JBSURFACE SOIL ABSORPTION OF WASTEWATER
ARTIFICIALLY DRAINED SYSTEMS
Richard J. Otis, P.E.
Rural Systems Engineering
Madison, Wisconsin
Introduction
Soils with high water tables or saturated conditions that limit
the use of conventional sa&surface disposal methods sometimes can be
drained to permit the use of trench or Bed systems. Curtain drains,
vertical drains or anderdrains may fre employed depending on the nature
of the drainage proBlem. Of the three, only curtain drains are used
to any great extent. Poorly drained soils often indicate other soil
and site conditions which drainage alone will not overcome. Therefore,
the stte usually is left undeveloped or other methods of onsite
wastewater disposal sucfi as mounds are used rather than to attempt
draining. If drainage is to Be used, careful site evaluation is
extremely important to determine which method will be appropriate and
whether it will be successful.
Application
A. Description
1.	Curtain Drains
Curtain drains are used to lower perched water that is moving
laterally across the site over a very slowly permeable barrier.
They are placed around the upslope perimeter of the soil absorption
site to intercept the groundwater moving into the area. The drains
consist of narrow trenches excavated down to the slowly permeable
Barrier in which drainage pipe is placed (See Figure II. A-l).
Gravel or crushed rock is placed around the pipe and to a level
above the high water table elevation so that the intercepted water
can readily flow to the drainage pipe. If sufficient slope exists
on the site, one end of the drain is brought to the ground surface
downslope to allow free drainage. If the slope is insufficient,
pumps must be used to remove the water.
2.	Vertical Drains
Vertical drains or barrier trenches are used to lower water
tables perched over a thin slowly permeable barrier that overlies

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w
CUI
OUTLET
FIGURE II. A-l
Curtain Drain System
VBRTICAL ORAIN
SAtRtfiR MATERIAL

UNSATURATED PCRMEAStfl
SOIL
A830PTION BID
FIGURE II. A-2
Vertical Drain System

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permeable soil. They are trench excavations dug deeo around the
upslope perimeter of the soil absorption site through the restric-
tive layer into the more permeable soil below. The excavation is
backfilled with porous material, No drainaqe piDe is used (See
Figure TI. A-2). Water intercepted by the drain is able to drain
into the underlying permeable soil.
3. Underdrains
Uriderdrains are used where true water tables exist in permeable
soils. The drains are similar to curtain drains in construction,
but several trenches uniformly spaced under the soil absorption
site may be necessary to lower the water table sufficiently
(See Figure II. A-3). Depth and spacing of the drains are determined
6y the soil and water table characteristics.
UNDERDRAINS
MfM/ '*|m
ABSORPTION
TRENCHES
- PILL
MATERIAL
GRAVEL
ORIGINAL WATER TABLE

0RAINAQ6 PIP6
FIGURE II. A-3
Underdrain System

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Site Considerations
Successful design of artificially drained systems depends upon
the correct diagnosis of the drainage problem. The source of the
groundwater and the way it moves through the soil absorption site
must be determined to select the proper method of drainage.
1. Subsurface Drainage Problems
The most common subsurface drainage problems can be
grouped into four general types: 1) free water tables;
2) water tables over leaky artesian aquifers; 3) perched
water tables; and 4) lateral groundwater flow problems
(Soil Conservation Service, 1973).
•	Free water tables
These typically are large slow-moving bodies of water
fed by surface waters, precipitation and subsurface
drainage from other areas. In lower elevations of the
basin, the groundwater is discharged into streams,
marshes or other aquifers. The elevation of its surface
fluctuates with the seasonal variation of the water
supply into the basin. The slope of the water table
surface is usually quite gentle, following the natural
surface topography.
•	Water tables over leaky artesian aquifers
Artesian aquifers are groundwater bodies confined by
an overlying impervious layer. Its pressure surface
(elevation to which the water will rise in a well
tapping the aquifer), is higher than the confining layer
and may rise above the ground surface. Pressure in
the aquifer is from .the weight of a continuous body of
water extending to a source higher than the pressure
surface. If there are weak points or breaks in the
confining layer, the aquifer may leak into the water table
above it.
•	Perched water tables
In stratified soils, a water table may develop which is
separated from the free water table by a slowly permeable
layer. Jhe soil above the layer may become saturated by
precipitation percolating throuqh the soil.
•	Lateral groundwater flow
Lateral groundwater flow usually occurs on sloping sites
with shallow barriers to vertical percolation.

