|FHWA > Engineering > Pavements > Drainage|
Summary of Federal Highway Administration's Drainage Efforts
February 5, 2002
Water in the pavement structure has long been recognized as a primary cause of distress. Within the past 15 years, drainage of pavements has received an increasing amount of consideration. This was evidenced by the American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures (1986) which includes drainage considerations.
In April of 1987, a study of retrofit longitudinal edge drains in ten States (Alabama, Arkansas, California, Illinois, Minnesota, New York, North Carolina, Oregon, West Virginia, and Wyoming) was started by the Federal Highway Administration (FHWA) to identify successful drainage practices. Experimental Project No. 12, "Concrete Pavement Drainage Rehabilitation" investigated different edge drain systems and instrumented field sites to determine the effect of the edge drain system on the drainage of the pavement structure. Basic drainage design philosophy and practices of the participating States were studied and discussed in Experimental Project No. 12, "State of the Practice Report" (April 1989). The Final Report, Experimental Project No. 12, "Effectiveness of Highway Edge drains" (April 1993) reported on the findings of the project. This report made a principal contribution in that it provided an excellent guide to any State interested in pavement drainage instrumentation. One of the major findings of this project was that the older dense graded aggregate base courses under concrete pavements do not drain adequately.
In 1988, the FHWA conducted a field survey (Demonstration Project, Permeable Base Design and Construction) of ten States (California, Iowa, Kentucky, Michigan, Minnesota, New Jersey, North Carolina, Pennsylvania, West Virginia, and Wisconsin) to determine design criteria and construction problems for constructing permeable bases. This study showed that permeable bases could be constructed with both the necessary permeability and stability.
In April 1990, a Technical Advisory Committee (TAC) for Demonstration Project No. 87, "Drainable Pavement Systems" was formed to review the various design parameters and construction practices for permeable bases and drainage systems.
As a result of these efforts, Technical Paper 90-01, "Subsurface Pavement Drainage" (October 1990) was prepared and transmitted to the field in October of 1990. This paper provides comprehensive technical guidance on the drainage problem. Design guidance is provided for development of the permeable base pavement cross section. Guidance for the design and construction of permeable bases using unstabilized aggregate or stabilized (asphalt or cement) aggregate is also provided. The importance of a separator layer (aggregate or geotextile) is stressed. The paper also discusses the importance of the edge drain system including the longitudinal edge drain, outlet pipe and ditch design.
The TAC provided meaningful input to the Pavement Division who developed the Demonstration Project No. 87, "Drainable Pavement Systems" (Demo 87) workshop which was a 12 hour slide lecture presentation on drainable pavement systems. Because the mechanics of moisture in Portland Cement Concrete (PCC) Pavements is well understood, Demo 87 concentrated on drainage of new or totally reconstructed permeable bases under PCC pavements. Established drainage design procedures were combined with the state-of-the-art in practical permeable base construction to provide a well balanced approach for the drainage of PCC pavements. The Participant Notebook for Demo 87 (March 1992) was the best, current collection of design guidance for permeable bases under PCC pavements.
Discussions on the relationship between the factors of construction traffic, edge drain location and pre- or post installation of edge drains are a significant contribution to the state of the art. Another significant contribution was made by the sensitivity analysis of the parameters used in the time to drain computation.
A Technical Working Group on Flexible Pavement Drainage Design (TWG) consisting of participants from FHWA, State highway agencies, Universities, and Industry was convened in June of 1994. The participants provided input as a TWG by drawing on their experience and expertise. Wide ranging discussions on the design and construction of flexible pavements revealed that there was no clear definition of the role of drainage in flexible pavements. The only point of consensus was that, if a permeable base was provided in a flexible pavement, it would primarily combat pavement infiltration water; it would not solve ground water problems. Proceedings of the meeting were prepared and distributed to the field.
