Section 905.60  Subsurface Seepage System Construction Requirements

 

a)         Seepage Field Requirements – Gravel, Gravelless and Chamber Systems.  Subsurface seepage fields shall be designed and constructed in accordance with Appendix A, Illustrations H, I, and J of this Part and the following:

 

1)         All subsurface seepage systems using soils information for sizing shall use the soil suitability table in Appendix A, Illustration M of this Part to determine the size requirements of the subsurface seepage system.  The least permeable soil profile between the top of the gravel or gravelless pipe or chamber system and the limiting layer shall be used to determine the size of the subsurface seepage system.

 

2)         The bottom of the subsurface seepage field, each trench and its distribution line shall be level.  Level for this Part shall mean plus or minus ½ inch in any direction over the entire area of the subsurface seepage system.

 

3)         There shall be a minimum of 6 inches and a maximum of 24 inches of earth backfill over the bedding materials, gravelless pipe or chamber system.

 

4)         There shall be a minimum of 5 feet of undisturbed earth between the septic tank and the nearest trench.

 

5)         If precipitation falls onto the excavation and evidence of soil washing into the excavation of the subsurface seepage system exists, that portion of the seepage system damaged shall be reconstructed to conform with this Section.

 

6)         The top of the gravel, gravelless pipe, or chamber system in the subsurface seepage field shall be at least one inch below the invert of the outlet pipe from the septic tank or distribution box in a gravity flow system.

 

7)         Site evaluation for subsurface seepage systems.  Subsurface seepage systems receiving septic tank effluent should have at least 2 feet of vertical separation distance between the bottom of the subsurface seepage system and the top of the limiting layer.  For soils in Design Group I-VI or with a loading rate of greater than .62 gallons per day per square foot, there should be a vertical separation of 3 feet between the bottom of the subsurface seepage system and the top of the limiting layer.

 

8)         Sizing of a seepage system in fill soil.

 

A)        The least permeable soil profile between the top of the gravel, gravelless pipe, or chamber system and the limiting layer shall be used to determine the size of the subsurface seepage system.

 

B)        The use of fill for installing subsurface seepage systems shall not be approved for lots platted after March 15, 1996.

 

C)        Fill soils may be used to cover a private sewage disposal system provided no part of the system is located in the fill and the fill material is at least equal to or better than the original soil or meets the requirements in subsection (a)(9) of this Section.

 

9)         Soil criteria for use of fill for subsurface seepage systems.

 

A)        Soils to be utilized for fill shall be identified by a soil classifier or licensed professional engineer and a report submitted to the Department or local authority.  The report shall contain specific information on the fill soil, including location, depth, permeability, and texture.  Soils that can be used as fill are those identified in Appendix A,  Illustration M of this Part as 2A, 2K, 3A, 3B, 3C, 3K, 3L, 4B and 4K (Design Group II, III and IV).

 

B)        In addition to the above requirements, fill soil shall not contain extraneous material such as tires, concrete, brick, reinforcing bar, demolition material, etc.

 

C)        All of the following conditions shall be met for a subsurface seepage system to be installed in fill.

 

i)          Satisfactory original soil shall be at least 3 feet above bedrock.

 

ii)         A maximum of 2 feet of fill soil shall be used.

 

iii)         Fill shall not be placed on original soil with a slope greater than 10%.

 

iv)        The fill shall be placed at the site so that a minimum of compaction occurs and the fill shall be allowed to settle undisturbed for a period of at least 12 months.  Soils in Design Group II, when used for fill, shall not be required to settle for a period of at least 12 months.

 

v)         After the fill has been settled, a percolation test shall be conducted in accordance with the procedure outlined in Appendix A,  Illustration G of this Part and a percolation rate of not greater than 270 minutes/6 inch fall or less than 60 minutes/6 inch fall shall be achieved.

 

10)         Site Preparation for use of fill soil.

 

A)        Excess vegetation shall be cut and removed.  The site shall be plowed with a mold board plow 7 to 8 inches deep with the plowing done perpendicular to the slope.  It shall not be done with the furrow running up and down the slope. Chisel plowing may be used in place of mold board.  Roto-tilling is prohibited.

 

B)        Once the site is plowed, all traffic must be kept off the site.  The fill material can be deposited on the top with a backhoe or pushed on from the side, preferably the upslope side, using a track type tractor, keeping 6 inches of fill beneath the tracks.  At no time shall ruts be made in the plowed area.  The fill shall be placed immediately after site preparation to avoid the possibility of precipitation falling on the plowed area.

 

C)        Traffic on the downslope side of the fill area shall be minimal to reduce compaction.  All work shall be performed from the ends and upslope side. Compaction of the natural soil downslope will reduce the lateral movement of the effluent.

 

D)        The fill shall not be placed on frozen ground or when the soil is wet. Moisture content of the soil is very important when filling.  Site preparation shall not take place when the soil is too wet.  To check moisture content, take a soil sample from the plow layer (7 to 8 inches) and roll it between the palms of the hands.  If the soil rolls into a ribbon, it is too wet to prepare.  If the soil crumbles, site preparation can then proceed.

 

b)         Gravel Seepage Field Requirements.

 

1)         Bedding Material.  The bedding material shall be clean gravel or clean stone that is free of mud, silt, or clay, with particle size ranging from ¾ inch minimum to 4 inches maximum.  The bedding material shall extend the full width of the trench and to a depth of at least 6 inches below the bottom of the distribution line.  The bedding material shall extend at least 2 inches above the top of the distribution line.

