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

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6 Early literature on road building offers some clues about when and how people became aware that excessive moisture in pavement structures is detrimental to pavement performance. MacAdam (1820), for example, is often mentioned (e.g., Moulton 1980) as having observed that, regard- less of the thickness of the structure, many roads in Great Britain deteriorated rapidly when the subgrade was saturated. However, instead of thinking that road building on a large scale began with the Roman Empire and that people only started observing the adverse effects of excessive moisture for pavements many centuries later, it may be more realistic to think of roads as being, from their very inception, engineering solutions to the problem of wet, weak, unimproved soils impeding the movement of people, goods, and armies. As Ksaibati and Kolkman (2006) observed, “They are called ‘high ways’ because early road builders knew it was necessary to construct roads above the ground and away from water.” Roman roads, which were substantial stone structures built to resist flooding and environmental damage and to move the armies and goods needed to sustain the empire, were considered to be the most advanced roads ever built until the 19th century, and some of them remained usable for more than 1,000 years. Post–Roman Empire road construction arguably originated in France in the mid-1700s, where Trésaguet oversaw the construction of multilayered gravel road surfaces on crowned subgrades with side ditches for drainage, first in Paris and later as standard practice throughout France (Lay 1992). However, to keep roadway surfaces at roughly the same height as the surrounding countryside, Trésaguet’s road design was typically built in a trench, which created drainage problems. These were addressed by design modifications that included digging deeper side ditches, making the surface layer as solid as possible, and constructing the road surface with a cross slope to hasten rainfall runoff. In the early 1800s, Telford applied Trésaguet’s road design approach to road building in Wales and developed it further, using a mixture of gravel and larger paving stones that became known as “Telford pitching.” To prevent pooling of water and erosion of the road surface, Telford raised the pavement structure above the ground level wherever possible, and where it was not, Telford drained the area around the roadside (Lay 1992). At around the same time, Metcalf emerged as a strong advocate of well-drained road structures in England (Lay 1992). Early modern road engineers such as Trésaguet, Telford, Metcalf, and MacAdam (1820) called attention to the need for stronger, more durable road surfaces that were less susceptible to the damaging effects of excess moisture in the road structure and foundation. MacAdam advo- cated less substantial gravel layer thicknesses in roads but more durable materials—specifically, carefully placed (MacAdam 1824) and carefully sized moisture- and frost-resistant crushed stone (Lay 1992). He favored the use of an adequate but not excessive cross slope, elevation of the road structure above the water table, and runoff into side ditches. C H A P T E R   2 Literature Review

Literature Review 7   The Industrial Revolution of the late 18th and early 19th centuries, which began in Great Britain, brought, among its many technological innovations, powerful new ways of moving people and goods, including the steam locomotive and gasoline-powered automobiles and trucks. The Industrial Revolution also brought about unprecedented population growth and an explosive growth in trade, both of which resulted in enormously increased volumes of traffic on roads. At the time of the Industrial Revolution, France was known for the quality of its roads, in no small part because of Trésaguet’s efforts mentioned earlier, but roads in other countries throughout Europe, including Great Britain, were in poor condition and were not fit to meet the challenges of the new era (Hunter 1985; Grübler 1990). Depending on local soil and rainfall conditions, roads that were sufficient to support animal-drawn carts bearing people and goods were often not sufficient to support the much heavier loadings of motorized vehicles and much higher traffic volumes of rapidly growing economies. As Ridgeway (1982) explained in NCHRP Synthesis 96: Pavement Subsurface Drainage Sys- tems, the soil–aggregate mixtures used as surfaces for roads built by the methods recommended by Trésaguet, Telford, Metcalf, and MacAdam were densely graded and compacted mixtures, intended to protect weaker layers below from both loads and water infiltration. As bituminous materials and Portland cement came into use as binders for road surfaces, these densely graded soil–aggregate mixtures became base materials, a use for which their low permeability may not have been a desirable characteristic. Despite the typically poor drainability of asphalt- and concrete-surfaced roads built in the United States during the first half of the 20th century, their performance was “satisfactory under the traffic and load conditions that existed before the Inter- state era” (Ridgeway 1982). However, concern about the role of excess water in exacerbating visible pavement distress and accelerating pavement failure increased as the weight and volume of traffic on roads increased. Cedergren et al. (1972) illustrated the main sources of water in pavement structures, as shown in Figure 1. Moulton (1980) grouped types of drainable subsurface water into two categories: • Groundwater, which is defined as water existing in the natural ground in the zone of satura- tion below the water table; and • Precipitation infiltration, which seeps into the pavement through joints and cracks in the pavement surface, through the pavement surface itself to whatever degree it is permeable, and from ditches along the side of the road. Moulton (1980) thus distinguished between subsurface drainage systems for ground- water control and subsurface drainage systems for infiltration control. Cedergren et  al. (1973) illustrated the main points of infiltration of water into pavement structures, as shown in Figure 2. “Pavement subsurface drainage systems,” as they are commonly understood in the context of pavement design rather than hydrological design, are intended primarily to address infiltration of water through joints and cracks and in some situations shallow groundwater movements, including lateral groundwater seepage, fluctuations in groundwater levels, and capillary action. Such systems typically consist of the following components: • A permeable base, either stabilized or unstabilized, that is either daylighted to a side ditch foreslope or drained by longitudinal edge drains; • A separator layer, of either untreated granular material or geosynthetic material, with a gradation (in the case of a granular separator) or design opening size (in the case of a geo- textile) selected to prevent fines from an underlying embankment fill or subgrade layer from infiltrating into the permeable base layer above and reducing its permeability;

8 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Source: Adapted from Cedergren et al. 1972. Notes: AC = asphalt concrete; PCC = Portland cement concrete. Figure 1. Sources of water in pavements. Source: Adapted from Cedergren et al. 1973. Figure 2. Points of inltration of water into AC and PCC pavements.

