The sand mound consists of a sand fill, an aggregate bed and
a soil cap.
(a) Selection Criteria:
The high groundwater level, bedrock, or impervious clay layer
is less than four (4) feet below the bottom of the soil absorption system
excavation.
(b) Site
Requirements:
(i) A minimum of one (1) foot of
vertical separation of the native soil is required between the bottom of the
sand fill and the top of the high groundwater level, any restrictive layer, or
any highly permeable material.
(ii)
The percolation rate of the native soil at the interface of the sand fill shall
be greater than five (5) and less than sixty (60) minutes per inch (5-60 mpi).
The percolation shall be measured in the top twelve (12) inches of native
soil.
(c) General Design
Requirements:
(i) Sand Layer
(A) Filter sand shall conform to ASTM C-33,
with less than two percent (2%) passing through the #200 sieve.
(B) The minimum depth of sand below the
aggregate bed surface shall be one (1) foot.
(C) The sand mound shall have a combination
of at least four (4) vertical feet of filter sand and unsaturated native soil
above the high groundwater level.
(I) For sand
mounds using pressure distribution systems, the depth to high groundwater shall
be three (3) feet below the bottom of the absorption surface if the percolation
rate of the soil is five (5) minutes per inch or greater (5-60 mpi).
(D) The top of the sand layer
under the aggregate bed shall be level in all directions.
(E) The sand layer shall fill around the
perimeter of and to the top of the aggregate bed.
(F) The slope of all sides shall be three (3)
horizontal to one (1) vertical or flatter. The side slopes shall be graded to
prevent seepage and/or ponding at the bottom of the slope.
(G) The infiltration area, which is the
bottom of the sand fill, shall be calculated by dividing the design flowrates
(gpd) from Table 1 or Table 2 by the loading rate
(gpd/ft2) found in Table 5.
(ii) Aggregate Bed
(A) The aggregate shall be crushed rock,
gravel or other acceptable, durable and inert material that is free from fines,
and has an effective diameter between one-half (1/2) inch and two and one half
(2 1/2) inch.
(B) The aggregate
bed depth shall not be less than nine (9) inches with a minimum of six (6)
inches of clean aggregate placed below the distribution pipe and two (2) inches
above the distribution pipe. The aggregate shall be covered with an approved
geotextile material after installation and testing of the pressure distribution
system.
(C) The design shall be a
long, narrow bed design with a maximum width of twenty-five (25)
feet.
(D) The infiltration area,
which is the bottom of the aggregate bed, shall be calculated by dividing the
design flowrates (gpd) from Table 1 and Table 2 by the loading rate of 0.8
gpd/ft2.
(iii) Soil Cover
(A) The soil cap shall be constructed of a
sandy loam, loamy sand, or silt loam. The depth of the soil cap shall be at
least six (6) inches at the edges to twelve (12) inches at the center. The
slope of all sides shall be three (3) horizontal to one (1) vertical or
flatter.
(B) A layer of top soil at
least six (6) inches thick shall be placed over the entire sand mound area. The
sand mound should be planted with vegetation that does not require watering and
will not establish deep roots. Native grasses are commonly used.
(d) A design package
for sand mound systems is provided online at the Division's website to assist
the applicant in submitting a completed application for coverage under the
general permit for small wastewater systems. The worksheet and calculations
were prepared by a registered professional engineer employed by the Wyoming
Department of Environmental Quality, Water Quality Division. The general design
requirements stated in this section are incorporated into the worksheets such
that by properly completing the forms and installing the components, the system
will comply with these requirements.
