UNI-COAT®—

Installation Guidelines for Installing Underground Duct

Detailed guidelines for designing and installing underground duct systems are provided in McGill AirFlow’s Engineering Report No. 155, Underground Duct Design and Engineering Report No. 95, Underground Duct Installation.

1. Trenches must be pitched to prevent the buildup of water around the ductwork.

2. Appropriate girth angle structural reinforcement must be installed on the ductwork. Such reinforcement should be designed by a structural engineer based on local conditions.

3. Ductwork encased in concrete must be tied down to avoid floating during pouring.

4. Fill or concrete must not be poured directly onto the ductwork. It should be poured in successive layers and tamped firmly around the ductwork. Pouring fill or concrete directly onto the ductwork will cause denting or collapse.

5. Vapor barriers must always be used between the ductwork and the fill or concrete.

6. Ductwork must always be buried above the water table.

7. Use the metal thicknesses listed in McGill AirFlow’s specifications for underground ductwork.

8. Use the following load specifications: (Refer to McGill AirFlow’s Engineering Report No. 155 for conditions other than those listed.)

Assumptions

Soil Modulus = 200 psi
Soil Density = 120 lb/ft3
Soil Depth = 5 ft

Loading specifications for duct larger than 36 inches in diameter have not been determined. Although duct as large as 60 inches in diameter has been used successfully in underground systems, it should be installed only by contractors experienced in this application. For any diameter duct, metal thickness can be increased to a maximum of 18 gauge. Angle rings or other bracing can be added for additional reinforcement. All external reinforcement should be primed and coated with UNI-COAT paint.

Under-Slab, Above Grade

The duct, either encased in concrete or buried directly below a concrete slab, is installed above the original line of undisturbed soil and above the water table. Encasing the duct in concrete with a porous fill beneath it is the best way to install UNI-COAT duct. An appropriate vapor barrier (e.g., 4 mil polyethylene) must be placed between the duct and the concrete.

If the duct is too large for concrete encasement, it should be buried in a porous fill, and a vapor barrier must be placed between the fill and the duct wall. A 2 1/2-inch minimum thickness concrete slab should be placed over the duct, with the top of the duct, near the bottom of the slab. A vapor barrier must also be placed between the slab and the fill.

Under-Slab, Below Grade

The duct, either encased in concrete or buried directly below a concrete slab, is installed below the original grade, but safely above the water table. This installation requires that the duct be buried in a trench with porous fill and an appropriate vapor barrier.

The first 12 inches of fill must be carefully shoveled on top of the duct. The fill must not be dumped or shoved directly into the trench and must not contain stones larger than 2 inches in diameter. Once the duct is properly covered, the fill must be firmly tamped under and around the duct, but must not be heavily impacted. Light equipment such as small bulldozers or backhoes must not be operated near the area until there is at least 2 1/2 feet of compacted cover.

Heavy equipment, such as bulldozers and loaded dump trucks, must not be operated near the site until there is at least 5 feet of compacted cover. Bridging is required if maximum loading is exceeded. Refer to McGill AirFlow’s Engineering Report No. 155 when determining maximum depth of burial.

Underground, Not Under-Slab

The duct is buried above the water table, avoiding groundwater infiltration, but is not beneath or encased in concrete. The installation procedure is the same as for under-slab, below grade. Make sure that at least 12 inches of compacted cover exists. Any equipment that can cause point loads above the ductwork (even light loads) must not be operated near the area.

UNI-COAT duct and fittings are not recommended for installations in which water infiltration might occur.

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