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

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20 Introduction A fill-in PDF survey was developed to document current DOT practices regarding the use of subsurface pavement drainage systems. The survey was distributed by email to all voting members of the AASHTO Committee on Pavement and Materials, representing all 50 state departments of transportation (DOTs) as well as the District of Columbia’s. As of June 2021, 41 DOTs (82%) had returned survey responses. The survey consisted of 20 multiple-choice questions covering issues related to subsurface pavement drainage system design criteria, construction activities and costs, inspection and maintenance, and effects on pavement performance. The survey questionnaire and supporting text are provided in Appendix A. For each question, boxes were included to allow respondents to provide “Other” responses (in the event that the multiple-choice options provided did not adequately represent a respondent’s desired response) and “Additional Comments/Clarifications” (in the event that a respondent wanted to expand on a response). The survey responses are tabu- lated in Appendix B. This chapter presents a summary of the survey responses, including the responses selected from among the multiple-choice options and the responses provided in the “Other” text boxes. For each pie and bar chart presented, the survey question number, total number of responding DOTs, and number of respondents that made multiple selections are also included. As each question allowed the selection of multiple response choices, the total number of responses for many of the questions exceeds the actual number of responding DOTs. Use of Subsurface Pavement Drainage Systems Of the 41 responding DOTs, 31 currently use subsurface pavement drainage systems, two (both in the desert Southwest) currently have no policy in place for the use of subsurface pavement drainage systems (AZ and NM), and eight have discontinued the use of subsurface pave- ment drainage systems because of problems with construction, maintenance, or poor per- formance (DE, ID, KS, MS, ND, OR, SC, and WY). The map shown in Figure 7 presents an overview of the current state of the practice of subsurface pavement drainage system use among the responding DOTs. Figure 8 presents schematic illustrations of the various types of subsurface pavement drainage systems currently in use. The type designations in this figure are included only for reference in this section of the synthesis. Individual DOTs use a wide variety of terms to describe these systems and their components, as detailed in their available design manuals and standard detail drawings. C H A P T E R   3 State of the Practice

State of the Practice 21   Created with Mapchart.net. Figure 7. State of the practice of subsurface pavement drainage system use. Subsurface Pavement Drainage System Design Subsurface pavement drainage systems are designed using various soware packages or standardized design charts, gures, and tables developed by individual DOTs. Of the 39 DOTs that have used or currently use subsurface pavement drainage systems, 31 have standardized the design process, three use in-house soware packages (including one that uses components of the Bentley OpenRoads soware package), and two use the DRIP soware. e ve other reported methods used for subsurface drainage design include guidance provided in drainage design manuals (one DOT), project-specic designs to address high groundwater (one DOT), and case-by-case assessments of surface inltration and performance of adjacent pavement systems (three DOTs). Figure 9 presents an overview of the design methods currently used. Subsurface pavement drainage systems may be installed as a matter of policy for new construc- tion or they may be installed only under certain circumstances or at certain pavement locations. Figure 10 presents an overview of the installation locations addressed by the various DOTs. Eight 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 specic project conditions or locations, including sag locations (eight DOTs), cut/ll transition locations (two DOTs), widening locations adjacent to existing

22 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Pavement surface Moderate to highly permeable base layer Filter aggregate separation layer Shoulder Porous aggregate-filled longitudinal trench with slotted pipe drain Transverse outlet pipe Type 1 Daylighted edge Pavement surface Permeable base Filter aggregate separation layer Type 2 Shoulder Pavement surface Moderate to highly permeable base layer Geotextile separation layer Shoulder Porous aggregate-filled longitudinal trench with slotted pipe drain Transverse outlet pipe Type 3 Moderate to highly permeable base layer Pavement surface Geotextile separation layer Shoulder Preformed geocomposite longitudinal drain Transverse outlet pipe Type 4 Outside edge of shoulder Edge of lane Centerline Edge of lane Downhill direction Uphill direction Outside edge of shoulder To outlet Aggregate-filled cross drain Type 5 Type 6 Outside edge of shoulder Edge of lane Centerline Edge of lane Outside edge of shoulder Discrete aggregate-filled side drain (typical) Figure 8. Schematic illustrations of subsurface pavement drainage types.

