Contractors persistently deal with an air bubble in the chamber during EPB tunneling; however, the cause of this is unclear. This paper addresses this problem through a series of unique mixing tests coupled with the study of foam-soil interaction at the bubble and soil grain level. Testing reveals that foam is very stable when properly mixed with soil. Soil water content and foam injection ratio play important roles in influencing bubble migration from conditioned soil. The foam-soil mixing test results suggest that the shearing action of mixing tools creates a pathway for bubble migration. These results suggest that soil conditioning parameters, excavated soil properties, and pressure conditions in the EPB chamber should be all considered in design to eliminate the air bubble in the EPB TBM chamber.
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