The main bearings of tunnel boring machines (TBMs) are crucial components, typically designed to operate reliably for around 15,000 hours under optimal conditions. However, the main bearing of an EPB-type TBM employed for excavating Lots 1 and 2 of the 18 km Ghomroud water transfer tunnel in Iran experienced failure after operating for less than 5,000 hours. This failure happened after only 9 km of excavation and caused serious damage to the TBM main drive system. To identify the cause of this failure, the relationship between the rock characteristics and the TBM’s main drive system was examined. Petrological studies were carried out on the rock specimens, including microscopic study of thin-sections, and geochemical tests using X-Ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). At the same time, lubricating oil from the TBM main drive system was tested with Rotating Disc Electrode Atomic Emission Spectrometry (RDE-AES) and Crackle experiments. The findings indicate that the failure of the TBM seals allowed rock-forming minerals, such as quartz, garnet, feldspar, biotite, chlorite, muscovite, and graphite, along with underground water, to enter the main drive system. The impact of these geological factors is linked to the mineralogical properties and the petrological history of the parent rocks. The results indicate that these geological factors initiate a destructive cycle involving surface scratching, localized loading, deformation, fatigue, and ultimately, failure of the main drive system components.