State of the Practice 23   Figure 9. State of the practice for subsurface pavement drainage design methods. 12 1 4 2 8 19 8 8 0 2 4 6 8 10 12 14 16 18 20 Other Decision to incorporate is pending based on current research Only on full-depth widening adjacent to roadway with existing subsurface drainage In all cut/fill transitions In all sag locations Only when usage meets established criteria For all new asphalt pavement construction For all new concrete pavement construction Number of Responses (Q2: 37 total responses, 15 with multiple selections) Figure 10. State of the practice for subsurface pavement drainage system installation locations. 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). Subsurface pavement drainage systems are installed to drain water that may infiltrate into the pavement structure from various sources and weaken the supporting base, subbase, and sub- grade materials. Figure 11 illustrates the sources of water addressed by the responding DOTs.

24 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Rainwater and snowmelt infiltration through joints and cracks and interception/drawdown of high groundwater were identified as the predominant sources of water in pavement struc- tures that subsurface pavement drainage systems were designed to address. Frost heave was also mentioned as a concern for one DOT. For locations where subsurface pavement drainage system installations may be considered, a variety of factors may be assessed during the decision process, as illustrated in Figure 12. Other reported criteria used to assess subsurface pavement drainage needs include traffic 28 27 29 8 20 31 0 5 10 15 20 25 30 35 Seasonal/long term fluctuations in groundwater levels Groundwater rise due to capillary action Groundwater entering horizontally Vapor movements Snowmelt entering through joints and cracks Rainwater entering through joints and cracks Number of Responses (Q5: 37 total responses, 36 with multiple selections) Figure 11. Sources of water addressed by subsurface pavement drainage systems. Figure 12. State of the practice for subsurface pavement drainage system needs assessments.

State of the Practice 25   4 5 24 4 34 26 19 13 3 25 0 5 10 15 20 25 30 35 40 Other Precipitation rates Subgrade properties Projected traffic Continuous maintenance needs Historical pavement distress Proposed base materials Pavement type Life-cycle cost analysis In-house decision criteria Number of Responses (Q6: 37 total responses, 35 with multiple selections) Figure 13. State of the practice for subsurface pavement drainage system decision factors. levels (one DOT), past pavement performance (two DOTs), and shallow water tables or springs exposed during construction (one DOT). A variety of design, material, cost, traffic, environmental, and performance factors may be considered when deciding whether to incorporate subsurface drainage into pavement repair and reconstruction projects. Figure 13 illustrates the decision factors reported by DOTs, with the vast majority of responding DOTs (35 of 37) selecting multiple decision factors. In addition to the selection choices provided, other reported decision factors included site-specific ground- water information (two DOTs), preparation needs for rubblization projects (one DOT), and project constraints that prohibit subbase daylighting (one DOT). Subsurface Pavement Drainage System Construction 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 accom- modate asymmetrical subbases sloped at 1% to the outside edge only. In Missouri, daylighted rock bases with thicknesses up to 18 in. have been used for subsurface pavement drainage, and after more than 25 years of service, these pavement sections are reported to be in good to excellent condition, with minimal weed growth in the daylighted shoulder areas. Figure 14 illustrates the reported permeable base thicknesses used, as well as the criteria used to establish permeable base thicknesses. Fourteen DOTs consider a permeable base layer to be a structural component of the pavement in the thickness design process.

