Subpart S - Underground Construction, Caissons, Cofferdams, and Compressed Air

Scope: This standard provides the requirements for underground construction.

Special Requirements: Warning signs (posted), check-in/check-out procedure, designated person, practice, familiar, respirators, rescue procedures, rescue services, air monitoring, procedure, communication systems, tested, informed, observation, environmental controls, communications, safety instruction, self-rescuers, rescue teams, qualifications, alarm, informed, records, recordkeeping, review, ventilation controls, competent person, demonstrated (employer), reasonable determination, MSHA-approved equipment, dust control measures, classified, history, references other standards

1926.800(b)(3)  - The employer shall control access to all openings to prevent unauthorized entry underground. Unused chutes, manways, or other openings shall be tightly covered, bulkheaded, or fenced off, and shall be posted with warning signs indicating "Keep Out" or similar language. Completed or unused sections of the underground facility shall be barricaded.

1926.800(c) - Check-in/check-out. The employer shall maintain a check-in/check-out procedure that will ensure that above-ground personnel can determine an accurate count of the number of persons underground in the event of an emergency. However, this procedure is not required when the construction of underground facilities designed for human occupancy has been sufficiently completed so that the permanent environmental controls are effective, and when the remaining construction activity will not cause any environmental hazard or structural failure within the facilities.

1926.800(d) - Safety instruction. All employees shall be instructed in the recognition and avoidance of hazards associated with underground construction activities including, where appropriate, the following subjects: [Reference paragraph (d)(1) - (d)(10)].

1926.800(e)(1) - Oncoming shifts shall be informed of any hazardous occurrences or conditions that have affected or might affect employee safety, including liberation of gas, equipment failures, earth or rock slides, cave-ins, floodings, fires or explosions.

1926.800(e)(2) - The employer shall establish and maintain direct communications for coordination of activities with other employers whose operations at the jobsite affect or may affect the safety of employees underground.

1926.800(f)(1) - When natural unassisted voice communication is ineffective, a power-assisted means of voice communication shall be used to provide communication between the work face, the bottom of the shaft, and the surface.

1926.800(f)(2) - Two effective means of communication, at least one of which shall be voice communication, shall be provided in all shafts which are being developed or used either for personnel access or for hoisting. Additional requirements for hoist operator communication are contained in paragraph (t)(3)(xiv) of this section.

1926.800(f)(3) - Powered communication systems shall operate on an independent power supply, and shall be installed so that the use of or disruption of any one phone or signal location will not disrupt the operation of the system from any other location.

1926.800(f)(4) - Communication systems shall be tested upon initial entry of each shift to the underground, and as often as necessary at later times, to ensure that they are in working order.

1926.800(f)(5) - Any employee working alone underground in a hazardous location, who is both out of the range of natural unassisted voice communication and not under observation by other persons, shall be provided with an effective means of obtaining assistance in an emergency.

1926.800(g)(1) - Hoisting capability. When a shaft is used as a means of egress, the employer shall make advance arrangements for power-assisted hoisting capability to be readily available in an emergency, unless the regular hoisting means can continue to function in the event of an electrical power failure at the jobsite. Such hoisting means shall be designed so that the load hoist drum is powered in both directions of rotation and so that the brake is automatically applied upon power release or failure.

1926.800(g)(2) - Self-rescuers. The employer must provide self-rescuers approved by the National Institute for Occupational Safety and Health under 42 CFR part 84. The respirators must be immediately available to all employees at work stations in underground areas where employees might be trapped by smoke or gas. The selection, issuance, use, and care of respirators must be in accordance with 29 CFR 1926.103 - respiratory protection.

1926.800(g)(3) - Designated person. At least one designated person shall be on duty above ground whenever any employee is working underground. This designated person shall be responsible for securing immediate aid and keeping an accurate count of employees underground in case of emergency. The designated person must not be so busy with other responsibilities that the counting function is encumbered.

1926.800(g)(5)(i) - On jobsites where 25 or more employees work underground at one time, the employer shall provide (or make arrangements in advance with locally available rescue services to provide) at least two 5-person rescue teams, one on the jobsite or within one-half hour travel time from the entry point, and the other within 2 hours travel time.

