Road Management & Engineering Journal
February 11, 1998
(U.S. and Canada)
Fax: (360) 335-6402
NCHRP Synthesis of Highway Practice 96: Pavement Subsurface Drainage Systems
Drainage of water from pavements has been an important consideration in road construction for more than 2000 years. However, modern processing, handling, and placement of materials frequently result in base courses that do not transmit water or drain; combined with increased traffic volumes and loads, this often leads to pavement distress caused by moisture in the structures.
Water is also present in pavement materials in the form of free water, capillary water, bound moisture, or water vapor. Free water is the form of most concern to the designer because it can decrease the strength of the pavement and is the only form of water that can be significantly removed by gravity drainage.
The primary source of water in pavements is atmospheric precipitation. This water can enter the pavement through several ways (e.g., cracks, infiltration, through shoulders and ditches, high groundwater) and is moved by an energy gradient, such as gravity, capillary forces, osmotic forces, and temperature or pressure differences. The drainage designer is primarily concerned with saturated gravity flow, which can be determined by application of Darcy's law.
To understand and analyze the conditions under which the pavement must function, the designer needs information on highway geometrics, surface drainage, nonpavement subsurface drainage, climate, and soil properties. These data enable the designer to predict the amount of free water that will enter the pavement structure, to predict the free water surface, and to establish the design subgrade moisture content. Two general types of subsurface drainage criteria are used: (a) a time for a certain percentage of drainage or (b) an inflow-outflow criterion.
The free water can be removed by draining vertically through the subgrade or laterally through a drainage layer. Several combinations of criteria and equations can be selected to calculate the required permeability of the drainage layer. The criterion selected has much more influence than the equation used; therefore, the drainage criterion should be selected carefully. Then the drainage layer and/or base can be designed to meet the selected criterion. The materials specifications should be checked to assure that permeability, strength, load-distribution, and construction stability requirements are met.
Among the conclusions of this synthesis are that Darcy's law is adequate for the design of subsurface drainage systems; subsurface drainage systems will only drain free water, for the primary source is infiltration; water held in the pavement structure by capillary forces cannot be removed by subsurface drainage systems; and permeability requirements for lateral flow are high because of low hydraulic gradient and small area of flow. The infiltration of free water into the pavement structure, its effect on material strength, and its removal by vertical flow or by a lateral subsurface drainage system should be an integral part of the pavement structural design process.
CHAPTER SEVEN: CONCLUSIONS
The observations that many pavements are subject to moisture-related problems has convinced many engineers that subsurface drainage design criteria and principles should be part of the pavement structural procedure. It is believed that better, more economical pavements can be designed and constructed if these criteria and principles become an integral part of the pavement design, construction, and maintenance.
The design of subsurface drainage for pavement structures is not difficult, but it is site- specific. Not understanding and/or not applying the basic concepts or principles can lead to uneconomic or poorly performing pavements. The main principles or concepts are as follows:
New pavement test sections incorporating subsurface drainage systems have been constructed in Kentucky, Michigan, New Jersey, and Pennsylvania. These pavements have both flexible and rigid surfaces. The drainage layer in these pavements is immediately beneath the pavement and is made of graded aggregate, asphalt-treated permeable material (ATPM), or porous concrete. Construction of these drainage layers was not overly difficult and the cost of the in-place material was competitive with dense- graded aggregate materials. To date these materials have served satisfactorily as drains and as structural support for the surfacing materials. California has adopted a standard design for subgrade drains, and has issued a memorandum instructing personnel to consider the need for longitudinal drains in both new and existing pavements for the purpose of discharging infiltrated surface water to reduce pavement failures. California requires the use of either asphalt-treated (ATPM) or cement-treated (porous concrete) permeable material for longitudinal drains.
Longitudinal drains have been installed as edge drains in existing pavements in California, Georgia, and Iowa, and other states. The results of using longitudinal drains on rehabilitation projects have been mixed. There are two particularly important conditions that affect the successful use of these drains: (a) the edge support for the pavement must not be damaged when the drain is installed, and (b) the material that is adjacent to the drain and needs to be drained must be sufficiently permeable to allow free water that is causing the problem to reach the longitudinal drain. Large rehabilitation projects incorporating longitudinal drains should be considered carefully. Cedergren recommends the installation of trial sections. Where longitudinal drains will not work, it is important that extra effort be made to seal all joints and cracks.
The infiltration of free water into the pavement structure, its effect on material strength, and its removal by vertical flow or a lateral subsurface drainage system should be an integral part of the pavement structural design process.
