Chapter 9 Mound Systems

User note:

About this chapter: Mound systems are another method for disposal of the nonpotable water from a septic tank. The existing soil at the site may not have the capacity to absorb the water because of seasonal ground water conditions or presence of bedrock just below the soil surface. Chapter 9 provides extensive design information and installation methods to build mound systems for absorbing the nonpotable water.

Section 901 General

901.1 Scope

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

Section 902 Soil and Site Requirements

902.1 Soil Borings

Not less than 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.

TABLE 902.2

MINIMUM SOIL DEPTHS FOR MOUND SYSTEM INSTALLATION

RESTRICTING FACTOR	MINIMUM SOIL DEPTH TO RESTRICTION (inches)
High ground water	24
Impermeable rock strata	60
Pervious rock	24
Rock fragments (50-percent volume)	24

For SI: 1 inch = 25.4 mm.

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

Not less than 24 inches (610 mm) of unsaturated natural soil shall be over creviced or porous bedrock.

902.7 Depth to High Ground Water

Not less than 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 of the 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

Not less than 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 except where there is not less than 24 inches (610 mm) of permeable, unsaturated soil with less than 50-percent coarse fragments located beneath this layer.

Section 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-10. 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 wastewater 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 wastewater flow shall be determined in accordance with Table 802.7.2.

TABLE 903.1(1)

DESIGN CRITERIA FOR A MOUND FOR A ONE-BEDROOM HOME ON A 0- TO 6-PERCENT SLOPE WITH LOADING RATES OF 150 GALLONS PER DAY FOR SLOWLY PERMEABLE SOIL

DESIGN PARAMETER		SLOPE (percent)
DESIGN PARAMETER		0	2	4	6
A	Trench width, feet	3	3	3	3
B	Trench length, feet Number of trenches	42 1	42 1	42 1	42 1
D	Mound height, inches	12	12	12	12
F	Mound height, inches	9	9	9	9
G	Mound height, inches	12	12	12	12
H	Mound height, inches	18	18	18	18
I	Mound width, feet^a	15	15	15	15
J	Mound width, feet^a	11	8	8	8
K	Mound length, feet	10	10	10	10
L	Mound length, feet	62	62	62	62
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of holes per distribution pipe^b Hole spacing, inches^b Hole diameter, inches^b	20 1 9 30 0.25	20 1 9 30 0.25	20 1 9 30 0.25	20 1 9 30 0.25
W	Mound width, feet	25	26	26	26

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L.

Additional width to obtain required basal area.
Last hole is located at the end of the distribution pipe, which is 15 inches from the other hole.

TABLE 903.1(2)

DESIGN CRITERIA FOR A TWO-BEDROOM HOME FOR A MOUND ON A 0- TO 6-PERCENT SLOPE WITH LOADING RATES OF 300 GALLONS PER DAY FOR SLOWLY PERMEABLE SOIL

DESIGN PARAMETER		SLOPE (percent)
DESIGN PARAMETER		0	2	4	6
A	Trench width, feet	3	3	3	3
B	Trench length, feet Number of trenches	42 2	42 2	42 2	42 2
C	Trench spacing, feet	15	15	15	15
D	Mound height, inches	12	12	12	12
E	Mound height, inches	12	17	25	25
F	Mound height, inches	9	9	9	9
G	Mound height, inches	12	12	12	12
H	Mound height, inches	18	18	18	18
I	Mound width, feet^a	12	20	20	20
J	Mound width, feet	12	8	8	8
K	Mound length, feet	10	10	10	10
L	Mound length, feet	62	62	62	62
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of holes per distribution pipe^b Hole spacing, inches^b Hole diameter, inches	20 1 9 30 0.25	20 1 9 30 0.25	20 1 9 30 0.25	20 1 9 30 0.25
R	Manifold length, feet Manifold diameter, inches^c	15 2	15 2	15 2	15 2
W	Mound width, feet	42	46	46	46

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L.

Additional width to obtain required basal area.
Last hole is located at the end of the distribution pipe, which is 15 inches from the other hole.
Diameter dependent on the size of pipe from pump and inlet position.