Literature Review 9   • Longitudinal edge drains, if present (i.e., if the permeable base layer is not daylighted to a side ditch), placed in backfilled trenches along the pavement edge on one or both sides, depending on the geometry of the pavement cross section; • Outlet pipes that drain the longitudinal edge drains to the ditches; and • Headwalls to protect the ends of the outlet pipes. According to Moulton (1980, 15), although free (drainable) water from melting ice lenses may exist above the water table, it is generally considered to be groundwater because “the water that feeds the growth of ice lenses originates at the base of the capillary fringe (i.e., at the water table), and no frost action can take place without water from this source.” Meltwater is sometimes treated as a component of the drainable water that is quantified as the design inflow in the design of pavement subsurface drainage systems. Excess subsurface moisture may cause or exacerbate the following problems: • Slope instability (which may lead to sliding of cut slopes above roadways and collapse of fill slopes below roadways); • Visible pavement distress (including rutting, cracking, and potholes in asphalt pavements; faulting, cracking, and corner breaks in jointed concrete pavements; punchouts in continuously reinforced concrete pavements; and pumping, blowholes, and settlement along lane–shoulder joints in all types of pavements); and • Diminished pavement strength, stability, and durability (including stripping of asphalt binder from aggregates in asphalt-bound layers, erosion of slab support under concrete pavements, D-cracking of concrete containing aggregates susceptible to freeze–thaw damage, expansive cracking of concrete containing reactive aggregates, and weakening of unbound base and foundation layers). The use of open-graded, and thus more permeable, granular base layers began to emerge as a way to improve pavement drainability in the 1940s. The term “drainage blanket” was often used to describe a very permeable base layer that had a length (in the direction of flow) and width that were large relative to its thickness. A rational approach to the estimation of the time required to drain a “pervious subbase,” based on the base permeability, slope, and amount of drainable water in the pavement struc- ture, was proposed by Izzard (1944). This approach assumes that water reaches the subbase through joints and cracks in the pavement and along the edge of the pavement. On flat grades, the capacity of the base to carry water longitudinally was assumed to control drainage, unless a continuous longitudinal underdrain was located at the pavement edge. Otherwise, drainage was assumed to be controlled by the capacity of outlets spaced along the shoulder. The maximum outlet spacing decreases as the grade of the roadway decreases. To improve pavement drainage, Izzard (1944) recommended that, on flat grades, either continuous longitudinal underdrains should be provided or the subbase should be extended across the shoulder (daylighted). Glossop (1947) examined the effects of soil types, soil profiles, and sources and movements of water in pavement structures on the bearing capacity, settlement, and frost heave suscep- tibility of road foundations. He also offered recommendations for the use of different types of drains (open ditches, French drains, horizontal drains, and interceptor drains) to maintain slope stability and adequate pavement drainage. Casagrande and Shannon (1952) observed that base courses beneath airport pavements at several air bases in the northern United States became saturated under some conditions, and they identified frost action and infiltration through the surface as the main sources of water responsible for base saturation. They developed a “time-to-drain” approach to permeable base design and compared the results obtained with the results of laboratory model tests and

10 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance full-scale field tests. The comparisons showed that the proposed formulas were satisfactory for use in the design of base drainage. Casagrande and Shannon (1952) also presented sets of equations—and a dimensionless chart in the form of a set of curves for solving the equations—for determining how long it takes for a base that becomes fully saturated by water inflow during a rainfall event to drain to a lower level of saturation by outflow from the base to ditches alongside the roadway, either through longitudinal edge drains and outlets, if present, or by seepage through the base to the ditch slope, if the base is daylighted. Similar time-to-drain equations and charts were developed by Barber and Sawyer (1952), Cedergren (1974), Moulton (1980), and Markow (1982). Casagrande and Shannon (1952) noted that as the slope of the pavement became flatter or the thickness of the base became greater, the predictions obtained from their time-to-drain equations differed more widely from observed values. Liu et al. (1983) re-examined the time- to-drain approach and concluded that the underlying assumptions of a linear phreatic surface (water level) in the base and an impermeable subgrade below the base were not as realistic as assumptions of a parabolic phreatic surface and some degree of permeability of the sub- grade. Liu et al. (1983) developed new time-to-drain equations based on these more realistic assumptions and showed that the results obtained were generally in good agreement with the field data obtained by Casagrande