.1956 MODIFICATIONS TO SEPTIC TANK SYSTEMS
The following are modifications to septic tank systems or sites which may be utilized singly or in combination to overcome selected soil and site limitations. Except as required in this Rule, the provisions for design and installation of Rule .1955 of this Section shall apply:
(1) SHALLOW SYSTEMS: Sites classified UNSUITABLE as to soil depth or soil wetness may be reclassified as PROVISIONALLY SUITABLE with respect to soil depth or soil wetness conditions by utilizing shallow placement of nitrification trenches in the naturally occurring soil. Shallow trenches may be used where at least 24 inches of naturally occurring soil are present above saprolite, rock, or soil wetness conditions and all other factors are PROVISIONALLY SUITABLE or SUITABLE. Shallow trenches shall be designed and constructed to meet the vertical separation requirements in Rule .1955(m) of this Section. The long-term acceptance rate shall be based on the most hydraulically limiting naturally occurring soil horizon within 24 inches of the ground surface ortoadepthofonefootbelowthetrenchbottom,whicheverisdeeper. Soilcoverabovetheoriginalgradeshall be placed at a uniform depth over the entire nitrification field and shall extend laterally five feet beyond the nitrification trench. The soil cover shall be placed over a nitrification field only after proper preparation of the original ground surface. The type and placement of soil cover shall be approved by the local health department.
(2) DRAINAGE AND RESTRICTIVE HORIZONS: Sites classified UNSUITABLE as to soil wetness conditions or restrictive horizons may be reclassified PROVISIONALLY SUITABLE as to soil wetness conditions or restrictive horizons when:
(a) Soils are Soil Groups I or II with SUITABLE structure, and clay mineralogy;
(b) Restrictive horizons, if present, are less than three inches thick or less than 12 inches from the soil surface;
(c) Modifications can be made to meet the requirements in Rule .1955(m) of this Section for the separation
between the water table and the bottom of the nitrification trench at all times and when provisions are made
for maintenance of the drainage systems;
(d) Easements are recorded and have adequate width for egress and ingress for maintenance of drainage
systems serving two or more lots;
(e) Maintenance of the drainage system is made a condition of any permit issued for the use or operation of
a sanitary sewage system; and
(f) Drainage may be used in other types of soil when the requirements of Rule .1957(c) in this Section are met.
(3) GRAVELLESS TRENCHES: Modified nitrification trenches or lines, including large diameter pipe (greater than four inches I.D.), and specially designed porous block systems may be permitted by the local health department.
(a) Gravelless nitrification trench systems may be substituted for conventional trench systems on any site found to be suitable or provisionally suitable in accordance with Rules .1940 to .1948 of this Section to eliminate the need for gravel, minimize site disturbance, or for other site planning considerations. Gravelless nitrification trench systems shall not be used, however, where wastes contain high amounts of grease and
oil, such as restaurants.
(i) Large diameter pipe systems shall consist of eight-inch or 10-inch (inside diameter), corrugated,
polythylene tubing encased in a nylon, polyester, or nylon/polyester blend filter wrap installed in
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ENR - ENVIRONMENTAL HEALTH T15A: 18A .1900
Sheet Grab Tensile: Trapezoid Tear:
2 Oz/yd = 1.0
MD - 23 lbs. MD - 6.2 lbs. XD - 5.1 lbs. PSI = 40 Kpa = 276
a nitrification trench, 12 or more inches wide and backfilled with soil classified as soil group I, II, or III. Nitrification area requirement shall be determined in accordance with Rules .1955(b) and .1955(c), or in Rule .1956(6)(b), Table III of this Section, when applicable, with eight-inch tubing considered equivalent to a two-foot-wide conventional trench and 10-inch tubing considered equivalent to a two and one-half-foot-wide conventional trench. The long-term acceptance rate shall not exceed 0.8 gallons per day per square foot. Tubing and fittings shall comply with the requirements of ASTM F-667, which is hereby incorporated by reference including any subsequent amendments and editions. Copies of the standards may be inspected in and copies obtained from the Divison of Environmental Health, P.O. Box 27687, Raleigh, NC 27611-7687 at no cost. The corrugated tubing shall have two rows of holes, each hole between three-eighths and one-half-inch in diameter, located 120 degrees apart along the bottom half of the pipe (each 60 degrees from the bottom center line) and staggered so that one hole is present in the valley of each corrugation. The tubing shall be marked with a visible top location indicator, 120 degrees away from each row of holes. Filter wrap shall be spun, bonded, or spunlaced nylon, polyester, or nylon/polyester blend nylon filter wrap meeting the following minimum requirements:
Corrugated Tubing shall be covered with filter wrap at the factory and each joint shall be immediately encased in a black polyethylene sleeve which shall continue to encase the large diameter pipe and wrap until just prior to installation in the trench. Large diameter pipe systems shall be installed in accordance with this Rule and the manufacturer's guidelines. The trench bottom and pipe shall be level (with a maximum fall of one inch in 100 feet). Filter wrap encasing the tubing shall not be exposed to sunlight (ultraviolet radiation) for extended periods. Rocks and large soil clumps shall be removed from backfill material prior to being used. Clayey soils (soil group IV) shall not be used for backfill. The near end of the large diameter pipe shall have an eight-inch by four-inch offset adaptor (small end opening at top) suitable for receiving the pipe from the septic tank or distribution device and making a mechanical joint in the nitrification trench.