TABLE 903.1(3)

DESIGN CRITERIA FOR A THREE-BEDROOM HOME FOR A MOUND ON A 0- TO 6-PERCENT SLOPE WITH LOADING RATES OF 450 GALLONS PER DAY FOR SLOWLY PERMEABLE SOIL

DESIGN PARAMETER		SLOPE (percent)
DESIGN PARAMETER		0	2	4	6
A	Trench width, feet	3	3	3	3
B	Trench length, feet Number of trenches	63 2	63 2	63 2	63 2
C	Trench spacing, feet	15	15	15	15
D	Mound height, inches	12	12	12	12
E	Mound height, inches	12	17	20	25
F	Mound height, inches	9	9	9	9
G	Mound height, inches	12	12	12	12
H	Mound height, inches	18	18	18	18
I	Mound width, feet^a	12	20	20	20
J	Mound width, feet^a	12	8	8	8
K	Mound length, feet	10	10	10	10
L	Mound length, feet	62	62	62	62
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of holes per distribution pipe^b Hole spacing, inches^b Hole diameter, inches	31 1¹/₄ 13 30 0.25	31 1¹/₄ 13 30 0.25	31 1¹/₄ 13 30 0.25	31 1¹/₄ 13 30 0.25
R	Manifold length, feet Manifold diameter, inches^c	15 2	15 2	15 2	15 2
W	Mound width, feet	42	46	46	46

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L.

Additional width to obtain required basal area.
First hole is located 12 inches from the manifold.
Diameter dependent on the size of pipe from pump and inlet position.

TABLE 903.1(4)

DESIGN CRITERIA FOR A FOUR-BEDROOM HOME FOR A MOUND ON A 0- TO 6-PERCENT SLOPE WITH LOADING RATES OF 600 GALLONS PER DAY FOR SLOWLY PERMEABLE SOIL

DESIGN PARAMETER		SLOPE (percent)
DESIGN PARAMETER		0	2	4	6
A	Trench width, feet	3	3	3	3
B	Trench length, feet Number of trenches	56 3	56 3	56 3	56 3
C	Trench spacing, feet	15	15	15	15
D	Mound height, inches	12	12	12	12
E	Mound height, inches	12	20	28	36
F	Mound height, inches	9	9	9	9
G	Mound height, inches	12	12	12	12
H	Mound height, inches	24	24	24	24
I	Mound width, feet^a	12	20	20	20
J	Mound width, feet^a	12	8	8	8
K	Mound length, feet	12	12	12	14
L	Mound length, feet	80	80	80	84
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of holes per distribution pipe^b Hole spacing, inches^b Hole diameter, inches	27.5 1¹/₄ 12 30 0.25	27.5 1¹/₄ 12 30 0.25	27.5 1¹/₄ 12 30 0.25	27.5 1¹/₄ 12 30 0.25
R	Manifold length, feet Manifold diameter, inches^c	30 2	30 2	30 2	30 2
W	Mound width, feet	57	61	61	61

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L.

Additional width to obtain required basal area.
Last hole is located at the end of the distribution pipe, which is 15 inches from the previous hole.
Diameter dependent on the size of pipe from pump and inlet position.

TABLE 903.1(5)

DESIGN CRITERIA FOR A ONE-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 150 GALLONS PER DAY FOR SHALLOW PERMEABLE SOIL OVER CREVICED BEDROCK

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		3 to 60				3 to less than 30
		0	2	4	6	8	10^a	12^a
A	Bed width, feet^b	10	10	10	10	10	10	10
B	Bed length, feet	13	13	13	13	13	13	13
D	Mound height, inches	24	24	24	24	24	24	24
E	Mound height, inches	24	26	29	31	34	36	38
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	12	13	14	17	18	21	26
J	Mound width, feet	12	11	10	10	9	9	9
K	Mound length, feet	12	12	12	13	13	13	15
L	Mound length, feet	37	37	37	39	39	39	43
P	Distribution pipe length, feet^c Distribution pipe diameter, inches Number of distribution pipes	12.5 1 6	12.5 1 6	12.5 1 6	12.5 1 6	12.5 1 6	12.5 1 6	12.5 1 6
R	Manifold length, feet Manifold diameter, inches^c	6 2	6 2	6 2	6 2	6 2	6 2	6 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^d Hole spacing, inches^d Hole diameter, inches	3 6 30 0.25	3 6 30 0.25	3 6 30 0.25	3 6 30 0.25	3 6 30 0.25	3 6 30 0.25	3 6 30 0.25
W	Mound width, feet	34	34	34	37	37	41	45

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

On sites with a 10- to 12-percent slope, the fill depth (D) shall be reduced to not less than 1.5 feet or the bed width shall be reduced to decrease E (downslope fill depth, feet).
Bed widths shall not be limited.
Use a manifold with distribution pipes on only one side.
Last hole is located at the end of the distribution pipe, which is 15 inches from the previous hole.