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2. Site Evaluation
Because each drainage problem requires a different solution,
the site evaluation must be done in sufficient detail to
differentiate between them. Where the need for subsurface
drainage is anticipated, topographic surveys, soil profile
descriptions and estimation of seasonally high groundwater
elevations and gradients should be emphasized. Evaluation
of these site characteristics must be done in addition to
other characteristics evaluated for subsurface disposal.
•	Topographic surveys
Topographic maps of the site with one or two foot
contour intervals are useful as base maps on which water
and soils information can be referenced. Water table
elevations, seep areas and areas with vegetation indicative
of wet areas should be located on the map. This information
is used to establish the source of the groundwater, its
direction of flow and the placement of the drainage
system.
•	Soil profile descriptions
Careful examination of the soil profile for stratifi-
cation and color helps to identify the type of drainage
problem and the seasonal water table fluctuations. Soil
texture, density, color, zones of saturation and root
penetration aid in identifying l-iyers of varying hydraulic
conductivity. The depth, thickness and slope of each layer
should be described. Stratified soils indicate that perched
or lateral groundwater flow may be the problem. Mottled or
qleyed soils indicate zones of periodic or continuous satura-
tion. Mottled soil colors help to locate the seasonally
hiqh water table elevation while the top of the qleyed
rone indicates the average water table elevation.
• Groundwater elevations and gradients
To accurately determine groundwater elevations and
gradients, observation wells and piezometers are used.
Observation wells use well screens which intercept the
water table surface while piezometers are solid pipes open
only at the bottom which penetrate the water table or
aquifer to different depths (See Figure II. B-l). Observation
wells are used to observe fluctuations in the water table
surface elevation. If several are strategically placed
about the area, the gradient of the water table surface can
also be established. Piezometers are driven to various
elevations to observe the hydrostatic pressure at different
depths within the water table. These are used to establish
vertical as well as horizontal hydraulic gradients. Fiqure
II. B-2 shows the interpretation of piezometer observations.

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WELL
piezometer

^ QIJOUND SURFACE
»aV/Aw/	'
WELL SCRSBM
•JtfSK.
QROUHO WATER TABLE

	
4 W.-W •
tew—A.
•SOLID WALL PIPE
J V

FIGURE II. B-l: Illustration of Observation Wells and Piezonmeters
v'AtV.^Li .-v.* Jlu —¦	-»* v • •
w..vxcc-V

¦*• **-* * ¦ r <-- ** -
FIGURE II. B-2: Interpretation of Piezometer Results: (a) some downward
percolation of groundwater; (b) some groundwater seeping upward from a'
deeper strata; (c) groundwater seeping both upward and downward from
stratum; (d) groundwater seeping into stratum and moving horizontally
from the area (After SCS, 1973).