26 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance 3 13 8 15 20 0 5 10 15 20 25 Other Permeable base layer not currently used Using cement-stabilized aggregates Using asphalt-stabilized aggregates Using unstabilized aggregates Number of Responses (Q8: 37 total responses, 16 with multiple selections) 5 12 14 2 5 9 0 2 4 6 8 10 12 14 16 Other Permeable base layers not used Based on agency design standards for structural layer components Based on infiltration quantities Standardized at 6 inches Standardized at 4 inches Number of Responses (Q7: 37 total responses, 11 with multiple selections) Figure 14. State of the practice for permeable base layer thickness. Figure 15. State of the practice for permeable base material stabilization. Permeable base layers may be constructed with unstabilized aggregates or with asphalt or cement stabilizers to improve stability during construction and to improve long-term perfor- mance. Figure 15 illustrates the current state of the practice for permeable base layer type use, with unstabilized and asphalt stabilization being predominantly used. One DOT reported using crushed concrete in permeable bases, another reported using cement stabilizers only rarely, and one reported using nonwoven geotextiles in lieu of permeable base layers. Four DOTs have discontinued the use of cement- and asphalt-treated permeable bases because of performance problems, and one DOT has eliminated the use of cement-treated permeable bases to avoid construction delays caused by the need for a 24-hour cure period prior to paving. Permeable base layer gradations may be established to achieve target values of in-place perme- ability and drainage time. Figure 16 illustrates the current state of the practice, which indicates limited use of specific permeability requirements (two DOTs) or drainage time requirements

State of the Practice 27   (one DOT). Other reported uses include a uniformly graded 2-in. reservoir stone layer and “free-draining” layers without assumed permeability values (one DOT) or with assumed per- meability values of 1,000 ft/day or greater (two DOTs). Five DOTs indicated that they have transitioned from using highly permeable aggregate gradations to using denser gradations with moderate to low permeability. A variety of permeable base layer modifications have been implemented by some DOTs to increase permeable base layer stability under traffic during construction, as illustrated in Fig- ure 17. Where modifications have been made, densifying the aggregate gradation at the expense 3 12 1 2 17 7 8 0 2 4 6 8 10 12 14 16 18 Other Permeable base layer not used Permeability based on time to 50% drainage With specified permeability > 1,000 ft/day No specifications for in-place permeability Using highly permeable aggregate gradations Using moderately permeable aggregate gradations Number of Responses (Q9: 36 total responses, 12 with multiple selections) Figure 16. State of the practice for permeable base material gradation use. 13 20 1 3 1 6 0 5 10 15 20 25 Permeable base layer not used No modifications required Cement stabilization required Asphalt stabilization required Higher aggregate fractured face count Denser aggregate gradation Number of Responses (Q11: 36 total responses, 8 with multiple selections) Figure 17. State of the practice for permeable base material stability requirements.

28 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance of permeability is the most common choice (six DOTs). Asphalt stabilization is currently required by three DOTs. One other DOT allows but does not require asphalt-treated permeable base under HMA pavements only. Another two DOTs allow its use at the contractor’s discretion. One DOT has also implemented a fractured face count requirement for permeable base materials to enhance stability under construction traffic. Water collected by the permeable base layer is transmitted away from the pavement struc- ture by various means. Of the subsurface pavement drainage system types shown in Figure 8, Types 1, 3, and 4 are the most commonly used; however, daylighting the drainage layer (Type 2) is also done to reduce costs and in-service maintenance needs. Figures 18 through 20 illustrate the responses to a single, multi-part survey question related to transmission of water away from the pavement structure. As illustrated in Figure 18, longitudinal trenches are the most widely used method (28 DOTs) for transmitting water. 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. Figure 19 illustrates the various longitudinal edge drain designs used, the predominant design being slotted pipe conduits and aggregate-backfilled trenches (28 DOTs). Of those 28 DOTs, preformed geocomposite edge drains (PGEDs) are used by eight DOTs, with five DOTs always using aggregate backfill, one DOT using no aggregate backfill, and two DOTs using PGEDs with or without aggregate backfill. Figure 20 illustrates the various methods used for transporting water collected in longitudinal edge drains to roadway ditches, the predominant method being the use of transverse pipes with outlets protected by concrete headwalls (19 DOTs). 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. Permeable base layer not used (9) Permeable base with geotextile daylighted to slope (12) With longitudinal edge drain (28) (Q10a: 36 total responses, 13 with multiple selections) Figure 18. State of the practice for collected water transmission.