TABLE 903.1(6)

DESIGN CRITERIA FOR A TWO-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 300 GALLONS PER DAY FOR SHALLOW PERMEABLE SOIL OVER CREVICED BEDROCK

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		3 to 60			3 to less than 30
		0	2	4	6	8	10^a	12^a
A	Bed width, feet^b	10	10	10	10	10	10	10
B	Bed length, feet	25	25	25	25	25	25	25
D	Mound height, inches	24	24	24	24	24	24	24
E	Mound height, inches	24	26	29	31	34	36	38
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	12	13	14	17	18	21	26
J	Mound width, feet	12	11	10	10	9	9	9
K	Mound length, feet	12	12	12	13	13	13	15
L	Mound length, feet	49	49	49	51	51	51	55
P	Distribution pipe length, feet^c Distribution pipe diameter, inches Number of distribution pipes	12 1 6	12 1 6	12 1 6	12 1 6	12 1 6	12 1 6	12 1 6
R	Manifold length, feet Manifold diameter, inches	6 2	6 2	6 2	6 2	6 2	6 2	6 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^d Hole spacing, inches^d Hole diameter, inches	3 5 30 0.25	3 5 30 0.25	3 5 30 0.25	3 5 30 0.25	3 5 30 0.25	3 5 30 0.25	3 5 30 0.25
W	Mound width, feet	34	34	34	37	37	41	45

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

On sites with a 10- to 12-percent slope, the fill depth (D) shall be reduced to not less than 1.5 feet or the bed width shall be reduced to decrease E (downslope fill depth, feet).
Bed widths shall not be limited.
This design is based on a manifold with distribution pipes on both sides. An alternative design basis is 24-foot distribution pipes, with manifold at the end.
Last hole is located 9 inches from the end of the distribution pipe.

TABLE 903.1(7)

DESIGN CRITERIA FOR A THREE-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 450 GALLONS PER DAY FOR SHALLOW PERMEABLE SOIL OVER CREVICED BEDROCK

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		3 to 60		3 to less than 30
		0	2	4	6	8	10^a	12^a
A	Bed width, feet^b	10	10	10	10	10	10	10
B	Bed length, feet	38	38	38	38	38	38	38
D	Mound height, inches	24	24	24	24	24	24	24
E	Mound height, inches	24	26	29	31	34	36	38
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	12	13	14	17	18	21	26
J	Mound width, feet	12	11	10	10	9	9	9
K	Mound length, feet	12	12	12	13	13	13	15
L	Mound length, feet	62	62	62	64	64	64	68
P	Distribution pipe length, feet^c Distribution pipe diameter, inches Number of distribution pipes	18.5 1 6	18.5 1 6	18.5 1 6	18.5 1 6	18.5 1 6	18.5 1 6	18.5 1 6
R	Manifold length, feet Manifold diameter, inches	6 2	6 2	6 2	6 2	6 2	6 2	6 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^d Hole spacing, inches^d Hole diameter, inches	3 8 30 0.25	3 8 30 0.25	3 8 30 0.25	3 8 30 0.25	3 8 30 0.25	3 8 30 0.25	3 8 30 0.25
W	Mound width, feet	34	34	34	37	37	41	45

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

On sites with a 10- to 12-percent slope, the fill depth (D) shall be reduced to not less than 1.5 feet or the bed width shall be reduced to decrease E (downslope fill depth, feet).
Bed widths shall not be limited.
Use a manifold with distribution pipes on only one side.
Last hole is located at the end of the distribution pipe, which is 27 inches from the previous hole.

TABLE 903.1(8)

DESIGN CRITERIA FOR A FOUR-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 600 GALLONS PER DAY FOR SHALLOW PERMEABLE SOIL OVER CREVICED BEDROCK

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		3 to 60				3 to less than 30
		0	2	4	6	8	10^a	12^a
A	Bed width, feet^b	10	10	10	10	10	10	10
B	Bed length, feet	50	50	50	50	50	50	50
D	Mound height, inches	24	24	24	24	24	24	24
E	Mound height, inches	24	26	29	31	34	36	38
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	12	13	14	17	18	21	26
J	Mound width, feet	12	11	10	10	9	9	9
K	Mound length, feet	12	12	12	13	13	13	15
L	Mound length, feet	74	74	74	76	76	76	78
P	Distribution pipe length, feet^c Distribution pipe diameter, inches Number of distribution pipes	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6
R	Manifold length, feet Manifold diameter, inches	6 2	6 2	6 2	6 2	6 2	6 2	6 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^d Hole spacing, inches^d Hole diameter, inches	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25
W	Mound width, feet	34	34	34	37	37	41	45

For SI: 1 inch 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

On sites with a 10- to 12-percent slope, the fill depth (D) shall be reduced to not less than 1.5 feet or the bed width shall be reduced to decrease E (downslope fill depth, feet).
Bed widths shall not be limited.
Use a manifold with distribution pipes on only one side.
Last hole is located 9 inches from the end of the distribution pipe.

