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4 Background The necessity and effectiveness of subsurface drainage are important considerations in achieving pavement performance expectations and preserving public investments. A drained pavement structure typically has a subsurface layer that is intended to evacuate water quickly, thereby minimizing or eliminating any detrimental effects of water on moisture-sensitive subbase or subgrade materials. The aggregate gradation and associated hydraulic conductivity are important considerations in the design, constructability, and performance of a drainage layer. A drainage layer may be constructed with or without an asphalt or cement stabilizer and typically requires a separation layer to prevent clogging by subgrade fines. Synthesis Objective The objective of this synthesis is to document the current state of the practice of departments of transportation (DOTs) in selecting, designing, constructing, and maintaining subsurface drainage systems for new pavement construction and reconstruction, pavement preservation, and pavement rehabilitation. Synthesis Scope and Approach The focus of this synthesis is on asphalt and concrete pavements with subsurface drainage systems that are designed to quickly and efficiently remove water entering a pavement struc- ture through infiltration of joints and cracks in the pavement surface, lateral groundwater seepage, fluctuations in groundwater levels, and capillary action. The literature on pavement subsurface drainage systems was reviewed and a survey of practice was distributed to the DOTs of all 50 U.S. states and the District of Columbia to identify current subsurface drainage system practices for new pavement construction and reconstruction, pavement preservation, and pave- ment rehabilitation. The survey addressed subsurface drainage system use, design, construction, testing, inspection, and maintenance, as well as the effects of subsurface drainage systems on pavement performance. Follow-up questions were sent to four state DOTs that use subsurface drainage and expressed their willingness to participate in the development of case examples. Telephone interviews were then conducted with representatives of those DOTs to obtain detailed information on project selection criteria, subsurface pavement drainage design methods, subsurface pavement drainage system construction issues, the effects of subsurface pavement drainage on pavement performance, and subsurface pavement drainage system maintenance needs. The information C H A P T E R Â 1 Introduction
Introduction 5  obtained from the literature review, survey of practice, and follow-up interviews formed the basis for this synthesis. Report Organization This synthesis is organized into the following chapters: ⢠Chapter 1âIntroduction. This chapter provides background information, states the syn- thesis objective, and summarizes the scope, approach, and organization of the synthesis. ⢠Chapter 2âLiterature Review. This chapter summarizes the findings from the literature review. ⢠Chapter 3âState of the Practice. This chapter summarizes the results of the survey of DOT practices regarding subsurface pavement drainage use, design, construction, inspection, and maintenance as well as the DOTsâ experiences with the effect of subsurface pavement drainage on pavement performance. ⢠Chapter 4âCase Examples. This chapter summarizes information provided by four state DOTs that currently use subsurface pavement drainage systems for new and reconstructed pave- ments as well as for pavement maintenance and preservation projects. Examples of project selection, system component design, and performance assessment are provided. ⢠Chapter 5âSummary of Key Findings and Topics for Future Research. This synthesis concludes with a summary of key findings and suggested areas for research to improve the performance and cost-effectiveness of pavements with subsurface drainage systems. Definitions of Terms The following definitions are used in this synthesis of practice: ⢠Subsurface drainageâa system used to intercept, collect, and discharge water that infiltrates into the pavement section; ⢠Pavement sectionâany combination of surface, base or subbase, and compacted or naturally occurring subgrade or embankment layers located within the right-of-way and intended to provide safe and efficient, long-term support to vehicular traffic; ⢠Surface infiltrationâwater that enters the pavement section through longitudinal and trans- verse joints and cracks in and between the travel lanes and the curbs or shoulders; ⢠Subsurface infiltrationâwater that migrates into the pavement section through lateral groundwater seepage, fluctuations in groundwater levels, or capillary action; ⢠Drainage layerâa porous aggregate layer within the pavement section that provides a means to remove infiltrated water. A drainage layer may be unstabilized, asphalt-stabilized, or cement- stabilized and may be referred to as a permeable layer, an open-graded layer, or a free-draining layer; ⢠Underdrainâa conduit to remove water from a drainage layer; ⢠Edge drainâan underdrain along a longitudinal edge of a pavement; and ⢠Filter layerâa layer below the drainage layer that is designed to allow water to drain through freely but prevents soil or aggregate particles in the underlying layer (subbase or subgrade/ embankment) from infiltrating into the drainage layer.