Mound Systems

This chapter governs the design and installation requirements for mound systems used in wastewater treatment.

901 — General

901.1 Scope. The provisions of this chapter shall govern the design and installation of mound systems.

902 — Soil and Site Requirements

902.1 Soil borings. A minimum of three soil borings per site shall be conducted in accordance with Chapter 4 to determine the depth to seasonal or permanent soil saturation or bedrock. Identification of a replacement system area is not required.

902.2 Prohibited locations. A mound system shall be prohibited on sites not having the minimum depths of soil specified in Table 902.2. The installation of a mound in a filled area shall be prohibited. A mound shall not be installed in a compacted area or over a failing conventional system.

A table appears here in the adopted code. Refer to the official source for the full table.

902.3 Slowly permeable soils with or without high ground water. Percolation tests shall be conducted at a depth of 20 inches to 24 inches (508 mm to 610 mm) from existing grade. Where a more slowly permeable horizon exists at less than 20 inches to 24 inches (508 mm to 610 mm), percolation tests shall be conducted within that horizon. A mound system shall be suitable for such site condition where the percolation rate is greater than 60 minutes per inch and less than or equal to 120 minutes per inch (2.4 min/mm to 4.7 min/mm).

902.4 Shallow permeable soils over creviced bedrock. Percolation tests shall be conducted at a depth of 12 inches to 18 inches (305 mm to 457 mm) from existing grade. Where a more slowly permeable horizon exists within 12 inches to 18 inches (305 mm to 457 mm), percolation tests shall be conducted within that horizon. A mound system shall be suitable for such site condition where the percolation rate is between 3 minutes per inch and 60 minutes per inch (0.12 min/mm and 2.4 min/mm).

902.5 Permeable soils with high ground water. Percolation tests shall be conducted at a depth of 20 inches to 24 inches (508 mm to 610 mm) from existing grade. Where a more slowly permeable horizon exists at less than 20 inches to 24 inches (508 mm to 610 mm), percolation tests shall be conducted within that horizon. A mound system shall be suitable for such site condition where the percolation rate is between 0 minutes per inch and 60 minutes per inch (0 min/mm and 2.4 min/mm).

902.6 Depth to pervious rock. A minimum of 24 inches (610 mm) ofunsaturated natural soil shall be over creviced or porous bedrock.

902.7 Depth to high ground water. A minimum of 24 inches (610 mm) of unsaturated natural soil shall be present over high ground water as indicated by soil mottling or direct observation of water in accordance with Chapter 4.

902.8 Slopes. A mound shall not be installed on a slope greater than 6 percent where the percolation rate is between 30 and 120 minutes per inch (1.2 and 4.7 min/mm). The maximum allowable slope shall be 12 percent where there is a complex slope (two directions).

902.9 Location of mound on sloping sites. The mound shall be located so the longest dimension ofthe mound and the distribution lines are perpendicular to the slope. The mound shall be placed upslope and not at the base of a slope. The mound shall be situated so the effluent is not concentrated in one direction where there is a complex slope (two directions). Surface water runoff shall be diverted around the mound.

902.10 Depth to rock strata or 50 percent by volume rock fragments. A minimum of 60 inches (1524 mm) of soil shall be present over uncreviced, impermeable bedrock. Where the soil contains 50 percent coarse fragments by volume in the upper 24 inches (610 mm), a mound shall not be installed unless there is at least 24 inches (610 mm) of permeable, unsaturated soil with less than 50-percent coarse fragments located beneath this layer.

Worked example

Scenario: A designer is planning a new single-family home that requires a mound system for wastewater treatment.

Explanation: Section 902.1 mandates that at least three soil borings be performed across the site, in accordance with Chapter 4, to accurately identify the depth to seasonal or permanent soil saturation or bedrock. A separate replacement system area is not required to be identified at this stage.

903 — System Design

903.1 Mound dimensions and design. For one- and two-family dwellings and other buildings with estimated waste water flows less than 600 gallons (2271 L) per day, the mound dimensions shall be determined in accordance with this section or Tables 903.1(1) through 903.1(12). Dimensions and corresponding letter designations listed in the tables and referenced in this section are shown in Appendix A, Figures A-6 through A-I 0. For buildings with estimated waste water flows exceeding 600 gallons (2271 L) per day, the mound shall be designed in accordance with this section. Daily waste water flow shall be estimated as 150 gallons (568 L) per day per bedroom for one-and two-family dwellings. For other buildings the total daily waste water flow shall be determined in accordance with Table 802.7.2.