NCHRP Synthesis 239: Pavement Surface Drainage System
Many premature pavement failures (occurring at less than 50 percent of expected life) have been traced to inadequate subsurface drainage. Although most state agencies recognize that water in pavement is not desirable, different philosophies exist on how to reduce the effects of this problem. Attempts range from completely sealing the pavement (including incorporating low permeable base with no drainage) to incorporating a fully drainable pavement section with permeable base and edgedrains. Numerous approaches fall somewhere in between (e.g., using edgedrains with dense- graded bases). This synthesis reviews practices in pavement subsurface drainage.
The differences in pavement drainage practices apparently relate to inconsistences in the reported performance of pavements with drainage systems. However, inadequate performance of pavements with drainage systems appears to be related more to inconsistences in design, construction, and maintenance than in the philosophy of positive pavement drainage. This synthesis focuses on the development of consistent practices in the drainage component of pavement design and discusses the effects of good and poor surface drainage. Also reviewed is the impact of decisions in planning, budgeting, procurement, construction, and maintenance on drainage performance.
Results of a survey of state transportation agencies on current pavement drainage strategies are interjected throughout the discussion to emphasize the important issues that influence design decisions. The drainage strategies currently used by state transportation agencies are presented, along with methods for evaluating performance. A team approach to decision making is proposed. This approach involves all functional groups during the design process, with feedback provided to the team throughout the life cycle of the pavement section.
This synthesis reviews design factors and appropriate design methods for pavement subsurface drainage systems, which should be considered as an update to NCHRP Synthesis of Highway Practice 96: Pavement Subsurface Drainage Systems. There has been significant activity in subsurface drainage in the areas of design, construction, and maintenance since Synthesis 96 was printed in 1982. Much of the design information in the present synthesis was obtained from the design methods proposed in the participant's notebook provided by Federal Highway Administration (FHWA) Demonstration Project 87: Drainable Pavement Systems. The proper use of and design details for edgedrains in both new and retrofit construction are included in the present synthesis, and existing standards and specifications are reviewed.
Poor construction techniques can destroy the best-designed subsurface drainage system. As a result, construction decisions and actions can have a significant impact on the design performance of a pavement section. This synthesis addresses how pavement design and construction affect each other and, more important, how they affect the long-term performance of the road system. Construction difficulties in the placement of permeable base and edgedrains do exist; but, as confirmed by the routine and successful installation experiences of many state departments of transportation (DOTs), all can be overcome with good training of and inspection by construction personnel.
Maintenance practices among state agencies vary as widely as their design philosophies. These practices range from no maintenance unless there are problems to full preventive maintenance with initial inspection starting at the time of construction. Unfortunately, maintenance-free pavement systems do not exist. Maintenance of subsurface drainage systems is essential to the long-term success of the drain system and, subsequently, the pavement. Support in both design and construction is necessary for an effective maintenance program. The requirements for a good maintenance program are reviewed. In fact, a major concern of many state agencies is consistency in the support of maintenance programs over the design life of the pavement system.
Difficulties were found to exist in the establishment of performance indicators, which stem from the elimination of factors that mask the effects of subsurface drainage (such as construction damage, poor materials, and lack of maintenance). The status of these performance indicators, along with the results of long-term performance studies, are examined. The opinions of state DOTs on the importance of pavement drainage are reviewed. Current research completed or underway in this area is identified, along with available performance information on drainage systems and their impact on pavement life. A preponderance of evidence was found supporting the philosophy that a combination of good sealing and good drainage, with a commitment to long-term maintenance, will lead to the optimum performance of a pavement system.
CHAPTER ONE: INTRODUCTION
Subsurface drainage is a key element in the design of pavement systems. Indiscriminate exclusion of this element will assuredly lead to the premature failure of pavement systems, thereby resulting in high life-cycle costs. Faulting and associated pumping in rigid pavement systems, extensive cracking from loss of subgrade support in flexible pavement systems, and distress from significant frost heave are clear signs of inadequate drainage. After years of unsuccessful sealing attempts, we have learned that we cannot prevent water from entering a pavement and that removal of that water is essential for the pavement elements to perform as predicted.
Most free water will enter the pavement through joints, cracks, and pores in the surface of the pavement. Water also will enter from backup in ditches and groundwater sources. Drainage prevents the buildup of free water in the pavement section, thereby reducing the damaging effects of load and environment. The gains in design life are significant.
