Experimental study on foamed sandy soil for EPBM tunnelling

Foam injection is becoming a standard conditioning agent when tunnelling with an earth-pressure balanced shield,resulting in an increasing interest in this technique. The tech-nique is mostly based on empirical rules and experience.To be able to determine what foam and how may foam has to be injected for different soils, it is necessary to knowthe mechanical parameters of the foam-water-soil mixture. This paper describes a comprehen-sive set of tests that hasbeen performed to evaluate the effect that foam, foam types, and foam parameters have on the shear strength of sandysoil. Four different foam agents and one type of polymer were used during the tests. The foam production processwas per-formed by a foam generator, which was constructed by the authors. Shear strength tests were carried out in aconventional direct shear box measuring 60x60 mm. The test results show a decrease in the internal friction angle andan increase in soil compressibility for foamed soil, where the changes are functions of soil gradation, foam types, andfoam pa-rameters. It appears that another agent (such as bentonite slurry) needs to be added to coarse-grained soils tocondition the soil effectively by increasing the fine content and re-ducing the number of large voids. The test resultsalso show that the effect of different foaming agents on the shear strength of fine sand is not noticeable.