// CODE SNIPPET
903.5 Mound Dimensions
JUMP TO FULL CODE CHAPTER
The mound height consists of the fill depth, bed or trench depth, the cap and topsoil 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 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.
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.
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.
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:
The mound width for a bed system shall be determined by the following equation:
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)SU
I = 3(E + F + G)SD
The upslope correction factor (SU) and the downslope correction factor (SD) shall be determined based on the slope in accordance with Table 903.5.5.
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:
(Equation 9-7)For SI: 1 foot = 304.8 mm.
where:
J = 3(D + F + G)SU
I = 3(E + F + G)SD
The upslope correction factor (SU) and the downslope correction factor (SD) shall be determined based on the slope in accordance with Table 903.5.5.
| SLOPE (percent) |
DOWNSLOPE CORRECTION FACTOR (SD) |
UPSLOPE CORRECTION FACTOR (SU) |
| 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 |