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

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39   Introduction Case examples were developed from responses to the initial survey, as well as from follow-up interviews conducted with DOT personnel. Follow-up interviews, lasting between 30 and 60 minutes, were conducted via telephone or videoconference. Nine of the 41 state DOTs that responded to the initial survey, all within or east of the Mississippi River valley, indicated their willingness to participate in the development of case examples. Online design documen- tation, survey responses, and referenced performance reports from those nine state agencies were reviewed for consideration, after which the current practices of four states (Alabama, Minnesota, Missouri, and Pennsylvania) were selected for inclusion in this synthesis. The selected states, or portions thereof, are located within four distinct climatic zones (see Figure 3): wet-freeze (Minnesota, Pennsylvania, Missouri), seasonally wet-freeze (Minnesota and Missouri), wet-freeze-thaw (Missouri and Alabama), and wet-non-freeze (Alabama). The following case examples present detailed summaries of current practices for the design, construction, and maintenance of subsurface drainage systems and experience with their performance and their effects on pavement performance. Case Example 1: Alabama Department of Transportation Introduction The Alabama Department of Transportation (ALDOT) uses subsurface drainage systems on new construction projects in areas with fine-grained soils, predominantly located in the northern portions of the state. Longitudinal edge drains are also retrofitted along existing concrete pavements. ALDOT design standards are based on recommendations developed under FHWA Demo Project 87 and are applied on a case-by-case basis. Working ALDOT documents and design manuals that were used in the development of this case example include the following: • Standard Specifications for Highway Construction, 2018 Edition (ALDOT Std. Specs.); • ALDOT Design Bureau Special Drawings (ALDOT Spec. Drwgs.); and • ALDOT Materials Report, Project No: IM-1059(407) (ALDOT Matls. Rep.). Design Factors ALDOT currently uses a permeable asphalt-treated base (PATB) mix under both concrete and asphalt driving surfaces. In areas that are determined to be suitable for subsurface drainage, a 4-in. PATB is specified. Table 1 provides the gradation used for the PATB, which is essentially C H A P T E R   4 Case Examples

40 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance an AASHTO #57 gradation with allowances for minimal amounts of materials passing the No. 200 sieve. PATB mixes are stabilized with a polymer-modified performance-graded (PG) 76-22 liquid asphalt binder at 2.0 to 3.0% by weight unless otherwise specified. The subsurface drainage system design typically incorporates a geotextile separation layer between the PATB and underlying crushed aggregate base course, continuous pavement edge drains, and lateral outlets spaced at 500 ft. Figure 34 provides detail drawings used for a recent alternative design/alternative bid reconstruction project along I-59 in Fort Payne, which was reconstructed using the asphalt alternative. Geotextile separation layers are used to prevent clogging of the PATB layer while allowing the passage of water. The geotextiles used are constructed of nonwoven synthetic fibers meeting the requirements of AASHTO M 288, with the exception that geotextiles manufactured with polyamide are not allowed. Geotextile-wrapped, aggregate-filled underdrains are provided with collection pipes and outlets. The aggregate fill material must meet the requirements for ALDOT Coarse Aggregates #4, #5, or #57, as provided in Table 2, with the additional requirement that not more than 1% of the aggregate may pass the No. 200 sieve. Perforated pipe underdrains of various types and sizes are allowed. Typically, 4-in.-diameter pipe underdrains are used. However, once installation starts, the pipe type must remain unchanged unless otherwise noted in the plans. Table 3 provides a listing of the approved under- drain pipe types. Unperforated 4-in.-diameter transverse underdrain pipes are installed every 500 ft. The pipe ends are protected with cast-in-place concrete headwalls and rodent screens. Figures 35 and 36 provide detail drawings for the outlets and headwalls. Subsurface Drainage System Construction PATB is a central-plant-mixed, hot-laid material with no requirements for density, air voids, voids in the mineral aggregate, or stability. The geotextile filter is installed immediately prior to the placement of the PATB and is placed full width between the inside and outside edge drains. PATB compaction is achieved using static steel-wheel rollers that apply 0.5 to 1.0 tons per foot of roller width. Rollers typically make 1–3 passes, as directed by the engineer, when the PATB tem- perature reaches approximately 150°F. The exposed PATB must be covered within five calendar days after laydown. No traffic is allowed to operate or park on the travel lanes or outside shoulder portion of the PATB. Limited operation of equipment may be permitted on the inside edge. Aggregate-filled underdrains are installed prior to final surfacing and in coordination with other work to prevent damage to the roadway. After trench excavation, the geotextile filter is Sieve Size Percent Passing by Weight 1 ½ in. 100 1 in. 95–100 ½ in. 25–60 No. 4 0–10 No. 8 0–5 No. 200 0–4 Source: ALDOT Std. Specs, Section 327.02. Table 1. ALDOT gradation specifications for PATB.

