Abstract: In this study, the current research shortcomings and future development of causes of secondary lining cracks in loess tunnels and countermeasures are discussed from three aspects: research methods, crack causes, and treatment measures. The main conclusions are drawn as follows. (1) The research methods of loess tunnel lining cracks mainly include investigation and analysis, numerical simulation, monitoring and measurement, theoretical analysis, model test, and field test. The investigation, analysis, monitoring, and measurement methods are usually used in the construction stage, the numerical analysis method is usually used in the operation stage, and theoretical analysis, model test, and field test are typically used as the extension of research depth. (2) The secondary lining cracks are classified. The secondary lining cracks in loess tunnels are mainly distributed at tunnel arches and sidewalls. The cracks on arches are mostly longitudinal, while those on sidewalls are mainly longitudinal, circumferential, and oblique. (3) Lining cracks are caused by five factors, i.e., engineering geological conditions, surrounding rock environment deterioration, improper design and construction, structural material deterioration, and natural (manmade) destruction. The structure and engineering characteristics of loess are the internal causes of lining cracks, and the water′s role is the key factor and the main external cause. (4) At present, crack treatment mainly includes two types: cause treatment and reinforcement. The keys to cause treatment are water control and performance improvement of surrounding rock to reduce the lining stress. Reinforcement measures mostly adopted measures such as internal grouting and external steel plate. Finally, some shortcomings are pointed out from the aspects of mechanism research, crack classification, and crack treatment technology of the secondary lining cracks of loess tunnels. A more comprehensive tunnel lining health monitoring system and theoretical model should be established in the future. The fine modeling analysis should be conducted to establish the intuitive relationship between crack morphology and causes. Attention should be paid to the innovation of crack treatment materials and means.

Key words: loess tunnel, secondary lining cracks, cause analysis; countermeasures