The step length needs to be determined based on factors such as the stability of the surrounding rock, the cross-sectional dimensions of the tunnel, the operating space of the construction machinery, and the requirements of the support structure. The following are some general principles and methods for controlling the step length:

　　1. Surrounding rock stability: A longer step length can be used for more stable surrounding rock, while a shorter step length is required for unstable surrounding rock to reduce disturbance to the surrounding rock and ensure construction safety.

　　2. Tunnel cross-sectional dimensions: When the cross-sectional dimensions are larger, the step length can be appropriately increased, but the operating space of the construction machinery and the difficulty of support should also be considered.

　　3. Construction machinery operating space: The step length should meet the operating requirements of the construction machinery, such as the operating space of excavators, loaders, etc., in order to improve construction efficiency.

　　4. Support structure requirements: Determine the appropriate step length based on the type and design requirements of the support structure. For example, when anchor support is used, the step length should be adapted to the length of the anchor.

　　5. Experience and field tests: Adjust the step length based on the experience of similar projects and the results of field tests. During the construction process, the rationality of the step length can be verified by monitoring the deformation of the surrounding rock and the stress of the support structure, and necessary adjustments can be made.

　　Generally speaking, the step length is between 1D (D is the tunnel span) and 2D, but the specific length needs to be comprehensively considered and optimized according to the actual situation. At the same time, during the construction process, operations should be strictly carried out in accordance with the construction plan, and support and monitoring should be strengthened to ensure the safety and quality of tunnel excavation. If special circumstances are encountered, such as changes in surrounding rock conditions, groundwater outburst, etc., the step length and construction method should be adjusted in time.