Earth pressure balance (EPB) tunnel boring machines have become increasingly common in the last decade for the construction of tunnels in soft soils. The performance of these machines is so different that they have a major effect on the outside conditions of drilling operations. They use the excavated soil in a pressurized head chamber to apply support pressure to the tunnel face during excavation. A machine may be designed to work in ideal ground conditions. However, natural soils rarely have these properties, and conditioning of the soil is usually necessary to alter its properties to suit the machine. Tunnel excavation is not impossible when the unconditioned soils around the tunnel are coarse and sticky. Effective soil conditioning significantly improves machine performance and control of the soil flow through the screw conveyor. This research presents experimental investigations of soil conditioning for soil/foam and foam/polymer/soil. Methods are also provided to establish a suitable conditioning procedure for determining the optimum foam injection ratio (FIR) for these soils and for studying the effective parameters in soil conditioning such as shear strength and plasticity paste of conditioned soils. Direct shear strength tests were conducted on a small scale, and the plasticity paste of soil was studied by slump tests. The slump tests show that the plasticity of sand/foam and sand/foam/polymer is dependent on the initial moisture content, type of soil, etc. Thus, these parameters are important for achieving optimum FIR and suitable conditioning. In addition, direct shear strength tests show that the injection of foam to sand Type C caused a reduction of 51% in the internal friction angle. Thus, foams are suitable for modifying soil. Increasing the FIR by more than a special value (optimum FIR) did not change the shear strength significantly.