ABSTRACT
Foam is produced in a predetermined concentration of foaming agent and water and injected to the excavation chamber for soil conditioning in Earth Pressure Balance (EPB) shield tunneling. This mixing process will induce the dilution of the foaming liquid and thus influence its surface tension. This paper presents an experimental study on the influence of dilution of foaming liquid as could be expected in EPB shield tunneling. The capillary rise method is employed for the measurement of surface tension at different concentrations. It is shown that in a typical tunnel application the surface tension can be twice as high as that in the original foam. Consequences from the increase in surface tension are theoretically analyzed. Volume change and compressibility of the foam bubbles are hardly influenced by the increased surface tension for the foam used in tunneling. While the influence of a higher surface tension is negative for the formation of a low permeable layer that is anticipated by engineers. The study suggests that the change in surface tension due to dilution in tunneling should be incorporated in the inspection of foam-soil interactions.
