
2018 Indiana Design and Installation Manual
15
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3.0 Subsurface Bed Design and Installation
3.0.1 ACCEPTABLE METHODS OF DISTRIBUTION:
Gravity, dosed and pressure distribution are acceptable.
3.0.2 MINIMUM DEPTH FROM ORIGINAL GRADE FOR SUBSURFACE SYSTEMS:
The minimum depth for
subsurface systems is 10 inches from original grade. That is the start of the sand/soil interface for the system.
3.0.3 GENERAL CROSS SECTION
FIGURE 8: SUBSURFACE BED CROSS SECTION
SPECIFIED SAND
12" MIN - 19" MAX
OF COVER SOIL
MATERIAL
13"
ORIGINAL GRADE
FINAL GRADE
All subsurface bed systems are required to have a minimum of:
6 inches of Specified Sand at the edges of the GSF module.
6 inches of Specified Sand at the beginning and end of each GSF Row.
6 inches of Specified Sand directly below the GSF module.
Minimum 12 inches of cover soil material above the 4-inch distribution pipe
Maximum excavation depth from final grade is 36 inches.
3.0.4 VERTICAL SEPARATION TO SEASONAL HIGH-WATER TABLE OR LIMITING LAYER:
Refer to section
Rule 410 IAC 6-8.3 70.b.5 for subsurface absorption systems or ISDH Rule 410 6-10.1 (61).
Vertical Separation Distance (VSD)
System Type
Daily Design Flow
Limiting
Layer
Seasonal High-Water
Table
Subsurface Bed
>450
30”
24”
<450
24”
Pressure Distribution
Any
24”
24”
VSD measured from the bottom of the excavated bed for the subsurface system.
3.0.5 DISTRIBUTION BOX:
Parallel distribution is preferred. Sequential distribution may be utilized for sloping
sites and must conform to 1.1.4. Distribution boxes are recommended to be 5 feet from the proximal end of each
soil absorption field row as per Rule 410 IAC 6-8.3 Section 75.i or ISDH Rule 410 6-10.1 Section 74. This may be
amended to shorter distance per the designer.
3.0.6 PARALLEL DISTRIBUTION:
Parallel distribution is the preferred method of application to a gravity or pump
to gravity system. It encourages equal flows to each of the lines in the system. It is recommended for most bed
systems.
3.0.7 ROWS REQUIRED:
All bed systems shall have a minimum of two rows of modules.
3.0.8 ROW LENGTH:
Rows will have a maximum of 100 ft in length unless pressure dosed per 410 IAC 6-8.3 (74)
(r) or ISDH Rule 410 6-10.1 (82) (r).
3.0.9 EQUAL LENGTH:
Rows must be of equal length in order to provide equal distribution.
Содержание GSF A42
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