Evaluation of the time course of the mechanical parameters of conditioned cohesionless soils

In the domain of mechanized tunneling, a substantial portion of the market is occupied by Earth Pressure Balance-Tunnel Boring Machines (EPB-TBM). Over recent decades, this technology has gained popularity due to several constraints such as limited space on the surface, noise and vibration concerns, and safety requirements. Ensuring efficient EPB-TBM operation necessitates a radical alteration in the rheological characteristics of excavated soil, achieving through soil conditioning. Soil conditioning is an indispensable aspect of EPB-TBM technology because it provides suitable soil for better pressure control within the bulk chamber and soil extraction with the screw conveyor. Furthermore, soil conditioning reduces both the internal friction angle and soil cohesion while enhancing material workability. This transformation is achieved by introducing additives, such as foam, at the excavation face and within the bulk chamber, thus altering the natural soil properties from solid-like to fluid-like with a pulpy consistency. Currently, the conditioning set parameters are usually determined through a trial-and-error procedure. Therefore, a contribution to reduce the errors and understand their causes may be helpful to reduce both time and costs of conditioning assessments required for each job site. The primary focus of this research is to evaluate how different foam generation methods influence the time-dependency of mechanical properties of conditioned soil. An essential aspect in characterization of soil conditioning involves evaluating the stability of both the conditioned soil and the foam used. Consequently, the core objective of the research is explored through the arrangement and implementation of various test campaigns, encompassing 2 half-life test campaigns, 2 preliminary test campaigns, and 17 primary test campaigns. The half-life test is employed to investigate the stability of generated foam. In the subsequent step, to assess the behavior and properties of the conditioned soil with different foam generation methods over time, a series of specific tests, including slump tests, density test, and vane shear test, were systematically conducted in time for distinct conditioning set parameters. These tests evaluate key characteristics of the conditioned soil, such as workability, compaction, and shear strength. Followingly, the potential variations between the abovementioned properties of conditioned soil with different foam generation method can be detected. In the final stage, semi-quantitative analyses of the results were performed, with the goal of gaining a more profound understanding of the potential correlations among distinct foam generation parameters, including foam generator flow rates, Foam Expansion Ratios (FER), and Foam Injection Ratios (FIR), and their influence on the time-dependency of mechanical properties of conditioned soil.