Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance (2022)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

66 Key Findings Pavement engineers have long known that an excess of water in pavement structures can cause or exacerbate slope instability above and below roadways, visible pavement distress, and diminished pavement strength, stability, and durability. Pavement subsurface drainage systems are intended primarily to remove water that infiltrates pavement structures in a timely manner so it will not weaken unbound base and subgrade layers and exacerbate pavement deterioration. The primary component of a pavement subsurface drainage system is a permeable base layer, either daylighted to a side ditch foreslope or drained by longitudinal edge drains with outlets to a side ditch or connected to a stormwater collection system. Research (Hall and Crovetti 2007; Bhattacharya et al. 2009) suggests that pavement sub- surface drainage systems are most likely to benefit pavement performance when they are used at locations where climate and soil conditions are such that water would otherwise be likely to accumulate to an excessive degree in a pavement structure; when they are designed to achieve timely lateral outflow of excess water; when they are constructed with appropriate materials, equipment, and methods; and when they are maintained over time to ensure that water outflow is not obstructed. Pavement subsurface drainage system design principles and construction practices have been well established for decades (Ridgeway 1982; Christopher and McGuffey 1997). There is less consensus concerning where subsurface drainage systems should be used, how much mainte- nance they require, and whether their initial and maintenance costs are balanced by pavement performance improvements—such as extension of the time to first rehabilitation and reduced pavement repair and maintenance costs—over the pavement life cycle. This synthesis was conducted to document the current state of the practice of departments of transportation with respect to the use, design, construction, inspection, and maintenance of subsurface pavement drainage systems and their effects on pavement performance. Infor- mation was gathered pertaining to subsurface pavement drainage use, assessment of pavement drainage needs, subsurface pavement drainage system design features and details, subsurface drainage system construction, subsurface pavement drainage maintenance policies and practices (including inspection practices), and the extent to which DOTs have assessed the effectiveness and benefits of subsurface pavement drainage. Use of Subsurface Pavement Drainage Systems Research (Hall and Crovetti 2007; Bhattacharya et al. 2009) suggests that subsurface pavement drainage systems are not likely to improve pavement performance and be cost-effective over the life of a pavement when used in places where they are not needed (i.e., where average and C H A P T E R   5 Summary of Key Findings and Topics for Future Research

Summary of Key Findings and Topics for Future Research 67   peak seasonal precipitation levels are low or where the natural soils in place before the pavement structure are moderately to well drained). The TMI and monthly precipitation data can be used to identify sites with year-round or seasonal excesses of available moisture. County soil reports and soil taxonomy information can be used to identify subgrade soils with poor natural drainage characteristics. Of the 41 responding state DOTs, 31 currently use subsurface drainage systems, two (both in the desert Southwest) currently have no policy in place for the use of subsurface pave ment 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 both asphalt and concrete, one uses them for all new construction of asphalt pavements, and one uses them for all new construction of concrete pavements. Others reported using subsurface drainage to address specific project conditions or locations, including sag locations (eight DOTs), cut/fill transition locations (two DOTs), widening locations adjacent to existing pavements with sub- surface 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). An example of climate consideration is that of Pennsylvania, which uses a map that divides the state into rainfall regions to determine the design storm (1-year frequency, 1-hour duration) rainfall to be used to calculate the design infiltration rate for a subsurface drainage system. Design of Subsurface Pavement Drainage Systems Design principles and procedures for subsurface pavement drainage systems were devel- oped beginning in the mid-1940s. Between 1980 and 2000, the FHWA, National Highway Institute, and NCHRP produced detailed guidance on design, construction, and maintenance of subsurface pavement drainage systems that has formed the basis of subsurface drainage system practice for most state DOTs. Specific design details and construction procedures that contribute to drainage system effectiveness, performance, and ease of future maintenance have been identified. Most of the DOTs that responded to the survey conducted for this synthe- sis (31 of 41) reported, however, that they use standardized subsurface drainage design details and material gradations rather than project-specific drainage design calculations. Fourteen DOTs consider a permeable base layer to be a structural component of the pave- ment in the thickness design process. Pennsylvania, for example, assigns structural coefficients for various types of permeable and dense-grade base layers for flexible pavements and loss of support factors for various types of concrete pavements in accordance with AASHTO-based pavement design procedures. Permeable Base Types, Thicknesses, and Gradations Among the 31 DOTs that use permeable base layers, untreated permeable aggregate layers are most commonly used (20 DOTs), followed by asphalt-treated permeable layers (15 DOTs) and cement-treated permeable layers (eight DOTs). The total exceeds the number of DOTs using permeable bases because several DOTs use multiple permeable base types. Permeable base layers, when used, range in thickness from 3 in. to 18 in. In addition to the most commonly used permeable base layer thicknesses of 4 in. and 6 in., other reported permeable base layer thicknesses were 3 in. (one DOT), 12 in. (one DOT), 16 in. (one DOT), and 18 in. (one DOT). One DOT reported varying the permeable base layer thickness as needed to accommodate asymmetrical subbases sloped at 1% to the outside edge only. In Missouri, daylighted rock