Based on documented case studies, Cedergren projects that pavement life can be extended up to three times if adequate subsurface drainage systems are installed and maintained. Forsyth et al. report a ratio of 2.4 to 1 for reduction of new crack information in Portland cement concrete (PCC) pavements with drainage, compared with pavements without drainage. Forsyth et al. also report at least a 33 percent increase in service life for asphalt pavements and a 50 percent increase for PCC pavements. Ray and Christory observed premature distress in pavement sections in France, inferring a reduction in service life of nearly 70 percent, compared with drained sections. The evidence is clear: the optimum performance of a pavement system is achieved by preventing water from entering the pavement system and removing any water that does enter by means of a well-designed subsurface drainage system.
This synthesis focuses on the drainage component of pavement design. Included are discussions on (1) the positive effects of good and negative effects of poor drainage; (2) the effects of design, construction, and maintenance decisions; and (3) the present state of the practice as identified by a nationwide survey and literature reviews. This synthesis is provided as an update to NCHRP Synthesis of Highway Practice 96: Pavement Subsurface Drainage Systems, by Hallis H. Ridgeway. Synthesis 96 is still a contemporary design reference because it focuses on the basic hydraulic considerations of design, based on classic works such as those by Moulton and Cedergren. This document provides a supplement to design, based on current issues concerning designers such as type and quality of aggregate, compaction requirements for open-graded aggregates, asphalt and cement binders, and use of geosynthetics, which are not covered in Synthesis 96. Other significant activities that have taken place since Synthesis 96 are reviewed, including the performance of different drainage strategies and their effect on pavement life.
The experiences of many state departments of transportation (DOTs) were collected through a nationwide survey and are summarize. Perspectives on various subsurface drainage strategies such as the use of permeable base, underdrains, edgedrains, filters, outlets, and prefabricated geocomposite edgedrains are included in the summaries. The best practices (as reported) are highlighted in cases in which there is consensus, and areas in which major controversies were exposed are identified.
Definition of Terms
An important element of this synthesis is the definition of terms used. A review of these terms is recommended to avoid confusion and misinterpretation of information. Definitions are based on existing standards from the American Association of State Highway and Transportation Officials (AASHTO), American Society for Testing and Materials (ASTM), and FHWA.
A drainable pavement contains the integral components shown in Figure 1. The primary components include the asphalt or concrete surface pavement, a permeable base, a separator/filter layer, the subgrade, and edgedrains. Table 1 shows the optional elements that can be selected for the design of each component. If any of these system components do not function properly, the system will not perform (e.g., a drainable pavement that does not drain will be a liability to the pavement system).
Rigid: Portland Cement Concrete
Flexible: Asphaltic Concrete
Asphalt Stabilized Granular
Cement Stabilized Granular
Dense-Graded Granular (Subbase)
Strength of Subgrade Soil
Location of Water Table
Edgedrains (including outlets with headwalls)
Aggregate Trench Drain w/ Geotextile Filter & Pipe
Prefabricated Geocomposite Edgedrain (PGED)
Terms associated with the pavement section as well as other terms used in this synthesis are defined as follows:
This synthesis was prepared in recognition of the changes in design philosophy and substantial developments that have taken place in the 14 years since publication of Synthesis 96. FHWA has defined the current design philosophy for rigid PCC pavements and provided guidance through Demonstration Project 87, Drainable Pavement Systems. Although the project is complete, the participant notebook is still available. The notebook, which is a source of the primary sources of information on PCC pavements, provides guidance on design, installation, and maintenance of drainable pavement systems.
AASHTO and FHWA are currently emphasizing longer life pavement designs. This emphasis is increasing the importance of subsurface drainage. FHWA has distributed Technical Paper 90-01 to inform the transportation community of its position on the importance of subsurface pavement drainage. The report on FHWA Experimental Project No. 12 shows how extensively water can infiltrate what appear to be good, well-sealed pavement systems.
Much experience has been gained with materials and techniques that were new or unavailable when Synthesis 96 was prepared. The national survey conducted for the present synthesis and published records demonstrate that drained and maintained pavements last up to twice as long as undrained pavements. Local transportation agencies have found that maintenance and overlays do not greatly improve the life of pavements that do not have good subsurface drainage. As a result, many agencies are now [more] willing to spend the extra money needed for subsurface drainage than they were in the past. Information supporting the good performance of subsurface drainage led to the use of more than 4 million linear m of edgedrains, crossdrains, and underdrains in new or reconstructed pavements at the time the national survey was conducted.