Case Examples 41   Source: ALDOT Matls. Rep. Figure 34. Design details for I-59 reconstruction project.

42 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Sieve Size Percent Passing by Weight #4 #5 #57 2 in. 100 1 ½ in. 90–100 100 100 1 in. 20–55 90–100 95–100 ¾ in. 0–15 20–55 ½ in. 0–10 25–60 0–5 0–5 No. 4 0–10 No. 8 0–5 No. 200 0–1 0–1 0–1 in. Source: ALDOT Std. Specs, Section 801.11. Type Kind Abbreviation 1 Concrete Pipe CP 2 Corrugated Steel CS 3 Coated Corrugated Steel CCS 4 Vitrified Clay VC 5 Corrugated Aluminum CA 6 Coated Corrugated Aluminum CCA Source: ALDOT Std. Specs, Section 606.01. 7 Bituminous Fiber BF 8 Polyvinyl Chloride PVC 9 Acrylonitrile Butadiene Styrene ABS 10 Polyethylene PE Source: ALDOT Spec. Drwgs. SUO-605-AB, Index 60501. Table 2. Selected ALDOT coarse aggregate gradation specifications. Table 3. ALDOT pipe underdrain types. Figure 35. Detail drawing of underdrain outlets.

Case Examples 43   installed immediately prior to the placement of the underdrain pipe and aggregate fill material. A maximum of 14 days is allowed between laydown and cover of the geotextile wrap to minimize the potential for damage. Subsurface Drainage System Testing and Maintenance No specific testing and maintenance protocols are currently in place for subsurface drainage systems. On occasion, contractors are requested to supply a water truck to flood the surface of the PATB after placement to ensure that the PATB readily accepts the water without ponding and that water flows from the outlets. Interstate maintenance review committees will ride projects to prioritize work to be completed, at which time local maintenance crews are urged to inspect and maintain outlets during grass-cutting operations. Case Example 2: Minnesota Department of Transportation Introduction The Minnesota Department of Transportation (MnDOT) addresses the need for subsurface drainage by providing alternate design guidelines and recommendations for its use. Aggregate base and subbase layers may be daylighted to the ditch, or subsurface drains may be installed to remove excess water and minimize or eliminate its detrimental effects on both HMA and PCC pavements. MnDOT uses drainage systems only where commitments are made by district staff to regularly inspect and maintain the system components or by daylighting to the ditch. The primary sources of subsurface water addressed by MnDOT originates as surface water entering through joints and cracks from rain, snowmelt, spring-thaw seepage, and groundwater movements. Working MnDOT documents and design manuals that were used in the devel- opment of this case example include the following: • MnDOT Pavement Design Manual, July 2019 (MnDOT PDM); • MnDOT Standard Specifications for Construction, 2020 Edition (MnDOT Std. Specs.); • MnDOT Geotechnical Engineering Manual, January 2017 (MnDOT GEM); • MnDOT Standard Plan Drawings (MnDOT Std. Drwgs.); and • MnDOT Standard Plates (MnDOT Std. Pl.). Subsurface Drainage Design MnDOT uses several types of drains to remove water from the pavement systems, including subcut drains, pavement edge drains, permeable aggregate base drains, pavement widening Source: ALDOT Spec. Drwgs. SUO-605-AB, Index 60501. Figure 36. Detail drawings of outlet headwalls.

