Blasting in Excavation Work Under Compressed Air: OSHA 29 CFR 1926.913 Q&A
Subterranean construction projects involving tunnels, shafts, and caissons frequently encounter groundwater and pressure conditions that require working under compressed air. When these operations demand rock fragmentation, conventional blasting methods give way to heightened regulatory controls. Estimators and project teams evaluating subsurface scopes alongside standard guidelines like pricing underground construction must account for the specialized procedures mandated by federal safety standards. Specifically, 29 CFR 1926.913 establishes strict protocols for blasting in excavation work under compressed air. Below is a detailed breakdown of how these rules govern underground site operations.
Can explosives or detonators be stored underground in tunnels, shafts, or caissons?
No. Under federal regulations, detonators and explosives shall not be stored or kept in tunnels, shafts, or caissons. The standard demands an immediate, direct chain of custody from external storage straight to the face of the excavation:
- Detonators and explosives for each round must be taken directly from the magazines to the blasting zone and immediately loaded.
- Any detonators and explosives left over after loading a round shall be removed from the working chamber before the connecting wires are connected up.
Furthermore, designated personnel maintain direct oversight. The blaster or powderman shall be responsible for the receipt, unloading, storage, and on-site transportation of all explosives and detonators. These requirements eliminate unattended underground explosive stockpiles and ensure that unexploded inventory is cleared prior to wiring, complementing broader site protocols found in bidding near blasting checklists.
How must explosives pass through an air lock into pressure working chambers?
Entering a pressurized environment introduces specific physical restrictions at the air lock. When detonators or explosives are brought into an air lock, no employee is permitted to enter except:
- The powderman,
- The blaster,
- The lock tender, and
- The employees necessary for carrying the materials.
In addition to strict personnel limits, no other material, supplies, or equipment shall be locked through with the explosives. Once past the lock, detonators and explosives shall be taken separately into pressure working chambers. This segregation reduces the possibility of premature contact or accidental detonation while moving materials into hyperbaric chambers.
What are the electrical bonding and grounding rules throughout the tunnel?
Stray electrical currents present severe risks during underground blasting operations. To counteract electrical hazards, 29 CFR 1926.913 mandates a continuous, bonded grounding network for all conductive lines and structures:
- All metal pipes, rails, air locks, and steel tunnel lining shall be electrically bonded together and grounded at or near the portal or shaft.
- These metal pipes and rails shall be cross-bonded together at not less than 1,000-foot intervals throughout the length of the tunnel.
- Each low air supply pipe shall be grounded at its delivery end.
Tying all conductive utilities into a single cross-bonded grid minimizes voltage differentials that could inadvertently trigger electric detonators along the tunnel line.
What explosive properties are mandated when blasting wet holes?
Water infiltration is common during caisson sinking and tunnel advancement under air pressure. When blasting in wet holes, the explosives selected for the work must satisfy two specific performance characteristics: they shall be water-resistant and shall be Fume Class 1. Using Fume Class 1 products minimizes the release of harmful toxic gases into an enclosed, pressurized working atmosphere where ventilation management is critical for crew safety.
What procedures apply when tunnel excavation approaches mixed face conditions?
Mixed face conditionsâwhere an excavation intersects both solid rock and softer groundâpose major stability challenges. Under the standard, when tunnel excavation in rock face is approaching mixed face, and when tunnel excavation is in mixed face, blasting shall be performed with light charges and with light burden on each hole.
To safely manage the transition between ground types, advance drilling shall be performed as tunnel excavation in rock face approaches mixed face. This exploratory drilling is required to determine the general nature and extent of rock cover, as well as the remaining distance ahead to soft ground as excavation advances. Identifying these ground transitions in advance prevents over-blasting and preserves ground stability under compressed air.
Sources
The rules detailed above are drawn directly from the following federal standard:
Published October 30, 2026. No affiliate links on this page. Facts come from the sources linked above; check them for updates.