and Shannon (1952) in full-scale tests. The second approach to permeable base design is the “depth-of-flow” approach described by Moulton (1980). This approach is based on the idea that the thickness of the base should be greater than or equal to the depth of water flow when the base is saturated, and that the steady flow capacity of the base should be greater than or equal to the rate of inflow. The required base thickness is determined from equations or a chart as a function of the rate of inflow, drainage length, slope, and base permeability. The approach taken to determine the required permeable base thickness (i.e., the time-to- drain approach or the depth-of-flow approach) dictates which set of equations is subsequently used to determine the design flow capacity for longitudinal edge drains of a specific type and diameter. This is done by setting the pipe capacity equal to the discharge from a unit length of pavement multiplied by the spacing between outlets (Babu et al. 2019). Moulton (1980) summarized in great detail the literature and practice on characterizing key inputs to pavement subsurface drainage design—namely, the geometry of the flow domain (the geometric design of the highway and prevailing subsurface conditions), the subgrade and pave- ment layer material properties (particularly their permeability, effective porosity, and frost susceptibility), and the characteristics of the climate (precipitation and temperature), that dictate how much water could infiltrate the pavement structure and how frost action might influence water movement in the pavement and foundation. Moulton (1980) also discussed how pavement subsurface drainage construction influenced the sequence of pavement con- struction operations and discussed economic considerations related to the use of subsurface drainage systems. Rainfall intensity–duration–frequency relations are commonly used to characterize precipi- tation for flood forecasting and drainage design. The U.S. Weather Bureau (Herschfield 1961) developed maps of rainfall intensity as a function of frequency and duration for any location within the 48 contiguous U.S. states. The U.S. National Oceanic and Atmospheric Administra- tion’s National Weather Service operates a website, the Precipitation Frequency Data Server, that can be used to obtain precipitation frequency estimates for any location in the 50 U.S. states, the District of Columbia, and the U.S. territories. The two main approaches to estimating the rate at which water infiltrates a pavement struc- ture are the infiltration ratio approach (Cedergren et al. 1972, Cedergren et al. 1973, Cedergren

Literature Review 11   1974) and the crack inltration approach (Ridgeway 1976). In Cedergren’s approach, the 1-hr- duration/1-yr-frequency inltration rate is multiplied by a coecient with a value less than 1 depending on the pavement surface type. Ridgeway, however, suggested that rainfall inltration depended more on duration than on intensity and was directly related to the degree of cracking. Procedures for identifying (a) existing moisture-accelerated distress and (b) conditions favorable to the development of moisture-accelerated distress were developed by Carpenter et al. (1979, 1981a, 1981b). e potential for excessive moisture in a pavement structure at any given location in the 48 contiguous U.S. states was characterized using ornthwaite’s climate classi- cation system (ornthwaite 1948). Carpenter et al. (1979, 1981a, 1981b) combined temperature and moisture measures, including ornthwaite’s moisture index (TMI), with soil deformation characteristics to produce a map of nine climatic zones of the United States (Figure 3), wherein the Roman numerals I, II, and III denote wet, seasonally wet, and dry subgrade conditions, respectively, and the letters A, B, and C denote freeze, freeze–thaw, and non-freeze winter soil conditions, respectively. is type of characterization can be useful in identifying areas where pavements may be more vulnerable to moisture-related damage because of subgrade moisture levels at or approaching saturation, either seasonally or throughout the year, as well as subgrade weakening due to thaw, either in the spring or repeatedly throughout the winter. A simplied version of this map, which divides the United States into four climatic zones (wet- freeze, wet-nonfreeze, dry-freeze, and dry-nonfreeze), was adopted for use in characterizing pavement test sites for the Strategic Highway Research Program’s (SHRP) Long-Term Pavement Performance (LTPP) studies and is shown in Figure 4. NCHRP Synthesis 96 (Ridgeway 1982) summarized the basic principles and then–state of the practice of subsurface pavement drainage system design, construction, and maintenance, including topics such as the assessment of the need for drainage, the sources of water in pave- ments, the ways in which water moves in pavements, subsurface drainage design inputs and criteria, the subsurface drainage design process, and the retrotting of longitudinal edge drains as part of existing pavement maintenance or rehabilitation. e synthesis attributed the increased attention toward pavement subsurface drainage in the 1970s and 1980s to both increased awareness of the adverse eects of excessive moisture on pavement performance (e.g., Carpenter et al. 1979) I-A I-B I-C II-A II-B II-C III-A III-B III-C Source: Adapted from Carpenter et al. 1979. Base map created at Mapchart.net. Figure 3. Nine climatic zones of the United States based on moisture and temperature.