State of the Practice 29   Figure 19. State of the practice for longitudinal collection system use. With unprotected pipe ends spaced at intervals > 250 ft (1) With unprotected pipe ends spaced at intervals up to 250 ft (3) With concrete headwalls spaced at intervals > 250 ft (10) (Q10c: 21 total responses, 2 with multiple selections) With concrete headwalls spaced at intervals up to 250 ft (9) Figure 20. State of the practice for transverse outlet spacing and protection.

30 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Longitudinal edge drains may be installed during new construction or retrofitted into existing pavement systems, as illustrated in Figure 21. One DOT reported that maintenance and perfor- mance issues have led to reduced use of longitudinal edge drains, and another DOT indicated that underdrains can only be installed in HMA pavements when there is a minimum of 9.5 in. of HMA in place. The good long-term performance of a permeable base layer depends on its being protected from contamination and clogging during the construction process and during service. Figure 22 summarizes strategies used by the reporting DOTs for protecting permeable base layers from contamination. One DOT reported that contractors are allowed to drive on the permeable base Figure 21. State of the practice for installation of longitudinal edge drains. Other (1) Permeable base layer not used (14) No specific requirements (5) Construction/local traffic prohibited prior to final surfacing (5) Construction/local traffic restricted prior to final surfacing (13) (Q12: 37 total responses, 1 with multiple selections) Figure 22. State of the practice for permeable base layer protection during construction.

State of the Practice 31   10 5 2 20 2 8 0 5 10 15 20 25 Filter layer not used Clogging resistance of geotextile based on soil gradation Piping resistance of geotextile based on soil gradation Standard geotextile fabric used Aggregate filter layer with size and gradation based on soil Aggregate filter layer with standard gradation Number of Responses (Q13: 37 total responses, 8 with multiple selections) Figure 23. State of the practice for filter layer protection during construction. Figure 24. Performance studies conducted by DOTs. only when an additional 2 in. of base material are provided and then removed prior to paving. Figure 23 illustrates the use of geotextiles and aggregate filter layers to protect the permeable base layer from contamination from subgrade soils. Of the 20 DOTs that use geotextiles, three consider soil gradation during selection to prevent clogging, and three consider soil gradation in determining resistance to clogging and piping. Subsurface Pavement Drainage System Performance The value of subsurface drainage systems, in terms of pavement performance enhancements, has been documented in published studies to a limited extent, as illustrated in Figure 24. Performance studies with published results were reported by eight DOTs (CA, FL, IL, LA, MN, NY, VA, and WI). The Virginia Transportation Research Council conducted a limited field investigation in 2004 (Diefenderfer et al. 2005) to study the effects of a drainage layer on the pavement structure and supporting subgrade materials by comparing drained and undrained flexible pavement systems on two high-priority routes. Based on backcalculated values of pavement