The recognition that good subsurface drainage can extend the life of a pavement also has led to a greater use of permeable base by DOTs. More than 6,000 lane km of permeable base were installed in 1993, with 34 states installing more than 16 lane km that year, compared with only 16 states that did in 1985 (see general trend in Figure 2). Many states have made use of permeable base under PCC pavements their standard. As indicated by the survey, several states (e.g., Florida, Oregon, and Virginia) use permeable base under all high-traffic roads.
The increased use of permeable base has helped to solve some problems previously associated with it and to identify applications in which permeable base should not be used. Many states have concentrated on using stabilized permeable base to avoid the constructibility and trafficabilty problems of unstabilized permeable base. Studies by New Jersey have led to the development of new gradations of materials for permeable bases that overcome construction stability problems and still provide adequate permeability. Pavements with subsurface drainage that have not been maintained have been found to perform as poorly as pavements without subsurface drainage. As a result, FHWA has recommended that permeable base not be installed unless there is a commitment to maintain the subsurface drainage system.
Some states (e.g., Minnesota) have reported success in improving the drainage of their less permeable, denser graded base by installing edgedrains during construction. In this case, the primary purpose of the edgedrain is to drain the infiltration that enters through the joints. Minnesota also has experimented with special crossdrains placed beneath the horizontal joints.
Postconstruction, retrofit edgedrains have been installed along most interstates in recent years in an attempt to decrease the rate of pavement deterioration. The survey indicates that more than 200 linear m of retrofit drains were installed in 1993. These attempts have been reasonably successful, with several states (e.g., Kentucky, Minnesota, and Virginia) reporting a significant increase in the performance and design life of the roadway. Many unsuccessful attempts occurred in poorly draining bases, emphasizing the importance of using free draining base and incorporating subsurface drainage into the initial design.
Corresponding with increased edgedrain use is an increase in the use of newer types of drains, such as PGEDs. The performance of PGEDs has been established through field and laboratory evaluation, as reported in NCHRP Report 367: Long-Term Performance of Geosynthetics in Drainage Applications. An important finding is that failures evaluated as part of the study were predictable and related to either the absence of design, misapplication, or improper construction of PGEDs. New installation equipment and procedures have reduced the unit cost of PGED installation, which makes its use very attractive. The national survey indicated that about 600,000 linear m of PGED was used on new or reconstructed pavements in 1993, and an additional 600,000 linear m of PGED was used for retrofit applications for existing pavements.
One agency, Minnesota DOT, has reduced the cost of its standard drain installation by using narrow trench drains. MinnDOT's drain installation cost is now equal to or less than that of a PGED.
Inspection also has improved. Small-diameter optical tube video cameras with closed circuit video systems placed inside subsurface drainage facilities have exposed weaknesses in construction and inspection procedures (see Figure 3). Iowa and Kentucky (from survey) found many instances of damage and improper construction and now make subdrain inspection by video camera a standard practice. Other states (e.g., Indiana) are considering requiring video camera inspections before acceptance of construction projects. Numerous other states have discovered flaws in their subsurface drains by using various types of video inspection cameras pushed into drain outlets. Minnesota indicated that most of its subsurface drainage problems were found between the edgedrain and the outlet. Maintenance activities usually can repair outlet pipe damage. The survey indicates that most maintenance departments do not have a routine inspection policy and therefore may not identify problem areas until damage is done and early pavement distress becomes visible on the surface.
Systematic inspection using appropriate performance indicators appears to insure the performance of drains. As a result, longer life pavements can be expected. The survey indicates that few agencies (approximately 7 percent of respondents) have set up systems of performance measures and only 20 percent have routine inspection procedures for pavement subsurface drainage. More than half of the respondents indicated that these are needed, and they are planning to emphasize subsurface drainage maintenance within their agencies. Comments from the survey indicate that more effort is needed in training maintenance staff on performance indicators and maintenance strategies. Survey results indicate that a more systematic approach is needed in many maintenance groups. The results of the survey conducted for this synthesis may help agencies develop a more unified approach to pavement subsurface drainage design, construction , and maintenance.
This synthesis is oriented around the tools and practices for design, construction, and maintenance of pavement subsurface drainage systems. The design approach is an extension of the procedures in Synthesis 96, which continues to be a valuable reference. In the present synthesis, the team approach to design is introduced in Chapter 2. The details for design are presented in Chapters 3, 4, and 5. Issues of performance measurement and the importance of performance data for planning and budget are included in Chapter 6. The findings and conclusions resulting from the survey conducted for this survey appear in Chapter 7.