12 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance and the emergence of new types of edge drains and lter materials, as reected by the develop- ment of prefabricated underdrains (e.g., Healy and Long 1971) and criteria for the use of lter fabrics (Bell and Hicks 1980). Among the main conclusions of NCHRP Synthesis 96 (Ridgeway 1982) were that Darcy’s law is suitable for use in the design of subsurface drainage systems; that the primary source of drainable water in pavement structures is inltration; that water held in the pavement structure by capillary forces cannot be removed by subsurface drainage systems; that the permeability requirements for lateral ow in permeable bases are high because of the low hydraulic gradients and small areas of ow that are characteristic of permeable base layers; and that appropriately designed lters (granular materials of suitable gradation or lter fabrics of suitable apparent opening size) are essential to prevent permeable base materials, trench backll materials, and longitudinal edge drains from becoming clogged. NCHRP Synthesis 96 (Ridgeway 1982) summarized information on pavements constructed with subsurface drainage systems in California, Kentucky, Michigan, New Jersey, and Pennsylvania. e treated and untreated permeable bases in these pavements were judged to be not overly dicult to construct, cost-competitive with dense-graded base layers, and performing satisfac- torily. e synthesis reported more mixed results for retrotted edge drains, citing examples from California, Georgia, and Iowa. One of the problems mentioned was that the quality of support at the pavement edge can be diminished during the process of retrotting edge drains. Another problem was that retrotted edge drains are not likely to be able to remove much water from an existing base layer that is densely graded. Dempsey et al. (1982) observed that base permeabilities of at least 25 /day are necessary for longitudinal edge drains to have some inuence on base moisture content. Cedergren et al. (1973) recommended the installation of edge drains in test sections to assess their eectiveness before making a decision to retrot edge drains in large pave- ment rehabilitation projects. Ring and Mottola (1984) referred to the ingress of moisture into pavement structures as the greatest contributor to accelerated deterioration. They discussed the range of adverse Wet-Freeze (WF) Wet-Nonfreeze (WNF) Dry-Freeze (DF) Wet-Nonfreeze (WNF) Dry-Nonfreeze (DNF) Source: Adapted from SHRP 1990a, b. Base map created at Mapchart.net. Figure 4. Four climatic zones of the United States as dened for LTPP studies.

Literature Review 13   effects that moisture can have on asphalt, concrete, granular base, and subgrade layers; described case studies of drainable systems constructed as experimental pavements; and offered rec- ommendations for the design of aggregate mixtures to achieve a balance between stability and drainability. In 1992, the FHWA published the results of a study titled Drainable Pavement Systems, commonly referred to as Demonstration Project 87, or more briefly as Demo 87, to provide guidance for the design of subsurface drainage systems to remove infiltrated water from pavement structures. A computer program titled Drainage Requirements in Pavements (DRIP 1.0) was developed as a companion to the participants’ notebook for a Demo 87 training course. In 1998, a National Highway Institute course, Pavement Subsurface Drainage Design (No. 131026), was developed to train state DOT personnel in the use of DRIP 1.0 for the design, construction, and maintenance of subsurface drainage systems according to the practices recommended in Demo 87. An updated version of the DRIP software (2.0) and user guide were released in 2002 (Mallela et al. 2002), and the corresponding National Highway Institute training course was updated in 2008. Hassan et al. (1996) reviewed field studies (Espinoza 1993; Zubair et al. 1993) on pavement subsurface drainage in Indiana and reported that the findings had led to several changes in subsurface drainage system practices in Indiana, including cessation of the use of geocomposite drains, recommendations for improvements to subdrain outlet protection and marking, imple- mentation of a program for routine inspection and maintenance, and requirements for contrac- tors to inspect edge drain installations and be responsible for any repairs needed. NCHRP Synthesis 239: Pavement Subsurface Drainage Systems (Christopher and McGuffey 1997) presented an update to NCHRP Synthesis 96 (Ridgeway 1982) on the state of the practice of pavement subsurface drainage system use, while describing NCHRP Synthesis 96 as a still- useful reference for the concepts and details of subsurface drainage system design. NCHRP Synthesis 239 examined design issues such as the type and quality of aggregate to be used in permeable bases, compaction requirements for open-graded aggregates, requirements for asphalt and cement binders, and the use of geosynthetics. It also summarized available information on the effects of design, construction, and maintenance decisions on the performance of pave- ment subsurface drainage systems. Much of the drainage system design information provided in NCHRP Synthesis 239 was obtained from