32 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance structural number and subgrade resilient modulus, determined using the 1993 AASHTO Design Guide procedures, the researchers concluded that the drainage layer appeared to have a positive impact on the in situ structural number at both locations and on the in situ subgrade resilient modulus at one location. No long-term performance data were provided; however, the researchers did recommend the continued use of bound permeable drainage layers and longitudinal under- drains for new construction on high-priority routes, provided that a maintenance program that included periodic visual or camera inspection of drainage outlet pipes was developed. Illinois examined six demonstration projects in which open-graded drainage layers were incorporated and found them to be more expensive and to have poorer performance than pavements without such layers, because of the intrusion of fines (Winkelman 2004). Further- more, continuously reinforced concrete pavements constructed over cement-treated open- graded drainage layers were observed to have poor performance. This poor performance was attributed to bonding between the layers, effectively creating a full-depth pavement, with the resulting steel placement being too high in the pavement and causing shorter crack spacings. In its survey response, the Florida DOT indicated that a performance life study of all interstate concrete pavements in Florida was conducted in 2013, with the life of well-drained concrete pavements, defined as those having either initial edge drains or special select embankments, being averaged separately from that of concrete pavements that did not meet those criteria. The average life to first rehabilitation was 17 years for all interstate concrete pavements, whereas well-drained concrete pavements had an average life to first rehabilitation of 25 years. The Wisconsin DOT examined the performance benefits of concrete pavement sections with open-graded base courses (OGBC) on numerous test sections throughout the state. One study found that over a 20-year period, the benefit of OGBC did not appear to justify the increased cost (Schmitt et al. 2010). However, in areas in which repair and rehabilitation after 20 years may not be viable, such as in large, urban settings in which traffic disruption causes incon- venience, the use of stabilized OGBC was judged to be potentially cost-effective. Of the OGBC types evaluated, asphalt-stabilized bases performed best. Key findings included the following: (1) for doweled unsealed concrete pavement, base differences had little impact on perfor- mance; (2) concrete pavements with asphalt-stabilized bases had no slab breakup or surface distress but exhibited more severe joint distress; (3) undoweled sections over asphalt-stabilized bases and interchannel drains outperformed other undoweled sections; (4) concrete pavements with fine-graded permeable bases offered the smoothest ride of the permeable base sections, whereas asphalt-stabilized bases produced the roughest ride; and (5) life-cycle cost analysis showed that dense-graded bases were the least costly of the base configurations. Untreated OGBC and asphalt-stabilized OGBC were more expensive, based on life-cycle cost, by 13 percent and 28 percent, respectively. Another Wisconsin study examined alternative drainage layer designs, including traditional two-way base drainage with longitudinal drains and transverse outlets along both the outer and median edges and one-way base layer drainage sloped toward the outer edge (Crovetti 2006). Test sections with one-way drainage exhibited better ride quality (a lower international rough- ness index) and were estimated to yield a construction cost savings of approximately $60,000 per lane mile as a result of the exclusion of median edge longitudinal/transverse drains. Minnesota (Ariza and Birgisson 2002) reported that underdrains by themselves did not signifi- cantly improve the drainability of dense-graded bases, whereas the introduction of either collector pipes or edge drains in combination with underdrains was very effective in reducing the amount of moisture in a Mn/DOT Class 6 special crushed granite base course material. Louisiana examined optimization of the aggregate gradation for permeable bases by means of laboratory testing of unbound aggregates commonly used in Louisiana highways and reported a trade-off between the structural stability and the permeability of unbound aggregates (Tao and Abu-Farsakh 2008). The findings indicated that an increase in permeability often comes at a cost

State of the Practice 33   Expected to have >30% longer service life to first rehab (4) Expected to have 21–30% longer service life to first rehab (1) Expected to have 11–20% longer service life to first rehab (1) Expected to have 1–10% longer service life to first rehab (3) Not expected to have longer service life to first rehab (10) (Q16a: 18 total responses, 1 with multiple selections) Figure 25. Performance comparisons for asphalt pavements. Figure 26. Performance comparisons for concrete pavements. in terms of structural stability. Criteria were developed for selecting an optimal gradation that would possess both (1) adequate permeability to drain infiltrated water from the pavement as quickly as possible and (2) sufficient structural stability to support traffic loadings. The perme- ability of the unbound aggregate was quantified by its saturated hydraulic conductivity, while its structural stability was characterized by various laboratory tests of the strength, stiffness, and permanent deformation of the material. Life-cycle cost analysis is commonly used to support the decision process during pavement type selection and component design. The decision to include a subsurface pavement drainage system must be based on consideration of the costs of each component of such a system, as well as the expected effects on the service life of the pavement. Figures 25 and 26 illustrate the

