South Carolina Code of Regulations › Chapter 61 - DEPARTMENT OF HEALTH AND ENVIRONMENTAL CONTROL › Subchapter 61-56 - ONSITE WASTEWATER SYSTEMS › Section 61-56.400 - Appendices of Standards for Permitted Systems › Appendix 61-56.400.K - System Standard 290/291 - Alternative Trench Width and Depth Systems with Fill Cap

Current through Register Vol. 48, No. 6, June 28, 2024
(1) Site/Permitting Requirements
(a) The depth to any restrictive horizon must be greater than
				  twelve (12) inches below the bottom of the proposed wastewater infiltration
				  trenches.
(b) The texture in the upper eighteen (18) inches of
				  naturally occurring soil must be no more limiting than Class III.
(c) Soil conditions, the depth to rock and other restrictive
				  horizons, the depth to the zone of saturation (ZOS), and the elevation
				  differential between the septic tank outlet and the highest wastewater
				  infiltration trench(es) must meet applicable standards for alternative onsite
				  wastewater systems.
(d) There shall be a replacement area equivalent to at least
				  fifty (50) percent in size of the original system area held in reserve for
				  system repair. This area shall have a suitable configuration and shall meet the
				  minimum soil and site conditions of this regulation.
(e) This system cannot be considered for facilities with peak
				  flow rates in excess of six hundred (600) gpd.
(f) The linear footage requirement for an alternative width
				  and depth system shall be determined by first figuring the conventional
				  (36-inch-wide with 14-inch aggregate depth) linear footage requirements and
				  then multiplying by the appropriate factor based on desired trench width and
				  aggregate depth as computed in the following table:
FACTOR (F) FOR MAINTAINING EQUIVALENT
								INFILTRATIVE SURFACE AREA
TRENCH WIDTH (ft.)
AGGREGATE DEPTH (in.)
6" *
9" **
# OF DISTRIBUTION PIPES
MULTIPLICATION FACTOR
4
'
1.20
1.06
1
5
'
1.00
0.89
1
6
'
0.85
0.78
2
7
'
0.74
0.68
2
8
'
0.66
0.61
2
9
'
0.59
0.55
3
10
'
0.54
0.51
3
* Factors reflect a twelve (12) percent
								increase
** Factors reflect a nine (9) percent
								increase
(g) This system may only be installed on sites where the
				  long-term acceptance rate (LTAR) for the system will be between 1.0 and 0.5 gpd
				  per foot squared (gpd/ft2). The bottom of the
				  absorption field cannot be installed in Class IV soils (0.4 to 0.1
				  LTAR).
(h) The distribution pipes in the bed shall be at least
				  eighteen (18) inches from the edge of the trench and located on three (3) foot
				  centers.
(i) This system must not be used on sloping sites that
				  require serial distribution unless it can be demonstrated that the entire
				  wastewater infiltration trench installation (i.e., side wall to side wall and
				  end to end) can meet the required textural limitations and the required offsets
				  to the zone of saturation and restrictive horizons.
(j) The long-term acceptance rate for system sizing shall be
				  based upon the most hydraulically limiting naturally occurring soil texture
				  from the ground surface to twelve (12) inches below the bottom of the proposed
				  wastewater infiltration trenches.
(2) Installation Requirements
(a) Trench widths shall not exceed ten (10) feet.
(b) Wide trenches should be installed by excavating from the
				  sides. Heavy equipment, such as backhoes, may not be placed in the trenches in
				  order to avoid soil compaction.
(c) The required fill cap must extend at least five (5) feet
				  beyond the limits of the wastewater infiltration trenches and must taper to the
				  original soil surface at a slope not to exceed ten (10) percent. On sloping
				  sites, where serial distribution has been incorporated into the system design,
				  on the lower side and ends of the wastewater infiltration trenches, the fill
				  cap must extend to at least ten (10) feet beyond the limits of the wastewater
				  infiltration trenches and must taper to the original soil surface at a slope
				  not to exceed five (5) percent. The required property line setback shall be
				  measured from the point at which the fill cap taper intersects with the
				  naturally occurring soil surface. The fill cap must be installed prior to
				  beginning trench installation when required on the construction permit.
(3) Final Landscaping and Drainage
(a) Installation of drainage swales, ditches, diversion
				  drains, or rain gutters may be required to divert or intercept water away from
				  the onsite wastewater system location. The septic tank and subsurface
				  wastewater infiltration area shall be backfilled and shaped to promote surface
				  water runoff.
(b) A barrier to preclude parking and vehicular traffic over
				  the area of the system may be required.
(c) Following final landscaping, seeding, or sodding may be
				  required to prevent erosion.
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