Making Edge Drains Work

Base edge drains can enhance pavement life — but only if you maintain them. Here’s what experts say you should be doing.

by Tom Kuennen, Contributing Editor

If water is the oft-cited enemy of pavements, longitudinal edge drains — when coupled with permeable base layers — can be a powerful weapon against the nemesis of roads.

But to work right, edge drain systems can’t be buried and forgotten, as they frequently are, according to experts. Years of research have shown that edge drains must be regularly inspected and maintained or they will succumb to forces of time and nature causing the road to fail prematurely and expensively.

Typical Edge Drain Installation

While data are still being collected and analyzed on the efficacy of permeable road bases and adjacent edge drains in prolonging pavement life under all circumstances, the weight of evidence shows that edge drains do work in keeping water out of pavement structures and help to preserve pavements. As such, the Federal Highway Administration has maintained that positive drainage systems be included in new pavement designs and reconstructions where appropriate.

“Consideration of drainage is recognized as one of the important factors in pavement design,” said former Federal Highway Administrator Tom Larson. “However, inadequate subsurface drainage continues to be identified as a major cause of pavement distress.

“Developments in technology for permeable bases and longitudinal edge drains make the provision of positive drainage of the pavement section possible and affordable,” Larson said. “Where the drainage analysis or past performance indicates the potential for reduced service life due to saturated structural layers or pumping, the design must include positive measures to limit that potential.”

Accentuate the positive

Essentially, a positive pavement drainage system as described by Larson will consist of a geotextile-wrapped pipe placed longitudinally along the edge of a pavement, buried in the shoulder a couple of feet down into the pavement structure and next to the pavement’s permeable base.

Outlets placed horizontally or laterally every 250 feet remove rain or meltwater collected by the edge drain from the permeable base of the roadbed, and move it into a roadside ditch or other outfall.

“A positive drainage system requires three main elements,” says Angel Correa, P.E., pavement design engineer, FHWA’s Southern Resource Center, Atlanta.

Correa describes them as a permeable base under the pavement; an aggregate separator layer under the permeable base to prevent fines from the subgrade from migrating into and plugging the permeable base; and a longitudinal pipe system that captures water from the permeable base and moves it to the outlets, ideally every 250 feet.

“The permeable base is an open gradation, such as AASHTO No. 67,” Correa says. “The coefficient of uniformity has to be from 2 to 8, and if it is below 4, the permeable base needs to be either cement- or asphalt-stabilized.” The lower the coefficient of uniformity, the greater the degree of drainability; however, its stability will be lower, hence its need for stabilization at the lower end.

A trench is dug along the outside pavement edge and backfilled with granular material equal to, or more permeable than, the permeable base itself. The longitudinal pipe is placed in this trench, wrapped with geotextile filter fabric to keep fines out. “We recommend using a 4-inch perforated, corrugated plastic pipe, the elephant trunk,” Correa says. “The minimum requirement is 2 inches of opening per lineal foot.”

Smooth, noncorrugated lateral outlet pipes are joined to the edge drain every 250 feet, Correa says. FHWA strongly recommends this junction not be a right-angled Tee, but a smooth curve connection with a 30- to 36-inch radius. “It has to be a smooth curve, because if it is a 90-degree connection, we will not be able to insert a video camera or maintenance flushing equipment around the corner,” explains Correa.

Depending on the cross-slope of the road, edge drains may be needed on both sides of the pavement. However, if the road slopes to one side only, that side should receive the edge drain.

Periodic maintenance needed

Their meticulous engineering design notwithstanding, positive drainage systems need periodic inspection and maintenance to keep in operating condition.

Typical Retrofit Edge Drain Installations

“Maintenance is required to keep pavement subsurface drainage open so the pavement structure will drain,” says Robert H. Baumgardner, P.E., Soils and Drainage Engineer, FHWA Office of Pavement Technology. “Vegetative growth around the pipe outlets, rodent nests, mowing clippings, and sediment collecting on rodent screens at head walls are common maintenance problems. Some outlets have been so plugged that water has gushed from the pipes when they were unplugged.”

“Most problems of pavement drainage are associated with lack of maintenance,” Correa adds. “Silty material can make its way into longitudinal edge drains or lateral outlets, and the material needs to be flushed periodically, perhaps once a year.”

Sometimes the pipe is damaged during construction and never gets a chance to perform. “Construction traffic may ride over the pipe, crushing it,” Correa told Better Roads.

“Video equipment must be used for inspection of those pipes right after construction, and once a year thereafter,” he says. “A video inspection can confirm the system is operating before we accept the project.”

Similarly, mowing equipment can crush lateral outlets or clog the outlets with clippings.

Clues to a malfunctioning system

State and local road agencies should be on the lookout for clues to a failing pavement edge drain system, says Jerry Daleiden, P.E., operations manager for Fugro-BRE based in Austin, Texas.

The first clue is that water no longer is flowing from the permeable base into a roadside ditch. “On a lot of the inspections we undertook for FHWA,” Daleiden told Better Roads, “we discovered we could not find the outlets any more, their having been crushed or covered after years of mowing. Or if they could be found, we could not get into them because they were plugged with silt, vegetation, or rodent nests.”