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III. Design
A. Selection of Drainage Method
A good site evaluation will, indicate whether it is practical to
provide subsurface drainage. In general, shallow perched water that
moves laterally across the site is the easiest subsurface drainage
problem to correct. Sites with slow-moving free water tables are
seldom practical to drain because closely spaced drains with pumped
discharges are often necessary. Table III. A-l presents recommended
drainage methods for various site characteristics.
TABLE III. A-l
Recommended Drainage Methods for Various Site Characteristics (after EPA, 1930)
Site Characteristics
Saturated or mottled soils
above a restrictive layer
with the source of water
located at a higher elevation.
Site is usually sloping.
Drainage Problem
Lateral flow
Drainage Method
Curtain drain
Vertical drain1
Saturated or mottled soils
above a restrictive layer
overlying unsaturated
permeable soils. Site is
level or only gently sloping.
Perched water
Vertical drain1
Deep uniform soils which are
mottled or saturated.
Free water table
Avoid3
Saturated soils above and
below a restrictive layer in
which hydraulic gradients
increase with depth.
Artesian fed water
table
Avoid2
'Use only where the restrictive layer is thin and the underlying soil is
reasonably permeable.
20ther alternatives such as mounds should be evaluated. Soils with greater
than 70 percent clay are difficult to drain and should be avoided.

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B. Curtain Drains
1.	Layout
Curtain drains are placed upslope from the site of the proposed
soil absorption system to intercept the groundwater flowing across
the site and around either end to prevent lateral intrusion. A
separation distance between the drain lines and the soil
absorption system is necessary to prevent wastewater from
entering the drain. The distance is dependent on the soil
permeability and the depth of the drain below the bottom of the
absorption system. A horizontal separation distance of
10 to 15 feet is commonly used. On sites with sufficient slope,
the drain is extended downslope until it surfaces to allow gravity
drainage.
2.	Depth
The drain must be placed at a sufficient depth to maintain
at least 3 feet of unsaturated soil below the bottom of the
absorption system during loading of the system. Where possible,
the drain should be placed on or slightly into the restrictive
layer to intercept all the groundwater flowing downslope. If
not, the depth necessary to maintain the saturated zone below
the required separation distance must be computed. An estimate
of the necessary depth can be determined simply by computing
the thickness of the saturated zone needed to conduct the liquid
discharged to the system. Darcy's law can be used assuming one
dimensional flow along the restrictive layer. The depth of the
drain below the ground surface is the sum of the depth of the
absorption system and the thickness of the saturated zone needed
plus 3 ft. (See Figure III. B-l) If the soil is reasonably
permeable, groundwater mounding immediately below the system can
be ignored.
Example: A trench system is to be installed near the base of a lone
4-percent slope. The soils are sandy loam and show the evidence of
seasonal saturation at a depth of 36 in. Impermeable bedrock is found
at a depth of 6 ft. with a slope parallel to the ground surface. It
is proposed to use a curtain drain to maintain a sufficient unsaturated
depth below the system for treatment. Ideally, the drain would be laid
just above the bedrock to removeall the water novino downslooe. However,
because of limited area, it is not possible to davlinht the curtain drain
on the owner's prooerty if it is put at that deoth. To dayliqht on t>e
owner's property, the drain can be no deeper than 5 ft.
The system will serve a 3-bedroom home. It will consist of two trenches
125 ft. long and 3 *t. wide snaced 6 ft. from sHewall to sidewall. The^
trenches will follow the slope contours. The vertical saturated conducti-
vity of the sandy loam is estimated to be 0.H2 ft/min. Can the curtain
drain he used?

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9V9TIM OBPTN
1/10
art.

FIGURE III. B-l
Determination of Curtain Drain Depth
The peak loading rate of the trenches will be 3 bdrm x 150 gpd/bdrm
or 450 gpd. Each linear ft. of trench will receive 450 gpd * 2 x 125 ft =
3.6 gpd. Therefore, each linear ft of slope width will receive 2 x 3.6 gpd
or 7.2 gpd.
Darcy's Law is used to determine the saturated thickness necessary to
conduct this liquid downslope.
Q = KA dh or A = Q
dl	„ dh
* dl
Where K is the horizontal saturated conductivity and is equal to the
slops of the bedrock. Since the horizontal saturated conductivity is
unknown, it is assumed to be equal to the vertical conductivity. This is
a conservative estimate since the horizontal saturated conductivity is
usually greater than the vertical due to soil forming processes.
	. 7-2 9Pd .	 = 0.84 ft/ft
7.48 gal/ft3x 0.02 ft/min x 0.04 x 1440 min/d

