Definitions for underground construction

A bulkhead is an airtight structure that separates the working chamber from free air or from another chamber at lower pressure and is used to maintain pressure differences between sections. See the definition in 1926.804(a).

• Typical uses: blocking off a flooded face, isolating a pressurized working area from a lower-pressure lock or the atmosphere, and protecting workers from sudden inrushes of water or free air.
• Important note: A bulkhead must be air- and water-tight to preserve the intended pressure differential and allow safe movement through locks or emergency procedures.

A caisson is an air- and water-tight wood, steel, concrete or reinforced concrete chamber that lets workers excavate below water level under air pressure greater than atmospheric pressure. See the definition in 1926.804(b).

• Core safety features include: airtight construction, means to control and monitor internal air pressure, suitable locks for entry/exit, and provisions for decompression and emergency treatment (medical lock or chamber).
• Use implication: Because workers are under increased pressure, employers must follow compressed-air work safety practices (decompression schedules, medical monitoring, and rescue capability).

Decanting is an emergency decompression method where employees are first brought to atmospheric pressure with high dissolved gas in their tissues and then immediately recompressed in a second, separate chamber or lock. See the definition in 1926.804(c).

• When used: only in emergency circumstances when standard decompression cannot be completed safely or rapidly enough.
• Safety implications: Decanting carries a high risk of decompression illness and therefore requires trained personnel, appropriate equipment (separate recompression chamber/lock), and medical oversight.

An emergency lock is a lock designed to hold and permit the quick passage of an entire shift of employees in an emergency. See the definition in 1926.804(d).

• Functional needs: large enough capacity to pass all workers at once, rapid operation under pressure differences, and reliable seals to maintain pressure after use.
• Practical considerations: Employers should ensure emergency locks are tested, accessible, and that workers are trained in emergency egress procedures using the lock.

"High air" means air pressure used to supply power to pneumatic tools and devices. See the definition in 1926.804(e).

• Practical point: High-air lines are separate from "low air" that pressurizes chambers and must be managed to prevent accidental over-pressurization of living spaces.
• Safety tip: Ensure high-air piping, couplings, and tools are rated for the pressures used and are maintained to avoid leaks or sudden pressure surges.

"Low air" is the air supplied to pressurize working chambers and locks, distinct from "high air" which powers pneumatic tools. See the definition in 1926.804(f).

• Key difference: Low air maintains a breathable, pressurized environment for workers; high air delivers mechanical power at higher pressures.
• Operational note: Systems must prevent cross-connection so tools do not pressurize chambers inadvertently and compromise decompression control.

A man lock is a chamber through which workers pass from one air-pressure environment into another. See the definition in 1926.804(g).

• Use: for controlled entry and exit between atmospheric pressure, low-pressure locks, and higher-pressure working chambers during normal shifts and decompression procedures.
• Safety practice: Workers must follow decompression schedules when transiting man locks and only use them under trained supervision to avoid decompression sickness.

A materials lock is a chamber through which materials and equipment pass between air-pressure environments and is intended for goods rather than personnel. See the definition in 1926.804(h).

• Operational difference: Materials locks are sized and operated for cargo transfer; they often have different cycling procedures and controls to avoid unnecessarily exposing workers to pressure changes.
• Practical tip: Keep materials lock operation distinct from man locks and ensure sealing and pressure-equalization procedures are followed to protect adjacent working chambers.

A medical lock is a special chamber for treating employees with decompression illness and may also be used in preemployment physicals to test adaptability to pressure changes. See the definition in 1926.804(i).

• When required: Employers conducting compressed-air operations should provide access to medical treatment facilities capable of recompression and treatment for decompression sickness.
• Additional use: Medical locks support medical screening procedures to confirm that prospective employees can tolerate pressure changes before assignment.

"Normal condition" means workers have a single continuous compressed-air work period followed by a single decompression in any 24‑hour period, and no return to compressed air until they have at least 12 consecutive hours at normal atmospheric pressure. See the definition in 1926.804(j).

• Practical effect: Employers must schedule shifts and decompressions so employees are not exposed to repeated pressurizations within 12 hours of decompression unless different protocols apply.
• Scheduling tip: Track individual workers’ pressure exposures to ensure compliance with the 12-hour rest-at-atmospheric rule.

Pressure is force per unit area (commonly pounds per square inch), absolute pressure (p.s.i.a.) equals atmospheric pressure plus gauge pressure (p.s.i.g.), and atmospheric pressure at sea level is about 14.7 p.s.i.a. See 1926.804(k), 1926.804(l), and 1926.804(m).