That lateral line from the edge drain itself is the main trouble spot, according to Daleiden. “Most of the damage that you will find is going to be in the laterals,” Daleiden says. “It’s the primary point of ingress and egress. If you can make it all the way in to the main line along the actual pavement, for the most part your system will be performing very well. But, if you can’t get into the laterals, that’s where most of your problems will lie. Problems will be rarer once into the main line system.”

One clue to a functioning system is the presence of deeper grass around a lateral outlet. “That’s a good sign that moisture is exiting those systems from the roadway,” observes Daleiden.

A malfunctioning pavement drainage system will also manifest itself by damage to the roadway, although this will take a while to appear. “If the moisture can’t get out, you’ll see the signs of pavement distress, such as pumping of subgrade,” Daleiden says. Look for pumped stains on portland cement concrete slabs where fines are coming up through joints.

Discontinuities or cracks in the pavement structure will show moisture coming through, he says, adding, “You’ll see more base failures and more weak spots in the pavement structure itself.”

First response

If signs of moisture-induced damage appear, a road agency should first inspect the drainage system for failures. “If you know you have a drainage system in place, the first thing to do is look for the laterals and assess their condition,” Daleiden says. “That’s something that any agency should be able to do.”

If you can see into the laterals, you may wish to call a video inspection team to confirm isolated problems within the system, he says. “A lot of the work a consultant might do is spent helping agency staff locate and clean out the laterals, which, in all honesty, could be done just as easily by agency staff itself.”

The video team actually photographs the interiors of the drains, to pinpoint the location of collapses or other blockages such as rodent nests. “The camera has a distance measuring function, so we know how far we went into the system before we encountered the obstacle,” Daleiden explains.

The video also is useful for agencies considering rebuilds of pavements, but wanting to save money by using as much of the existing system as they can. “This allows them to determine the need to replace a limited amount of footage of an edge drain from one station to another,” Daleiden points out. “Or they can specify replacement of a number of laterals at different locations and create a table they can place in their plans or specs.”

Once located internally, a blockage can be removed mechanically or hydraulically. “There is equipment not unlike a power-wash system that will wash the silt out under pressure,” says Daleiden, who likens the equipment to the snakes used to clean out lines from houses into storm and sanitary sewer lines.

“It’s best to backwash at the outlet itself,” he adds. “If you see a washing out elsewhere, but don’t see a pipe, you probably will have to replace a lateral.”

Daleiden’s firm was able to put its expertise to work for the benefit of state and local road agencies across the country. “We did a study for FHWA which evaluated technology for inspecting edge drains with a video camera,” Daleiden says. “We conducted demonstrations in as many as 26 states.” Many of the video captures from that video inspection program illustrate this article.

Weary of lugging electronic equipment, power cable, and video snake from roadside to roadside, Fugro-BRE employees suggested the equipment be mounted on a utility vehicle that can be driven from spot to spot.

“We used to lug that equipment around by hand,” Daleiden laughs. “Our technicians said, if you can give us an ATV to haul it all, we can do the surveys a lot faster. The technician can drive right up to a lateral and feed the camera right off the back end of the ATV.”

Fugro-BRE’s efforts notwithstanding, in truth, public works agencies can’t leave inspection and maintenance to engineering firms alone. Maintenance and upkeep of pavement drainage systems have to start with the agency itself, with timely assistance when needed from consulting experts. Anything less, the experts warn, will result in the agency’s pavement preservation efforts pipes going down the drain. 

Edge Drain Evaluation to Enhance LTPP Database

The Long-Term Pavement Performance program will be adding data from an edge drain evaluation study to the LTPP database, according to Focus Magazine, a publication of the Federal Highway Administration.

Experiments conducted under the LTPP program provided for installation of edge drains at 45 test sites across the United States. Under the evaluation performed by the engineering firm Fugro-BRE, inspectors inserted video cameras into the drainage pipes to record the condition of the drainage systems. This data in the LTPP database will allow researchers to draw correlations about when and in what situations edge drains function the best.

“Designers know that edge drains can work, and the database is there to provide assistance in identifying the most appropriate situation for edge drain use,” says Jack Springer, an LTPP highway research engineer.

The LTPP team was to have the site-specific drainage data reports integrated into its database in the summer of 2002. The team will also be releasing a CD containing much of the study’s video footage. For more information on the edge-drain study, contact Springer at the FHWA (202-493-3144, or jack.springer@fhwa.dot.gov).

Previous FHWA edge-drain studies include Experimental Project No. 12, Concrete Pavement Drainage Rehabilitation, which began in 1987; and Demonstration Project No. 87, Drainable Pavement Systems, conducted in different states through the early 1990s.

Now, a National Highway Institute course entitled Pavement Subsurface Drainage Design (Course No. 131026) is available to state highway agencies. The course provides detailed information concerning pavement subsurface drainage design for new or reconstructed portland cement concrete or asphalt concrete pavements and retrofit edge drains.

