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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2022. Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance. Washington, DC: The National Academies Press. doi: 10.17226/26569.
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Page 4
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2022. Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance. Washington, DC: The National Academies Press. doi: 10.17226/26569.
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Page 5

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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   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 1—Introduction. This chapter provides background information, states the syn- thesis objective, and summarizes the scope, approach, and organization of the synthesis. • Chapter 2—Literature Review. This chapter summarizes the findings from the literature review. • Chapter 3—State 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 DOTs’ experiences with the effect of subsurface pavement drainage on pavement performance. • Chapter 4—Case 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 5—Summary 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 drainage—a system used to intercept, collect, and discharge water that infiltrates into the pavement section; • Pavement section—any 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 infiltration—water 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 infiltration—water that migrates into the pavement section through lateral groundwater seepage, fluctuations in groundwater levels, or capillary action; • Drainage layer—a 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; • Underdrain—a conduit to remove water from a drainage layer; • Edge drain—an underdrain along a longitudinal edge of a pavement; and • Filter layer—a 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.

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Subsurface drainage is known to pavement professionals to be an important factor affecting pavement performance and preservation of public investment, and numerous studies have emphasized its importance.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 579: Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance documents the current state of practice by departments of transportation (DOTs) for subsurface drainage, which is not always considered as a critical component when it comes to design, construction, and maintenance of pavement.

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