Construction Concerns: Underground Confined Spaces

Article and photos by Gregory Havel

When we think of undergroundconfined spaces, we usually think of sewer manholes, the most common type. Although they meet the definition in OSHA 29 CFR 1910.146, “Permit-Required Confined Spaces,” they are usually assembled of precast concrete sections when they are built and pose limited hazards to the workers (and to firefighters andEMSpersonnel responding to work sites) besides the usual hazards in working in and around excavation. These usual hazards include access to the site on foot and in vehicles, unstable ground, excavations, and spoil piles and access into the space underconstruction. According to the proposed revisions to OSHA’s 29 CFR 1926 Subpart P, excavations and trenches will be consideredconfined spaces, and under some conditions under both present and proposed regulations, they may be permit-required confined spaces even though they are open to the atmosphere.

Other types of underground confined spaces are larger, more complex, and more hazardous; they can become Permit-Required Confined Spaces (PRCS) even before they are completed. They include tanks and other underground structures that are fabricated in place at water and wastewater utility plants and at industrial locations.

(1)

Photo 1 shows the excavation for a water utility process tank being built next to an existing pumping station. This excavation is as deep and as large as the basement for an ordinary-sized single-family residence.

(2)

Photo 2 shows this underground tank under construction. The foundations, floor, and walls of reinforced concrete have been completed. The falsework, forms, and reinforcing steel have been installed, and it is almost ready for the concrete to be poured for the top of the tank. At some time before the plywood form for the concrete top was completed, this tank became a PRCS instead of a simpler confined space. This is because of the more restricted access to the interior, reduced ventilation, and the presence of concrete form-release agents on the forms, which are often kerosene-based and which can produce flammable vapors that will accumulate inside the tank. Note the forms for the three 30-inch round openings in the top of the tank, two of which will be used by pipes and process equipment. The third will be the manhole with ladder to the interior and will be posted as a PRCS when it is complete.

After the concrete top had cured to the minimum strength specified by the engineers, the falsework and formwork inside the tank needed to be removed. Photo 3 shows the tank with its top complete, with temporary covers attached to one of the three openings. The interior of this tank was considered a PRCS until all three holes were opened, mechanical ventilation was established, and continuous air monitoring with a four-gas meter proved that the oxygen content was normal and that hydrogen sulfide, carbon monoxide, and flammable vapors were absent. At that time, the interior of this tank was declassified to a non-permit-required space, since the only potential hazard was atmospheric. Mechanical ventilation and air monitoring continued until work was completed inside the tank. Workers were trained for this type of confined-space entry, equipped with harnesses and lifelines, and retrieval equipment was set up for use if needed. Control of hazardous energy (lock out-tag out) as required by OSHA’s 29 CFR 1910.147 was not needed at this time, since the permanent connection to the sanitary sewer had not been completed (lower left in photo 3).

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After the formwork was removed from inside the tank, the temporary covers were replaced the permanent connection to the sanitary sewer system was made (lower left in photo 3). and the excavation was backfilled (photo 4).

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At this time, the interior of the tank can no longer be declassified from PRCS to non-permit space because of its limited access and permanent connection to the wastewater system.

Underground confined spaces like the tank shown in the photos, and also reservoirs that are many times larger, can pose challenges during response to fires and EMS incidents:
  • Access to the site is often limited or awkward because of excavations, spoil piles, rough temporary roadways, ramps, and ladders.
  • 进入the confined space is limited by the phase of construction, the size of access openings, and the need for ladders into excavations and onto new structures.
  • Combustible materials are used to support concrete that has not cured enough to be self-supporting.
  • Flammable vapors, toxic gases, or oxygen-deficient atmospheres can be present inside the spaces under construction.
  • Technical rescue procedures are needed, including lifelines, retrieval devices, and use of SCBA and air monitoring.
  • The ground in the area of the new structure can become unstable because of the weight of apparatus parked in the area or because of saturation with firefighting water.

A working relationship with the building department and facility managers can provide emergency services agencies with the information they need for preincident planning construction job sites like these to reduce the risk to their personnel if a response is needed.

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Gregory Havel is a member of the Burlington (WI) Fire Department, a retired deputy chief and training officer, and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College, has more than 30 years of experience in facilities management and building construction, and has presented classes at FDIC.

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