44 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance drainage systems, and interceptor drains. Subcut drains are used to remove water from subcut sections that are at a high risk of collecting water in bathtubs because of relatively impermeable grades. Subcut drains typically include a longitudinal perforated pipe with a geotextile wrap, as shown in Figure 37. Edge drains are installed along each side of the mainline pavement to drain portions of the pavement structure and are classified as pavement edge drain–type drains and permeable aggre- gate base–type drains. An edge drain–type drain, which may be installed during new construction as well as retrofitted into existing pavements, consists of a geotextile-wrapped perforated pipe placed in the bottoms of a fine filter aggregate-filled trench. Figure 38 provides a typical detail for the edge drain–type drain. Table 4 provides MnDOT gradation specifications for fine filter aggregates. Edge drain–type drains are recommended for use on rubblized or crack-and-seat Source: MnDOT Std. Drwgs. 5-297.430. Source: MnDOT Std. Drwgs. 5-297.432. Figure 37. Typical MnDOT subcut subsurface drain type. Figure 38. Typical MnDOT edge drain type.

Case Examples 45   projects, for unbonded PCC overlays with geotextile interlayers, or in areas where the pavement has a history of pumping. A permeable aggregate base (PAB)–type edge drain consists of an unwrapped perforated pipe placed in the bottom of a trench filled with permeable aggregate. This drain type provides a high rate of flow and is used in combination with an open-graded aggregate base (OGAB), a drainable stable base (DSB), a permeable asphalt-stabilized stress relief course (PASSRC), or a permeable asphalt-stabilized base (PASB). Figures 39 and 40 provide typical details for PAB edge drain types. Table 5 provides MnDOT gradation and quality specifications for OGAB and DSB materials. Table 6 provides MnDOT gradation specifications for PASB and PASSRC materials. When narrow pavements are widened, a drainage system may be installed to facilitate the drainage of water that may be trapped in the existing pavement, as illustrated in Figure 41. Either OGAB or PASB may be used for the drainage layer. Interceptor drains are used with unbonded concrete overlays to collect water from joints and major cracks in the existing concrete pavement, as illustrated by the cross-hatched region with the encircled 5 in Figure 40. The interceptor drains are typically connected to the permeable base drain for discharge. MnDOT typically uses drainable base layers for new and reconstructed PCC pavements and PASSRC interlayers for unbonded PCC overlays of existing PCC pavements where existing joint faulting exceeds 0.25 in. and where pumping and free water are present. When existing faulting is less than 0.25 in., a geotextile interlayer may be used as a replacement for the PASSRC layer and drained by daylighting or by edge drains. Sieve Size Percent Passing by Weight 100 No. 4 90–100 No. 10 45–95 No. 40 5–35 No. 200 0–3.5 Source: MnDOT Std. Specs., Section 3149.2-I.2. in. Table 4. MnDOT gradation specifications for fine filter aggregates. Source: MnDOT Std. Drwgs 5-297.432. Figure 39. Typical MnDOT PAB-type edge drain with OGAB or PASB.