903.1.1 Symbols. The following symbols and notations shall apply to the provisions of this section. A = Bed or trench width, feet (mm). AA = Required absorption area, square feet (m2). B = Bed or trench length, feet (mm). BA = Basal area, square feet (m2). C = Trench spacing, feet (mm). C1 = Infiltration capacity of natural soil, gallons per foot per day (L/mm/day). D = Fill depth, feet (mm). E = Downslope fill depth, feet (mm). F = Bed or trench depth, feet (mm). G = Minimum cap and topsoil depth, feet (mm). H = Cap and topsoil depth at center of mound, foot (mm). I = Downslope width, feet (mm). J = Upslope width, feet (mm). K = End slope length, feet (mm). L = Total mound length, feet (mm). N = Number of trenches. P = Distribution pipe length, feet (mm). R = Manifold length, feet (mm). 5 = Distribution pipe spacing, feet (mm). 5D = Downslope correction factor. Su = Upslope correction factor. Tw = Total daily waste-water flow, gallons per day (L/day). W = Total mound width, feet (mm). X = Slope, percent.

903.1.1 Symbols. The following symbols and notations shall apply to the provisions of this section.

A = Bed or trench width, feet (mm).

AA = Required absorption area, square feet (m2).

B = Bed or trench length, feet (mm).

BA = Basal area, square feet (m2).

C = Trench spacing, feet (mm).

C1 = Infiltration capacity of natural soil, gallons per foot per day (L/mm/day).

D = Fill depth, feet (mm).

E = Downslope fill depth, feet (mm).

F = Bed or trench depth, feet (mm).

G = Minimum cap and topsoil depth, feet (mm).

H = Cap and topsoil depth at center of mound, foot (mm).

I = Downslope width, feet (mm).

J = Upslope width, feet (mm).

K = End slope length, feet (mm).

L = Total mound length, feet (mm).

N = Number of trenches.

P = Distribution pipe length, feet (mm).

R = Manifold length, feet (mm).

5 = Distribution pipe spacing, feet (mm).

5D = Downslope correction factor.

Su = Upslope correction factor.

Tw = Total daily waste-water flow, gallons per day (L/day).

W = Total mound width, feet (mm).

X = Slope, percent.

903.2 Size of absorption area. The absorption area shall be sized based on the daily waste-water flow and the infiltrative capacity of the medium sand texture fill material, equaling 1.2 gallons per square foot (0.03 L/m2) per day. The required absorption area shall be determined by the following equation:

For SI: 1 square foot = 0.0929 m2, 1 gallon = 3.785 L.

903.3 Trenches. Effluent shall be distributed in the mound through a trench system for slowly permeable soils with or without high ground water. Trench length shall be selected by determining the longest dimension perpendicular to any slope on the site. Trench width and spacing is dependent on specific site conditions. Trenches shall be 2 feet to 4 feet (610 mm to 1219 mm) wide. Trench length (B) shall be not more than 100 feet (2540 mm). Trenches shall be of equal length where more than one trench is required. A mound shall not have more than three trenches. Trench spacing (C) shall be determined by the following equation:

For SI: 1 gallon = 3.785 L, 1 square foot = 0.0929 m2.

The calculated trench spacing (C) shall be measured from center to center of the trenches. Facilities with more than 1,500 gallons (56 775 L) per day shall be specifically engineered and approved for use with a trench system.

903.4 Beds. A long, narrow bed design shall be used for permeable soils with high water tables. The bed shall be square or rectangular for shallow permeable soils over bedrock. The bed length (B) shall be set after determining the longest dimension available and perpendicular to any slope on the site.

903.5 Mound dimensions. The mound height consists of the fill depth, bed or trench depth, the cap and topsoil depth.

903.5.1 Fill depth. The fill depth (D) shall be not less than 1 foot (305 mm) for slowly permeable soils and permeable soils with high water tables and not less than 2 feet (610 mm) of fill shall be required for shallow permeable soils over bedrock. Additional fill shall be placed at the downslope end ofthe bed or trench where the site is not level so the bottom of the bed or trenches is level. The downslope fill depth for bed systems shall be determined by the following equation: E=D+XA (Equation 9-3) For SI: 1 foot = 304.8 mm. The downslope fill depth for trench systems shall be determined by the following equation: E=D+X(C+A) (Equation 9-4) For SI: 1 foot = 304.8 mm. 903.5.2 Bed or trench depth. The bed or trench depth (F) shall be not less than 9 inches (229 mm) and not less than 6 inches (152 mm) of aggregate shall be placed under the distribution pipes and not less than 2 inches (51 mm) of aggregate shall be placed over the top of the distribution pipes. 903.5.3 Cap and topsoil depth. The cap and topsoil depth (H) at the center of the mound shall be not less than 18 inches (457 mm), which includes 1 foot (305 mm) ofsubsoil and 6 inches (152 mm) of topsoil. Outer edges of the mound, G (the minimum cap and topsoil depth), shall be not less than 1 foot (305 mm), which includes 6 inches (152 mm) of subsoil and 6 inches (152 mm) of topsoil. The soil used for the cap shall be topsoil or finer textured subsoil. 903.5.4 Mound lengths. The total mound length (L) shall be determined by the following equation: L=B+2K (Equation 9-5) For SI: 1 foot = 304.8 mm. where:

903.5.1 Fill depth. The fill depth (D) shall be not less than 1 foot (305 mm) for slowly permeable soils and permeable soils with high water tables and not less than 2 feet (610 mm) of fill shall be required for shallow permeable soils over bedrock. Additional fill shall be placed at the downslope end ofthe bed or trench where the site is not level so the bottom of the bed or trenches is level. The downslope fill depth for bed systems shall be determined by the following equation:

E=D+XA (Equation 9-3)

For SI: 1 foot = 304.8 mm.

The downslope fill depth for trench systems shall be determined by the following equation:

E=D+X(C+A) (Equation 9-4)

For SI: 1 foot = 304.8 mm.

903.5.2 Bed or trench depth. The bed or trench depth (F) shall be not less than 9 inches (229 mm) and not less than 6 inches (152 mm) of aggregate shall be placed under the distribution pipes and not less than 2 inches (51 mm) of aggregate shall be placed over the top of the distribution pipes.

903.5.3 Cap and topsoil depth. The cap and topsoil depth (H) at the center of the mound shall be not less than 18 inches (457 mm), which includes 1 foot (305 mm) ofsubsoil and 6 inches (152 mm) of topsoil. Outer edges of the mound, G (the minimum cap and topsoil depth), shall be not less than 1 foot (305 mm), which includes 6 inches (152 mm) of subsoil and 6 inches (152 mm) of topsoil. The soil used for the cap shall be topsoil or finer textured subsoil.

903.5.4 Mound lengths. The total mound length (L) shall be determined by the following equation:

L=B+2K (Equation 9-5)

For SI: 1 foot = 304.8 mm.

where:

A table appears here in the adopted code. Refer to the official source for the full table.

903.5.5 Mound widths. The mound width for a bed system shall be determined by the following equation:

W=J+A+I (Equation 9-6)

For SI: 1 foot = 304.8 mm.

The mound width for a trench system shall be determined by the following equation:

For SI: 1 foot = 304.8 mm.

where:

J=3(D+F+G) Su

1=3(£+ F+ G)SD

The upslope correction factor (Su) and the downslope correction factor (5D) shall be determined based on the slope in accordance with Table 903.5.5.

A table appears here in the adopted code. Refer to the official source for the full table.

903.6 Basal area. The minimum basal area required shall be determined by the following equation:

For SI: 1 square foot = 0.0929 m2.

The infiltrative capacity of natural soil shall be determined on the percolation rate in accordance with Table 903.6.

A table appears here in the adopted code. Refer to the official source for the full table.

903.6.1 Basal area available in bed system. The available basal area for a bed system shall be determined by one of the following equations: BA = B(A + 1) for sloping sites (Equation 9-9) BA = BW for level sites (Equation 9-10) For SI: 1 square foot = 0.0929 m2. 903.6.2 Basal area available in trench system. The available basal area for a trench system shall be determined by one of the following equations: BA = BW for level sites (Equation 9-12) For SI: 1 square foot = 0.0929 m2. 903.6.3 Adequacy of basal area. The downslope width (1) on a sloping site shall be increased or the upslope width (}) and downslope (1) widths on a level site shall be increased until sufficient area is available if the basal area available is not equal to or greater than the basal area required.

903.6.1 Basal area available in bed system. The available basal area for a bed system shall be determined by one of the following equations:

BA = B(A + 1) for sloping sites (Equation 9-9)

BA = BW for level sites (Equation 9-10)

For SI: 1 square foot = 0.0929 m2.

903.6.2 Basal area available in trench system. The available basal area for a trench system shall be determined by one of the following equations:

BA = BW for level sites (Equation 9-12)

For SI: 1 square foot = 0.0929 m2.

903.6.3 Adequacy of basal area. The downslope width (1) on a sloping site shall be increased or the upslope width (}) and downslope (1) widths on a level site shall be increased until sufficient area is available if the basal area available is not equal to or greater than the basal area required.

903.7 Dose volume and pump. The dose volume and pump shall conform to the requirements of Chapters 7 and 8.

Figure from IPC 2009 Section 903.