68 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance bases up to 18 in. in thickness have been used for subsurface pavement drainage, and after more than 25 years of service, the pavement sections are reported to be in good to excellent condition, with minimal weed growth in the daylighted shoulder areas. Permeable base layer gradations may be established to achieve target values of in-place per- meability and drainage time. A majority of the responding DOTs (17 of 31) have no specific requirements for in-place drainage layer permeability. Five DOTs indicated that they have transitioned from using highly permeable aggregate gradations to using denser gradations with moderate to low permeability. Edge Drains, Outlets, and Headwalls Slotted longitudinal pipes in aggregate-backfilled trenches are the most widely used means of transmitting water away from permeable bases (28 DOTs). Preformed geocomposite edge drains are used by eight DOTs, with five DOTs always using aggregate backfill, one DOT using no aggregate backfill, and two DOTs using preformed geocomposite edge drains with or without aggregate backfill. There is also substantial use of daylighting via geotextiles, with 10 DOTs using both methods and two DOTs exclusively using wicking geotextiles. Three DOTs use longitudinal edge drains without permeable base layers. Water collected in longitudinal edge drains is transmitted to roadway ditches predominantly by transverse pipes with outlets protected by concrete headwalls (19 DOTs). For example, the Alabama DOT uses cast-in-place concrete headwalls for edge drain outlets, rather than precast headwalls, to match the ditch side slope and minimize damage to both mowing equipment and pipe outlets. Unprotected transverse pipe outlets are used by three DOTs, while one DOT uses a combination of protected and unprotected pipe outlets. Three DOTs use cast-in-place concrete headwalls with lower profiles to eliminate damage to headwalls and equipment during mowing operations. Transverse outlet pipe spacing intervals of 250 ft or less are used by 11 DOTs, spacing intervals greater than 250 ft are used by 10 DOTs, and unprotected outlets are used as needed by two DOTs. Edge Drain Retrofitting According to a previous NCHRP synthesis on subsurface pavement drainage systems (Christopher and McGuffey 1997), the retrofitting of longitudinal edge drains to existing pave- ment structures saw a growth in interest in the 1980s, together with the development of various geocomposites that were designed to be easier to install in narrow trenches alongside existing pavements. Twenty-four of the responding DOTs reported that they had retrofitted edge drains in existing pavement structures. Widening existing roadways (e.g., from four lanes to six lanes in each direction) offers another opportunity to improve subsurface drainage for existing pavements. Minnesota and Missouri, for example, construct permeable bases and edge drains when lanes are added to existing routes. Acceptance Testing of Subsurface Pavement Drainage Systems Alabama reports that contractors are occasionally asked to supply a water truck to flood the surface of a permeable asphalt-treated base after placement to ensure that the base readily accepts water without ponding and that water flows from the outlets. Minnesota performs quality assurance testing of subdrains during construction using a probe mounted on the end of a flexible fiberglass rod. Inspections are conducted through the discharge pipe, radius con- nection, and at least 3 ft into the main discharge line. Crushed or deformed pipes or connec- tions must be replaced at no additional cost to the DOT. Four DOTs use contractors for video inspection of edge drains and outlets during construction. The New York DOT has devel- oped a special specification for video inspection of edge drains and outlets during construction.

