Drainage Design Assistance
A subsurface drainage system is used to intercept water before it
can infiltrate the subgrade under a paved surface or to remove it
once it is there. A saturated subbase becomes unstable and no longer
provides a reliable foundation for the pavement. The importance of
drainage is documented in a recent FHWA survey that reveals a strong
correlation between the quality of pavement drainage and the life
expectancy of that pavement:
of drainage Average
*Christopher, Zhao, and Hayden (FHWA)
discussion assumes that attention has been given to grading considerations
as well as to the strategic placement of surface inlets. It will address
only the subsurface drainage system.
drainage system consists of two components:
The collector system
Collectors are placed in a pattern beneath the surface to collect
water from saturated soils under the pavement. The collector system
in this discussion is assumed to be a Multi-Flow water collection
The transport system
Once water has been collected, it is transferred to the transport
system. The transport system has the sole purpose of delivering
water to the exit point(s).
Determine the outlet capacity.
the outlet in a single fixed location?
Does the outlet have a limited capacity?
is its capacity?
ideal situations, there are no restrictions on the amount of
water that can be discharged and the rate at which it can be
discharged. However, this is frequently not the case. Consequently,
it is essential to discover the outlet capacity before designing
the system. Outlet capacity will significantly determine the
structure of the collection system.
Determine the amount of water that should be evacuated.
The site must be studied for the source and volume of water. Unwanted
water in the base and subbase will originate from one or more
of these three sources: infiltration, encroachment, and/or water
surprising amount of water infiltrates from the surface.
FHWA studies show typical infiltration rates:
• Concrete 50%
rates can be reduced by:
the amount of time water stands on the surface by ensuring
proper grading and the strategic placement of surface inlets.
a uniform, non-porous surface through seal coating, etc.
for 30% - 50% infiltration. Attempt to evacuate all surplus
subsurface water within 12 hours of a rain event.
second source of water is from lateral movement. This results
when the paved surface is adjacent to an area with a higher
elevation and an impermeable layer that prevents the natural
downward movement of the water. Under some circumstances, water
can travel long distances laterally. Borings can help determine
the amount of water moving laterally.
Water table elevation
planning must take into account the possibility that water may
invade from below. Soil structures and rainfall amounts will
dictate the rate at which water table levels change.
Design a transport system capable of carrying water from the Multi-Flow
collector system to the outlet. The capacity of the transport
system at the outlet should be:
Less than or equal to the capacity of the outlet
If possible, greater than or equal to the capacity of the collector
In most cases, Schedule # 40 PVC pipe should be specified for
the transport system. Under certain conditions PVC sewer pipe
or dual wall HDPE pipe is adequate if engineered properly. In
all cases, the transport system must have load bearing capabilities
equal to or greater than that of the collector system.
3. A minimum of .5% slope must be maintained on the transport
4. For purposes of calculating the correct size PVC for transport
system assume these nominal flow rates:
4” PVC 82
6” PVC 240
8” PVC 494
10” PVC 902
rates do not take into account the effect of head, which may be
substantial. Head pressure will vary depending on the amount of
rainfall and the depth of the transport system relative to the
depth of the collector system.
selecting the size and location of collector lines, the use of
the site and/or the load-bearing demands placed on the site need
to be accounted for. It is also essential to identify the areas
of the site that have greater potential for accumulating excess
water. For this reason two parameters must be established:
2. Physical factors
establishing the parameters for designing the collector layout,
consider these two categories in the broadest sense.
features include the presence, location, amount, source, and
movement of water. Examples of hydrologic features might be:
moving under the paved surface from an adjacent area with
a higher elevation. e.g. a hillside
Water encroachment from a heavily irrigated area. e.g. a boulevard
or an island
Water present under the pavement due to a high water table
or underground spring.
The pavement, and/or the subbase, slope to a “bowl”
concentrating the water beneath the pavement in a given area.
Each feature will require a unique response.
water is moving to the site laterally, it can be intercepted.
water is concentrated in restricted areas, these spots can
be targeted with intensive drainage.
When the saturation is widespread, a more systematic comprehensive
approach may be required.
Physical factors include both the intended use and the load bearing
capacity of the pavement. Both of these factors will contribute
to collector system design decisions. Saturated bases and subbases
exhibit dramatically reduced load bearing capabilities and do
not provide an acceptable foundation for the pavement.
Physical factors can be divided into two parts: a) Usage factors
and b) Structural factors.
Usage factors The use of a paved surface may range
from light pedestrian traffic to heavy truck traffic. The heavier
the anticipated usage, the closer the collector lines will need
to be located to each other.
