities and therefore has a significant environmental impact in terms of CO2 emissions. These emissions can and should be reduced. Furthermore, the backfilling is usually in contact with the surrounding groundwater, which could be affected by the release of alkaline compounds from cement hydration. The tender documents for the E6 Clear Water Tunnel, part of the New Water Supply Oslo project, are an exemplary case where the technical specifications for the backfill grout were provided based on both the physical-mechanical properties of the fresh and hardened grout and its environmental profile in terms of carbon footprint and pH. This paper presents the research conducted by GEEG on behalf of Ghella to meet these requirements. The main objective was to identify possible mix designs for a backfill grout that would achieve high performance in terms of strength, low carbon footprint and low leachate pH. Keeping these targets in mind, the experimental activity was designed and carried out by applying innovative products and alternative solutions. Several mix designs were tested using cement-free binders or alternative cement and binder mixtures to investigate the effectiveness of additional cementitious materials such as slag and silica fume. All proposed solutions were able to achieve satisfactory performances in terms of strength and pH values of the sample leach water. The carbon footprint of each tested mix shows low CO2 emissions both by completely and partially replacing cement with other binders.
