Section 410IAC6-10.1-87. Elevated sand mound on-site sewage systems: design of the aggregate bed

   (a) The size of the aggregate bed shall be determined from the following:

(1) The minimum area of the aggregate bed shall be calculated as:

minimum aggregate bed area (ft2) (AB)	=	DDF (gpd)
		1.2 gpd/ft2

(2) The dimensions of the aggregate bed shall be as long and narrow as the site allows, while not exceeding the maximum bed width calculated in subdivision (3)(A).

(3) The maximum width of the aggregate bed shall meet the following requirements:

	(A) The maximum aggregate bed width (ft.)(ABW) = 0.83 ft2/gpd
	where 0.83 is a conversion factor (ft2/gpd)	DDF (gpd)	n
	SLR is soil loading rate, and	≤ 1500	3
	where: DDF is design daily flow, and	1501-3000	4
	n is determined by the DDF in this chart	3001-4000	5
	This number may be rounded down to the nearest whole number.

(B) For on-site sewage systems with a DDF of seven hundred fifty (750) gallons per day or less, the width of the aggregate bed shall be at least four (4) feet and not greater than ten (10) feet. The aggregate bed width shall not exceed the maximum bed width calculated in clause (A).

(C) For on-site sewage systems with a DDF of greater than seven hundred fifty (750) gallons per day, the following apply:

(i) If the soil loading rate is fifty-hundredths (0.50) gallons per day per square foot (gpd/ft²) or less, the width of the aggregate bed shall be not greater than fifteen (15) feet, and shall not exceed the maximum bed width calculated in clause (A).

(ii) If the soil loading rate is greater than fifty-hundredths (0.50) gallons per day per square foot (gpd/ft²), the width of the aggregate bed shall be not greater than twenty (20) feet, and shall not exceed the maximum bed width calculated in clause (A).

(4) The minimum length of the aggregate bed shall be calculated as:

Minimum length of the aggregate bed (ABL)	=	Minimum aggregate bed area (AB)
		Maximum aggregate bed width (ABW)

(5) The depth of the aggregate bed shall be at least the sum of:

(A) at least six (6) inches of aggregate below the pressure distribution lateral;

(B) the outside diameter of the pressure distribution lateral; and

(C) at least two (2) inches of aggregate above the pressure distribution lateral.

  (b) The aggregate bed shall be installed on the INDOT Specification 23 sand in the basal area, as listed in Table XVI in section 88(j) of this rule.

  (c) The location of the aggregate bed shall be:

(1) for sites with slopes of one-half percent (1/2%) or less, with its length positioned along the long axis in the center of the basal area; and

(2) for sites with slopes greater than one-half percent (1/2%) and less than or equal to six percent (6%), with its length positioned along the long axis at the upslope side of the basal area.

  (d) The design of the aggregate bed shall comply with the following:

(1) The long axis of the aggregate bed shall be constructed along the contours of the absorption system site.

(2) The bottom of the aggregate bed shall be level along its length and width.

(3) Aggregate used in the aggregate bed shall comply with the requirements of section 76 of this rule.

(4) If more than one (1) aggregate bed is constructed, each of the aggregate beds shall be equal in area.

(5) A one (1) foot wide border of INDOT Specification 23 sand, level with the top of the aggregate bed, shall surround the aggregate bed.

Figure 1, as follows, presents a visual depiction of the location of the aggregate bed within the basal area:

(Indiana State Department of Health; 410 IAC 6-10.1-87; filed Oct 19, 2012, 2:07 p.m.: 20121114-IR-410120157FRA; errata filed Apr 23, 2013, 11:44 a.m.: 20130508-IR-410130165ACA)