Earth pressure balance tunnel boring machines (EPB TBMs) may sometimes face, during excavation of soft soils, with some challenges of which one of them occurs when mixture of the excavated soil and water generates a sticky muck that adheres to the metal sections of the TBM such as cutterhead, cutting tools and transportation system. This phenomenon commonly referred as clogging leading to blockages and stoppages that decrease the advance rate and increase the project costs, since the excavation process could only be continued after cleaning operation of the blocked parts. This study was conducted in Istanbul Akfirat Waste Water Tunnel to determine the effects of clogging and foam injection ratio (FIR) on the EPB TBM excavation performance (torque, EPB TBM thrust, specific energy, unit penetration rate and/or net cutting rate). The studied tunnel section was divided in three zones with respect to clogging behavior of soils such as high, medium and low clogging zones. Soil sample could only be collected from the tunnel face belonging to high clogging zone during excavation of the tail tunnel. Particle size distribution was analyzed and Atterberg limit tests were conducted on the soil sample. In order to investigate the consistency behavior of the soil sample, slump and flow table tests were performed with changing water contents. Flow table test was performed with two different anticlay foams to investigate the effect of soil conditioning on consistency behavior of the soil sample. Field observations indicated that increasing clogging potential decreased the overall performance (daily advance rates) due to muck transportation problems and frequent stoppages to clean up the clogged cutterhead. Increased FIR reduced the transportation problems and stoppages. However, data analysis indicated that increasing FIR did not improve the penetration rate for open mode (without face pressure) working conditions since the soil on the face was quite firm for the anticlay foam to penetrate into. The mean FIR values were measured three times higher for the high clogging zone than for the low clogging zone. The mean specific energy values for the low and medium clogging zones were almost the same and half of the value obtained during excavation of high clogging zone. The EPB TBM thrust and cutterhead torque values did not exhibit any correlation with adhesion potential.