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.G - System Standard 250/251 - Reservoir Infiltration System for Soils with Expansive Clay

Current through Register Vol. 48, No. 6, June 28, 2024
(1) Site/Permitting Requirements
(a) Rock formations must be greater than four (4) feet below
				  the naturally occurring soil surface.
(b) For standard installations (see Typical Design
				  Illustration A), the wastewater infiltration trenches must penetrate the
				  saprolite at least six (6) inches. There must be an offset greater than twelve
				  (12) inches between the bottom of the trenches and any rock formations. (i.e.,
				  there must be greater than eighteen (18) inches of clean, unconsolidated
				  saprolite below the expansive clay layer.)
(c) If the unconsolidated saprolite layer is greater than
				  sixty (60) inches below the naturally occurring soil surface (see Typical
				  Design Illustration B), paragraph (1)(b) (above) shall apply and clean medium
				  sand shall be added to the trenches so that the top of the aggregate will be
				  twelve (12) inches below finished grade.
(d) There must be no evidence of a zone of saturation (ZOS)
				  in the unconsolidated saprolite layer.
(e) The long-term acceptance rate shall not exceed 0.25
				  gpd/sq. ft.
(f) 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.
(g) Sites to be considered for this system shall be evaluated
				  using backhoe pits to describe the soil profile.
(h) This system cannot be considered for facilities with peak
				  flow rates in excess of fifteen hundred (1500) gpd.
(i) Clean, unconsolidated saprolite shall be defined as soft,
				  friable, thoroughly decomposed rock that has formed in place by chemical
				  weathering, retaining the fabric and structure of the parent rock, and being
				  devoid of expansive clay. Unconsolidated saprolite can be dug using a hand
				  auger or knife. Consolidated saprolite cannot be penetrated with a hand auger
				  or similar tool and must be dug with a backhoe or other powered
				  equipment.
(ii) Expansive clay shall be defined as soils containing
				  significant amounts of expansible-layer clay minerals or smectites as evidenced
				  in the field by classifications of Very Sticky and Very Plastic and Structure
				  Grades of Weak or Structureless when evaluated in accordance with the Field
				  Book. Such soils are considered to be unsuitable for onsite wastewater
				  systems.
(2) Installation Requirements
(a) The depth of approved aggregate shall be twenty-four (24)
				  inches.
(b) The depth of medium sand will vary between zero (0) and
				  one hundred twenty (120) inches, depending upon the depth to the saprolite
				  layer.
(c) The trench width shall be thirty-six (36) inches.
(d) Where gravity flow from the septic tank to the subsurface
				  wastewater infiltration area is utilized, the invert elevation of the septic
				  tank outlet shall be installed at an elevation at least equal to or higher than
				  the top of the aggregate in the highest wastewater infiltration
				  trench(es).
(e) The backfill shall range from twelve (12) inches to
				  thirty-six (36) inches for standard installations (see Typical Design
				  Illustration A) and shall be twelve (12) inches where the depth to saprolite is
				  greater than sixty (60) inches below the naturally occurring soil surface (see
				  Typical Design Illustration B).
(3) Final Landscaping and Drainage
(a) On sites where there is evidence of a zone of saturation
				  at the soil-expansive clay interface, a curtain drain must be placed upslope
				  along a contour and must extend the entire length of the subsurface wastewater
				  infiltration area. The curtain drain shall extend a minimum of six (6) inches
				  into the expansive clay layer. The septic tank and subsurface wastewater
				  infiltration area shall be backfilled and shaped to promote surface water
				  runoff.
(b) Following final landscaping, seeding or sodding may be
				  required to prevent erosion.
(c) Final approval shall be withheld until all landscaping
				  and drainage improvements have been satisfactorily completed.
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