How many steel frames can a mining tunnel dig at a time?

2. Mining method is a construction method to build tunnels by digging underground tunnels. When going down the steps, the excavation shall be carried out after the shotcrete reaches more than 70% of the design strength, and the excavation length of the lower steps shall not exceed 2 arch distances at a time. The construction method of tunnel and underground engineering is originally the method of mining roadway, so it is called mining method.