Under the "Video Inspection of Highway Edge drains" contract, the Contractor inspected highway edge drains. This project provided SHA's with a qualitative video picture of edge drain conditions. A secondary purpose of the project was to demonstrate the technology associated with the inspection equipment. Upon request of the SHA, the video contractor, using a closed circuit video camera investigated the in situ conditions of edge drains in a State. Inspection progress was viewed on the video monitor; photographs could be taken using a 35-mm printer, and the entire inspection was recorded on video tapes. The inspection equipment had the capability of inspecting up to 150 meters of edge drain. Both existing edge drains and new construction could be viewed.
In the final report, "Video Inspection of Highway Edge drain Systems," (January 1998), a tabulation of the inspections revealed that: 35 percent of the outlet pipes were non-functional (crushed or silted-in); 13 percent of the mainlines were not reviewable (geocomposites or unable to make 100 mm x 100 mm tee turn); 17 percent of the mainlines were non-functional; only 35 percent of the edge drain systems could be rated as functional.
Video inspection provides good quality assurance for acceptance of newly constructed edge drains; video inspection will also indicate the need for maintenance or repair of the edge drain.
The National Highway Institute (NHI) has developed a training course entitled NHI Course No. 131026A "Pavement Subsurface Drainage Design" (April 1999). Drainage guidance for PCC and flexible pavements, along with retrofit edge drains, was compiled into a comprehensive pavement drainage training course. Cost effectiveness of drainage, need for drainage, drainage of flexible pavements, retrofit edge drains, and geocomposites are new subject areas covered by the training course. The role of drainage in the structural design of flexible and PCC pavements is also covered. The length of the course is 3 days and follows a slide-lecture format. To date, the course has been presented in nine States. This training course is available to all SHA's though NHI.
The "Drainage Requirements In Pavements (DRIP)" drainage microcomputer program replicates design procedures contained in the Demo 87 Participant Notebook. The microcomputer program allows engineers to calculate the necessary pavement subsurface drainage design quickly and accurately. Program capabilities include time to drain, steady state flow, separator layer design, edge drain pipe flow, and geocomposite flow. Sensitivity plots of the various parameters provide the engineer with a vivid, visual picture to aid in decision making.
The program provides three new contributions to the state of the art in drainage design. First, the program allows user-specified aggregate gradations to be entered in either a single value or range mode. Second, the program performs the necessary calculations for aggregate separator layer design and then provides a graphic plot showing the design to the engineer. Third, using coefficients developed by laboratory testing for different types of geocomposites, the program will calculate the capacity of geocomposite edge drains.
Version 2.0 "Drainage Requirements In Pavements (DRIP)" (2001) was necessitated by the need to make the program compatible with Windows NT technology. Virtually every aspect of the program was improved and made more user friendly. All of the graphics have been significantly improved. Sensitivity plots of the various design parameters are most helpful in understanding the workings of the design procedures.
Tabbed property pages provide the user with a more intuitive navigation through the various analysis screens. Each screen is a module of the computer program. This allows the user to see the entire scope of the program.
An improved gradation library makes the importing and saving of aggregate gradations much easier. The program allows a more descriptive file name when saving a user generated gradation. Particle sizes less than 0.075 mm can now be included in the gradation analysis. Both FHWA Power 0.45 and semi-log plots can be produced.
Version 2.0 of DRIP is now being tested for possible bugs. The final program will be available through PCTRANS or McTrans.
The Drainage Construction Workshop - "Construction of Pavement Subsurface Drainage System - Workshops" has been fully developed. This project emphasizes the need for quality in the construction of drainage systems. The main thrust of the project is to provide good guidance for the construction of permeable bases, aggregate separator layer and edge drain systems. Each participant will receive a Reference Manual, which acts as a text, and a Participant Notebook that shows all of the graphics presented along with space adjacent to the graphics for note taking. The workshop is 8-hours in length.
The main contribution of the Reference Manual is the development of Guide Specifications for the various types of permeable bases. The merits of video inspection of completed edge drain systems for better quality assurance is stressed along with the need of video inspection to determine if any maintenance is necessary. Again, the need for maintenance is stressed.
Workshop sessions are available on request on a first-come basis. Please contact John D'Angelo, 202.366-0121