The new Istanbul Airport is being constructed over an old coal field area, 35 km away from the city center. The construction of a rail link between the city and the airport entails the excavation of tunnels in the areas of complex geology, creating several problems for tunnel construction. This paper summarizes the effect of different grades of weathering of an interbedded sandstone-mudstone-siltstone sequence and transition zones on the performance of an earth pressure balance tunnel boring machine (EPB-TBM) during this work. The variation of EPB pressure, TBM thrust, torque, penetration, and specific energy values were analyzed in different lithologies and weathering zones and with respect to the attitude of transition zones between different materials. Normalized thrust and torque values considered in respect of penetration rate and EPB pressure showed that the attitude of transition zone on TBM performance is strictly dependent on the position of each geologic formation in the respect to the other, inclined or horizontal. To be specific, in an inclined transition zone, the thrust and torque values were almost double compared to the values obtained in a horizontal transition zone in the same lithology. This was probably due to increased over-break in inclined transition zones, which created loose material that accumulated in front of the cutter head. However, further case studies would be required definitely to confirm this. Mean normalized thrust and torque values were almost half in moderately/highly weathered sandstone compared to fresh/slightly weathered sandstone. The highest mean penetration of 28 mm/rev and mean lowest specific energy of 2.4 kWh/m(3) were obtained in transition zone between claystone with silty sand and highly weathered interbedded sandstone-mudstone-siltstone. Due to impact effects, increased disc consumption was experienced where passage was made from low strength due to high strength formations, with up to one disc per 115 m(3) of excavated material in the transition zones included in the study. The relatively high production rates were achieved, which attributed to a combination of logging daily the face conditions accompanied by appropriate adjustments of soil conditioning agents (FIR-foam injection ratio and FER foam expansion ratio values) and anti-clogging agents to control the sticking behavior of the spoil. The impact of different ground conditions on EPB-TBM performance in different projects is also discussed in relation to the results obtained in this study. It is believed that this research will help to manage better mechanized tunneling activities in similar conditions.