Long-term characterization of innovative backfilling grout for mechanized 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 adopted the ultrasonic pulse velocity (UPV) method to estimate at laboratory scale the dynamic elastic parameters of an innovative backfilling grout that has a mortar stabilizer agent different from bentonite. The geophysical campaign aimed at retrieving the dynamic elastic and shear moduli and Poisson’s ratio in the long term, that is, from one week until one year after the casting, to ensure that the grout obeyed with time the technical specifications of a given project or regulation. The results of our analysis demonstrated that, after one year of curing time, the temporal variation was moderate for the dynamic moduli and negligible for the Poisson’s ratio. The most significant variation of the inspected parameters was appreciated between 7 and 14 days of curing time, while a flat trend was observed between 28 and 90 days of curing time. Future work will be devoted to the long-term analysis of cement-free two-component grouts with added bentonite.