the training materials for FHWA’s Demo 87 project. Design details and state standards and specifications were reviewed for both subdrainage systems in new pavement construction and subdrain retrofitting for existing pavements. The synthesis identified various difficulties that can arise with the use of permeable base layers, edge drains, and outlets, as well as ways in which those difficulties can be overcome, including proper material selection (e.g., permeable base gradation, quality, and durability, and the content of asphalt or cement binder, if used), traffic control during construction, video inspection and flow testing prior to accep- tance, and training of construction and inspection personnel. Several DOTs reported that unstabilized, highly permeable base layers are difficult to construct and are unable to support construction traffic, and therefore the DOTs adopted gradation changes and the use of asphalt or cement binders to improve base stability at the expense of some degree of base permeability. As in FHWA’s Demo 87 training course, NCHRP Synthesis 239 strongly emphasizes the need for appropriately designed, constructed, and maintained subsurface drainage systems to achieve acceptable long-term pavement performance. The synthesis cites several studies (e.g., Cedergren 1974; Forsyth et al. 1987; Ray and Christory 1989; Christory 1990; Smith et al. 1994; Hagen and Cochran 1996), as well as the results of the synthesis’ survey of state DOTs, in support of the conclusion that “although there may be a real or perceived lack of cost–benefit data on the use

14 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance of subsurface drainage, the preponderance of data show a direct relationship between improved performance and extended pavement life.” NCHRP Synthesis 239 does, however, acknowledge that the degree to which subsurface drainage systems may benefit pavement performance and be cost-effective also depends on whether their use is warranted, given decision factors that include climate conditions, subgrade per- meability, and traffic levels. Cited research suggests that pavement subsurface drainage systems may not be beneficial at locations with subgrade permeabilities of more than about 10 ft/day (Grogan 1992) or with rainfall of less than about 16 in./yr (Wells and Nokes 1993). The synthe- sis suggests developing decision matrices to guide agencies in determining when subsurface drainage systems are warranted, based on factors such as the following: • Functional classification; • Design life; • Pavement and shoulder type and width; • Hydraulic considerations, including infiltration and time to drain; • Longitudinal and transverse grades; • Climate, including rainfall, temperature, and frost action; • Subgrade characteristics, including permeability; • Contribution of drainage layer to pavement structural capacity; • Constructability of drainage system components; • Maintenance requirements, capabilities, and commitment; and • Initial and life-cycle costs. NCHRP Synthesis 285: Maintenance of Highway Edgedrains (Christopher 2000) was con- ducted as an extension of NCHRP Synthesis 239 (Christopher and McGuffey 1997). According to NCHRP Synthesis 285, most of the state DOTs that responded to the NCHRP Synthesis 239 survey agreed that maintenance was the most important factor contributing to the long-term performance of pavement subsurface drainage systems. Nonetheless, many respondents (most of whom were designers) had little information on drainage system maintenance activities conducted by their departments or the effects of such maintenance. NCHRP Synthesis 285 was therefore conducted to identify (a) design and construction details and procedures that facili- tate edge drain maintenance and reduce the amount of maintenance required and (b) practices for effective maintenance of edge drain system components. A subsurface drainage system’s effectiveness in removing water from a pavement structure and in contributing to good long-term pavement performance can be enhanced, as NCHRP Syn- thesis 285 points out, by taking future maintenance needs and techniques into consideration in the design and construction phases. Among the design details and construction procedures that were identified as contributing to drainage system effectiveness, performance, and ease of future maintenance were the following: • Slotted longitudinal pipes in open-graded aggregate backfill in geotextile-lined trenches, all sized appropriately (pipe slot size, backfill gradation, and geotextile apparent opening size) to prevent migration of aggregate and fines into pipes; • Pipe, connection, and outlet dimension and configuration details (pipe diameter, pipe slope, connection radius, outlet spacing, and outlet height above the design flow level of the ditch) to facilitate water flow, video inspection, and pipe flushing; • Large cast-in-place or precast headwalls sloped flush with the ditch slope to facilitate mowing; • Careful pipe trench backfill compaction control to avoid both pipe crushing and future densi- fication of edge drain trench backfill; and • Permanent (painted or stamped) pavement markings to indicate outlet locations, as opposed to vertical delineator posts that are likely to be damaged by mowing equipment.