1926.800(g)(5)(ii) - On jobsites where less than 25 employees work underground at one time, the employer shall provide (or make arrangements in advance with locally available rescue services to provide) at least one 5-person rescue team to be either on the jobsite or within one-half hour travel time from the entry point.

1926.800(g)(5)(iii) - Rescue team members shall be qualified in rescue procedures, the use and limitations of breathing apparatus, and the use of firefighting equipment. Qualifications shall be reviewed not less than annually.

1926.800(g)(5)(iv) - On jobsites where flammable or noxious gases are encountered or anticipated in hazardous quantities, rescue team members shall practice donning and using self-contained breathing apparatus monthly.

1926.800(g)(5)(v) - The employer shall ensure that rescue teams are familiar with conditions at the jobsite.

1926.800(h)(1) - Potentially gassy operations. Underground construction operations shall be classified as potentially gassy if either:

• 1926.800(h)(1)(i) - Air monitoring discloses 10 percent or more of the lower explosive limit for methane or other flammable gases measured at 12 inches (304.8 mm) ±0.25 inch (6.35 mm) from the roof, face, floor or walls in any underground work area for more than a 24-hour period; or
• 1926.800(h)(1)(ii) - The history of the geographical area or geological formation indicates that 10 percent or more of the lower explosive limit for methane or other flammable gases is likely to be encountered in such underground operations.

1926.800(h)(2) - Gassy operations. Underground construction operations shall be classified as gassy if:

• 1926.800(h)(2)(i) - Air monitoring discloses 10 percent or more of the lower explosive limit for methane or other flammable gases measured at 12 inches (304.8 mm) ±0.25 inch (6.35 mm) from the roof, face, floor or walls in any underground work area for three consecutive days; or
• 1926.800(h)(2)(iii) - The underground construction operation is both connected to an underground work area which is currently classified as gassy and is also subject to a continuous course of air containing the flammable gas concentration.

1926.800(h)(3) - Declassification to potentially gassy operations. Underground construction gassy operations may be declassified to Potentially Gassy when air monitoring results remain under 10 percent of the lower explosive limit for methane or other flammable gases for three consecutive days.

1926.800(i)(2) - Mobile diesel-powered equipment used in gassy operations shall be either approved in accordance with the requirements of 30 CFR part 36 (formerly Schedule 31) by MSHA, or shall be demonstrated by the employer to be fully equivalent to such MSHA-approved equipment, and shall be operated in accordance with that part.

1926.800(i)(3) - Each entrance to a gassy operation shall be prominently posted with signs notifying all entrants of the gassy classification.

1926.800(i)(5) - A fire watch as described in 1926.352(e) - fire prevention, shall be maintained when hot work is performed.

1926.800(i)(6) - Once an operation has met the criteria in paragraph (h)(2) warranting classification as gassy, all operations in the affected area, except the following, shall be discontinued until the operation either is in compliance with all of the gassy operation requirements or has been declassified in accordance with paragraph (h)(3) of this section:

1926.800(j)(1) - General. Air quality limits and control requirements for construction are found in 1926.55 - gases, vapors, fumes, dusts, and mists, except as modified by this section.

• 1926.800(j)(1)(i)  
  • 1926.800(j)(1)(i)(A) - The employer shall assign a competent person who shall perform all air monitoring required by this section.
  • 1926.800(j)(1)(i)(B) - Where this paragraph requires monitoring of airborne contaminants “as often as necessary,” the competent person shall make a reasonable determination as to which substances to monitor and how frequently to monitor, considering at least the following factors: [Reference paragraph (j)(1)(i)(B(1) - (4)].

1926.800(j)(1)(ii)(A) - The atmosphere in all underground work areas shall be tested as often as necessary to assure that the atmosphere at normal atmospheric pressure contains at least 19.5 percent oxygen and no more than 22 percent oxygen.

1926.800(j)(1)(ii)(B) - Tests for oxygen content shall be made before tests for air contaminants.

1926.800(j)(1)(iii)(A) - The atmosphere in all underground work areas shall be tested quantitatively for carbon monoxide, nitrogen dioxide, hydrogen sulfide, and other toxic gases, dusts, vapors, mists, and fumes as often as necessary to ensure that the permissible exposure limits prescribed in 1926.55 - gases, vapors, fumes, dusts, and mists, are not exceeded.

