S.C. Code Regs. § ch. 61, 61-56, 61-56.400, app 61-56.400.G
(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.
S.C. Code Regs. ch. 61, 61-56, 61-56.400, app 61-56.400.G