• Formula: p.s.i.a. = p.s.i.g. + atmospheric pressure (~14.7 p.s.i.a. at sea level).
• Example: If a gauge reads 10 p.s.i.g., absolute pressure = 10 + 14.7 = 24.7 p.s.i.a.

Gauge pressure (p.s.i.g.) is the pressure measured by a gauge and indicates the pressure exceeding atmospheric pressure. See the definition in 1926.804(n).

• Importance: Decompression schedules and chamber integrity are based on absolute pressure, but gauges commonly report p.s.i.g., so operators must convert gauge readings to absolute pressure when applying decompression tables.
• Operational reminder: Always account for local atmospheric pressure variation (elevation) when converting or applying decompression protocols.

A safety screen is an air- and water-tight diaphragm placed across the upper part of a compressed-air tunnel between the face and bulkhead to prevent flooding of the tunnel crown and to provide a refuge and exit from a flooding or flooded tunnel. See the definition in 1926.804(o).

• Function: It limits water intrusion into the tunnel crown so workers have a protected route to a bulkhead or lock during a flood event.
• Installation note: Safety screens must be airtight and positioned so they create a usable refuge space and escape path above the working face.

A special decompression chamber is provided to give greater comfort to employees when the total decompression time exceeds 75 minutes. See the definition in 1926.804(p).

• Practical trigger: If decompression schedules for the work exceed 75 minutes, employers should furnish chambers equipped for worker comfort (seating, lighting, ventilation) during lengthy decompression.
• Worker welfare: Longer decompressions increase the need for safe, habitable space to reduce stress and medical complications.

The working chamber is the space or compartment under air pressure in which the work is being done. See the definition in 1926.804(q).

• Operational implications: The working chamber must be maintained at the required low-air pressure, equipped with safe egress via man locks, and have monitoring for pressure and gas composition.
• Safety controls: Employers must manage decompression, emergency procedures, and access to medical assistance for personnel in the working chamber.

Employers must convert gauge readings to absolute pressure and use absolute pressures when applying decompression criteria because decompression depends on absolute pressure (atmospheric + gauge). See 1926.804(l) and 1926.804(n).

• Practical step: Add the local atmospheric pressure (approx. 14.7 p.s.i.a. at sea level) to the p.s.i.g. reading to get p.s.i.a. before consulting decompression tables.
• Note on elevation: Adjust atmospheric pressure for site elevation when precision is required.

No — Returning to compressed air within 10 hours does not meet "normal condition" because the definition requires at least 12 consecutive hours at atmospheric pressure before a second exposure. See 1926.804(j).

• Compliance implication: Employers scheduling shifts must ensure at least 12 continuous hours at normal atmospheric pressure between compressed-air exposures for the same worker under normal-condition rules.
• Exceptions: If operations require different regimes, alternative decompression or medical controls must be in place per applicable standards and guidance.

Materials should be moved through a materials lock rather than a man lock because 1926.804(h) defines a materials lock specifically for equipment and supplies, while 1926.804(g) defines the man lock for personnel.

• Why: Materials locks are designed and operated for cargo transfer and help avoid unnecessary decompression cycles or contamination of personnel locks.
• Best practice: Keep procedures and control systems separate for man locks and materials locks to maintain safety and operational efficiency.

Yes — A medical lock may be used both to treat decompression illness and to perform preemployment physical examinations to assess an applicant's adaptability to pressure changes. See 1926.804(i).

• Operational note: Use for medical screening should be conducted under qualified medical supervision and follow appropriate medical confidentiality and consent practices.
• Emergency readiness: Maintain medical locks so they are immediately available and staffed or reachable for treatment when compressed-air operations are in progress.

A safety screen should be used whenever there is a risk of flooding the tunnel crown between the face and bulkhead so that workers have a safe refuge and escape route; the definition emphasizes this protective purpose. See 1926.804(o).

• Considerations: Assess geological and water-inflow risks at the face; install a safety screen when those risks could flood the crown and block egress.
• Design requirement: The safety screen must be air- and water-tight and positioned to provide a usable refuge area.

When total decompression time exceeds 75 minutes, a special decompression chamber should be provided to improve employee comfort during decompression. See 1926.804(p).

• Comfort features: These chambers typically include seating, lighting, ventilation, and facilities to make long decompressions tolerable and safe.
• Operational planning: Incorporate the use of special chambers into shift and decompression planning when decompression times near or exceed 75 minutes.