NATURAL RESOURCES CONSERVATION SERVICE

CONSERVATION PRACTICE STANDARD

SUBSURFACE DRAIN

(Ft.)

Code 606

# DEFINITION

A conduit, such as corrugated plastic tubing, tile, or pipe, installed
beneath the ground surface to collect and/or convey drainage water.

# PURPOSE

-   Improve the soil environment for vegetative growth, reduce erosion,
    and improve water quality by:

-   Regulating water table and ground water flows,

-   Intercepting and preventing water movement into a wet area,

-   Relieving artesian pressures,

-   Removing surface runoff,

-   Leaching of saline and sodic soils,

-   Serving as an outlet for other subsurface drains, and

-   Regulating subirrigated areas or waste disposal areas.

-   Collect ground water for beneficial uses.

-   Remove water from heavy use areas, such as around buildings, roads,
    and play areas; and accomplish other physical improvements related
    to water removal.

-   Regulate water to control health hazards caused by pests such as
    flukes, flies, or mosquitoes.

# CONDITIONS WHERE PRACTICE APPLIES

This standard applies to areas having a high water table where the
benefits of lowering the water table or controlling ground water or
surface runoff justify installing such a system.

This standard applies to areas suitable for the intended use after
installation of required drainage and other conservation practices. The
soil shall have enough depth and permeability to permit installation of
an effective and economically feasible system.

In areas where an outlet is available, either by gravity flow or by
pumping, the outlet shall be adequate for the quantity and quality of
effluent to be discharged.

# CRITERIA

The design and installation shall be based on adequate surveys and
investigations.

**Capacity.** One or more of the following shall determine the required
capacity:

-   Application of a locally tried and proven drainage coefficient to
    the acreage drained, including added capacity required to dispose of
    surface water entering through inlets.

-   Yield of ground water based on the expected deep percolation of
    irrigation water from the overlying fields, including the leaching
    requirement.

-   Comparison of the site with other similar sites where subsurface
    drain yields have been measured.

-   Measurement of the rate of subsurface flow at the site during a
    period of adverse weather and ground water conditions.

-   Application of Darcy's law to lateral or artesian subsurface flow.

-   Estimates of lateral or artesian subsurface flow.

**Size.** The size of subsurface drains shall be computed by applying
Manning's formula. The size shall be based on the required capacity and
computed by using one of the following assumptions:

1\. The hydraulic gradeline is parallel to the bottom grade of the
subsurface drain with the conduit flowing full at design flow.

2\. The conduit flowing partly full where a steep grade or other
conditions require excess capacity.

3\. Conduit flowing under pressure with hydraulic gradeline set by site
conditions on a grade that differs from that of the subsurface drain.
This procedure shall be used only if surface water inlets or nearness of
the conduit to outlets with fixed water elevations permit satisfactory
estimates of hydraulic pressure and flows under design conditions.

All subsurface drains shall have a nominal diameter that equals or
exceeds 3 inches.

**Depth, Spacing, and Location.** The depth, spacing, and location of
the subsurface drain shall be based on site conditions, including soils,
topography, ground water conditions, crops, land use, outlets, and
saline or sodic conditions.

The minimum depth of cover over subsurface drains in mineral soils shall
be 2 feet. This minimum depth shall apply to normal field levels and may
exclude sections of line near the outlet sections laid through minor
depressions where the conduit is not subject to damage by frost action
or equipment travel.

The minimum depth of cover in organic soils shall be 2.5 feet for normal
field levels, as defined above, after initial subsidence. Structural
measures shall be installed if it is feasible to control the water table
level in organic soils within the optimum range of depths.

The maximum depth of cover for standard duty corrugated plastic tubing
shall be 10 feet for trench widths of 2 feet or less (measured at tubing
and to 1 foot above top of tubing). Heavy-duty tubing shall be specified
for depths greater than 10 feet, trench widths more than 2 feet, or in
rocky soils.

For computation of maximum allowable loads on subsurface drains, use the
trench and bedding conditions specified and the crushing strength of the
kind and class of drain. The design load on the conduit shall be based
on a combination of equipment loads and trench loads. Equipment loads
are based on the maximum expected wheel loads for the equipment to be
used, the minimum height of cover over the conduit, and the trench
width. Equipment loads on the conduit may be neglected when the depth of
cover exceeds 6 feet. Trench loads are based on the type of backfill
over the conduit, the width of the trench, and the unit weight of the
backfill material. A safety factor of not less than 1.5 shall be used in
computing the maximum allowable depth of cover for a particular type of
conduit.

