From the ornamental stone production chain to tunnelling: Marble sludge and gneiss powder in two-component grout applications in the view of a sustainable backfilling phase

Sustainability is becoming more and more important in engineering works, and the underground world is also involved. When shield machines are used, the backfilling phase plays a crucial role in lining stabilisation and surface settlement control, and it is commonly carried out using the two-component grout technology. This study presents the outcomes of extensive research carried out at Politecnico di Torino, aimed at enhancing the sustainability of the backfilling process using marble sludge and gneiss powder as alternative constituents for component A of the grout. The research was conducted in two phases. The first focused on identifying optimal mix designs and characterizing the grouts according to established standards from the scientific literature. The second involved testing grout samples produced using a specially developed injection prototype, designed to replicate the actual injection system of tunnel boring machines. The results underscore the environmental advantages of utilising these alternative materials and highlight the critical need for simulating the injection process with the injection prototype. Since these materials have never been tested before in this context, their behaviour within the backfilling system should be studied in deep and it is essential to validate the material performance through a rigorous mixing and injection simulation to ensure reliability and effectiveness of the results.