1926.800(j)(1)(iii)(B) - The atmosphere in all underground work areas shall be tested quantitatively for methane and other flammable gases as often as necessary to determine:

1926.800(j)(1)(iii)(D) - Testing shall be performed as often as necessary to ensure that the ventilation requirements of paragraph (k) of this section are met.

1926.800(j)(1)(v)(A) - Whenever air monitoring indicates the presence of 5 ppm or more of hydrogen sulfide, a test shall be conducted in the affected underground work area(s), at least at the beginning and midpoint of each shift, until the concentration of hydrogen sulfide has been less than 5 ppm for 3 consecutive days.

1926.800(j)(1)(v)(B) - Whenever hydrogen sulfide is detected in an amount exceeding 10 ppm, a continuous sampling and indicating hydrogen sulfide monitor shall be used to monitor the affected work area.

1926.800(j)(1)(v)(C) - Employees shall be informed when a concentration of 10 ppm hydrogen sulfide is exceeded.

1926.800(j)(1)(v)(D) - The continuous sampling and indicating hydrogen sulfide monitor shall be designed, installed, and maintained to provide a visual and aural alarm when the hydrogen sulfide concentration reaches 20 ppm to signal that additional measures, such as respirator use, increased ventilation, or evacuation, might be necessary to maintain hydrogen sulfide exposure below the permissible exposure limit.

1926.800(j)(1)(vi) - When the competent person determines, on the basis of air monitoring results or other information, that air contaminants may be present in sufficient quantity to be dangerous to life, the employer shall: [Reference paragraph (j)(1)(vi)].

1926.800(j)(1)(vii) - Whenever five percent or more of the lower explosive limit for methane or other flammable gases is detected in any underground work area(s) or in the air return, steps shall be taken to increase ventilation air volume or otherwise control the gas concentration, unless the employer is operating in accordance with the potentially gassy or gassy operation requirements. Such additional ventilation controls may be discontinued when gas concentrations are reduced below five percent of the lower explosive limit, but shall be reinstituted whenever the five percent level is exceeded.

1926.800(j)(2) - Additional monitoring for potentially gassy and gassy operations. Operations which meet the criteria for potentially gassy and gassy operations set forth in paragraph (h) of this section shall be subject to the additional monitoring requirements of this paragraph.

1926.800(j)(3) - Recordkeeping. A record of all air quality tests shall be maintained above ground at the worksite and be made available to the Secretary of Labor upon request. The record shall include the location, date, time, substance and amount monitored. Records of exposures to toxic substances shall be retained in accordance with 1926.33 - access to employee exposure and medical records, of this chapter. All other air quality test records shall be retained until completion of the project.

1926.800(k)(1)(ii) - Mechanical ventilation shall be provided in all underground work areas except when the employer can demonstrate that natural ventilation provides the necessary air quality through sufficient air volume and air flow.

1926.800(k)(7) - When ventilation has been reduced to the extent that hazardous levels of methane or flammable gas may have accumulated, a competent person shall test all affected areas after ventilation has been restored and shall determine whether the atmosphere is within flammable limits before any power, other than for acceptable equipment, is restored or work is resumed.

1926.800(k)(8) - Whenever the ventilation system has been shut down with all employees out of the underground area, only competent persons authorized to test for air contaminants shall be allowed underground until the ventilation has been restored and all affected areas have been tested for air contaminants and declared safe.

1926.800(k)(9) - When drilling rock or concrete, appropriate dust control measures shall be taken to maintain dust levels within limits set in 1926.55 - gases, vapors, fumes, dusts, and mists. Such measures may include, but are not limited to, wet drilling, the use of vacuum collectors, and water mix spray systems.

1926.800(k)(10)(ii) - Mobile diesel-powered equipment used underground in atmospheres other than gassy operations:

• 1926.800(k)(10)(ii)(A) - Shall comply with MSHA provisions in 30 CFR 57.5067; or
• 1926.800(k)(10)(ii)(B) - If purchased on or before July 15, 2019, may alternatively comply with MSHA provisions under 30 CFR part 32 (revised as of July 1, 1996) (formerly Schedule 24), or be demonstrated by the employer to be fully equivalent to such MSHA-approved equipment, and be operated in accordance with that part.

