Subsurface Drain (Ft) 606

Field Office Technical Guide Section IV

Natural Resources Conservation Service Conservation Practice Standard

SUBSURFACE DRAIN (Ft.) CODE 606

DEFINITION This practice is used on areas having a high water table where the benefits of lowering the A conduit, such as corrugated plastic tubing, water table or controlling ground water or tile, or pipe, installed beneath the ground surface runoff justify installing such a system. surface to collect and/or convey drainage water. This standard applies to areas suitable for the intended use after installation of required PURPOSE drainage and other conservation practices. The soil shall have enough depth and The purpose of subsurface drainage is to: permeability to permit installation of an effective and economically feasible system. 1. Improve the soil environment for The ability to drain and treat saline and sodic vegetative growth, reduce erosion, and soils shall be considered where this is a improve water quality by: problem.

a. regulating water table and ground In areas where an outlet is available, either by water flows, gravity flow or by pumping, the outlet shall be b. intercepting and preventing water adequate for the quantity and quality of effluent movement into a wet area, to be discharged. Consideration shall be given c. relieving artesian pressures, to possible damages above or below the point d. removing surface runoff, of discharge that might involve legal actions e. leaching of saline and sodic soils, under state or local laws. Consideration shall f. serving as an outlet for other be given to maintaining or enhancing subsurface drains, and environmental values. g. regulating subirrigated areas or waste disposal areas. CRITERIA

2. Collect ground water for beneficial uses. General

3. Remove water from heavy use areas, such Subsurface drain installations shall be planned, as around buildings, roads, and play designed, and constructed in compliance with areas; and accomplish other physical Federal, State and Local laws and regulations. improvements related to water removal. Design 4. Regulate water to control health hazards caused by pests such as live fluke, flies, or The design and installation shall be based on mosquitoes. adequate surveys and investigations.

CONDITIONS WHERE PRACTICE APPLIES Capacity. The required capacity shall be determined by one or more of the following: This standard is used for the design and installation of conduits placed beneath the 1. Application of a locally tried and proven surface of the ground to provide drainage. drainage coefficient to the acreage drained, including added capacity required to dispose of surface water entering through inlets.

Conservation practice standards are reviewed periodically, and updated if needed. To obtain the current version of this standard, contact the Natural Resources Conservation Service.

NRCS, Mississippi January 2000 606 - 2

2. Yield of ground water based on the levels and may exclude sections of line near expected deep percolation of irrigation the outlet or sections laid through minor water from the overlying fields, including depressions where the conduit is not subject to the leaching requirement. damage by frost action or equipment travel.

3. Comparison of the site with other similar The minimum depth of cover in organic soils sites where subsurface drain yields have shall be 2.5 ft for normal field levels, as defined been measured. above, after initial subsidence. Structural measures shall be installed if it is feasible to 4. Measurement of the rate of subsurface control the water table level in organic soils flow at the site during a period of adverse within the optimum range of depths. weather and ground water conditions. The maximum depth of cover for standard duty 5. Application of Darcy’s law to lateral or corrugated plastic tubing shall be 10 ft for artesian subsurface flow. trench widths of 2 ft or less (measured at tubing and to 1 ft above top of tubing). Heavy 6. Estimates of lateral or artesian subsurface duty tubing shall be specified for depths flow. greater than 10 ft, trench widths more than 2 ft, or in rocky soils. Size. The size of subsurface drains shall be computed by applying Manning’s formula. The For computation of maximum allowable loads size shall be based on the required capacity on subsurface drains, use the trench and and computed by using one of the following bedding conditions specified and the crushing assumptions: strength of the kind and class of drain. The design load on the conduit shall be based on a 1. The hydraulic gradeline is parallel to the combination of equipment loads and trench bottom grade of the subsurface drain with loads. Equipment loads are based on the the conduit flowing full at design flow. maximum expected wheel loads for the equipment to be used, the minimum height of 2. The conduit flowing partly full where a cover over the conduit, and the trench width. steep grade or other conditions require Equipment loads on the conduit may be excess capacity. neglected when the depth of cover exceeds 6 ft. Trench loads are based on the type of 3. Conduit flowing under pressure with backfill over the conduit, the width of the hydraulic gradeline set by site conditions trench, and the unit weight of the backfill on a grade that differs from that of the material. A safety factor of not less than 1.5 subsurface drain. This procedure shall be shall be used in computing the maximum used only if surface water inlets or allowable depth of cover for a particular type of nearness of the conduit to outlets with conduit. fixed water elevations permit satisfactory estimates of hydraulic pressure and flows Minimum velocity and grade. In areas where under design conditions. sedimentation is not a hazard, the minimum grades shall be based on site conditions and a All subsurface drains shall have a nominal velocity of not less than 0.5 ft/s. If a hazard diameter that equals or exceeds 3 in. exists, a velocity of not less than 1.4 ft/s shall be used to establish the minimum grades if site Depth, spacing, and location. The depth, conditions permit. Otherwise, provisions shall spacing, and location of the subsurface drain be made for preventing sedimentation by use shall be based on site conditions, including of filters or by collecting and periodically soils, topography, ground water conditions, removing sediment from installed traps, or by crops, land use, outlets, and saline or sodic periodically cleaning the lines with high- conditions. pressure jetting systems or cleaning solutions.

