New Jersey Administrative Code|Section 7:9A-10.7 - Interceptor drains

                                                
 § 7:9A-10.7. Interceptor drains 
 
(a) Interceptor drains may be used on sloping sites to improve site suitability by intercepting laterally moving ground water which is perched above a hydraulically restrictive horizon provided that the requirements of (b) through (k) below are met.
 
(b) Interceptor drains shall be oriented parallel to the length and width of the disposal field and shall be installed on all sides except for the downslope side, as shown in Figure 25 of Appendix A.
 
(c) Interceptor drains designed to intercept ground water which is perched above a hydraulically restrictive horizon shall extend to the top but not through the entire thickness of the hydraulically restrictive horizon.
 
(d) The minimum distance between the disposal field and an interceptor drain shall be as prescribed in (d)1 and 2 below. The only exceptions to these requirements shall be where the bottom of the drain is at an elevation which is higher than the bottom of the disposal field or where the drain is set at the top of a restrictive horizon which is penetrated by the excavation for a soil replacement or mounded soil replacement installation, in which cases the minimum setback distance between the disposal field and the drain shall be 20 feet.
 
1. The minimum distance between a disposal field and any portion of an interceptor drain which is downslope of the disposal field shall be 50 feet.
 
2. The minimum distance between a disposal field and those portions of the interceptor drain which are upslope of the disposal field's downslope side shall be 50 feet unless a shorter distance is calculated using the formula given in (d)3 below. In no case shall this distance be less than 10 feet.
 
3. Calculate the minimum required horizontal separation distance, D, using the equation, D = Q/(LKI[LESS THAN]2[GREATER THAN]), where: 
 
Q is the volume of sanitary sewage, determined as prescribed in 7:9A-7.4, in gallons per day, multiplied by a unit conversion factor of 1 ft[LESS THAN]3[GREATER THAN]/7.48 gallons.
 
L is the total length of the disposal field, in feet, measured parallel to the topographic contour.
 
K is the horizontal saturated permeability above the restrictive horizon, in inches per hour, determined as prescribed in 7:9A-6.5 or 6.6, multiplied by unit conversion factors of (1 foot/12 inches) and (24 hours/1 day).
 
I is the slope, in feet/foot, measured perpendicular to the topographic contour and described based on appropriately located subsurface explorations.
 
(e) Excavation shall be carried out as follows:
 
1. The excavation for the interceptor drain shall be made to the exact depth required in (c) above, a minimum two feet wide, and shall extend for the entire length of the drain, around the upslope side of the disposal field and down both ends of the field to the downslope side, as shown in Figure 25 of Appendix A.
 
2. To accommodate the drain discharge pipes, the excavation shall extend, on each end of the disposal field, beyond the extent of the drain, from the downslope side of the disposal field to free-flowing outlet meeting the requirements of (f) below.
 
3. The part of the excavation in which the drain discharge pipe will be laid shall have a slope which is steep enough to carry away the intercepted ground water.
 
(f) That portion of the excavation which will accommodate the drain shall be filled with filter material to a depth which is a minimum of one foot higher than the top of the perched zone of saturation which is to be drained. Filter material used for this purpose shall be washed gravel or crushed stone, free of fines, dust, ashes or clay, and shall conform in size and gradation with one of the following New Jersey Department of Transportation standard sizes for coarse aggregate as shown in Figure 26 of Appendix A: size number four, size number five, size number 56 or size number six.
 
(g) Barrier material shall consist of continuous layers of drainage fabric and shall be placed throughout the entire length of the drain, above, below and along the sides of the filter material. The following requirements shall be met: 
 
1. The edges of adjacent sheets shall be overlapped by a minimum of six inches.
 
2. The type of drainage fabric used shall be specified in the engineering design and shall have adequate tensile strength to prevent ripping during installation and backfilling, adequate permeability to allow unimpeded passage of water, and adequate particle retention to prevent migration of soil particles into the filter material.
 
(h) Drainage pipe shall be laid throughout the entire length of the excavation and shall be placed immediately above the barrier material at the bottom of the excavation and midway between the sides. The type of drainage pipe used shall be as follows: 
 
1. Upslope of the downslope side of the disposal field, where the excavation is filled with filter material, the pipe shall be perforated or laid with open joints.
 
2. Downslope of the downslope edge of the disposal field, and beyond the extent of the filter material, the pipe shall be non-perforated and laid with tight joints.
 
3. The size of the pipe shall be large enough to handle the expected amount of flow and in no case shall the pipe diameter be less than four inches.
 
4. Materials used for drainage pipe shall be as allowed in 7:9A-9.5(b).
 
(i) Free-flowing outlets shall be provided downslope of the drain, on each end of the disposal field. Outlets shall meet the following requirements: 
 
1. Outlets may empty into a surface water body, a drainage swale discharging to a surface water body, a storm sewer, a groundwater recharge basin, a gravel bed, dedicated seepage pit, or dry well.
 
2. Outlets shall be designed, constructed, located and maintained in a manner which does not cause soil erosion, surface flooding or damage to adjacent properties, does not create a public nuisance, and does not violate any applicable Federal, State or local laws or regulations.
 
3. Adequate measures shall be taken to protect each outlet from entry of rodents or other small animals.
 
(j) Backfill over the drain and the drain discharge pipes shall be of earth similar to that found at the site and free of large stones, broken masonry, stumps or other waste construction material.
 
(k) Where an interceptor drain is proposed to divert laterally moving perched ground water away from the area of the disposal field, the drain shall be installed and its satisfactory performance confirmed prior to granting of final approval, as follows. 
 
1. After installation of the drain has been completed, borings or pits shall be excavated to the top of (but not penetrating) the hydraulically restrictive horizon, hydraulically restrictive substratum or massive rock substratum above which the perched zone of saturation is located. This shall be done on the upslope and downslope sides of the drain and during a time of year when the presence of the perched zone of saturation is anticipated. Piezometers may also be used for this purpose provided that they do not penetrate through the hydraulically restrictive horizon and provided that the requirements of 7:9A-5.9(e) are met.
 
2. The drain shall be considered to be performing adequately if no perched zone of saturation is observed on the downslope side of the drain at the same time that a perched zone of saturation is observed on the upslope side of the drain. This test shall be witnessed by the administrative authority or its authorized agent.
 (Amended by R.1999 d.314, effective September 20, 1999. 
 See: 31 New Jersey Register 1416(a), 31 New Jersey Register 2741(a). 
 In (i)1, added a reference to dedicated seepage pits and dry wells.)