Current through Register Vol. 48, No. 12, March 22, 2024
a) A mound
system shall include a septic tank for pre-treatment of sewage. The septic tank
and piping between the septic tank and the pumping chamber shall conform to the
applicable rules in the Private Sewage Disposal Code (77 Ill. Adm. Code
905).
b) The design of the mound
shall be based upon the expected daily waste water volume using the data
contained in Appendix A, Illustration A, of the Private Sewage Disposal Code,
(77 Ill. Adm. Code 905) and the soil percolation rate. Mounds shall be sized
such that they can accept the daily waste water flow without surface seepage,
and the basal area, which is the natural soil area beneath the mound, shall be
sufficiently large to conduct the effluent into the underlying topsoil. The
system shall also be designed to avoid encroachment of the water table into the
mound.
c) For homes having up to
and including 4 bedrooms or for flows less than 800 gallons per day the mound
shall be designed in accordance with
906.60.
For homes with 5 or more bedrooms or for flows greater than or equal to 800
gallons the mound shall be designed in accordance with
906.70.
d) Design of the Absorption Area
1) Sizing the absorption area. The size of
the absorption area is dependent upon the daily waste water flow. The design
infiltration capacity of the fill material shall be 1.2
gal/ft2/day.
2) Absorption Area Design
A) System configuration. The absorption area
within the mound shall be constructed as trenches or beds. An illustration of
construction using trenches and bed is shown in Appendix A, Illustration A
through D. The location of the water table and soil permeability will dictate
whether a trench or bed shall be used. In slowly permeable soils, two or three
narrow parallel trenches shall be used instead of a bed. Trench widths shall be
between 24 and 48 inches. For permeable soils either a narrow rectangular bed
or two or three narrow parallel trenches may be used. Bed widths shall not be
greater than 10 ft.
B) On sloping
sites, the trenches and beds shall be situated perpendicular to the slope in
order to prevent the concentration of effluent into a small area as it moves
laterally downslope. Sufficient basal area shall be provided so all the
effluent infiltrates into the natural soil before it reaches the toe of the
mound. With a trench system, the trench spacing shall be such that the effluent
from an upslope trench shall be absorbed by the natural soil before reaching
the area under the next trench downslope.
C) The bottom of the absorption area within
the bed and trenches shall be level and at the same elevation.
e) Mound Dimensions
1) Mound height. The mound height shall
consist of the fill depth (D & E), the trench or bed depth (F), and the cap
and topsoil depth (G & H) as shown in Appendix A, Illustration A through D
for trench and bed construction respectively. A minimum of 1 foot of fill is
required under the bed or trenches. For sites where the soil depth is less than
3 feet over creviced bedrock, the fill depth (D) shall be a minimum of 2
feet.
2) Bed or trench depth (F).
The depth of the bed or trenches shall be at least 10 inches. A minimum of 6
in. of aggregate shall be placed beneath the distribution pipe. Clean, 1/2-2
inch stone shall be used. The use of soft limestone is prohibited.
3) Cap and topsoil (H & G). The depth of
soil over the aggregate at the apex (H) shall be a minimum of
1.5 ft. For a 3 parallel
trench system, the depth shall be a minimum of 2 ft. At the outer edge of the
gravel the cap and topsoil shall be at least 1 ft. deep. The cap shall be
topsoil or finer textured subsoil. A minimum of 6 inches of topsoil shall be
placed over the entire mound. The topsoil shall be seeded with grass seed to
control erosion.
4) Side and end
slopes. Side and end slopes shall be no steeper than one foot vertical rise in
3 feet horizontal.
f)
Basal Area
1) The basal area is the natural
soil-fill interface of the mound. The basal area required shall be dependent
upon the soil and site conditions. For level sites, the total basal area
beneath the mound can be used. For sloping sites the only basal area which may
be considered for design is the area beneath and downslope of the bed or
trenches (see Appendix A, Exhibit C). The percolation rate of the natural soil
shall determine the mound area required. For the percolation rates shown the
following design loading rates shall be used:
A) 60 min - 1.2
gal/ft2/day
B) 180 min - .74
gal/ft2/day
C) 360 min - .24
gal/ft2/day
2) If sufficient basal area is not available
for the given design and site conditions, additional fill shall be used to make
the mound wider for a level site or the fill used to extend the downslope width
on a slope site until sufficient area is available.