The minimum depth of cover over subsurface Maximum velocity without protection. drains in mineral soils shall be 2 ft. This Excessive flow velocity in the drain may induce minimum depth shall apply to normal field piping of soil material into the drain line.

be installed during the dry season when the Maximum velocities by soil texture: water table is low and the iron is in its insoluble Soil texture Velocity form. ft/s Sand and sandy loam 3.5 Where possible, in areas where the potential Silt and silt loam 5.0 for ochre problems is high, protection against Silt clay loam 6.0 ochre development can be provided by Clay and clay loam 7.0 designing an outlet facility to ensure Coarse sand or gravel 9.0 permanent submergence of the drain line.

Maximum grade and protection. On sites Protection against root clogging. Problems where topographic conditions require that drain may occur where it is necessary to place lines be placed on steep grades and design drains in close proximity to perennial velocities will be greater than indicated under vegetation. Roots or water-loving trees, such “Maximum velocity without protection,” special as willow, cottonwood, elm, and soft maple, or measures shall be used to protect the conduit some shrubs and grasses growing near or surrounding soil. These measures shall be subsurface drains may enter and obstruct the specified for each job according to the flow. particular conditions of the job site. The first consideration is to use nonperforated The protective measure shall include one or tubing or closed joints through the root zone more of the following: area. Where this is not possible, water-loving trees should be removed from a distance of at 1. Enclose continuous perforated pipe or least 100-ft on each side of the drain. A tubing with fabric-type filter material or distance of 50 ft should be maintained from property graded sand and gravel. other species of trees except for fruit trees. Orchards can often be drained by drains 2. Use nonperforated continuous tubing, a located close to the fruit trees. watertight pipe, or seal joints. Where corps and grasses may cause trouble 3. Place the conduit in a sand and gravel on drain lines, facilities may be installed to envelope or blinding with the least erodible provide a means for submerging the line to soil available. terminate the root growth as desired or to maintain a water table above the drainlines to 4. Select rigid butt end pipes or tile with prevent growth into the system. straight smooth sections and squares ends to obtain tight fitting joints. Materials. Subsurface drains include conduits of plastic, clay, concrete, bituminized fiber, 5. Wrap open joints of the pipe or tile with tar metal, or other materials of acceptable quality. impregnated paper, burlap, or special fabric-type filter material. The conduit shall meet strength and durability requirements of the site. All conduits shall 6. Install open-air risers for air release or meet or exceed the minimum requirements entry. indicated in the Materials section of the specifications. Iron ochre considerations. If drains are to installed in sites where iron ochre problems are Foundation. If soft or yielding foundations are likely to occur, provisions should be made to encountered, the lines shall be stabilized and provide access for cleaning the lines. Each protected from settlement by adding gravel or drain line should outlet directly into an open other suitable materials to the trench, by ditch and/or should have entry ports as needed placing the conduit on a treated plank that will to provide access for cleaning equipment. not readily decompose or on other rigid Drain cleaning provisions should be installed in supports, or by using long sections or such a way that the drains can be cleaned in perforated or watertight pipe having adequate an upstream or rising grade direction. If strength to insure satisfactory subsurface drain possible, drains in ochre-prone areas should performance. The use of a flat treated plank is