**Minimum Velocity and Grade.** In areas where sedimentation is not a
hazard, the minimum grades shall be based on site conditions and a
velocity of not less than 0.5 feet per second (ft/s). If a hazard exits,
a velocity of not less than 1.4 ft/s shall be used to establish the
minimum grades if site conditions permit. Otherwise, provisions shall be
made for preventing sedimentation by use of filters or by collecting and
periodically removing sediment from installed traps, or by periodically
cleaning the lines with high-pressure jetting systems or cleaning
solutions.

**Maximum Velocity without Protection.** Design velocities shall not
exceed those given in Table 1 unless special protective measures are
installed.

  ----------------------------------- -----------------------------------
  Table 1. Maximum velocities by soil 
  texture                             

  Soil Texture                        Velocity, ft/s

  Sand and sandy loam                 3.5

  Silt and silt loam                  5.0

  Silty clay loam                     6.0

  Clay and clay loam                  7.0

  Coarse sand or gravel               9.0
  ----------------------------------- -----------------------------------

**Maximum Grade and Protection.** On sites where topographic conditions
require that drain lines be placed on steep grades and design velocities
will be greater than indicated in Table 1, special measures shall be
used to protect the conduit or surrounding soil. These measures shall be
specified for each job according to the particular conditions of the job
site.

The protective measure shall include one or more of the following:

1\. Enclose continuous perforated pipe or tubing with fabric-type filter
material or properly graded sand and gravel.

2\. Use non-perforated continuous tubing, a watertight pipe, or seal
joints.

Place the conduit in a sand and gravel envelope or blinding with the
least erodible soil available.

Select rigid butt end pipe or tile with straight, smooth sections and
square ends to obtain tight fitting joints.

Wrap open joints of the pipe or tile with tar impregnated paper, burlap,
or special fabric-type filter material.

Install open-air risers for air release or entry.

**Iron Ochre Control.** If drains are to be installed in sites where
iron ochre and manganese dioxide problems are likely to occur,
provisions should be made to provide access for cleaning the lines. Each
drain line should outlet directly into an open ditch and/or should have
entry ports as needed to provide access for cleaning equipment. Drain
cleaning provisions should be installed in such a way that the drains
can be cleaned in an upstream or rising grade direction. If possible,
drains in ochre-prone areas should be installed during the dry season
when the water table is low and the iron and manganese dioxide is in its
insoluble form.

Where possible, in areas where the potential for such problems is high,
protection against their development can be provided by designing an
outlet facility to ensure permanent submergence of the drain line.

**Protection against Root Clogging.** Problems may occur where it is
necessary to place drains in close proximity to perennial vegetation.
Roots of water-loving trees, such as willow, cottonwood, elm, and soft
maple, or some shrubs and grasses growing near subsurface drains may
enter and obstruct the flow.

Where possible, use non-perforated tubing or closed joints through the
root zone area. Where this is not possible, water-loving trees should be
removed from a distance of at least 100 feet on each side of the drain.
A distance of 50 feet should be maintained from other species of trees
except for fruit trees. Orchards can often be drained by drain lines
located close to the fruit trees.

Where crops and grasses may cause trouble on drain lines, facilities may
be installed to provide a means for submerging the line to terminate the
root growth as desired or to maintain a water table above the drain
lines to prevent growth into the system.

**Materials.** Subsurface drains include conduits of plastic, clay,
concrete, bituminized fiber, metal, or other materials of acceptable
quality.

The conduit shall meet strength and durability requirements for the
site. All conduits shall meet or exceed the minimum requirements of the
appropriate specifications published by the American Society for Testing
and Materials (ASTM), American Association of State Highway and
Transportation Officials (AASHTO), and the American Water Works
Association (AWWA).

**Foundation.** If soft or yielding foundations are encountered, the
lines shall be stabilized and protected from settlement by adding gravel
or other suitable materials to the trench, by placing the conduit on a
treated plank that will not readily decompose or on other rigid
supports, or by using long sections or perforated or watertight pipe
having adequate strength to ensure satisfactory subsurface drain
performance. The use of a flat treated plank is not recommended for
corrugated plastic tubing.