Worked example

Scenario: An architect is designing a duplex with an estimated wastewater flow of 500 gallons (1893 L) per day and needs to determine the mound system dimensions.

Explanation: According to Section 903.1, since the estimated wastewater flow is less than 600 gallons (2271 L) per day, the architect can determine the mound dimensions using the criteria specified within this section or by referencing the values provided in Tables 903.1(1) through 903.1(12).

904 — Construction Techniques

904.1 General. Construction shall not commence where the soil is so wet a soil wire forms when the soil is rolled between the hands. Installation of mound systems where the soil on the site is frozen shall be prohibited for new construction.

904.2 Site preparation. Excess vegetation shall be cut and removed from the mound area. Small trees shall be cut to grade surface, leaving the stumps in place.

904.3 Force main. The force main from the pumping chamber shall be installed before the mound site is plowed. The force main shall be sloped uniformly toward the pumping chamber so the force main drains after each dose.

904.4 Plowing. The site shall be plowed with a moldboard plow or chisel plow. The site shall be plowed to a depth of 7 inches to 8 inches (178 mm to 203 mm) with the plowing perpendicular to the slope. Rototillers shall not be used. The sand fill shall be placed immediately after plowing. All foot and vehicular traffic shall be kept off the plowed area.

904.5 Sand fill material. The fill material shall be medium sand texture defined as 25 percent or more very coarse, coarse and medium sand and a maximum of 50 percent fine sand, very fine sand, silt and clay. The percentage of silt plus one and one-half times the percentage of clay shall not exceed 15 percent. Fill materials with higher content of silt and clay shall not be used.

904.5.1 Placement of sand fill. The medium sand fill shall be moved into place from the upslope and side edges of the plowed area. Vehicular traffic shall be prohibited in the area extending to 25 feet (7620 mm) beyond the downslope edge of the mound. The sand fill shall be moved into place with a track-type tractor and not less than 6 inches (152 mm) of sand shall be kept beneath the tracks at all times.

904.5.1 Placement of sand fill. The medium sand fill shall be moved into place from the upslope and side edges of the plowed area. Vehicular traffic shall be prohibited in the area extending to 25 feet (7620 mm) beyond the downslope edge of the mound. The sand fill shall be moved into place with a track-type tractor and not less than 6 inches (152 mm) of sand shall be kept beneath the tracks at all times.

904.6 Installation of the absorption area. The bed or trenches shall be formed within the sand fill. The bottom of the trenches or bed shall be level. The elevation ofthe bottom ofthe trenches or bed shall be checked at the upslope and downslope edges to ensure that the fill has been placed to the proper depth.

904.7 Placement of the aggregate. A minimum of 6 inches (152 mm) of coarse aggregate ranging in size from ½ inch to 2½ inches (12.7 mm to 64 mm) shall be placed in the bed or trench excavation. The top of the aggregate shall be level.

904.8 Distribution system. Distribution systems shall be placed on the aggregate, with the holes located on the bottom of the distribution pipe. The ends of all distribution pipes shall be marked at the surface, and an observation pipe shall be placed to the bottom of the bed or each trench.

904.9 Cover. The top of the bed or trenches shall be covered with not less than 2 inches (51 mm) of aggregate ranging in size from ½ inch to 2½ inches (12.7 mm to 64 mm) and not less than 4 inches to 5 inches (102 mm to 127 mm) of uncompacted straw or marsh hay or approved synthetic fabric shall be placed over the aggregate. Cap and topsoil covers shall be in place and the mound shall be seeded immediately and protected from erosion.

904.10 Maintenance. When the septic tank is pumped, the pump chamber shall be inspected and pumped to remove any solids present. Excess traffic in the mound area shall be avoided.

Worked example

Scenario: A construction crew arrives on site to install a mound system after a period of heavy rain and overnight freezing temperatures.

Explanation: Section 904.1 prohibits the crew from commencing construction if the soil is too wet to form a wire when rolled between their hands, or if the soil on the site is frozen, ensuring the proper preparation and stability required for mound system installation.

Frequently asked questions

How many soil borings are required for a mound system installation?

A minimum of three soil borings must be conducted per site to determine the depth to seasonal or permanent soil saturation or bedrock.

What are the restrictions on when mound systems can be installed?

Construction is prohibited when the soil is too wet to form a soil wire when rolled between hands, or when the soil on the site is frozen for new construction.

How are mound system dimensions determined for typical dwellings?

For one- and two-family dwellings and other buildings with wastewater flows less than 600 gallons per day, mound dimensions are determined by this section or Tables 903.1(1) through 903.1(12).