NRCS, Mississippi January 2000 606 - 4 not recommended for corrugated plastic Envelopes and envelope material. Envelopes tubing. shall be used around subsurface drains if they are needed for proper bedding of the conduit Filters and filter material. Filters will be used or to improve the characteristics of flow of around conduits, as needed, to prevent ground water into the conduit. movement of the surrounding soil material into the conduit. The need for a filter will be Materials used for envelopes do not need to determined by the characteristics of the meet the gradation requirements of filters, but surrounding soil material, site conditions, and they must not contain materials that will cause the velocity of flow in the conduit. A suitable an accumulation of sediment in the conduit or filter should be specified if: that will render the envelope unsuitable for bedding of the conduit. a. local experience indicated a need, b. soil materials surrounding the conduit are Envelope materials shall consist of sand- dispersed clays, low plasticity silts, or fine gravel, organic, or similar material, Sand- sands (ML or SM with P.I. less than 7), gravel envelope materials shall all pass a c. where deep soil cracking is expected, or 1.5-in. sieve; not more than 30 percent shall d. where the method of installation may result pass a No. 60 sieve; and not more than in voids between the conduit and backfill 5 percent shall pass the No. 200 sieve. ASTM- material. C-33 fine aggregate for concrete has been satisfactorily used and is readily available. If a sand-gravel filter is specified, the filter gradation will be based on the gradation of the Where organic or other compressible materials base material surrounding the conduit within are used, they shall be used only around a the following limits: rigid wall conduit and above the centerline of flexible tubing. All organic or other

 D size smaller than 7 times d85 size but compressible material shall be of a type that not smaller than 0.6 mm, will not readily decompose.

 D15 size larger than 4 times d15 size,  Less than 5% passing No. 200 sieve, Placement and bedding. The conduit should  Maximum size smaller than 1.5 inches, not be placed on exposed rock or stones more where D represents the filter material than 1.5 in. in diameter. Where such and d represents the surrounding base conditions are present the trench must be material. overexcavated a minimum of 6 in. and refilled to grade with a suitable bedding material. The number following each letter is the percent of the sample, by weight, which is finer than The conduit must be place on a firm foundation to insure proper alignment. If installation will that size. For example, D15 size means that 15 percent of the filter material is finer than that be below a water table or where unstable soils size. are present, special equipment, installation procedures, or bedding materials may be Specified filter material must completely needed. These special requirements may also encase the conduit so that all openings are be necessary to prevent soil movement into covered with at least 3 in. on filter material the drain or plugging of the envelope if except that the top of the conduit and side filter installation will be made in such materials as material may be covered by a sheet of plastic quicksand or a silt slurry. or similar impervious material to reduce the quantity of filter material required. For trench installations or corrugated plastic tubing 8 in. or less in diameter, one of the Artificial fabric or mat-type filter materials may following bedding methods will be specified: be used, provided that the effective opening size, strength, durability, and permeability are 1. A shaped groove or 90o V-notch in the adequate to prevent soil movement into the bottom of the trench for tubing support and drain throughout the expected life of the alignment. system.

2. A sand-gravel envelope, at least 3 in. thick, to provide support Junction boxes, manholes, catch basins, and sand traps must be accessible for 3. Compacted soil-bedding material beside maintenance. A clear opening of not less than and to 3 in. above the tubing. 2 ft will be provided in either circular or rectangular structures. For trench installations of corrugated plastic tubing larger than 8 in., the same bedding The drain system must be protected against requirements will be met except that a semi- velocities exceeding those provided under circular or trapezoidal groove shaped to fit the “Maximum velocity without protection” and conduit will be used rather than a V-shaped against turbulence created near outlets, groove. surface inlets, or similar structures. Continuous or closed-joint pipe must be used For rigid conduits installed in a trench, the in drain lines adjoining the structure where same requirements will be met except that a excessive velocities will occur. groove or notch is not required. Junction boxes shall be installed where three All trench installations should be made when or more lines join or if two lines join at different the soil profile is in its driest possible condition elevations. In some locations it may be in order to minimize problems of trench desirable to bury junction boxes. A solid cover stability, conduit alignment, and soil movement should be used, and the junction box should into the drain. have a minimum of 1 ½ ft of soil cover.

