(a) Boreholes shall be drilled in each advancing working place when the working place approaches -
(1) To within 50 feet of any area located in the mine as shown by surveys that are certified by a registered engineer or registered surveyor unless the area has been preshift examined;
(2) To within 200 feet of any area located in the mine not shown by surveys that are certified by a registered engineer or registered surveyor unless the area has been preshift examined; or
(3) To within 200 feet of any mine workings of an adjacent mine located in the same coalbed unless the mine workings have been preshift examined.
(b) Boreholes shall be drilled as follows:
(1) Into the working face, parallel to the rib, and within 3 feet of each rib.
(2) Into the working face, parallel to the rib, and at intervals across the face not to exceed 8 feet.
(3) At least 20 feet in depth in advance of the working face, and always maintained to a distance of 10 feet in advance of the working face.
(c) Boreholes shall be drilled in both ribs of advancing working places described in paragraph (a) of this section unless an alternative drilling plan is approved by the District Manager in accordance with paragraph (g) of this section. These boreholes shall be drilled -
(1) At an angle of 45 degrees to the direction of advance;
(2) At least 20 feet in depth; and
(3) At intervals not to exceed 8 feet.
(d) When a borehole penetrates an area that cannot be examined, and before mining continues, a certified person shall, if possible, determine -
(1) The direction of airflow in the borehole;
(2) The pressure differential between the penetrated area and the mine workings;
(3) The concentrations of methane, oxygen, carbon monoxide, and carbon dioxide; and
(4) Whether water is impounded within the penetrated area.
(e) Unless action is taken to dewater or to ventilate penetrated areas, boreholes shall be plugged with wooden plugs or similar devices when -
(1) Tests conducted at the boreholes show that the atmosphere in the penetrated area contains more than 1.0 percent methane, less than 19.5 percent oxygen, or harmful concentrations of carbon monoxide, carbon dioxide or other explosive, harmful or noxious gases;
(2) Tests for methane, oxygen, carbon monoxide, and carbon dioxide cannot be made because air from mine workings is flowing into the penetrated area; or
(3) Water is discharging through the boreholes from the penetrated area into the mine workings.
(f) If mining is to be conducted within 50 feet above or below an inaccessible area of another mine, boreholes shall be drilled, as necessary, according to a plan approved by the district manager.