(ii) A Prefabricated, Permeable Block Panel System (PPBPS), untilizing both horizontal and vertical air chambers and special construction to promote downline and horizontal distribution of effluent, may be used under the following conditions:
(A) the soil and site criteria of this Section shall be met;
(B) in calculating the required linear footage for a PPBPS's nitrification field, the linear footage for the nitrification line as determined in Rule .1955 (b) and (c), or in Rule .1956 (6)(b), Table III of this Section when applicable, shall be multiplied by 0.5 for a 16 inch PPBPS;
(C) installation of the PPBPS shall be in accordance with these Rules except:
(I) the PPBPS trench shall be located not less than eight feet on centers;
(II) the installation shall be in accordance with the manufacturer's specifications; and
(III) the sidewalls of nitrification trenches placed in Group IVa soils shall be raked to
open pores which were damaged or sealed during excavation;
(D) where design sewage flow is more than 480 gallons per day, the system shall be
(E) the long-term acceptance rate shall not exceed 0.8 gallons per day per square foot.
(b) Other types of nitrification trenches or lines may be approved by the local health department on a site-specific basis in accordance with Rule .1969 of this Section.
(4) INTERCEPTOR DRAINS: Sites classified as UNSUITABLE as to soil wetness conditions because of the presence of lateral water movement may be reclassified PROVISIONALLY SUITABLE as to soil wetness conditions when such water is intercepted and diverted to prevent saturation of the soil absorption system.
(5) STEEP SLOPES: Stable slopes greater than 30 percent may be reclassified as PROVISIONALLY SUITABLE
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(a) The soil characteristics can be classified as SUITABLE or PROVISIONALLY SUITABLE to a depth of
at least one foot below the bottom of the nitrification trench at the upslope side of the trench;
(b) Surface water runoff is diverted around the nitrification field if necessary to prevent scouring or erosion
of the soil over the field; and
(c) The finished grade over the nitrification field site is returned to the original topography and adequately
seeded, unless otherwise specified by the local health department.
(6) SAPROLITE SYSTEM: Sites classified UNSUITABLE as to soil depth, with saprolite present, may be reclassified PROVISIONALLY SUITABLE as to soil depth when the provisions of this Paragraph are met.
An investigation of the site using pits at locations specified by the local health department shall be conducted. The following physical properties and characteristics shall be present in the two feet of saprolite below the proposed trench bottom:
(i) the saprolite texture shall be sand, loamy sand, sandy loam, loam, or silt loam; (ii) clay mineralogy shall be suitable;
(iii) greater than two-thirds of the material shall have a moist consistence of loose, very friable, friable, or firm;
(iv) the saprolite wet consistence shall be nonsticky or slightly sticky and nonplastic or slightly plastic; (v) the saprolite shall be in an undisturbed, naturally occurring state; and
(vi) the saprolite shall have no open and continuous joints, quartz veins, or fractures relic of parent rock to a depth of two feet below the proposed trench bottom.
(b) Table III shall be used in determining the long-term acceptance rate for septic tank systems installed pursuant to Paragraph (6) of this Rule. The long-term acceptance rate shall be based on the most hydraulically limiting, naturally occurring saprolite to a depth of two feet below trench bottom.
If a low pressure pipe system is used, the long term acceptance rate in Table III shall be reduced by one-half and the system shall be designed in accordance with Rule .1957(a) of this Section, except that Rule .1957 (a)(2)(B) and Rule .1957(a)(3) shall not apply. Saprolite textural classifications shall be determined from disturbed materials and determined by Rule .1941(a)(1) of this Section. Low-pressure distribution shall be used when the total length of nitrification lines exceeds 750 feet in a single system.
The design daily flow shall not exceed 1000 gallons.
The nitrification field shall be constructed using nitrification trenches with a maximum width of three feet and a maximum depth of three feet on the downslope side of the nitrification trench. The bottom of a nitrification trench shall be a minimum of two feet above rock or saprolite that does not meet the requirements of Subparagraph (6)(a) of this Rule. However, where SUITABLE or PROVISIONALLY SUITABLE soil underlies the trench bottom, this separation distance may be reduced by subtracting the actual soil depth beneath the trench bottom from 24 inches to establish the minimum separation distance from the trench bottom to rock.
The bottom of any nitrification trench shall be a minimum of two feet above any wetness condition. Surface and subsurface interceptor drains shall be required on sites with more slowly permeable horizons above the usable saprolite to intercept laterally flowing waters or perched waters.
Exceptions to the provisions of Rule .1950(a) found in Rule .1950 and .1951 of this Section shall not apply to systems installed pursuant to this Paragraph [Rule .1956(6)].
Other saprolite systems may be approved on a site-specific basis in accordance with Rule .1948(d) of this Section.
Authority G.S. 130A-335(e) and (f); Eff. July 1, 1982;
Amended Eff. July 1, 1995; April 1, 1993; January 1, 1990; August 1, 1988.