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Max
Depth = 6 ft - (6 ft-5 ft) - (0.84 ft x cos 4°) - 3 ft - 0.84 ft - 3 ft = 1.15 ft
Trenches
Therefore, the drain can be used if the trench bottoms are no more than about
1 ft. below the natural ground surface.
3. Size
The size of the drain is dependent on the soil permeability,
size of the area contributing water upslope from the drain
and the gradient of the pipe. In the case of curtain drains,
the amount of water to be removed by the drain is equal to the
amount of precipitation that soaks into the ground upslope from
the drain. Figure III. B-2 is useful in determining the required
"pipe diameter for given pipe gradients and drainage areas. The
drainage coefficient is the amount of water to be reiroved over
a 24-hour period. It will vary with climate and soil permeability.
Unless the drainage coefficient is known for an area, 3/8 to 1/2
inch may be used for mineral soils (Soil Conservation Service,
1973). It is important to note that the area drained is the
area upslope from the drain, not the area of the absorption
system.
Example: The area upslope of the curtain drain in the example in Sec. III.
B-2 is 10 acres. The slope of the discharge pipe can be no more than
0.33 percent or 0.33 ft/100 ft to daylight on the owner's property. Assuming
a drainage coefficient for the sandy loam of 3/8-in, what size drainaqe
pipe must be used? Corrugated plastic drainage tubing will be used.
In Figure III. B-2, find the 10 acres-mark in the 3/8-in "Drainage
Coefficient" column. Move horizontally to the left from this point until
the 0.33 ft/100 ft vertical qrade line is intersected. This point lies between
the 4-in and 5-in diameter lines. Therefore, a 5-in pipe must be used.
C. Vertical Drain
1.	Layout
The layout of vertical drains or barrier trenches is similar
to curtain drains except that no outlet is necessary. A
horizontal separation distance between the drain and the soil
absorption system 1s not critical because there is no outlet.
2.	Depth
The drain must be excavated through the restrictive layer
and into the permeable soil below.

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GRADE IN CENTIMETERS PER METER
0.2 0.3 0.40.5	t.O 2.0 3.0 4.0 5.0
ACRES
DRAINED
0.05
•«ooi'«soor)oofl
-woof"000!-"**. "00Q
45oor"00
i I ! '|l'
4000K>000
2000 **300
k-2000
f'OOO
1200 '100
23O0
2000 Usoo
(000
too r'°°
<00 >*00
TOO i
isoo r
[-1000
itOO LfOO
.oooh00 tjoo f400
~00	.
•00 Uoo (-500
ISO
430 OOO I
400
• *«a
*00
no r,4fl
Ito H20
I!
£
3.0 4jQ 5.0
0.1	0.2 03 0 ~ 0 5	10
GRADE IN FEET PER 100 FEET
DRAINAGE
COEPFlCiEN
Space between lines is the range of drain
capacity for the sue shown between lines
V= velocity m feet per second
n= 0.015
FIGURE III. B-2
Capacity Chart for Subsurface Drains. V is Velocity
in fps. (n3 0.015", corrugated plastic tubing) (SCS, 1977

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3. Size
Sufficient infiltrative surface at the bottom of the trench
within the permeable soil must be provided to absorb all the
water entering the drain. The volume of water to be absorbed
must be estimated by multiplying the drainage coefficient (the
number of inches of precipitation per acre which is absorbed by
the soil), times the upslope area drained. The width and depth
of the drain below the restrictive layer is calculated by
assuming an infiltration rate of the underlying soil.
D. Underdrains
Experience with underdrains is lacking. Because of concern
for the effects on groundwater quality and continued performance of
the system, dther alternatives should be explored. Underdrains are
not recommended. See Soil Conservation Service (1973) for design
of these systems.
IV. Construction
A.	Curtain Drains
1.	Porous media
Porous media such as gravel, crushed rock, cinders, coral,
etc. is placed around and over the pipe to an elevation above
the seasonally high water table.
2.	Barrier material
A barrier material is placed over the media to prevent back-
fill from infiltrating the media. If siltation is expected
to be a problem, the pipe may be wrapped with drainage fabric
to keep the pipe clean.
3.	Outlet
The outlet should be about 1 to 2 feet above the hiqh water
level of the drainaqe ditch. It should be covered with rock or
gravel to protect against bank erosion and small animals.
B.	Vertical Drains
Vertical drains are constructed similarly to curtain drains
except that the porous media may not be necessary if the restrictive
layer is sufficiently broken up.
V. Maintenance
A well constructed drainage system should require little maintenance.
The outlet should be inspected routinely, however, to see that it is
kept clean and clear.

