Reuse of waste mud from Slurry Shield TBM in two-component backfilling grout: A preliminary feasibility study

The sustainability of underground structures has gained increasing importance in recent years. In this context, the optimization of materials and excavation techniques plays a key role, particularly in mechanized tunnelling. Given more sustainable tunnel construction, this research explores the potential reuse of bentonite from waste mud of slurry shields as a component of backfilling grout, tested at the laboratory scale. Indeed, two-component grout, widely used for backfilling in mechanized tunnelling, typically requires a significant amount of bentonite, which could be sourced from treated waste rather than fresh material. A preliminary testing campaign was conducted to evaluate samples in which the standard bentonite was replaced both with the bentonite usually used in SS-TBM operations and the exhausted bentonite. The exhausted bentonite was directly employed in the production of the two-component grout. A series of laboratory tests were carried out on component A, gel time, and compressive strength of the hardened grout. The results were then compared to those obtained with a standard mix design. Preliminary findings suggest that the proposed grout, produced with waste mud, can achieve performance levels comparable to those of traditional formulations and standard bentonite. However, an increase in both the density and viscosity of component A was observed. While these initial results are promising, further investigation is required to validate the integration of recycled slurry into industrial-scale applications.