Effects of foam on soil conditioned behaviour

Earth Pressure Balanced Shields have experienced a remarkable increase in the number of applications throughout the world and can be considered the most commonly used mechanized tunnelling technology due to both the mechanical developments and more effective use of soil conditioning. Conditioning modifies the soil properties, mainly thanks to the use of foam and polymers (EFNARC, 2005) by making the soil suitable for both the pressure control in the bulk chamber and for extraction with the screw conveyor (Peila et al., 2007; Vinai et al., 2008; Merrit & Mair, 2006). As a first assessment, the soil features required for good EPBs operations are: a certain degree of “plasticity” that makes the conditioned soil suitable for pressure transmission in the bulk chamber and pressure control along the screw conveyor; a low inner friction that reduces both the power requirement of the cutting head and the wearing of the machine parts that are in contact with the soil; the persistence of the above-mentioned characteristics over time to allow safe control of the face stability during the whole excavation step (excavation and segment lining installation) and when the machine has to be stopped for any reason (maintenance, working problems, etc..). The conditioning quality and the control of the conditioned soil behaviour can be done above all with slump tests and using tests carried out by means of devices that are able to extract the soil from a tank by using a screw conveyor, as discussed in the following.