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1Â Â An excess of water in a pavement structure can cause or exacerbate pavement distress, slope instability alongside the roadway, and loss of pavement strength, stability, and dura- bility. Pavement subsurface drainage systems are intended primarily to remove water that infiltrates pavement structures through joints and cracks in the pavement surface or through lateral groundwater seepage, fluctuations in groundwater levels, and capillary action. The two types of subsurface drainage systems used to remove infiltrated water are those with a permeable base layer, longitudinal edge drains, and transverse outlets to the roadside ditch, and those with a permeable base layer daylighted to the roadside ditch. Pavement subsurface drainage systems benefit pavement performance most when they are used at locations where they are warranted by climate and soil conditions and when they are designed, constructed, and maintained to ensure timely and unobstructed water outflow. Subsurface pavement drainage systems are not likely to improve pavement performance or be cost-effective when used in locations where rainfall is minimal or where subgrades have sufficient porosity to permit rapid vertical drainage. The objective of this synthesis is to document the current state of the practice of depart- ments of transportation (DOTs) with respect to the design, construction, and maintenance of subsurface pavement drainage systems. Information pertaining to the use of subsurface pavement drainage, the assessment of pavement drainage needs, the design features and details of subsurface pavement drainage systems, the construction of subsurface drainage systems, the maintenance policies and practices for subsurface pavement drainage, and the extent to which DOTs have assessed the effectiveness and benefits of subsurface pavement drainage was gathered through a review of the literature and a survey of practice. The survey was sent to the U.S. state and District of Columbia representatives that are currently serving as voting members of the AASHTO Committee on Materials and Pavements. The survey addressed subsurface drainage system use, design, construction, inspection, maintenance, and performance. Forty-one of the 51 DOTs responded to the survey. Interviews were conducted with personnel from four DOTs to develop detailed case examples of approaches to subsurface pavement drainage use, design, construction, inspection, maintenance, and performance. Thirty-one of the DOTs that responded to the survey currently use subsurface drainage systems, two (both in the desert Southwest) currently have no policy in place for the use of subsurface pavement drainage systems, and eight have discontinued the use of subsurface pavement drainage systems because of problems with construction, maintenance, or poor performance. Seven DOTs reported using subsurface pavement drainage systems for all new construction of asphalt and concrete. One of the DOTs uses them for all new construc- tion of asphalt pavements, and another uses them for all new construction of concrete pave- ments. Others reported using subsurface drainage to address specific project conditions or S U M M A R Y Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance
2 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance locations, including sag locations (eight DOTs), cut/fill transition locations (two DOTs), and widening locations adjacent to existing pavements with subsurface drainage (four DOTs), or on the basis of other established criteria (19 DOTs). The 12 DOTs that reported other conditions or locations for subsurface pavement drainage use cited high groundwater (two DOTs) and site-specific conditions related to climate, soil type, and pavement performance (10 DOTs). Detailed guidance on design, construction, and maintenance of subsurface pavement drainage systems has been available for decades and has formed the basis of subdrainage system practice for most state DOTs. Currently, standardized subsurface drainage design details and material gradations are more widely used (by 31 of 41 responding DOTs) than project-specific drainage design calculations. Fourteen DOTs consider a permeable base layer to be a structural component of the pavement in the thickness design process. Untreated permeable aggregate layers are most commonly used (20 DOTs), followed by asphalt-treated permeable aggregate layers (15 DOTs) and cement-treated permeable aggregate layers (eight DOTs). Permeable base layers, when used, range in thickness from 3 to 18Â in. The most commonly used permeable base layer thicknesses were 4Â in. and 6Â in., while other reported thicknesses were 3Â in. (one DOT), 12Â in. (one DOT), 16Â in. (one DOT), and 18Â in. (one DOT). A variety of measures are used by the responding DOTs to ensure the proper construc- tion, maintenance, and longevity of subsurface drainage system components. Four DOTs have policies in place for video inspection of edge drains prior to opening to traffic, but video inspection is not commonly used for routine inspection throughout the service lives of pavements. One DOT reported that it conducted video inspections at intervals of 2 to 3Â years, and nine reported that they conducted video inspections at intervals of more than 5Â years. DOT practices vary with respect to in-service pavement drainage inspection and maintenance practices, such as visual inspection of outlets, headwalls, and rodent screens; video inspection of longitudinal pipes and outlets; observation of water flow from outlets after storm events or during flow testing; checking that headwalls and outlet openings are not obscured by vegetation, blocked by sedimentation, damaged, or settled; and flushing pipes with high-pressure water jets. One DOT reported that edge drains were only used in districts that were committed to regular maintenance of edge drains. For proper inspection and maintenance of outlets, maintenance crews must be able to readily locate them. Fifteen DOTs reported that they use vertical delineator posts to mark outlet locations, while seven reported that they use markings on pavement or shoulder sur- faces. Of the 19 DOTs that use concrete headwalls to protect transverse outlet pipes, three use cast-in-place concrete headwalls with lower profiles to eliminate damage to headwalls and equipment during mowing operations. Unprotected transverse pipe outlets are used by three DOTs, while one DOT uses a combination of protected and unprotected pipe outlets. Information remains limited on two aspects of subsurface pavement drainage systems: (a) whether and to what degree subsurface pavement drainage systems extend pavement service life and reduce pavement maintenance and repair requirements and (b) whether pavements with longitudinal edge drains and outlets that are installed but not subsequently maintained perform as well as or worse than pavements with edge drains that are main- tained. Performance studies with published results were reported by eight DOTs. Two other DOTs have conducted performance studies and used the results to establish pavement life expectancies, and one also used the results to establish design costs. Of the 18 DOTs that expressed views on whether asphalt pavements with subsurface drainage systems performed better than asphalt pavements without them, 10 did not expect drained asphalt pavements
Summary 3Â Â to have longer service lives to first rehabilitation, while the other eight expected them to have service lives that ranged from less than 10Â percent longer to more than 30Â percent longer. Similarly, of the 16 DOTs that expressed views on whether concrete pavements with subsurface drainage systems performed better than concrete pavements without them, nine did not expect drained concrete pavements to have longer service lives to first reha- bilitation, while the other five expected them to have service lives that ranged from less than 10Â percent longer to more than 30Â percent longer. The limited information on those two topics is a barrier to improving the effectiveness of subsurface pavement drainage systems and their contributions to pavement perfor- mance in locations where they are warranted. Efforts to obtain more information on those topics could include comparative performance analyses of drained and undrained pave- ment sections, flow testing of daylighted and edge-drained permeable base layers to confirm their functionality, analysis of cost and performance data for drained and undrained pave- ment sections from pavement management system databases, and examination of the effect of edge drain and outlet maintenance on pavement performance.
