Depressurization systems in sands or other granular soils shall as a minimum and within the practical limits posed by the building, meet the following requirements.
Within the practical limits posed by the building, suction points shall be distributed as nearly equally as possible, and as follows:
- A maximum of 1,300 square feet (121 m2) per suction point, and
- Each required suction point shall be located not less than 6 feet (1829 mm) nor more than 18 feet (5486 mm) from the perimeter; and
- Multiple suction points shall be located within 36 feet (10 973 mm) of each other.
Suction pipe should be of a size appropriate to the air-flows of the system, a minimum of 1/2 inches (38 mm) in diameter at the fan, and shall not be reduced between the fan outlet and the final termination point.
Suction point pits excavated below the slab shall be sized to provide adequate pressure distribution beneath the slab. Dimensions of 22 inches (559 mm) in diameter and 11 inches (279 mm) deep, or excavation of 1 cubic foot (.02832 m3) of soil, shall be presumed to meet this requirement. Further the pit shall be filled with 1 inch (25 mm) size gravel.
Suction fans must be capable of developing minimum flows appropriate to the system at 1 inch water column pressure. Fans producing 100 cubic foot per minute (cfm) (.047 m3/s) at 1-inch water column pressure are presumed to meet this requirement.
Submembrane soil depressurization systems are essentially the same as subslab depressurization systems, but without the cover of a concrete slab. The membrane shall be protected from wind uplift in accordance with locally adopted codes. Systems may be of suction pit or continuous ventilation mat design.
A membrane soil-gas retarder shall consist of a 8 mil or thicker single ply polyethylene sheet or other sheeting material of equal or lower permeability and equal or greater strength. Place sheeting to minimize seams and to cover all of the soil below the building floor. Retarders must provide excellent environmental stress crack resistance, impact strength and high tensile strength including additives to retard polymer oxidation and UV degradation. Where pipes, columns or other objects penetrate the soil-gas retarder, it shall be cut and sealed to the pipe, column or penetration. All seams of the membrane shall be lapped at least 12 inches (305 mm). Punctures or tears in the membrane shall be repaired with the same or compatible material.
Submembrane soil depressurization systems covering sand or other granular soils shall meet the requirements of Section C602.1, with the suction pits filled with 1-inch (25 mm) size gravel which shall be covered by 1/8-inch (3.2 mm) thick steel plate, 16 gage corrugated sheet metal, or equivalent sheets of other termite resistant structural materials, in compliance with existing and applicable codes.
C603.2.4 Depressurization Systems in Sands or Granular Soils With Continuous Ventilation Mat(s) Design
Depressurization systems in sands or other granular soils and utilizing a continuous ventilation mat shall have at least 216 square inches (0.14 m2) of suction area per lineal foot and shall meet the following requirements.
Suction points shall be equally distributed as follows:
- The suction point should be centrally located along the length of each unconnected strip of mat; and
- Mat strips should be oriented along the central axis of the longest dimension of the crawlspace; and
- A minimum of one strip shall be used for crawlspaces having widths up to 50 feet (15 240 mm) [additional strips should be added for each additional crawlspace width of up to 50 feet (15 240 mm) width]; and
- The mat strip shall extend to not closer than 6 feet (1828 mm) of the inner stemwall at both ends of the building; and
- A separate suction point and fan shall be installed for each 100 feet (30 480 mm) linear length of ventilation mat.
Suction pipe shall be a minimum 3-inch (76 mm) diameter and shall be carried full size to the final termination point.
Suction fans must be capable of developing minimum flows of at least 100 cfm (.047 m3/s), at 1-inch water column (.2488 kPa) pressure.