Plasticity behavior of the foam-conditioned soil in the chamber is critical in the earth pressure balanced shield tunneling operation. An experimental study was conducted to evaluate the factors affecting the plastic fluidity of the air-foamed sand under pressures close to the actual field, and to clarify the relationship with that under atmospheric pressure. Plastic fluidity of the air-foamed sand was highly dependent on the difference between the overburden pressure and foam injection pressure, which is normalized by the overburden pressure. Another critical factor was the foam expansion ratio (FER). Foam-conditioned sand with FER = 16 maintained higher plasticity than that with FER = 8, when subjected to an equivalent increment of effective stress. However, the higher FER led to the larger residual volume change after a loading-unloading cycle. The table flow test is widely employed as an index of the plastic fluidity of foam-conditioned soil. The table flow values measured under atmospheric pressure do not completely correspond to the plastic fluidity under high-pressure conditions.
