In-line foam quality control for EPB-TBM: A conductivity-based FER monitoring system.

Earth Pressure Balance Tunnel Boring Machines (EPB-TBMs) rely on foam conditioning to maintain excavation chamber pressure and ensure ground stability. The Foam Expansion Ratio (FER) – defined as the ratio between foam volume and foaming liquid volume – is one of the primary parameters governing conditioning effectiveness. Current practice relies of FER measurement on machine PLCs calculating FER based on measured volumes of foaming agent and compressed air, assuming ideal mixing conditions, but this approach cannot detect phase segregation within injection lines, when foaming agent and compressed air flow separately, with liquid gravitating toward the bottom and gas rising to the top, no actual foam formation occurs, despite correct volumetric proportions. This segregation creates air pockets in the excavation chamber that may compromise pressure distribution and potentially trigger face instability and surface settlements. This research proposes a novel real-time monitoring system for direct FER measurement based on electrical conductivity principles. As air content increases, the conductive liquid fraction decreases proportionally, reducing overall foam conductivity. The system employs sensors positioned at upper and lower injection line sections, enabling simultaneous foam quality detection and phase segregation identification through comparative analysis. Preliminary laboratory investigations demonstrate promising correlations between electrical conductivity measurements and independently verified FER values. The system will provide robust information to the EPB-TBM operators with continuous, real-time feedback on actual foam quality at injection points for immediate corrective interventions when segregation occurs, defining a paradigm shift toward predictive maintenance and operational optimization in mechanized tunnelling.