Evaluation of foam application in providing required conditioning for EPB hard rock TBMs

Utilizing the earth pressure balancing technique on hard rock tunnel boring machines (TBMs) is an efficient way to overcome engineering geological hazards that disrupt the application of these machines. Considering that geomechanical and engineering geological conditions of soil and rock are dramatically different, proposing a suitable conditioning design for earth pressure balance (EPB) hard rock TBMs requires particular attention to the characteristics of the excavated material and ground conditions. In this paper, foam application providing a favourable support medium for EPB hard rock TBMs is evaluated based on the experience of the Tehran metro line 6 southern expansion sector project, using laboratory and actual data. TBM muck, with different water contents, was used as the test specimen after granulometry correction. The trend of changes in the workability of foam conditioned mix was obtained by performing 350 slump tests. The pressurized permeability cell test (PPCT), a testing device capable of applying high water pressures to the conditioned material, was used to evaluate the impermeability of workable mixes. Results indicate that although foam can efficiently increase the workability of dry and moistened specimens, the presence of a separated liquid phase in the test specimen disrupts its application; moreover, foam conditioned mix is permeable even against low water pressures.