TABLE 903.1(9)

DESIGN CRITERIA FOR A ONE-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 150 GALLONS PER DAY FOR PERMEABLE SOIL WITH A HIGH WATER TABLE

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		0 to 60				0 to less than 30
		0	2	4	6	8	10	12
A	Bed width, feet	4	4	4	4	4	4	4
B	Bed length, feet	32	32	32	32	32	32	32
D	Mound height, inches	12	12	12	12	12	12	12
E	Mound height, inches	12	13	14	14	16	17	18
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	9	10	11	12	13	14	15
J	Mound width, feet	9	9	8	8	7	7	6
K	Mound length, feet	10	10	10	10	10	11	11
L	Mound length, feet	52	52	52	52	52	53	53
P	Distribution pipe length Distribution pipe diameter, inches Number of distribution pipes Number of holes per distribution pipe^a Hole spacing, inches^a Hole diameter, inches	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25	15.5 1 2 7 30 0.25
W	Mound width, feet	22	23	23	24	24	25	25

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

Last hole is located at the end of the distribution pipe, which is 21 inches from the previous hole.

TABLE 903.1(10)

DESIGN CRITERIA FOR A TWO-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 300 GALLONS PER DAY FOR PERMEABLE SOIL WITH A HIGH WATER TABLE

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		0 to 60				0 to less than 30
		0	2	4	6	8	10	12
A	Bed width, feet	6	6	6	6	6	6	6
B	Bed length, feet	42	42	42	42	42	42	42
D	Mound height, inches	12	12	12	12	12	12	12
E	Mound height, inches	12	13	14	17	18	19	22
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	9	10	11	12	13	15	16
J	Mound width, feet	9	9	8	8	7	7	6
K	Mound length, feet	10	10	10	10	10	11	11
L	Mound length, feet	62	62	62	62	62	64	64
P	Distribution pipe length, feet^a Distribution pipe diameter, inches Number of distribution pipes	20 1 4	20 1 4	20 1 4	20 1 4	20 1 4	20 1 4	20 1 4
R	Manifold length, feet Manifold diameter, inches	3 2	3 2	3 2	3 2	3 2	3 2	3 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^b Hole spacing, inches^b Hole diameter, inches	3 9 30 0.25	3 9 30 0.25	3 9 30 0.25	3 9 30 0.25	3 9 30 0.25	3 9 30 0.25	3 9 30 0.25
W	Mound width, feet	24	25	25	26	26	28	29

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

Use a manifold with distribution pipes only on one side.
Last hole is located at the end of the distribution pipe, which is 15 inches from the previous hole.

TABLE 903.1(11)

DESIGN CRITERIA FOR A THREE-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 450 GALLONS PER DAY FOR PERMEABLE SOIL WITH A HIGH WATER TABLE

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		0 to 60				0 to less than 30
		0	2	4	6	8	10	12
A	Bed width, feet	8	8	8	8	8	8	8
B	Bed length, feet	47	47	47	47	47	47	47
D	Mound height, inches	12	12	12	12	12	12	12
E	Mound height, inches	12	12	16	18	19	22	24
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	9	11	12	13	15	17	18
J	Mound width, feet	9	9	8	8	7	7	6
K	Mound length, feet	10	10	10	10	10	11	12
L	Mound length, feet	67	67	67	67	69	69	71
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of distribution pipes	23 1 6	23 1 6	23 1 6	23 1 6	23 1 6	23 1 6	23 1 6
R	Manifold length, feet Manifold diameter, inches	64 2	64 2	64 2	64 2	64 2	64 2	64 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^a Hole spacing, inches^a Hole diameter, inches	32 10 30 0.25	32 10 30 0.25	32 10 30 0.25	32 10 30 0.25	32 10 30 0.25	32 10 30 0.25	32 10 30 0.25
W	Mound width, feet	26	28	28	29	30	32	32

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

Last hole is located at the end of the distribution pipe, which is 21 inches from the previous hole.