As indicated previously, this synthesis is supported by a national survey, the results of which are discussed throughout the document. The survey questionnaire, along with a summary of responses, appear in Appendix A. The survey was sent to the 50 DOTs in the spring of 1994. Forty-two agencies responded.
CHAPTER SEVEN: CONCLUSIONS
Although there are several existing philosophies on pavement design, the study conducted for this synthesis found a preponderance of evidence supporting the philosophy that good sealing and good drainage, along with a commitment to long-term maintenance, will lead to optimum performance of a pavement system. From this study, it was found that the design principles of pavement subsurface drainage systems for both structural and hydraulic requirements are well established in FHWA Demonstration Project 87, as supported by Synthesis 96.
One of the most important design elements appears to be the quality (i.e., durability and gradation) of the permeable aggregate. Construction difficulties concerning placement of permeable base and edgedrains do exist; however, as confirmed by the routine, successful installation experiments of many state DOTs, all can be overcome by resourceful contractors and inspection by well-trained construction personnel.
Long-term maintenance also was found to be essential to successful long-term pavement performance. Because the design, construction, and maintenance groups are interrelated, the team approach has been proposed. In this approach, communication between all functional groups is established at the design phase, with feedback provided throughout construction and long-term maintenance.
Several other significant conclusions were drawn from this study, including the following:
Although several significant research projects on permeability pavement systems are ongoing, as reviewed in Chapter 6, this study identified many needs that must be addressed to advance this technology. It is hoped that the following needs will support ongoing research to help engineers design, build, and maintain pavement systems with confidence.
One of the survey questions asked respondents to identify areas in which more study would help them select the best design strategies for pavement subsurface drainage. There was a unanimous response as to which topic would be most useful to study-- Cost-Effectiveness of Edgedrains and Permeable Base. Comments obtained from notes on the survey and telephone conversations indicated that there is uncertainty about whether improved subsurface drainage is the solution for prolonging pavement life. Permeable base and drainage systems add cost to a highway project, and there is little documented data on the costs and benefits of anticipated performance improvements.
Respondents selected Long-Term Pavement Monitoring as the second most useful topic for study, which supports the concern about the cost and benefits of drainable pavements.
Third was the desire to learn more about the topic Effects of Installation, followed closely by Effects of Low Maintenance, Alternate Construction Strategies, and Alternate Maintenance Strategies. These are followed by Life-Cycle Costs and Effects of Shoulder Detail on Performance.
It appears that respondents have an adequate understanding of the following items because they were low in total score and few respondents indicated a need for a study:
The results of the study conducted for this synthesis support the opinions of state DOTs about the need for more documented cost-benefit studies to help define appropriate subsurface drainage strategies (e.g., use of PCC shoulders, stabilized or unstabilized permeable base, pipe flushing, and preventative maintenance). Respondents also indicated the desire for additional cost-benefit information on retrofit edgedrains.
Better performance indicators and performance monitoring schemes are required to fully explore cost-benefit decisions. Information is required on changes in roadway support so that it can be compared with historical information on other undrained sections, joint behavior, and shoulder behavior. The national effort to improve pavement assessment methods (e.g., using radar to predict changes in support and using geographic information systems with other data gathering methods) could help provide this information. Remote methods of collecting inflow and outflow data and rapid assessment of drainage backup also are required to demonstrate drainage effectiveness.
A clear indicator of the cost and benefits of maintenance is needed. In addition, national and local training programs for construction and maintenance personnel are needed to improve drainable pavement performance.
Although not identified as a significant research need in the survey, the structural contribution of permeable base to the pavement section is not fully understood and needs further study. More study is also required to evaluate the effectiveness of permeable base compared with that of dense-graded base for asphaltic pavement. Design guidelines are needed to determine when construction of a drainage system is cost-effective for special climatic conditions (e.g., arid and semi-arid climates with significant snow melt and the positive and negative effects during freeze-thaw events).
The team approach, in which all functional groups are involved in making design, construction, and maintenance decisions, is introduced in this synthesis as a method to fully evaluate and establish the most appropriate subsurface drainage strategy. The team approach requires the development of formal lines of communication to get key information to decision makers before the design has been completed. This approach works if changes are continuously fed back into the system. It is difficult for decision makers to delay projects and recycle information back through the process if the impact of the change is not evident. An excellent method of handling the communication process may be through a quality steering committee, as outlined in Chapter 2.
The team approach appears to be the best method for obtaining true life-cycle cost- benefit assessment of drained and undrained pavements systems and for providing the information necessary for continuous improvements.