For trench installations where a sand-gravel or If not connected to a structure, the upper end compacted bedding is not specified, the of each subsurface drain line will be capped conduit should be blinded with selected with a tight-fitting cap of the same material as material containing no hard objects larger than the conduit or other durable materials. 1.5 in. in diameter. Blinder should be carried to a minimum of 3 in. above the conduit. The outlet must be protected against erosion and undermining of the conduit, entry of tree Auxiliary structures and protection. Structures roots, damaging periods of submergence, and installed in drain lines must not unduly impede entry of rodents or other animals into the the flow of water in the system. Their capacity subsurface drain. A continuous section of rigid must be no less than that of the line or lines pipe without open joints or perforations will be feeding into or through them. The use of used at the outlet end of the line and must internal couplers for corrugated plastic tubing discharge above the normal elevation of low will be allowed. flow in the outlet ditch. Corrugated plastic tubing is not suitable for the outlet section. If the drain system is to carry surface water Minimize the visual impact of projecting outlets. flow, the capacity of the surface water inlet shall not be greater than the maximum design The outlet pipe and its installation will conform flow in the drain line or lines. Covers, orifice to the following requirements: plated, and/or trash racks should be used to ensure that no foreign materials are allowed in 1. If burning vegetation on the outlet ditch the drain lines. band is likely to create a fire hazard, the material from which the outlet pipe is The capacity of a relief well system will be fabricated must be fire resistant. If the base on the flow from the aquifer, the well likelihood is great, the outlet pipe must be spacing, and other site conditions and will be fireproof. adequate to lower the artesian waterhead to the desired level. 2. Two-thirds of the pipe will be buried in the ditch bank, and the cantilever section must The size of relief wells is generally based on extend to the toe of the ditch side slope or the available materials rather than on hydraulic the side slope protected from erosion. The considerations. Such wells will not be less minimum length of the pipe will normally than 4 in. in diameter. be 8 ft. Under certain conditions shorter

sections are appropriate; e.g., steep-sided drains through depressed or low areas and main and laterals (1:1 or less) with a near outlets must be protected from damage narrow bottom width of 3 ft, commonly caused by farm machinery and other referred to as “minimum ditches,” for equipment and from freezing and thawing. outletting individual subsurface drain laterals. For conduits 10 in. in diameter CONSIDERATIONS and greater, longer outlet sections should be considered, such as: Water quantity

10 in. and 12 in. in diameter, use 12 ft.  Effects on the water budget.  Effects on baseflow and runoff to water 15 in. and 18 in. in diameter, use 16 ft. uses and users.  Effects on ground water recharge. Use 20-ft outlet pipe for all diameters  The volume of soil water needed to larger than 18 in. improve plant growth.

3. If ice or floating debris may damage the Water quality outlet pipe, the outlet shall be recessed to the extent that the cantilevered part of the  Effects on the delivery of sediment and pipe will be protected from the current in dissolved and sediment-attached the ditch.  Effect of changes in the delivery of dissolved salts, such as nitrates, on 4. Headwalls used for subsurface drain downstream water uses and users. outlets must be adequate in strength and  In areas of ground water recharge, design to avoid washouts and other changes in the delivery of dissolved failures. substances to the aquifer.  Effect on downstream water temperatures. Watertight conduits strong enough to withstand the expected loads will be used if subsurface  Effects on the visual quality of downstream drains cross under irrigation canals, ditches, or water. other structures. Conduits under roadways must be designed to withstand the expected PLANS AND SPECIFICATIONS loads. Shallow subsurface Plans and specifications for installing subsurface drains shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose.

