Hydraulic impact hammers have been used widely in mining industry and civil engineering applications since 1960 (Rodford 1974; Pelizza et al. 1994). Almost 20 km of metro tunnels were driven in Istanbul with impact hammers, since the initial capital investment was relatively lower and rock formations were highly fractured in some zones; rock quality designation (RQD) values ranging from 0 to 80. The impact hammers may be mounted in any type of excavator and operated easily. A typical impact hammer working in a tunnel is seen in Figure 8.1.