 

2)         Distribution Lines.  Distribution lines shall be constructed of materials as approved in Section 905.20(f).  The lines shall be perforated or open-joint tile.  Where open joint tile is used, the tile sections shall be spaced not less than ¼ inch or more than ½ inch apart.  Perforated piping with the exception of 8 inch or 10 inch gravelless seepage beds shall have ½ to ¾ inch diameter openings on 3 to 5 inch centers with a minimum of 2 rows. The openings in the pipe shall be placed downward.

 

3)         Separation Material.  Bedding materials shall be covered by straw, newspaper, untreated building paper, geotextile fabric or other permeable or biodegradable material to support the backfill as the laying of the distribution line proceeds.  Tar paper, plastic, or other impervious material shall not be used between the bedding material and the earth backfill.

 

4)         The ends of a gravel seepage field shall be looped except in serial distribution systems.

 

c)         Gravelless Seepage Field Requirements.  In addition to Section 905.20(f), 8 or 10 inch gravelless seepage systems shall comply with the following specifications:

 

1)         8 and 10 inch inside diameter (I.D.) corrugated polyethylene tubing shall meet the requirements of ASTM F667-84, Standard Specification for Large Diameter Corrugated Polyethylene Tubing with the following exceptions:

 

A)        Perforations shall be uniformly spaced along the length of the tubing as follows:  2 rows of holes ⅜ inch in diameter for 8 inch tubing and ½ inch in diameter for 10 inch tubing, located 120° to 140° apart along the bottom half of the tubing, each row 60° to 70° up from the bottom center line.  The perforations shall be staggered so that there is at least one hole in each corrugation.

 

B)        The pipe shall be marked to indicate the top of the pipe.

 

2)         All gravelless drainfield pipe shall be encased at the point of manufacture with a filter wrap having the following characteristics:

 

Physical Properties	Minimum Value
Grab Strength, lbs. (ASTM D1682-64 – Reapproved 1975 or ASTM D4632)
Machine Direction	19
Traverse Direction	11
Burst strength, psi (ASTM D3786-80a)	26
Air Permeability, cfm per sq. ft. (ASTM D737-75, Reapproved 1980)	500
Particle Size Distribution (ASTM F662-80) Polyethylene particles in water and alcohol solution, coulter counter analysis, single pass:
Particle Size (Microns)	% Retained
70	80
60	68
50	56
40	40
30	22
20	5

 

3)         8 or 10 inch gravelless seepage trenches shall comply with all requirements that apply to standard gravel trench systems as stated in Appendix A unless otherwise stated in this Part.

 

4)         Bedding Material.  8 and 10 inch gravelless seepage systems or chamber systems may be bedded with material excavated to construct the system.  The backfill material shall not contain large clods of earth, demolition material or other extraneous material.

 

5)         Separation Material. No straw, newspaper or untreated building paper shall be placed between the gravelless seepage system or chamber system and the earth backfill.

 

6)         Bending.  8 inch and 10 inch gravelless pipe shall not be bent around corners on a radius of less than 5 feet.  If a sharper radius is required, a tee shall be used.

 

7)         Gravelless seepage systems or chamber systems are not required to be looped.  Gravelless seepage systems or chamber systems that are not looped shall be capped on the end.

 

d)         Serial Distribution.  Serial distribution shall be used in areas where the slope of the terrain prohibits the installation of conventional subsurface seepage systems.  The following criteria shall be used in the design and construction of a serial distribution system: (Appendix A,  Illustration K of this Part)

 

1)         The bottom of each trench and its distribution line shall be level.

 

2)         There shall be a minimum of 6 inches of earth backfill over the bedding material or chamber system or the gravelless pipe in the trenches.

 

3)         The trench shall follow the ground surface contours so that variation in trench depth will be minimized.

 

4)         There shall be a minimum of 5 feet of undisturbed earth between the septic tank and the nearest trench.

 

5)         Adjacent trenches shall be connected with a relief line or a drop box arranged so that each trench is completely filled to the full depth of the gravel or gravelless pipe or chamber system before effluent flows to the succeeding trench.

 

6)         The relief lines connecting the trenches shall have watertight joints and direct connections to the distribution lines in adjacent trenches.  Tight joint T's and 45° ells, or a drop box arrangement shall be used to connect adjacent trenches.

 

7)         Where the relief pipe trench connects with the higher trench, it shall not be deeper than the top of the gravel or gravelless pipe or chamber system in the higher trench.  Relief lines shall rest on undisturbed earth and the backfill shall be carefully tamped.

 

8)         The invert of the first relief line shall be at least one inch lower than the invert of the septic tank or aerobic treatment plant outlet.  (See Appendix A,  Illustration K of this Part.)

 

9)         All other construction features of the serial distribution field shall comply with subsections (a) through (d) of this Section.

 

e)         Seepage Beds.  The total bottom area of the seepage bed shall be 1½ times the area specified in Appendix A,  Illustration H of this Part. Construction features shall conform to subsections (a) and (b) of this Section. Distribution lines shall be spaced no further than 6 feet center to center and shall be equally spaced.  Lines adjacent to the bed sidewalls shall be 18 inches from the bed sidewall.  (See Appendix A,  Illustration L of this Part.) Seepage beds shall be constructed so that construction equipment does not drive over the bottom of the bed.

 

f)          Chamber Systems.  Chamber systems shall be sized and installed in accordance wit the following:

 

1)         The minimum center to center spacing of chambers shall be 7 feet.

 

2)         Chamber systems shall be sized in accordance with Appendix A, Illustration I, Exhibit E.

 

3)         Chamber systems shall be designed to support all weight of earth backfill without collapsing.

 

4)         Chamber systems shall be designed to prevent earth backfill from restricting flow within the chamber.

 

(Source:  Amended at 27 Ill. Reg. 3074, effective February 10, 2003)