Characterisation of Innovative Backfilling Grout Containing by-Products for Mechanised Tunnelling via Ultrasonic Pulse Velocity

Building tunnels with the shield machines involves the adoption of innovative materials that harden fast while the machine advances. To date, the fastest material used in tunnelling for backfilling is the two-component grout but its direct characterization at the construction site may result unfeasible. We present the ultrasonic pulse velocity (UPV) test campaign carried out on three new two-component grouts that incorporate waste material from industrial processes as aggregate. By partially replacing the cement with by-products as inert material, our approach not only promotes the reuse of useless material obtained as secondary product from the ornamental stones cutting process but also minimizes land requirements for disposal and reduces CO₂ emissions associated with cement production. We assessed the elastic behavior of the new grouts from one hour to 28 days after the casting and compared the results with those obtained from a standard two-component grout mix design. The results of our analysis demonstrated that at 28 days from casting the dynamic elastic and shear moduli have higher values than those of the standard mix design.