Literature Review 15   Video inspection prior to acceptance was identified as the only effective means of detecting construction-related problems with edge drains, such as crushing of pipes. Among the recom- mended post-acceptance inspection and maintenance practices were the following: • Regularly scheduled visual inspection of outlets and headwalls; • Periodic video inspection of longitudinal pipes and outlets; • 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); • Checking that rodent screens, if present, are not blocked; • Checking that the headwalls and outlet openings are not obscured by vegetation, blocked by sedimentation, damaged, or settled; and • Regularly scheduled pipe flushing using high-pressure water jets. Communication between design, construction, and maintenance groups within state DOTs was emphasized as an important part of coordinating the design, construction, and mainte- nance of subsurface drainage systems that will perform as expected in removing excess water from pavement structures. Training construction and inspection personnel was identified as an important step to ensuring proper edge drain functioning because, according to the synthesis, most edge drain system failures can be traced to poor construction and inadequate inspection. NCHRP Synthesis 285 also examined the costs of edge drain maintenance, strategies to reduce maintenance costs, and ways to increase edge drain maintenance effectiveness. A commitment to edge drain maintenance was concluded to be crucial for edge drains to perform as expected and contribute to good long-term pavement performance. The costs of edge drain maintenance efforts were judged to be far outweighed by the benefits of effective pavement drainage in terms of increased pavement life and performance. Conversely, according to the synthesis, edge drains that are installed but not adequately maintained may result in additional costs associated with poorer pavement performance (e.g., shorter life to first rehabilitation and increased repair needs). However, the synthesis acknowledged the limited availability of quantitative informa- tion to substantiate the cost savings achievable by installing and maintaining edge drains versus installing edge drains but not maintaining them. This was identified as a subject warranting future research. Daleiden (1998) conducted 287 video inspections of highway edge drain systems in 29 states and found that only one-third of the inspected edge drain systems were performing as intended. One-third of the inspected systems had non-functional outlets, and the other third had longi- tudinal edge drainpipes that were either non-functional or could not be inspected because of physical obstructions. While subsurface pavement drainage systems (e.g., a permeable base layer and edge drains) were increasingly used by many U.S. states by the mid-1990s, controversy persisted over the design and benefits of subsurface drainage (Harrigan 2002). NCHRP conducted a series of studies between 1995 and 2007 to address the following two key questions concerning sub- surface pavement drainage: (1) Do the various design features of subsurface drainage systems contribute to improved performance of asphalt concrete (AC) and Portland cement concrete (PCC) pavements? (2) Are the features cost-effective, and if so, under what conditions? Obser- vations and critical analyses of the performance of in-service pavements with and without subsurface drainage, in all of the major climatic zones of the United States, were judged to be the only way that credible evidence could be gathered concerning the true effects of subsurface drainage on performance (Harrigan 2002). The first of these studies, NCHRP Project 1-34 (Yu et al. 1998), was conducted using data obtained from surveys of 91 AC pavement sections at 22 sites in 10 U.S. states and one Cana- dian province, and 46 PCC pavement sections at 16 sites in seven U.S. states and one Canadian

16 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance province, as well as additional data on about 300 AC and PCC pavement sections in the FHWA’s Rigid Pavement Performance and Rehabilitation database and the SHRP’s LTPP database. However, pavement sections from the LTPP Specific Pavement Studies (SPS) AC pavement (SPS-1) and PCC pavement (SPS-2) experiments, the only LTPP experiments developed to permit comparison of drained and undrained pavement sections at the same sites, were not included because they were not of sufficient age at the time the project was underway. The key findings of NCHRP Project 1-34, summarized in NCHRP Research Results Digest 268 (Harrigan 2002), encompassed a variety of observations of differences in distress and roughness for AC and PCC pavement sections with and without edge drains and with different base types (including unbound dense-graded, asphalt- or cement-treated dense-graded, unbound permeable, and asphalt- or cement-treated permeable), as well as daylighted bases of various types. The findings were judged to be limited, however, by the small number of sites and their lack of distribution throughout the United States, the fact that most of the drained (permeable base) pavement sections were relatively young and had carried relatively little traffic, and the lack of information about whether and how well the subsurface drainage systems in the drained pavement sections were functioning (Harrigan 2002). Three follow-up studies were commissioned by the NCHRP Project 1-34 panel. NCHRP Project 1-34B (Hall et al. 2000) critically reviewed the results of NCHRP Project 1-34 and developed an experimental plan to evaluate and test NCHRP Project 1-34’s key findings, using data from the LTPP SPS-1 and SPS-2 experiments, under NCHRP Project 1-34C (Hall