1926.800(l)(1) - Illumination requirements applicable to underground construction operations are found in Table D-3 of 1926.56 - Illumination, of this part.

1926.800(m) - Fire prevention and control. Fire prevention and protection requirements applicable to underground construction operations are found in subpart F - fire protection and prevention, of this part, except as modified by the following additional standards.

1926.800(m)(2)(ii) - Readily visible signs prohibiting smoking and open flames shall be posted in areas having fire or explosion hazards.

1926.800(m)(5)(ii) - Acetylene, liquefied petroleum gas, and Methylacetylene Propadiene Stabilized gas may be used underground only for welding, cutting and other hot work, and only in accordance with subpart J - welding and cutting, of this part, and paragraphs (j), (k), (m), and (n) of this section.

1926.800(m)(9)(ii) - Lighting fixtures in storage areas, or within 25 feet (7.62 m) of underground areas where oil, grease, or diesel fuel are stored, shall be approved for Class I, Division 2 locations, in accordance with subpart K -  electrical, of this part.

1926.800(n) - Welding, cutting, and other hot work. In addition to the requirements of subpart J - welding and cutting of this part, the following requirements shall apply to underground welding, cutting, and other hot work.

1926.800(o)(2) - Subsidence areas. The employer shall ensure ground stability in hazardous subsidence areas by shoring, by filling in, or by erecting barricades and posting warning signs to prevent entry.

1926.800(o)(3)(i)(A) - A competent person shall inspect the roof, face, and walls of the work area at the start of each shift and as often as necessary to determine ground stability.

1926.800(o)(3)(i)(B) - Competent persons conducting such inspections shall be protected from loose ground by location, ground support or equivalent means.

1926.800(o)(3)(ii) - Ground conditions along haulageways and travelways shall be inspected as frequently as necessary to ensure safe passage.

1926.800(o)(3)(iv)(B) - A competent person shall determine whether rock bolts meet the necessary torque, and shall determine the testing frequency in light of the bolt system, ground conditions and the distance from vibration sources.

1926.800(p) - Blasting. This paragraph applies in addition to the requirements for blasting and explosives operations, including handling of misfires, which are found in subpart U - blasting and use of explosives, of this part.

Scope: This standard provides the requirements for caissons.

Caisson - A wood, steel, concrete or reinforced concrete, air- and water-tight chamber in which it is possible for men to work under air pressure greater than atmospheric pressure to excavate material below water level.

Special Requirements: Tests, stamped, references other standards

1926.801(b) - Shafts shall be subjected to a hydrostatic or air-pressure test, at which pressure they shall be tight. The shaft shall be stamped on the outside shell about 12 inches from each flange to show the pressure to which they have been subjected.

1926.801(f) - In caisson operations where employees are exposed to compressed air working environments, the requirements contained in 1926.803 - compressed air, shall be complied with.

Scope: This standard provides the requirements for cofferdams.

Special Requirements: Warning signals, posted, references other standards

1926.802(b) - Warning signals for evacuation of employees in case of emergency shall be developed and posted.

1926.802(c) - Cofferdam walkways, bridges, or ramps with at least two means of rapid exit shall be provided with guardrails as specified in subpart M -fall protection, of this part.

Scope: This standard provides the requirements for compressed air.

Special Requirements: Competent person (familiar), rules, regulations, instruction, records, copy to OSHA, designated, licensed physician, medical supervision, physically qualified, first aid station, lock attendant, station/field office, observation port, decompression tables, identification badges, posted signals/time, communication, instruction, ambulance, analyze air, tests, training, medical lock, maintenance, methods, rated capacity (posted), procedures, fire watch, approved extinguisher, taped programs, cams, references other standards

1926.803(a)(1) - There shall be present, at all times, at least one competent person designated by and representing the employer, who shall be familiar with this subpart in all respects, and responsible for full compliance with these and other applicable subparts.

1926.803(a)(2) - Every employee shall be instructed in the rules and regulations which concern his safety or the safety of others.