The NHI course teaches cost-effective design methods, including permeable bases and edge drains where appropriate, to prevent or minimize moisture-

related distress to pavements. The course is aimed at federal, state, and local highway engineers, designers, and personnel involved in hydraulic design, materials control, pavements design, research, construction, and maintenance of pavement subsurface drainage systems. For more information, contact Danielle Mathis-Lee at FHWA (703-235-0528, or danielle.mathis-lee @fhwa.dot.gov).

For more information on the LTPP database, contact the LTPP help desk by telephone at 865-481-2967, or by e-mail at LTPPinfo@fhwa.dot.gov, or visit the web site at www.tfhrc.gov/pavement/ltpp/ltpp.htm.

The above material was adapted from the June 2002 issue of FHWA’s Focus Magazine.

Four Levels of Service for Edge Drain Systems

Four levels of service for pavement drainage are outlined in the publication, Catalog of Recommended Flexible Pavement Design Features authored by Harold L. Von Quintus, P.E. and Brian M. Killingsworth of Brent Rauhut Engineering Inc. (predecessor to Fugro-BRE); and Mike Darter, Emmanuel Owusu-Antwi, and Jane Jiang of ERES Consultants Inc.

They say many early distresses of flexible pavements can be blamed on excess moisture in the pavement section, called moisture-accelerated damage.

“Most of this water enters from the surface through cracks in flexible pavements,” they write. “Wet subgrades and ground water can also cause significant problems in the subgrade and underlying pavement layers including loss of support (subgrade rutting), pumping of subgrade fines up into base course, and frost heave.”

They list four levels of solutions for pavement drainage problems, in order of effectiveness and cost:

Level 1: Seal joints and cracks and proper geometrics. Reduce the amount of moisture entering the structural section through continual sealing of cracks (that develop over time) and joints, and provide side ditches on both sides of pavement section.

Level 2: Use non-moisture-sensitive and non-erodible materials. Level 1 recommendations plus provision of materials that are not moisture sensitive or will not erode or disintegrate in the presence of excess moisture for the level of heavy traffic loads over the design period.

Level 3: Install edge drains. At Level 3, the authors include all the steps in Levels 1 and 2, plus the removal of excess moisture that enters the pavement and seeps along the layer interfaces to the longitudinal edge drain.

Level 4: Full subdrainage system with permeable base. The most extensive — and expensive — fix the authors outline is a combination of Level 1 and Level 2 (including full consideration of permeable base material durability) plus a subdrainage system to rapidly remove excess moisture that enters the pavement section before it can cause damage.

This will require a permeable drainage layer beneath the concrete slab for rigid pavements, or beneath the lowest asphalt bound layer for flexible pavements, with a granular separation layer between the permeable layer and the subgrade [with geotextile fabric between the permeable base and granular separation layer], plus longitudinal permeable trenches with edge drains and horizontal outlets.

Successful Edge Drains Start with Correct Installation

Agencies need to watch the installation of edge-drain systems very carefully because correct installation is critical to a system’s success. So advises Bjorn Birgisson, Ph.D., P.E., University of Florida-Gainesville, and Ruth Roberson, Minnesota Road Research Project of the Minnesota DOT, co-authors of the paper Drainage of Pavement Base Material: Design and Construction Issues for the 79th annual meeting of the Transportation Research Board, January 2000.

Birgisson and Roberson studied sections consisting of edge drains that were introduced into a dense graded base material to simulate retrofitting of pavements, and one reproducing a traditional edge drain design. Data on water movement were collected.

In one configuration, they found the influence of edge drains in dense graded bases might be of very limited extent. This indicates that feasibility of retrofitting existing pavements with drainage schemes needs to be evaluated in context of the overall pavement system. “Edge drains may be justified only in drainable materials where they can be used to their full advantage,” they wrote.

In the other configurations, they found inadequate compaction of the soil above the edge drain during construction may have adversely influenced edge drain performance in moving infiltrated volumes of water out of the drainable base in a timely fashion.

The performance of pavement drainage systems are impacted by the degree of compaction around and on top of the drainage pipe, layering of the pavement, and the connection between the edge drain and the shoulder, they said.

Resources Available

A variety of resources are available to the road construction and maintenance community to assist in ascertaining and meeting road drainage needs.

New in 2002, the Federal Highway Administration has developed a reference manual on pavement drainage. Construction of Pavement Subsurface Drainage Systems is a 96-page volume now available from FHWA’s Office of Pavement Technology, 400 Seventh Street, SW, Washington, DC 20590, (202) 366-1324.

A software program to help determine the drainage needs of pavements is available at no charge from FHWA. DRIP 2.0 (Drainage Requirements In Pavements) can be downloaded at no charge as a 5-megabyte zip file from the FHWA Pavement Technology page at www.fhwa.dot.gov/pavement/index.htm.

Summaries of current FHWA drainage efforts may be downloaded at www.fhwa.dot.gov/pavement/ drain.htm.

Lastly, an informative guide to edge-drain maintenance is available from the Office of Pavement Technology in .html or .pdf formats at www.fhwa.dot.gov/ pavement/edge.htm.

Reprinted from Better Roads Magazine
January 2003

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