TABLE 903.1(12)

DESIGN CRITERIA FOR A FOUR-BEDROOM HOME FOR A MOUND ON A 0- TO 12-PERCENT SLOPE WITH LOADING RATES OF 600 GALLONS PER DAY FOR PERMEABLE SOIL WITH A HIGH WATER TABLE

DESIGN PARAMETER		PERCOLATION RATE (minutes per inch) SLOPE (percent)
		0 to 60				0 to less than 30
		0	2	4	6	8	10	12
A	Bed width, feet	10	10	10	10	10	10	10
B	Bed length, feet	50	50	50	50	50	50	50
D	Mound height, inches	12	12	12	12	12	12	12
E	Mound height, inches	12	14	17	19	22	24	26
F	Mound height, inches	9	9	9	9	9	9	9
G	Mound height, inches	12	12	12	12	12	12	12
H	Mound height, inches	18	18	18	18	18	18	18
I	Mound width, feet	9	11	13	14	17	18	19
J	Mound width, feet	9	9	8	8	7	7	6
K	Mound length, feet	10	10	10	10	11	11	12
L	Mound length, feet	70	70	70	70	72	72	74
P	Distribution pipe length, feet Distribution pipe diameter, inches Number of distribution pipes	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6	24.5 1 6
R	Manifold length, feet Manifold diameter, inches	6 2	6 2	6 2	6 2	6 2	6 2	6 2
S	Distribution pipe spacing, feet Number of holes per distribution pipe^a Hole spacing, inches^a Hole diameter, inches	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25	3 10 30 0.25
W	Mound width, feet	28	29	31	32	34	35	36

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon = 3.785 L, 1 minute per inch = 2.4 s/mm.

Last hole is 9 inches from the end of the distribution pipe.

903.1.1 Symbols

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

A	=	Bed or trench width, feet (mm).
A_A	=	Required absorption area, square feet (m²).
B	=	Bed or trench length, feet (mm).
B_A	=	Basal area, square feet (m²).
C	=	Trench spacing, feet (mm).
C_I	=	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).
S	=	Distribution pipe spacing, feet (mm).
S_D	=	Downslope correction factor.
S_U	=	Upslope correction factor.
T_W	=	Total daily wastewater 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 wastewater 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:

(Equation 9-1)

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:

(Equation 9-2)

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 of the bed or trench where the site is not level so the bottom of the bed or trench is level. The downslope fill depth for bed systems shall be determined by the following equation:

(Equation 9-3)

For SI: 1 foot = 304.8 mm.

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

(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) of subsoil 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:

(Equation 9-5)

For SI: 1 foot = 304.8 mm.

where:

903.5.5 Mound Widths

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

(Equation 9-6)

For SI: 1 foot = 304.8 mm.

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

(Equation 9-7)

For SI: 1 foot = 304.8 mm.

where:

J = 3(D + F + G)S_U

I = 3(E + F + G)S_D

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

TABLE 903.5.5

DOWNSLOPE AND UPSLOPE WIDTH CORRECTIONS FOR MOUNDS ON SLOPING SITES

SLOPE (percent)	DOWNSLOPE CORRECTION FACTOR (S_D)	UPSLOPE CORRECTION FACTOR (S_U)
0	1	1
1	1.03	0.97
2	1.06	0.94
3	1.10	0.915
4	1.14	0.89
5	1.18	0.875
6	1.22	0.86
7	1.27	0.83
8	1.32	0.80
9	1.38	0.785
10	1.44	0.77
11	1.51	0.75
12	1.57	0.73

903.6 Basal Area

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

(Equation 9-8)

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.

TABLE 903.6

INFILTRATIVE CAPACITY OF NATURAL SOIL

PERCOLATION RATE (minutes per inch)	INFILTRATIVE CAPACITY (gallons per foot per day)
Less than 30	1.2
30 to 60	0.74
More than 60 to 120	0.24

For SI: 1 gallon per foot per day = 0.012 L/mm/day,1 minute per inch = 2.4 s/mm.

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:

(Equation 9-9)

(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:

(Equation 9-11)

(Equation 9-12)

For SI: 1 square foot = 0.0929 m².

903.6.3 Adequacy of Basal Area

The downslope width (I) on a sloping site shall be increased or the upslope width (J) and downslope (I) 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.

Section 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. 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 not more than 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.

904.6 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 of the bottom of the 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

Not less than 6 inches (152 mm) of coarse aggregate ranging in size from ¹/₂ inch to 2¹/₂ 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 ¹/₂ inch to 2¹/₂ 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.

International Private Sewage Disposal Code 2018 (IPSDC 2018)