34 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance performance expectations for drained asphalt and concrete pavements, respectively, in com- parison to their counterpart undrained pavements. 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 seven expected them to have service lives that ranged from less than 10 percent to more than 30 percent longer. Some DOT respondents indicated that they expected drained pavements to perform better than undrained pavements but that they did not have performance data upon which to quantify the degree of improvement possible. Four DOTs indicated that the decision to include a subsurface drainage system in a pavement design was based on site factors and that a life-cycle cost analysis of drained versus undrained pavements was not justified. Subsurface Pavement Drainage System Inspection and Maintenance The integrity of subsurface pavement drainage system components must be validated by various means throughout the life of the pavement system. Figures 27 through 30 illustrate methods used by DOTs for this purpose, as well as the frequency of use of each method. As these figures show, there is a notable lack of inspection of subsurface drainage components during the life of the pavement, with only visual inspections being routinely conducted. Four DOTs indicated that inspections occur on an as-needed basis, as established by local mainte- nance crews or in the event of localized pavement failures. Four DOTs use contractors for video inspection of edge drains and outlets during construction. One of these four, the New York DOT, has developed a special specification for video inspection of edge drains and outlets during construction. 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. Figure 27. State of the practice for visual inspections of drainpipes.

State of the Practice 35   Figure 28. State of the practice for probing of outlets. Figure 29. State of the practice for camera inspections of drainpipes. Transverse outlets are the terminal points of subsurface pavement drainage systems and are the most convenient locations for inspection, provided that they can be readily located in the field. Figure 31 illustrates the methods currently used for marking the locations of transverse outlets and headwalls. One DOT reported marking outlet locations with depressions in asphalt shoulders, and another DOT reported using “O” castings at concrete pavement edges. One DOT indicated that it inventoried outlet locations and stored the location information in a geographic information system database. Figure 32 illustrates the reported types of problems observed with subsurface drainage system components. One DOT reported the removal of headwall rodent screens because of blockage caused by the formation of calcium carbonate (tufa), and another DOT reported that

36 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance 13 4 1 0 0 0 2 4 6 8 10 12 14 Inspections not conducted At intervals > 5 years At 4- to 5-year intervals At 2- to 3-year intervals On an annual basis Number of Responses (Q17d: 18 total responses, 0 with multiple selections) Figure 30. State of the practice for water flow testing to confirm functionality. 2 6 16 15 7 0 2 4 6 8 10 12 14 16 18 Other Transverse outlets not used No location markers used Delineator posts adjacent to outlet Paint marks on shoulder/pavement edge Number of Responses (Q18: 38 total responses, 6 with multiple selections) Figure 31. State of the practice for outlet location marking. headwall rodent screens may soon be eliminated to limit plugging with debris. Figure 33 illus- trates current methods used by DOTs to assess the functional and structural integrity of pave- ments with subsurface pavement drainage systems. One DOT indicated that it is developing a ground-penetrating radar (GPR) testing procedure for use in structural analysis, while another DOT reported the use of highway-speed falling weight deflectometer (FWD) test- ing to determine the extent of pavement failure associated with underdrains or the pervious pavement layer failing.

State of the Practice 37   27 28 32 19 27 0 5 10 15 20 25 30 35 Transverse outlets missing rodent screens Transverse outlets/headwalls damaged or missing Transverse outlets/headwalls buried Longitudinal drainpipes collapsed Transverse outlet pipes collapsed Number of Responses (Q19: 34 total responses, 33 with multiple selections) Figure 32. Reported problems with subsurface drainage system components. Figure 33. Reported methods used for drained pavement assessment.

38 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Summary This chapter has presented an overview of the responses received from 41 DOTs (an 82% response rate) to a survey consisting of 20 multiple-choice questions covering issues related to subsurface pavement drainage system design criteria, construction activities and costs, inspection and maintenance, and effects on pavement performance. Thirty-one DOTs reported using subsurface pavement drainage systems with new and reconstructed pavements, as well as for selected rehabilitation projects. Seven DOTs reported having discontinued the use of subsurface pavement drainage systems because of construction, maintenance, or performance issues. The survey questionnaire and supporting text are provided in Appendix A, and the DOTs’ responses to the individual questions, as well as their comments provided in the “Other” and “Additional Comments/Clarifications” boxes, are tabulated in Appendix B.