What requirements should be met for shotcrete operations during tunnel support？

The spraying operation shall be carried out in sections, pieces and layers, from the bottom to the top in sequence, and shall be pre-filled when there are large depressions in the rock surface.

When spraying in layers, the thickness of one shot can be determined according to the site and design thickness.

Repeat spraying (2 times possible) to the design thickness with a rebound rate of not more than 25% for arches and 15% for sidewalls. Minimize concrete resilience when applying

Should follow the excavation work surface in a timely manner shotcrete, follow the "short excavation, strong support, fast cycle", the principle of in the end of the shot within 4 hours of the next cycle of blasting operations shall not be carried out.

Mixed table material should be mixed with the spray, not mixed with quick-setting agent dry mix, storage time should not be greater than 2 hours, mixed with quick-setting agent dry mix, storage time should not be greater than 20 minutes.

The dosage of quick-setting agent should be accurate and added evenly, and should not be increased or decreased arbitrarily.

The nozzle should be perpendicular to the rock surface, and at the same time should maintain an appropriate distance (0.6 to 1.2 meters is appropriate) and spraying capacity. The nozzle should move continuously and slowly in a circular pattern in order to make the thickness of the spray layer uniform.

Wet curing shall be carried out 2 hours after final setting of sprayed concrete, and the curing time shall not be less than 14 days.