503 EM 385-1-1 Sections 20-26

Air monitoring devices must be inspected, calibrated, maintained, and used in accordance with the manufacturer's instructions. Back-up monitoring devices must be maintained in calibrated and working condition at the worksite. > See Section 6.

It's important to have air monitoring equipment during Underground tunnel construction.

When air monitoring is required "as often as necessary", the CP must determine which substances to monitor and how frequently to monitor. Such determination must be based on:

(1) The location of the job site and proximity to fuel tanks, sewers, gas lines, old landfills, coal deposits, and swamps;
(2) The geology of the job site, particularly the soil types and their permeability;
(3) Any history of air contaminants in nearby job sites or any changes in air quality monitored during a previous shift; and
(4) Work practices and job site conditions (use of diesel engines, explosives, or fuel gas, ventilation characteristics, visible atmospheric conditions, decompression of the atmosphere, welding, cutting, or hot work, etc.).

A record (including location, date, time, substance, monitoring results, and name of person conducting the test) of all air quality tests must be maintained at the job site.
The atmosphere in all underground work areas must be tested as often as necessary to assure that the atmosphere at normal atmospheric pressure contains at least 19.5% oxygen and no more than 22% oxygen.
The atmosphere in all underground work areas must be tested quantitatively for CO, nitrogen dioxide, hydrogen sulfide, and other toxic gases, dusts, vapors, mists, and fumes as often as necessary to ensure that the PEL are not exceeded.

The atmosphere in all underground work areas must be tested quantitatively for methane and other flammable gases as often as necessary to determine whether action is to be taken under 26.C.02.f-h and to determine whether an operation is to be classified gassy or potentially gassy under 26.B.01.
The atmosphere in all underground work areas must be tested as often as necessary to ensure that the ventilation requirements of Section 26.C.03-05 are met.
If diesel-engine or gasoline-engine driven ventilating fans or compressors are used, an initial test must be made of the inlet air of the fan or compressor, with the engine operating, to ensure that the air supply is not contaminated by engine exhaust.
When rapid excavation machines are used, a continuous flammable gas monitor must be operated at the face with the sensor(s) placed as high and close to the front of the machine's cutter head as possible.
Operations that meet the criteria for potentially gassy or gassy operations must be subjected to the following monitoring:

(1) Tests for oxygen content must be conducted in all affected work areas and work areas immediately adjacent to such areas at least at the beginning and midpoint of each shift;
(2) When using rapid excavation machines, continuous automatic flammable gas monitoring equipment must be used to monitor the air at the heading, on the rib, and in the return air duct. The continuous monitor must signal the heading and shut down electric power in the affected underground work area, except for acceptable pumping and ventilation equipment, when 20% or more of the LEL for methane or other flammable gases is encountered.
(3) A manual flammable gas monitor must be used as needed, but at least at the beginning and midpoint of each shift, to ensure that the limits prescribed in 26.B 01 and 26.C.01.d and f are not exceeded. In addition, a manual electrical shut down control must be provided near the heading.
(4) Local gas tests must be made prior to and continuously during any welding, cutting, or other hot work.
(5) In underground operations driven by drill-and-blast methods, the air in the affected area must be tested for flammable gas prior to re-entry after blasting and continuously when employees are working underground.

26-6. What is the normal safe oxygen level that should exist at normal atmospheric pressure in all underground work areas?

At least 16.5% oxygen and no more than 20% oxygen

At least 18.5% oxygen and no more than 21% oxygen

At least 19.5% oxygen and no more than 22% oxygen

At least 21.5% oxygen and no more than 23% oxygen

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