and Correa 2003). Field testing of the functionality of the subsurface drainage features in the LTPP SPS-1 and SPS-2 field sections and further analyses of the relationship between subsurface drainage, features, and pavement performance (using distress, roughness, and deflection data from the SPS-1 and SPS-2 sites) were conducted under NCHRP Project 1-34D (Hall and Crovetti 2007). Possible reasons for the lack of consensus on the cost-effectiveness and benefits of sub- surface pavement drainage systems for pavement performance were summarized by Hall and Crovetti (2007, p. 1) as follows: The reasons often mentioned for why subsurface pavement drainage systems do not always yield improve- ments in performance include inadequate design, improper construction, and inadequate maintenance. If, however, these were the only reasons, then they could be countered by—and improvements in pavement performance and pavement life could be consistently achieved by—adequate design, proper construction, and adequate maintenance. Yet there are at least two other reasons why drained pavements do not con- sistently perform better than undrained pavements. First, subsurface drainage systems are sometimes used in locations where they are not needed (e.g., places with low amounts of rainfall or with subgrade soils that have sufficient natural drainage characteristics so that water rarely, if ever, collects in the constructed pavement layers long enough to contribute to any damage). Second, subsurface drainage systems are sometimes used in pavements where they are not needed, such as pavements with other design features (such as thickness or dowels) that make them unlikely to develop the types of damage that would be exacerbated by excess water. NCHRP Project 1-34D was conducted using distress, roughness, and deflection data retrieved from Release 19 (January 2005) of the LTPP database for all SPS-1 and SPS-2 sites, along with the results of drainage outlet inspections and subdrainage system flow time tests conducted at all of the sites to assess the functioning of the subsurface drainage systems. The flow meter used in the flow time testing is shown in Figure 5. The same flow meter and other equipment used to conduct the flow time testing are shown in Figure 6. The AC and PCC pavements in the SPS-1 and SPS-2 experiments were located in 18 and 14 states, respectively (SPS-1 AC sites in AL, AZ, AR, DE, FL, IA, KS, LA, MI, MT, NE, NV, NM, OH, OK, TX, VA, and WI; SPS-2 PCC sites in AZ, AR, CA, CO, DE, IA, KS, MI, NV, NC, ND, OH, WA, and WI), in both cases distributed across all of the four main climatic zones of the United States and across a wide range of annual average rainfall amounts and hydrological soil

Literature Review 17   Source: Hall and Crove 2007. ll Figure 5. Flow meter used in ow testing at LTPP SPS-1 and SPS-2 sites. Figure 6. Water truck and equipment used in ow testing at LTPP SPS-1 and SPS-2 sites. types (from high to very low inltration rates). e SPS-1 sites were predominantly located on more lightly tracked routes than the SPS-2 sites, with more than half of the SPS-2 sites having carried more truck trac than all but two of the SPS-1 sites. e results of the testing and analyses conducted under NCHRP Project 1-34D did not identify any aspect of the behavior or performance of the AC and PCC pavement structures in the SPS-1 and SPS-2 experiments that was improved by the presence of subsurface pavement drainage. Instead, the measures of pavement behavior and performance analyzed for these

18 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance pavements—namely, deflection response, roughness, rutting, faulting, and cracking—were found to be influenced by the stiffness, not the drainability, of the base layers. Overall, the best- performing AC pavements in the SPS-1 experiment were those with the stiffest bases (dense- graded asphalt-treated base layers), whether drained or undrained. The best-performing PCC pavements in the SPS-2 experiments were those with bases that were neither too weak (untreated aggregate) nor too stiff (lean concrete). These included both the sections with drained permeable asphalt-treated bases and the sections with undrained hot-mix asphalt (HMA) bases and cement-aggregate-mixture bases. NCHRP Project 1-34D recommended that the need for subsurface pavement drainage should be assessed using the TMI and precipita- tion data to identify sites with year-round or seasonal excesses of available moisture, as well as using county soil reports and soil taxonomy information to identify subgrade soils with poor natural drainage characteristics. Bhattacharya et al. (2009) reported on an evaluation of the performance of 24 projects in 15 counties in California where concrete pavements were constructed with subsurface edge drains. California has constructed subsurface drainage systems according to a wide range of designs, but many of them were later found to have become ineffective because of deficien- cies in design, materials, construction, and maintenance. Of the 24 projects surveyed, fewer than 30 percent had functioning edge drains. These projects were typically in areas of higher rainfall. At most of the remaining sites, little or no maintenance had been done, and the drainpipes were clogged with soil. Bhattacharya et al. (2009) concluded that subsurface drainage systems installed as part of new construction, with larger diameter drainpipes, deep trenches, and treated permeable bases, contributed to better pavement performance than that of shallow edge drains with slotted pipes retrofitted to existing pavements. On the basis of the study results, Bhattacharya et al. (2009) recommended that the use of subsurface drainage systems should be based on an assessment of the