Embodiments in accordance with the present disclosure include a meta-stable detergent based foam generating device of a turbine cleaning system includes a manifold configured to receive a liquid detergent and an expansion gas, a gas supply source configured to store the expansion gas, and one or more aerators fluidly coupled with, and between, the gas supply source and the manifold. Each aerator of the one or more aerators comprises an orifice through which the expansion gas enters the manifold, and wherein the orifice of each aerator is sized to enable generation of a meta-stable detergent based foam having bubbles with bubble diameters within a range of 10 microns (3.910.sup.4 inches) and 5 millimeters (0.2 inches), having a half-life within a range of 5 minutes and 180 minutes, or a combination thereof.

The invention relates to a foam generator (14) for an earth-pressure-balance-shield tunnel-boring machine comprising: a mixing chamber, which has a first inlet opening (22) for a foamable liquid and a second inlet opening (23) for a gas and a foam outlet opening (24); a liquid-feeding device connected to the first inlet opening (22); and a gas-feeding device connected to the second inlet opening (23). The mixing chamber contains a tubular flow chamber (28) having the first inlet opening (22) at one end and the foam outlet opening (24) at the other end. A segment of the flow chamber (28) is designed as a gassing section having a gas-permeable porous wall (26) and adjoins a pressure chamber (29) having the second inlet opening (23) in such a way that the gas fed through the second inlet opening (23) under a pressure enters the flow chamber (28) through the porous wall (26) and, in the flow chamber, mixes with the liquid in such a way that foam is formed. The gas-feeding device and the liquid-feeding device are designed in such a way that the pressure of the fed gas can be set in such a way that the pressure of the fed gas is greater than the pressure exerted on the porous wall (26) by the liquid and that a desired ratio of fed gas to fed liquid is achieved. In a method for conditioning removed soil material as a supporting medium for an earth-pressure balance shield, removed soil is fed to an excavation chamber. Depending on the soil quality, foam is provided in that a foam generator having a gassing section of a specified length, of a specified flow cross-section, and of a specified pore size and pore density is provided and the ratio of fed gas to fed liquid is set in such a way that a desired structure and size of the foam bubbles result. The exiting foam is fed to the excavation chamber and mixed with removed soil.