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• The process of clogging can be divided into three stages. • Clogging can be detected by monitoring torque and excavation speed during tunnelling. • Soil conditioning can reduce clogging, but excess injected foam could be harmful. • The opening at the centre of the cutterhead is of vital importance. Clogging may occur inside the shield machine during earth pressure balance (EPB) tunnelling in cohesive soils. Soil conditioning can be used to reduce clogging potential and ensure the efficiency of the shield tunnelling. The current methods used to assess the clogging potential mainly focus on pure clayey soils or artificial mixed soils, the clogging in natural weathered soils is less studied. This paper studied the clogging when tunnelling in completely decomposed granite by laboratory tests, followed by field validation. In the laboratory tests, the torque and excavation speed were recorded and the soil distribution on the cutting device were analyzed. It appeared that when clogging happens, the excavation speed drops and the fluctuation in speed increases, while the torque rises as well as the fluctuations in the torque. Clogging first covers the cutters, then fill in the intervals between cutters, and finally blocked the openings on the cutting device. The opening at the central part of cutting device is of vital importance to prevent clogging. The process of clogging can be divided into three stages, the initial stage, the stabilized stage and the clogging stage. In-situ data from Guangzhou Metro Line 21 showed that clogging on cutterhead was effectively avoided when using the recommended soil conditioning scheme. Furthermore, the clogging evaluation method and process of clogging proposed are consistent with field validation and with current major clogging assessment method. [ABSTRACT FROM AUTHOR]