Starting in 2018, Musaimeer Pumping Station and Outfall project (MPSO) was constructed to manage ground and storm water received from 270 km2 area of southern Doha Qatar. The Project consists of a pumping station, subsea tunnel, (constructed through Rus formation, Midra shale and simsima limestone), riser shaft and diffuser bed structure. An Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM) was used for subsea tunnelling where ground water inflows, mixed ground condition with the presence of vertical and lateral fractures connected to the seabed were encountered during tunnelling activity. This paper analyses the reliability of geotechnical investigation results and interpretations by comparison with encountered conditions. This will be beneficial to design a reliable geotechnical investigation programme for future projects of similar scope in the state of Qatar. Results and interpretations from drilled bore holes and geophysical survey provided an overall picture of underground conditions along the tunnel alignment, which identified the critical tunnelling areas for cutter head intervention and maintenance. These results were then compared with as built geotechnical conditions accessed by collecting data through the onboard TBM monitoring equipment, collecting rock mass samples, and performing geological face mappings during tunnelling activity. Comparison of pre-tendered investigation results with as built geotechnical conditions concluded that investigation results and encountered rock mass conditions were in line with each other. Geotechnical investigation programme followed for this project and interpretation as a result, made the tunnelling under sea to be carried out in more careful and lower risk manner.