1926.803(b)(1) - There shall be retained one or more licensed physicians familiar with and experienced in the physical requirements and the medical aspects of compressed air work and the treatment of decompression illness. He shall be available at all times while work is in progress in order to provide medical supervision of employees employed in compressed air work. He shall himself be physically qualified and be willing to enter a pressurized environment.

1926.803(b)(2) - No employee shall be permitted to enter a compressed air environment until he has been examined by the physician and reported by him to be physically qualified to engage in such work.

1926.803(b)(3) - In the event an employee is absent from work for 10 days, or is absent due to sickness or injury, he shall not resume work until he is reexamined by the physician, and his physical condition reported, as provided in this paragraph, to be such as to permit him to work in compressed air.

1926.803(b)(4) - After an employee has been employed continuously in compressed air for a period designated by the physician, but not to exceed 1 year, he shall be reexamined by the physician to determine if he is still physically qualified to engage in compressed air work.

1926.803(b)(5) - Such physician shall at all times keep a complete and full record of examinations made by him. The physician shall also keep an accurate record of any decompression illness or other illness or injury incapacitating any employee for work, and of all loss of life that occurs in the operation of a tunnel, caisson, or other compartment in which compressed air is used.

1926.803(b)(6) - Records shall be available for the inspection of the Secretary or his representatives, and a copy thereof shall be forwarded to OSHA within 48 hours following the occurrence of the accident, death, injury, or decompression illness. It shall state as fully as possible the cause of said death or decompression illness, and the place where the injured or sick employee was taken, and such other relative information as may be required by the Secretary.

1926.803(b)(7) - A fully equipped first aid station shall be provided at each tunnel project regardless of the number of persons employed. An ambulance or transportation suitable for a litter case shall be at each project.

1926.803(b)(10) - The medical lock shall: [Reference paragraph (b)(10)(i) - (xvi)].

1926.803(b)(11) - Identification badges shall be furnished to all employees, indicating that the wearer is a compressed air worker. A permanent record shall be kept of all identification badges issued. The badge shall give the employee's name, address of the medical lock, the telephone number of the licensed physician for the compressed air project, and contain instructions that in case of emergency of unknown or doubtful cause or illness, the wearer shall be rushed to the medical lock. The badge shall be worn at all times—off the job, as well as on the job.

1926.803(c)(1) - Effective and reliable means of communication, such as bells, whistles, or telephones, shall be maintained, at all times between all the following locations: [Reference paragraph (c)(1)(i) - (viii)].

1926.803(d)(1) - The time of decompression shall be posted in each man lock as follows: [Reference paragraph (d)(1)].

1926.803(d)(2) - Any code of signals used shall be conspicuously posted near workplace entrances and such other locations as may be necessary to bring them to the attention of all employees concerned.

1926.803(d)(3) - For each 8-hour shift, a record of employees employed under air pressure shall be kept by an employee who shall remain outside the lock near the entrance. This record shall show the period each employee spends in the air chamber and the time taken from decompression. A copy shall be submitted to the appointed physician after each shift.

1926.803(e)(1) - Every employee going under air pressure for the first time shall be instructed on how to avoid excessive discomfort.

1926.803(e)(4) - If any employee complains of discomfort, the pressure shall be held to determine if the symptoms are relieved. If, after 5 minutes the discomfort does not disappear, the lock attendant shall gradually reduce the pressure until the employee signals that the discomfort has ceased. If he does not indicate that the discomfort has disappeared, the lock attendant shall reduce the pressure to atmospheric and the employee shall be released from the lock.

1926.803(f)(1) - Decompression to normal condition shall be in accordance with the Decompression Tables in appendix A of this subpart.

1926.803(f)(2) - In the event it is necessary for an employee to be in compressed air more than once in a 24-hour period, the appointed physician shall be responsible for the establishment of methods and procedures of decompression applicable to repetitive exposures.

1926.803(f)(3) - If decanting is necessary, the appointed physician shall establish procedures before any employee is permitted to be decompressed by decanting methods. The period of time that the employees spend at atmospheric pressure between the decompression following the shift and recompression shall not exceed 5 minutes.