Structural factors The condition and makeup of the
pavement, base, and subbase will further determine the line
spacings. Perhaps the largest single factor is the hydraulic
conductivity of the base material. Densely packed clay will
require closer line spacings than free draining fill.
Line spacing guidelines
effective line spacings range from 10 to 50 feet. In calculating
appropriate line spacings for any given area, it is crucial
to assess and rate both categories of physical factors. Hydrologic
features will have less impact on line spacing. They will more
likely affect pattern, positioning, and product size selection.
factors can be rated:
Light (e.g. pedestrian and bike traffic)
Light (e.g. car parking)
Heavy (e.g. car traffic)
Very Heavy (e.g. truck traffic)
factors can be rated:
(e.g. free draining soil)
Poor (e.g. compacted clay)
lightly used pavement with an excellent structure might be adequately
drained using 45 foot spacings. A heavily used pavement with
a poor structure might require 10 foot spacings. The attached
charts give examples of suggested spacings in light water volume
situations and in heavy water volume situations.
spacing in light water volume situations
spacings in heavy water volume situations
accounting for hydrologic features and physical factors, drainage
collector lines can be placed where they are most needed and
with the concentration that is required. This provides affordable
and effective drainage.
the outlet is insufficient for rapid evacuation, spacing the
lines further apart and deeper will widen the drawdown gradient
and slow down the desaturation process.
The drainage pattern will be dictated by the surface profile.
- Relatively flat surfaces are most effectively drained using
a grid pattern.
- Sloping surfaces are best drained by orienting the collectors
at a 45 degree angle to the slope. This will place the collectors
in position to intercept the natural flow of the water and
minimize the need to increase the trench depth
to maintain grade.
5. Product size
The product size (6, 12, or 18-inch) will be determined by three
Length of run
If the outlet capacity is sufficient, use larger versions
of Multi-Flow in runs that are longer or deeper.
purposes of planning assume that:
each line of 6-inch Multi-Flow can deliver 17 gpm
each line of 12-inch Multi-Flow can deliver 29 gpm
each line of 18-inch Multi-Flow can deliver 45 gpm
Overall system capacity
Select product size that is consistent with the overall outlet
capacity of the system. Do not deliver water to the outlet faster
than it can leave the site.
Volume of water to be removed
In some situations where there are large amounts of water to
be removed such as near an underground spring or when intercepting
large amounts of laterally moving water, a larger product size
6. A perimeter drain or a curtain drain should
be specified wherever there is an intrusion of water under the
pavement from an adjacent slope and wherever large amounts of
water are anticipated to rush off of the pavement.
Collector pipes must maintain a minimum slope of .5%, 1% is preferred
Backfill material should be clean, very coarse sand. Predominantly
#10 to #30 sieve size is optimal. (See Selecting
Do not design the trench to be lined or covered
with filter fabric. These fabrics are prone to blinding. The
fabric on the Multi-Flow is protected by the sand backfill but
trench liners are exposed to surrounding soil without protection.
Plot the collector system, transport system, and outlet(s)
onto the site plan. Require excavation lines be painted on to existing
paved surface if it exists.
The illustration below demonstrates several design considerations.
Collector lines are placed at three different spacings reflecting
three intended uses. They are placed such that they will intercept
the water flow direction. Curtain drains are placed to intercept
water invading from the hillside to the right as well as from the
heavily irrigated islands near the top. Transport pipes running
along the top and bottom from right to left and along the left side
from the bottom to the top take water from the collectors and removes
it from the site.
Excavation must take place in a manner that results in a 4-6 inch
wide trench with a clean, sharp edge. The bottom of the trench will
be free from loose material. All excavated spoil is to be removed
from the site.
Connection of collection system to transport system
If the elevation of the outlets allows for it, locating the transport
system below the collector system has several benefits. This method
increases flow performance and reduces the risk of rehydration
of low-lying areas.
the transport system is located below the collector system, Multi-Flow
multi-purpose connectors are used to outlet the Multi-Flow collectors.
ending in 09 and 0M are most commonly utilized)
e.g. see 1200M
If the site includes elevation constraints the transport system
can be located level with the bottom of the collector system.
the transport system is located at the same level as the bottom
of the collector system, end outlets or side outlets are used
to outlet the Multi-Flow collectors.
method should not be used unless it is dictated by the elevations.
Backfill and consolidation
Backfill will be accomplished using select, washed, very coarse
sand. See Selecting Backfill Material
for more information on this topic.
Thorough and effective consolidation is required.
Replacement pavement should take place after consolidation is complete.
Replacement pavement should exceed the thickness of surrounding
Paved Surfaces Applications
How Artificial Drainage Works