This paper summarizes the results of probe drilling carried out ahead of TBMs in two difficult tunneling projects in Turkey. The tunnels have completely different geological characteristics which necessitated two different methods of data analysis. Melen Water Tunnel was excavated under Istanbul Bosphorus within sedimentary rocks which are cut frequently by andesitic dykes, fracturing the surrounding rocks and creating a potential risk for water ingress into the tunnel. At the beginning of Melen Project, pioneering probe drillings with petrographic analysis and strength tests were performed on samples collected from TBM muck. This analysis allowed identifying some critical normalized probe drilling rate values for predicting potential weak zones created by andesitic dykes. These studies gave a sound basis for further interpretation of TBM and geologic data for the same tunnel. The second set of probe drilling analysis was from Kargi Tunnel. The North Anatolian Fault highly affected the tunnel excavation by fractured rock formations. Although the change in normalized probe drilling data was a good indicator of fractured zones, the diversity of rock formations made it difficult to interpret the data.