Tunneling in areas where two or more materials with significantly different geotechnical properties are simultaneously encountered on the tunnel face has often been described as one of the most challenging tunneling scenarios. Some but not all of these challenges are discussed in this paper. Mixed face conditions in alluvial sediments are one of the challenges that occur in mechanized tunneling. These conditions occur when soil mixed with cobble and boulder. Boulder is well known as one of the obstruction material and problematic to the underground excavations and constructions. In this situation, the main challenges include abnormal abrasion of cutting tools and cutterhead, instability of tunnel face, mud system transfer and subsidence. Abrasion and high consumption of cutting tools lead to successive tool replacement and ultimately reduce tunnel excavation efficiency. Since the abrasion of the cutterhead alloy is greater than that of the cutting tool, so if there is clay in the excavation face and it combines with quartz mineral, it will form a corrosive adhesive which will clog the TBM cutterhead face and then abrasion as well as other parts involved with It also becomes. On the other hand, the cutterhead\\\\\\\\\\\\\\\'s continuous impact with hard and soft materials and the constant change of the place of impact of the force may cause the cutterhead to deform and crack and aggravate long-term failure. This paper will draw from real field experiences, including mix face conditions in the eastern tunnel of the Mashhad Metro Line 3 project, which has caused challenges such as abrasion of cutting tools and the need for frequent replacement of cutting tools reduced tunnel excavation performance. Mashhad Metro Line 3, 28.5 km long, excavating by EPB-TBM with a diameter of 9.4 m in two routes of the east and west tunnels. The eastern tunnel route\\\\\\\\\\\\\\\'s length is 13933 meters, and currently, 10773 meters have been excavated. The boulder\\\\\\\\\\\\\\\'s existence starts from about 8400 meters, and the tunnel excavation is in mixed face conditions with cobble and boulder presence. The route\\\\\\\\\\\\\\\'s boulders have been subsurface investigated by the pit excavation, and metamorphic rocks cobbles and boulders, including Amphibolite, Schist, Serpentine, Slate and Diorite, have been observed.

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