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VI. Questions
1.	How should observation wells and piezometers be placed to determine
the horizontal and vertical groundwater gradients? How many of
each are necessary?
2.	What information must be gathered during the site evaluation to
determine whether a curtain drain or vertical drain is necessary?
VII. References
Soil Conservation Service. 1973, Drainage of agricultural land. Water
Information Center, Inc. Port Washington, New York,
Soil Conservation Service. 1977. Drainage quide for Wisconsin. 'JSDA,
Madison, Wisconsin.
U.S. Environmental Protection Agency. 1980. Onsite wastewater treatment
and disposal systems. Design manual, Tecfinoloay Transfer. Cincinnati,
Ohio.

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VIII. Problems
A. Statements
1.	Use of Figure III. B-2
•	What is the maximum area a 4-in pipe laid on a 2 percent
grade can drain assuming a 3/8 in drainage coefficient;
a 1/2 in drainage coefficient?
•	What is the rate of discharge for these two conditions?
•	What size pipe is necessary if a 0.4 percent tile grade
is used?
•	What is the drainage coefficient? What factors would
affect it?
2.	Stump Creek Subdivision
The area shown in the attached topographic map is to be
used for a subsurface soil absorption system. Periodic
saturation of the soil occurs to within 2 ft of the ground
surface. A curtain drain is to be used to maintain the zone
of saturation below 5 ft from the surface. A 25 ft min
separation distance between the drain and the system is
required. The drain will daylight at Stump Creek. The soils
are loamy with a grass cover.
•	What is the area which will contribute water to the drain?
•	What is the slope of the drain?
•	What size drain is required?

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B. Solution
1. Use of Figure III. B-2
Drainage Coefficient
3/8 .in
1/2 in
• Maximum area
drained at 2%
7 acres
5 acres
•	Discharge rate
•	Pipe size at
0.11 ft3/sec 0.11 ft3/sec
0.4% tile grade
6-in
5-i n

Drainage coefficent
The drainage coefficient is the amount of water that must be
removed in a 24-hour period. It is an estimate of the volume of
precipitation in inches/acre which is absorbed by the soil and
percolates down to the saturated zone. Soil texture, soil struc-
ture, soil Hens-ity, slooe anH veoetation all affect the coefficient.
The coefficient does not take into account groundwater from other
sources such as surface bodies of water and other aquifers.
imp Creek Subdivision
Contributing area
Roads bound the site to the north and east. It is
assumed that the drainage ditches will remove nearly all
the water from the area beyond the roads. Therefore,
approximately 25 acres will contribute to the drain (See
the attached figure).
Drain slope
To meet the 25 ft separation distance, the drain will
be located on the east side of Stump Creek Road. Here,
the ground elevation is approximately 676 ft. The drain
elevation will be 671 ft with the outlet at 652 ft. The
discharge line will be approximately 1150 ft long.
Therefore,
Drain size
A drainage coefficent of 3/8 in is assumed. Using
Figure IV. B-2, a 6-in min pipe is required. (If a 1/4-in
drainage coefficient is assumed, a 5-in pipe could be used.
This probably would be adequate since the development will
increase the amount of impervious areas in the drainage basin).
0.015 ft/ft or 1.5%

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