NRCS, Mississippi January 2000 Field Office Technical Guide Section IV

Natural Resources Conservation Service Construction Specification

SUBSURFACE DRAIN

1. SCOPE fittings 4-8 in.

This specification covers the minimum requirements for materials and installation Type Specification of subsurface drains. Polyvinyl chloride (PVC) sewer pipe and fittings ASTM-D-27291 2. MATERIALS AND INSTALLATION Polyvinyl chloride (PVC) ASTM-D-30331 pipe or D-3034 type Inspecting and handling materials. Material PSM or PSP for subsurface drains shall be carefully Clay inspected before the drains are installed. Clay drain tile ASTM-C-41 Plastic pipe and tubing shall be protected Clay drain tile, perforated 1 from hazard-causing deformation or ASTM-C-498 warping. Plastic pipe and tubing with Clay pipe, perforated, standard and extra ASTM-C- physical imperfections shall not be installed. 1 A damaged section shall be removed and a strength 700 suitable joint made connecting the retained Clay pipe, testing ASTM-C-3011 sections. Clay and concrete tile shall be Concrete checked for damage from freezing and Concrete drain tile ASTM-C-41 thawing before it is installed. All material Concrete pipe for ASTM-C-1181 shall be satisfactory for its intended used irrigation or drainage and shall meet applicable specifications and Concrete pipe or tile, requirements. determining physical properties of ASTM-C-4971 Materials. The following specifications Concrete sewer, storm pertain to products currently acceptable for drain, and culvert pipe ASTM-C-141 use as subsurface drains. These Reinforced concrete specifications are also to be applied in culvert, storm drain, and ASTM-C-4441 determining the quality of materials sewer pipe referenced by other standards: Perforated concrete pipe ASTM-C-761 Portland cement ASTM-C-1501 Type Specification Other Styrene rubber plastic Plastic drain pipe and fittings ASTM-D-28521 Pipe, corrugated Federal Corrugated polyethylene (aluminum alloy) Specification 1 (PE) tubing and fittings 3-6 ASTM-F-405 WW-P-4022 in. Pipe, corrugated (iron or Specification Corrugated polyethylene steel, zinc Federal 2 1 WW-P-405 (PE) tubing and fittings 8- ASTM-F-667 coated) 24 in. Corrugated polyvinyl 1 Specifications can be obtained from the chloride (PVC) tubing and American Society for Testing and Materials, compatible fittings ASTM-F-8001 1916 Race Street, Philadelphia, PA 19103 Polyvinyl chloride (PVC) 2 Specifications can be obtained from the corrugated sewer pipe with Superintendent of Documents, U.S. Government a smooth interior and Printing Office, Washington, D.C. 20402 ASTM-F-9491

needs to be exercised relative to grade General. The installing contractor shall control and bedding conditions. certify that the installation complies with the requirements of these specifications. The IV. Bedding contractor shall also name the source of materials. The trench bottom shall be smooth and free of clods and loose or exposed rock. Where Perforated conduit. Where perforated a gravel envelope is not specified, the conduit is required, the water inlet area bottom of the trench shall be shaped to shall be at least 1 in.2/ft of conduit length. conform to the pipe. The groove may be Round perforations shall not exceed 3/16- semi-circular, trapezoidal, or a 90 degrees in. in diameter except where filters, “V”-shape (90 degree “V” suitable for 3-8 in. envelopes, or other protection is provided or only) and shall be of such dimensions that for organic soils, where a maximum hole the bottom quarter of the pipe is below the diameter of ½ in. may be used. Slotted contact points of the groove. perforations shall not exceed 1/8 in. in width. In unstable soils a firm foundation shall be provided by overexcavation and backfilling 3. FLEXIBLE CONDUIT with processed stone or gravel, suitably graded so as to act as a mat into which I. General requirements unstable soil will not penetrate.

All conduits shall be laid to line and grade in V. Filters and envelopes such a way that the sidewalls are continuously and uniformly supported with If a sand-gravel filter is specified, it shall be suitable bedding material. Such material clean, hard, durable material and of the shall be properly placed and compacted to gradation specified. provide lateral restraint against deflection and to protect the conduit against collapse When sand-gravel envelopes are used they during backfilling. will be of clean, hard, durable material with less than 5 percent passing the No. 200 II. Trenching sieve, not more than 30 percent passing the No. 60 sieve, and with a maximum size of 1 Trench widths must be adequate for proper ½ in. installation of the conduit, allow proper joining of sections, and allow proper VI. Placement placement of filter, envelope, or blinding materials. The trench bottom shall be Conduit will be placed in such a way that constructed to proper grade before maximum stretch does not exceed 5 placement of the conduit. percent.

Where rock is encountered the trench will Fittings shall be installed in accordance with be overexcavated a minimum of 6 in. and instructions furnished by the manufacturers. refilled to proper grade with a suitable Couplers are recommended at all joints and bedding material. fittings, at all changes in direction (where the centerline radius is less than three times Provisions for safety during trenching tubing diameter), at changes in diameter, operations shall be in compliance with the and at junction with another line. applicable safety and health regulations for construction. Caps are needed at the ends of lines. All fittings shall be compatible with the tubing. III. Plow installation Where certain fittings are not available, handcut holes are acceptable provided care Plow installation has been satisfactorily is taken when making the connection not to used in many situations. Special care create a means of obstructing flow, catching debris, or allowing soil to enter the line.