rainfall characteristics of the site, the permeability of the natural soil, and whether there is a long-term commitment to maintenance of the edge drain system. They also suggested that the use of edge drains should be considered only in isolated locations of poor drainage and not necessarily throughout the entire length of a project. They noted that video inspection was facilitated by the use of larger- diameter (4-in.) drainpipes and that maintenance was facilitated by the use of dual-outlet systems (i.e., where edge drains and outlets form sequential open loops along the roadway, with the end of each loop and the beginning of the next one forming a pair of adjacent drainage outlets). Bhattacharya et al. (2009) observed that the use of edge drain systems did not yield an evident improvement in long-term concrete pavement performance beyond those already offered by load transfer devices (dowel bars and tie bars), daylighted permeable bases, and AC interlayers. An NCHRP IDEA study (Stormont et al. 2009) yielded an innovative method for draining water from a pavement base layer and preventing water from reaching the subgrade by using a geocomposite capillary barrier drain (GCBD). A GCBD consists of three layers: a transport layer (a specially designed geotextile) beneath the base, a capillary barrier (geonet) beneath the transport layer, and a separator (geotextile) beneath the geonet and above the subgrade. Water that infiltrates into the pavement and enters the base is prevented from moving into the under- lying subgrade by the capillary barrier formed by the geonet. The transport layer (a special geotextile) becomes increasingly hydraulically conductive as it gets wet. Water drains along the slope of the transport layer, and if the transport layer does not become saturated, no water will break through into the capillary barrier. The bottom separator protects the geonet from becoming contaminated with subgrade soil. A GCBD also cuts off capillary rise of water in the underlying soil, and if the overlying base and transport layer become saturated owing to an extraordinary infiltration event, it provides saturated drainage in the geonet. Ceylan et al. (2013) conducted field investigations of the effects of drainage outlet conditions on 56 jointed plain concrete pavements and eight AC pavements in Iowa. They found that (a) while

Literature Review 19   less than 20 percent of the edge drain outlets of the concrete pavements surveyed were damaged, approximately 65 percent were blocked to some degree; and (b) while less than 10 percent of the edge drains of the asphalt pavements surveyed were damaged, approximately 45 percent were blocked to some degree. In the case of concrete pavements built with granular bases consisting of recycled concrete, blockage occurred in some subdrain outlets due to the for- mation of precipitated calcium carbonate, known as tuff or tufa. Concrete pavement bases constructed with a blend of recycled concrete aggregate and virgin aggregate were found to exhibit fewer outlet blockages due to tufa formation than bases constructed entirely with recycled concrete aggregate. In their field investigations, Ceylan et al. (2013) saw little evidence of outlet blockages due to rodents but did see evidence of blockages due to rodent screens, so they advised against the use of rodent screens. Ceylan et al. (2013) observed that, although greater degrees of blockage reduce the flow rate of water in outlet pipes, they did not necessarily stop the flowing water from exiting the drainage system through the outlet pipes unless the outlet was completely blocked. They also found that little pavement distress was observed near blocked subsurface drainage outlet spots, although shoulder distresses (shoulder drop off and cracking) were observed more often near blocked drainage outlets than near clear outlets. Both the field observations and the limited perfor- mance analysis from this study led Ceylan et al. (2013) to conclude that drainage outlet condi- tions did not have a significant effect on pavement performance in Iowa. Summary Centuries of road building experience, going as far back as the Roman Empire, have shown that excess moisture in pavement structures and foundations can adversely affect pavement behavior and performance. Early modern road building practices recognized the importance of protecting road structures from excess moisture, and the growth in road traffic weights and volumes brought about by the Industrial Revolution intensified the need for roads that were resistant to moisture-related damage. Among the problems that excess moisture can cause or exacerbate are slope instability, visible pavement distress of various types, and diminished pave- ment strength, stability, and durability. In the United States, roads built during the first half of the 20th century were typically poorly drained but performed satisfactorily under the traffic and load conditions of the time. Con- cepts and procedures for the design of subsurface pavement drainage systems were developed in the 1940s and improved through the 1990s, including implementations using nomographs and computer software. Such systems are intended primarily to address infiltration of water through joints and cracks, and in some situations, shallow groundwater movements (including lateral groundwater seepage, fluctuations in groundwater levels, and capillary action). The state of the practice of subsurface drainage system use, design, construction, and maintenance has been summarized in three prior NCHRP syntheses published between 1982 and 2000. This current synthesis was conducted to provide a more current assessment of state DOT practices and experiences with regard to subsurface pavement drainage systems.