**Filters and Filter Material.** Filters will be used around conduits,
as needed, to prevent movement of the surrounding soil material into the
conduit. The need for a filter will be determined by the characteristics
of the surrounding soil material, site conditions, and the velocity of
flow in the conduit. A suitable filter should be specified if:

1.  Local experience indicated a need.

2.  Soil materials surrounding the conduit are dispersed clays, silts
    with a plasticity index less than 7, or fine sands with a plasticity
    index less than 7.

3.  Deep soil cracking is expected, or

4.  The method of installation may result in voids between the conduit
    and backfill material.

If a sand-gravel filter is specified, the filter gradation shall be
designed in accordance with National Engineering Handbook (NEH) Part
633, Chapter 26, Gradation Design of Sand and Gravel Filters.

Specified filter material must completely encase the conduit so that all
openings are covered with at least 3 inches of filter material except
that the top of the conduit and side filter material may be covered by a
sheet of plastic or similar impervious material to reduce the quantity
of filter material required. In all cases the resulting flow pattern
through filter material shall be a minimum of 3 inches.

Artificial fabric or mat-type filter materials may be used, provided
that the effective opening size, strength, durability, and permeability
are adequate to prevent soil movement into the drain throughout the
expected life of the system.

**Envelopes and Envelope Material.** Envelopes shall be used around
subsurface drains if they are needed for proper bedding of the conduit
or to improve the characteristics of flow of ground water into the
conduit.

Materials used for envelopes do not need to meet the gradation
requirements of filters, but they must not contain materials that will
cause an accumulation of sediment in the conduit or that will render the
envelope unsuitable for bedding of the conduit.

Envelope materials shall consist of sand-gravel, organic, or similar
material. Sand-gravel envelope materials shall all pass a 1.5-inch
sieve; not more than 30 percent shall pass a No. 60 sieve; and not more
than 5 percent shall pass the No. 200 sieve. ASTM-C-33 fine aggregate
for concrete has been satisfactorily used and is readily available.

Where organic or other compressible materials are used, they shall be
used only around a rigid wall conduit and above the centerline of
flexible tubing. All organic or other compressible material shall be of
a type that will not readily decompose.

**Placement and Bedding.** The conduit should not be placed on exposed
rock or stones more than 1.5 inches in diameter for 6 inch or larger
tile and stones no more than ¾ inch diameter for tile less than 6
inches. Where such conditions are present the trench must be
over-excavated, a minimum of 6 inches and refilled to grade with a
suitable bedding material.

The conduit must be placed on a firm foundation to ensure proper
alignment. Prevent runoff and surface water from entering the trench.

If installation will be below a water table or where unstable soils are
present, special equipment, installation procedures, or bedding
materials may be needed. These special requirements may also be
necessary to prevent soil movement into the drain or plugging of the
envelope if installation will be made in such materials as quicksand or
a silt slurry.

For trench installations of corrugated plastic tubing 8 inches or less
in diameter, one of the following bedding methods will be specified:

1.  A shaped groove or 90° V-notch in the bottom of the trench for
    tubing support and alignment.

2.  A sand-gravel envelope, at least 3 inches thick, to provide support.

3.  Compacted soil bedding material beside and to 3 inches above the
    tubing.

For trench installations of corrugated plastic tubing larger than 8
inches, the same bedding requirements will be met except that a
semi-circular or trapezoidal groove shaped to fit the conduit will be
used rather than a V-shaped groove.

For rigid conduits installed in a trench, the same requirements will be
met except that a groove or notch is not required.

All trench installations should be made when the soil profile is in its
driest possible condition in order to minimize problems of trench
stability, conduit alignment, and soil movement into the drain.

For trench installations where a sand-gravel or compacted bedding is not
specified, the conduit should be blinded with selected material
containing no hard objects larger than 1.5 inches in diameter. Blinding
should be carried to a minimum of 3 inches above the conduit.

All installations shall meet the minimum requirements of the appropriate
ASTM specification.

**Auxiliary Structures and Protection.** Structures installed in drain
lines must not unduly impede the flow of water in the system. Their
capacity must be no less than that of the line or lines feeding into or
through them. The use of internal couplers for corrugated plastic tubing
will be allowed.

If the drain system is to carry surface water flow, the capacity of the
surface water inlet shall not be greater than the maximum design flow in
the drain line or lines. Covers or trash racks should be used to ensure
that no foreign materials are allowed in the drain lines.

The capacity of a relief well system will be based on the flow from the
aquifer, the well spacing, and other site conditions and will be
adequate to lower the artesian water head to the desired level.

