Soil conditioning is often critical to high performance earth pressure balance (EPB) tunnel boring machine (TBM) tunneling. The injection of foam into the cutterhead tool gap and the subsequent mechanical mixing with scraped soil in the tool gap, cutter-head openings and in the excavation chamber, serves to transform the formation soil into a low permeability, low shear strength, compressible chamber material for steady face support and muck extraction. A significant body of research and literature exists on conditioned soil behavior, primarily based on laboratory studies (e.g., Vinai et al. 2007, Bezuijen et al. 2012, Budach and Thewes 2015, Williamson et al. 1999, Mori et al., 2016). However, very few if any published studies have focused on how foam is mechanically produced on board the TBM, what foam generation system characteristics are important, and how in-situ conditions such as ground pressure influence foam generation and system design. The lack of documented studies in the literature leaves many contractors 'foaming in the dark'. The first part of this paper presents an overview of on-board foam generating systems for EPB TBMs, discussing the desired foam properties and quantities, how they are estimated, and key system parameters that deliver and influence these properties. The second part of the paper presents the results of foam generation testing in a laboratory-simulated EPB TBM soil conditioning system, focusing on the influence of a number of key parameters such as pressure, air velocity, generator fillings, elbows, etc. on foam properties.
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