Place selected bedding material, containing The use of concrete tile in acid and sulfate no hard object larger than 1 ½ in. in soils shall be with the following limitations: diameter in the trench to a minimum depth of 6 in. over the conduit. The conduit will be Acid soils: held in place mechanically until secured by Lower permissible limits blinding. of pH values Organic Medium VII. Backfilling and and Class of tile sandy heavy- Place backfill material so that displacement soils textured or deflection of the conduit will not occur. soils This is preferably on an angle, so the ASTM-C-412 material flows down the front slope. Avoid large stones, frozen material, and dry clods Standard quality 6.5 6.0 that cause concentrated point loads on the Extra quality 6.0 5.5 Heavy duty extra tubing. The trench should be backfilled as quality soon as practical. When installing the 6.0 5.5 tubing on a hot day, backfilling should be Special quality 5.5 5.0 delayed until tubing temperature cools to ASTM-C-14, the soil temperature. C-118, C-444 5.5 5.0

4. CLAY AND CONCRETE TILE NOTE: Figures represent the lowest reading of pH values for soil or soil water at subsurface drain depth. I. Clay and concrete drain tile special requirements Sulfate soils: Permissible maximum limit If clay tile will not be exposed to freezing Type of tile and cement of sulfates, and thawing before or during installation singly or in and if the average frost depth will be less (minimum) combination ppm than 18 in., the freezing and thawing and Tile: ASTM-C-412 7,000 adsorption tests may be modified or waived. Special quality C-14, C-118, Bell and spigot, tongue and groove, and C-444 other types of pipe that meet the strength, Cement: ASTM-C-150, absorption, and other requirements of clay Type V or concrete tile as specified in the preceding Tile: ASTM-C-412 3,000 paragraphs, except for minor imperfections Extra quality, Heavy-duty in the bell, the spigot tongue, or the groove, extra quality and ordinarily classed by the industry as C-14, C-118, “seconds,” may be used for drainage C-444 conduits, provided that the pipe is otherwise Cement: ASTM-C-150, Type II or V adequate for the job. Tile: ASTM-C-412 1,000 Standard quality C-14, C-118, C-444 Cement: ASTM-C-150, any type NOTE: Figures represent the highest reading of sulfates for soil or soil water at subsurface drain depth.

II. Trenching When sand-gravel envelopes are used they will be of clean, hard, durable material with Trench widths must be adequate for proper less than 5 percent passing the No. 200 installation of the conduit; must allow proper sieve, not more than 30 percent passing the joining of sections; and must allow proper No. 60 sieve, and with a maximum size of 1 placement of filter, envelope, or blinding ½ in. ASTM-C-33 fine aggregate for materials. The trench width will be a concrete will meet these requirements. minimum of 3 to 6 in. on both sides of tubing. The trench bottom shall be V. Placement constructed to proper grade and shape before placement of the conduit. All conduits shall be laid to line and grade and covered with the specified blinding, Where rock is encountered the trench will envelope, or filter material to a depth of not be overexcavated a minimum of 6 in. and less than 3 in. around the drain. Blinding refilled to proper grade with a suitable material shall contain no hard objects larger bedding material. than 1 ½ in. in diameter.

Provisions for safety during trenching When a sand-gravel filter is specified, all operations shall be in compliance with the openings in the conduit must be covered applicable safety and health regulations for with at least 3 in. or filter material except construction. that the top of the conduit and the side filter material may be covered with a sheet of III. Bedding plastic or similar impervious material. The impervious sheet will be covered with at If unstable soil conditions are encountered, least 3 in. of blinding material. the trench bottom must be stabilized before placement of conduit. Where necessary the Joints between drain tile shall not exceed unstable material will be removed and 1/8 in. except in sandy soils, where the replaced with sand-gravel or a similar closest possible fit must be obtained, and in suitable stabilizing material. Where an organic soils where some of the more envelope is not specified, the bottom of the fibrous types make it desirable to increase trench shall be shaped to ensure good slightly the space between tile. alignment of the conduit. VI. Backfill Where the conduit is to be laid in a rock trench, or where rock is exposed at the Backfill will be placed in such a manner as bottom of the trench, the rock shall be to avoid displacement of the conduit. removed below grade enough that the Backfill should be moved into the trench at trench may be backfilled, compacted, and an angle so that material slows down the bedded; and when completed, the conduit front slope of previously placed material. shall be a minimum of 6 in. from rock. Backfill shall not contain frozen material, stones, clods, or objects large enough to IV. Filters and envelopes damage the conduit. The trench should be backfilled as soon as possible after blinding. If a sand-gravel filter is specified, it shall be of clean, hard durable material and of the gradation specified.

NRCS, Mississippi January 2000