Summary of Key Findings and Topics for Future Research 69   Missouri conducts video inspections of up to 500 ft of the mainline pipe at up to 10% of the lateral outlets after all paving is completed. Repair or replacement of damaged or deficient portions of the outlets or longitudinal pipes must be performed at the contractor’s expense. In-Service Inspection and Maintenance of Subsurface Pavement Drainage Systems Among the in-service pavement drainage inspection and maintenance practices that have been described as contributing to subsurface drainage system effectiveness are (1) regularly scheduled visual inspection of outlets and headwalls; (2) periodic video inspection of longi- tudinal pipes and outlets; (3) observation of water flow from outlets after storm events or during flow testing (e.g., pouring water on the pavement surface using a water truck); (4) checking that rodent screens, if present, are not blocked; (5) checking that headwalls and outlet openings are not obscured by vegetation, blocked by sedimentation, damaged, or settled; and (6) flushing pipes with high-pressure water jets. The results of the survey of DOT practices indicate that visual inspections are the only type of inspections routinely conducted during the service life of pavements with subsurface drainage systems. Four DOTs indicated that inspections occur on an as-needed basis, as estab- lished by local maintenance crews or in the event of localized pavement failures. One DOT indicated that it used cameras to inspect 10% of underdrains every 2 years and 100% of the underdrains during the 10th year of service. Although vertical marker posts to indicate outlet locations may be damaged by mowing equipment, they are used more (14 DOTs) than markings on pavement or shoulder surfaces (seven DOTs). Minnesota, for example, uses 6-in. × 18-in. strips of white marking tape on pavement edges or shoulders to indicate drainage outlet locations. Michigan reported marking outlet locations with depressions in asphalt shoulders, and North Dakota reported using “O” castings at concrete pavement edges. Indiana indicated that it inventoried outlet locations and stored the location information in a geo- graphic information system database. Even more (16 DOTs) reported that they do not use any outlet location markers. The U-shaped edge drain layout, in which individual edge drain segments turn to form outlets with headwalls pairing them to outlets for adjacent segments, was developed to facilitate video inspection and pipe flushing. Large-radius connections between longitudinal drainpipes and outlet pipes are also intended to facilitate video inspection and pipe flushing. Ten of the responding DOTs reported that they conduct annual visual inspection of drains, seven reported that they conduct visual inspections at intervals ranging from 2 to more than 5 years, and 14 reported that they did not conduct visual inspections of drains. Most (14 of 23 who responded to the question) do not conduct probing of edge drains, and most (14 of 24 who responded to the question) do not conduct video inspection of edge drains. Four DOTs reported that they conduct video inspection during construction, while nine reported that they conduct video inspections at intervals of more than 5 years. Effects of Subsurface Pavement Drainage Systems on Pavement Performance Only eight of the responding DOTs indicated that they had conducted studies on the effects of subsurface drainage systems on pavement performance. Instances of both improved and worsened pavement performance were reported. Florida published study results that showed that the overall average life to first rehabilitation of all concrete interstate pavements in the state was 17 years, whereas the average for concrete interstate pavements judged to be well drained was 25 years. Another DOT examined the gradations of unbound aggregates typically used in

70 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance pavement bases in the state and concluded that gradation adjustments were sometimes required to achieve adequate stability at the expense of some degree of permeability. Notably, however, only two DOTs reported that they had used performance study results to establish cost esti- mates or pavement life estimates for design purposes. 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 to have longer service lives to first rehabilitation, while the other eight expected them to have service lives that ranged from less than 10 percent to more than 30 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 rehabilitation, while the other five expected them to have service lives that ranged from less than 10 percent to more than 30 percent longer. Topics for Future Research Information on two aspects of subsurface pavement drainage systems remains limited: (a) whether and to what degree they extend pavement service life or reduce pavement mainte- nance 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 maintained. The limited information on these two topics is a barrier to improving the effectiveness of subsurface pavement drainage systems and their contributions to pavement performance in locations where they are warranted. The following research efforts could address these knowledge gaps: (1) monitoring and analysis of field experiments with drained and undrained pavement sections, including, for example, selected Long-Term Pavement Performance (LTPP) SPS-1 and SPS-2 sites that are still in service; (2) retrieval and analysis of cost and performance data for drained and undrained pavement sections from pavement management system databases; (3) examination of the rela- tion between pavement performance and effort expended on edge drain and outlet mainte- nance; (4) examination of when and how to replace subsurface drainage system components; and (5) establishment of up-to-date subdrainage system-related cost inputs and pavement life estimates for use in life-cycle cost analyses of drained pavements. Specific testing efforts that could be conducted to assess drainage system functionality are (1) water flow testing of in-service pavements to observe whether and how quickly water drains from daylighted bases or outlets, (2) laboratory testing to assess the permeabilities of base gradations currently used by DOTs, with and without asphalt or cement binders, and (3) field testing of innovative geocomposite materials, such as a geocomposite capillary barrier drain, to assess their con- tributions to pavement subsurface drainage system effectiveness and pavement performance. Better guidance for estimating the stability and structural contribution of permeable base layers in relation to the gradation and binder types and contents used would also be beneficial in material selection and thickness design for pavements with subsurface drainage systems. An additional concern that may warrant further research is whether climate changes (in either annual averages or seasonal extremes) might contribute to heightened risk of damage to newly constructed or in-service pavements because of inadequate subsurface drainage, and thus whether (1) design infiltration rates traditionally used by DOTs for subsurface pavement drainage design purposes need to be updated using more current climate data sources, such as the Modern-Era Retrospective Analysis for Research and Applications database developed by NASA, as described by Schwartz et al. (2015), and (2) whether subsurface drains might need to be retrofitted in local- ized areas susceptible to damage.