The size of relief wells is generally based on the available materials
rather than on hydraulic considerations. Such wells will not be less
than 4 inches in diameter.

Junction boxes, manholes, catch basins, and sand traps must be
accessible for maintenance. A clear opening of not less than 2 feet will
be provided in either circular or rectangular structures.

The drain system must be protected against velocities exceeding those
given in Table 1 and against turbulence created near outlets, surface
inlets or similar structures. Continuous or closed-joint pipe must be
used in drain lines adjoining the structure where excessive velocities
will occur.

Junction boxes shall be installed where three or more lines join or if
two lines join at different elevations. In some locations it may be
desirable to bury junction boxes. A solid cover should be used, and the
junction box should have a minimum of 1.5 feet of soil cover.

If not connected to a structure, the upper end of each subsurface drain
line will be capped with a tight-fitting external cap of the same
material as the conduit or other durable materials.

The outlet must be protected against erosion and undermining of the
conduit, entry of tree roots, damaging periods of submergence, and entry
of rodents or other animals into the subsurface drain. A continuous
section of rigid pipe without open joints or perforations will be used
at the outlet end of the line and must discharge above the normal
elevation of low flow in the outlet ditch. Standard corrugated plastic
tubing is not suitable for the outlet section. Minimize the visual
impact of projecting outlets.

Continuously submerged outlets will be permitted for water table control
systems if planned and designed according to the standard for Drainage
Water Management (code 554).

The outlet pipe and its installation will conform to the following
requirements:

1.  If burning vegetation on the outlet ditch bank is likely to create a
    fire hazard; the material from which the outlet pipe is fabricated
    must be fire resistant. If the likelihood is great, the outlet pipe
    must be fireproof.

2.  Two-thirds of the pipe will be buried in the ditch bank, and the
    cantilever section must extend to the toe of the ditch side slope or
    the side slope protected from erosion. The minimum length of the
    pipe will normally be 8 feet. Under certain conditions shorter
    sections are appropriate; e.g., steep-sided main and laterals (1
    (horizontal) : 1 (vertical) or less) with a narrow bottom width of 3
    feet, commonly referred to as "minimum ditches," for outletting
    individual subsurface drain laterals. For conduits 10 inches in
    diameter and greater, longer outlet sections shall be considered,
    such as:

-   10 inches and 12 inches in diameter, use 12 feet.

-   15 inches and 18 inches in diameter, use 16 feet.

-   Use 20 feet outlet pipe for all diameters larger than 18 inches.

1.  If ice or floating debris may damage the outlet pipe, the outlet
    shall be recessed to the extent that the cantilevered part of the
    pipe will be protected from the current in the ditch.

2.  Headwalls used for subsurface drain outlets must be adequate in
    strength and design to avoid washouts and other failures.

Watertight conduits strong enough to withstand the expected loads will
be used if subsurface drains cross under irrigation canals, ditches, or
other structures. Conduits under roadways must be designed to withstand
the expected loads. Shallow subsurface drains through depressed or low
areas and near outlets must be protected from damage caused by farm
machinery and other equipment and from freezing and thawing.

# CONSIDERATIONS

When designing subsurface drainage systems, consider the effects the
system will have on water quantity and quality.

Effects on quantity to consider include: water budget, base flow and
runoff to water uses and users, groundwater recharge, and volume of soil
water needed to improve plant growth.

Water quality effects that should be considered include: delivery of
sediment, changes in the delivery of dissolved salts, such as nitrates,
on downstream water uses and users, changes in delivery of dissolved
substances to the aquifer, downstream water temperatures, and the
effects on the visual quality of downstream water.

If a concern exists of tile lines picking up polluted water from manure
spreading, consider installing tile blocks, stoppable catch basins, or
other temporary flow blocking devices.

Consideration should be given to using subsurface drainage to control
high water tables in areas where septic tanks and leach fields exist.

Consider adding collector mains to minimize the visual impact, potential
fear from ice or debris damage, and to facilitate maintenance of the
grassed ditch bank.

The ability to drain and treat saline and sodic soils shall be
considered where this is a problem.

Consideration shall be given to possible damages above or below the
point of discharge that might involve legal actions under federal,
state, or local laws. Consideration shall be given to maintaining or
enhancing environmental values.

Considerations must be given to preventing adverse impacts to delineated
wetlands regulated by State and Federal regulations.

# PLANS AND SPECIFICATIONS

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.