4.Safety technologies for coal exca

4.Safety technologies for coal excavation in shafts
To achieve economical, safe, efficient and fast coal uncovering in mine shafts, we divide outburst prevention and control measures into three types of measures: the primary technical measures (e.g., drainage or extraction boreholes), accelerated gas drainage measures (e.g., coal sluicing and hole cleaning) and safety protection measures (e.g., coal seam strengthening using metal skeletons and grout injection).
4.1. The primary technical measures
The main outburst prevention measures include gas dilution by ventilation and drainage boreholes [17,18]. Its principle of eliminating outbursts is to remove the coal-rock mass in concentrated areas of stress by drilling, which causes pressure relief and the deformation of coal surrounding the drilling space due to the action of inelastic deformation energy, thereby lowering the stress concentration peak and increasing the coal seam permeability. By extracting the coal seam gas in the vicinity of the borehole, the gas outburst energy and gas pressure are reduced, the coal strength is enhanced, and the outburst risk is eliminated. A last a protection barrier around shaft wall is formed for preventing coal and gas outbursts.
Drainage boreholes must satisfy the following conditions:
(1) The controlling range is in accordance with the range requirements.
(2) The diameter of borehole should be in the range of 75–120 mm, and the space between each hole is determined by the coal seam permeability and the drainage time that is allowed in the production plan (typically not more than 3 m at the final hole bottom).
(3) Penetration boreholes should attempt to drill through the full-thickness of the coal seam in one drilling.
(4) Continuous drainage or natural discharge before uncovering the coal seam should be kept.
(5) Natural drainage is suitable for coal seams with relatively high permeability and sufficient time to discharge. For low-permeability seams and poorly-drained working faces in the shaft, coal sluicing and hole cleaning can be used to improve the gas drainage.
A sketch of the drainage borehole arrangement is shown in Fig. 5.
4.2. Accelerated gas drainage measures
In the shaft, the gas drainage boreholes are nearly vertical downward-facing holes which are easy for hydrops, and the holes are prone to collapse along the section crossing the coal seam. Therefore, an innovative method for improving and accelerating gas drainage, i.e., coal sluicing and hole cleaning technology, is used. Generally, the method is thought to be effective when the quantity coal obtained by sluicing is greater than 2% of the total amount of coal in the seams being controlled.
Coal sluicing and hole cleaning technology accelerates gas drainage through two main processes. First, these methods can be used to flush out hydrops in holes and reduce the resistance to gas emission. Second, collapsed and accumulated coal in the drilling holes, as well as a certain volume of coal-rock mass around the holes, can be flushed out. Thus, coal sluicing and hole cleaning provides space for the movement and deformation of the follow-ing coal mass, thereby transferring the deep-seated concentrated stress, enlarging the pressure relief range, and increasing the coal permeability to promote gas emission. Specifically, we set a metal DN15 pipe at the bottom of the drilling hole connecting to air compressor with 4–5 MPa pressure and continuously sweep the drilling holes for a period time (as shown in Fig. 6). For severe hole collapses and shrinkage, we should use a drill rig and metal pipe with small holes through main boreholes that can be used to protect the hole in areas of roof or floor collapse.

4.3. Safety protection measures
After identification of the effectiveness, we can use metal skeletons, grout injection and other assurance measures to enhance coal strength for guaranteeing the safe excavation of coal in shafts. Skeleton materials can choose 8 kg m