Reasons for tunnel grouting: 

Tunnel shrinkage cracks are mostly surface-level, with fine textures and irregular orientations. The formation of these cracks is highly dependent on construction techniques, including the choice of excavation method, concrete vibration, water-cement ratio determination, formwork removal, arch backfilling, and curing. Premature formwork removal, before the concrete reaches its design strength, causes cracks. In particular, if the arch backfill is not dense, the pressure is dispersed to the arch sides, resulting in a "saddle-shaped" load distribution pattern. This deviates significantly from the original lining design, and the drying and cooling shrinkage of the concrete itself is also a major cause of cracking when the arch is opened.