EPB-TBMs counterbalance the earth and water pressures in unstable grounds by excavated muck, which is usually conditioned by foaming agents generating homogeneous and plastic material (easy to transport) with low internal friction and permeability, enabling higher excavation performance. Basic aim of this study is to determine the effects of chemical additives for muck conditioning on the performance of an EPB-TBM used to excavate the Selimpasa Sewerage Tunnel in Istanbul. All conditioning tests for characterization of ground, foam and ground-foam mixture are performed on samples of claystone, which represents the most common lithology through the alignment, in a mobile laboratory. The results of the laboratory tests as well as those of the field studies, which were performed to validate the findings of the laboratory tests, are presented. The results indicate that the optimum conditioning is obtained at c(f) of 1.0%, FER of 10, water content of 40 to 45% (minus natural water content) and FIR of 20% for claystone. It is also indicated that 5% of anti-clay agent can be used for reducing sticking problems and power consumption without using foam. Foaming agent at correct dosage also reduces the sticking problem, but not totally. Field trials validate the suggested conditioning design.