1926.803(g)(1)(i) - Man locks - Except in emergency, no employees employed in compressed air shall be permitted to pass from the working chamber to atmospheric pressure until after decompression, in accordance with the procedures in this subpart.

1926.803(g)(1)(iii) - Except where air pressure in the working chamber is below 12 p.s.i.g., each man lock shall be equipped with automatic controls which, through taped programs, cams, or similar apparatus, shall automatically regulate decompressions. It shall also be equipped with manual controls to permit the lock attendant to override the automatic mechanism in the event of an emergency, as provided in paragraph (g)(1)(viii) of this section.

1926.803(g)(1)(v) - A clock, thermometer, and continuous recording pressure gauge with a 4-hour graph shall be installed outside of each man lock and shall be changed prior to each shift's decompression. The chart shall be of sufficient size to register a legible record of variations in pressure within the man lock and shall be visible to the lock attendant. A copy of each graph shall be submitted to the appointed physician after each shift. In addition, a pressure gauge, clock, and thermometer shall also be installed in each man lock. Additional fittings shall be provided so that test gauges may be attached whenever necessary.

1926.803(g)(2)(i) - The headroom in the special decompression chamber shall be not less than a minimum 7 feet and the cubical content shall provide at least 50 cubic feet of airspace for each employee. For each occupant, there shall be provided 4 square feet of free walking area and 3 square feet of seating space, exclusive of area required for lavatory and toilet facilities. The rated capacity shall be based on the stated minimum space per employee and shall be posted at the chamber entrance. The posted capacity shall not be exceeded, except in case of emergency.

1926.803(g)(2)(ii) - Each special decompression chamber shall be equipped with the following:

• 1926.803(g)(2)(ii)(e) - Effective means of oral intercommunication between the attendant, occupants of the chamber, and the air compressor plant; and
• 1926.803(g)(2)(ii)(f) - An observation port at the entrance to permit observation of the chamber occupants.

1926.803(h)(1) - Compressor plant and air supply. At all times there shall be a thoroughly experienced, competent, and reliable person on duty at the air control valves as a gauge tender who shall regulate the pressure in the working areas. During tunneling operations, one gauge tender may regulate the pressure in not more than two headings: Provided, That the gauge and controls are all in one location. In caisson work, there shall be a gauge tender for each caisson.

1926.803(h)(4) - The capacity, arrangement, and number of compressors shall be sufficient to maintain the necessary pressure without overloading the equipment and to assure maintenance of such pressure in the working chamber during periods of breakdown, repair, or emergency.

1926.803(h)(10) - Gauges indicating the pressure in the working chamber shall be installed in the compressor building, the lock attendant's station, and at the employer's field office.

1926.803(i)(2) - The air in the workplace shall be analyzed by the employer not less than once each shift, and records of such tests shall be kept on file at the place where the work is in progress. The test results shall be within the threshold limit values specified in subpart D - occupational health and environmental controls, of this part, for hazardous gases, and within 10 percent of the lower explosive limit of flammable gases. If these limits are not met, immediate action to correct the situation shall be taken by the employer.

1926.803(j)(3) - All electrical equipment and wiring for light and power circuits shall comply with the requirements of subpart K - electrical, of this part for use in damp, hazardous, high temperature, and compressed air environments.

1926.803(l)(1) - Firefighting equipment shall be available at all times and shall be maintained in working condition.

1926.803(l)(2) - While welding or flame-cutting is being done in compressed air, a firewatch with a fire hose or approved extinguisher shall stand by until such operation is completed.

1926.803(l)(14) - All explosives used in connection with compressed air work shall be selected, stored, transported, and used as specified in subpart U - blasting and the use of explosives, of this part.

1926.803(m)(2) - In tunnels 16 feet or more in diameter, hanging walkways shall be provided from the face to the man lock as high in the tunnel as practicable, with at least 6 feet of head room. Walkways shall be constructed of noncombustible material. Standard railings shall be securely installed throughout the length of all walkways on open sides in accordance with subpart M  - fall protection, of this part. Where walkways are ramped under safety screens, the walkway surface shall be skidproofed by cleats or by equivalent means.

1926.803(m)(3)  - Bulkheads used to contain compressed air shall be tested, where practicable, to prove their ability to resist the highest air pressure which may be expected to be used.