46 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Source: MnDOT Std. Drwgs 5-297.432. Figure 40. Typical MnDOT PAB-type edge drain with PASSRC. Sieve Size Percent Passing by Weight OGAB DSB 1 ½ in. 100 100 1 in. 95–100 — ¾ in. 65–95 75–100 30–65 45–75 No. 4 10–35 30–60 No. 10 3–20 10–35 No. 40 0–8 5–20 No. 200 0–3.5 0–6.5 for Class A 0–5.0 for Class B or C Class per specification 3137 Quality Requirements D60 / D10 ≥ 4.0 ≥ 8.0 Minimum Crushing (Two Faces) 85% 60% Maximum LA Rattler Loss 40% 40% Maximum Acid Insoluble Residue Minus No. 200 sieve 10% 10% Maximum Spall–Total Sample 5.0% 5.0% in. Source: MnDOT Std. Specs, Section 3136.2. Notes: D60 = sieve size that 60 percent of material passes through; D10 = sieve size that 10 percent of material passes through. Table 5. MnDOT gradation specifications for OGAB and DSB aggregates. Required PCC thicknesses are determined using the MnPAVE-Rigid program, which uses transverse cracking as the controlling distress. Three drainable base type options are available for new construction: OGAB, DSB, or PASB with edge drains or geocomposite joint drains that drain into either edge drains or daylighted DSB layers. The OGAB layer is designed to have a permeability of approximately 1,000 ft/day but lacks the stability to support construction traffic. DSB layers use smaller aggregates and are designed to be driven on during subsequent pavement construction. PASB and PASSRC layers are designed to be stable after construction to avoid rutting by haul trucks, pavers, and rollers during construction or the placement of overlying

Case Examples 47   Sieve Size Percent Passing by Weight PASB PASSRC 1 ½ in. 100 1 in. 95–100 ¾ in. 85–95 — 100 ½ in. — 85–100 30–60 50–100 No. 4 10–30 0–25 No. 8 0–10 0–5 No. 30 0–5 No. 200 0–3 in. in. Source: MnDOT Std. Specs, Section 3139.2-B.2. Table 6. MnDOT gradation specifications for PASB and PASSRC aggregates. Source: MnDOT Std. Drwgs 5-297.432. Figure 41. Typical MnDOT drainage details for widened pavements. pavement layers. PASB and PASSRC materials are stabilized with PG 64S-22 binders at a rate of 2.5% by total weight of the mixture. Edge drain systems use 4-in. wrapped or unwrapped perforated thermoplastic pipes or corrugated polyethylene (PE) drainage tubing, depending on the subsurface drainage details. Non-perforated lateral outlets are spaced between 300 and 500 ft, depending on the longitudinal grade. Outlet locations are permanently marked with a 6 in. × 18 in. strip of white marking tape placed at the outside edge of the HMA shoulder or on the edge of the HMA pavement if no paved shoulder is in place. White paint strips are provided on PCC pavement edges. Figures 42 and 43 provide standard plan drawings for single and low-point “Y” discharge outlets. All discharges use 4-in. precast headwalls and pipe ends protected with rodent shields, as indicated in Figure 44. Subsurface Drainage System Construction DSB construction requirements include having adequate moisture for compaction and ensuring that the placement equipment does not rut the in-place surface or displace or damage

48 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Source: MnDOT Std. Drwgs. 5-297.433. Source: MnDOT Std. Drwgs. 5-297.433. Figure 42. MnDOT standard drawing for single discharge outlets. Figure 43. MnDOT standard drawing for “Y” discharge outlets.

Case Examples 49   Source: MnDOT Std. Pl. 3131C. Figure 44. MnDOT standard drawings for concrete headwalls.

50 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance any geotextile materials. DSB compaction should be conducted using non-vibratory rollers, and no traffic is allowed after final placement and compaction. PASSRC and PASB must be mixed and compacted at temperatures recommended by the binder supplier, which may not be more than 30oF above the recommended maximum mixing temperature during production. Layer compaction must be achieved with self-propelled non- vibratory steel-wheeled rollers weighing at least 8 tons. Temperatures must not fall below 110oF during compaction. There are no specific density requirements other than that the PASSRC and PASB layers must be dense and stable after construction so they will not rut when the over- lying pavement is placed. Concrete hauling units, loaded or empty, are allowed on the PASSRC. Only pavers, rollers, and HMA hauling trucks are allowed to drive on the PASB, and HMA hauling trucks can only drive on the PASB immediately in front of the paver to unload and must leave the PASB as soon as the HMA materials are unloaded. Pavement edge drains are installed at various times in the construction sequence, depending on the pavement type. For new PCC pavements, edge drains are installed after constructing the pavement, with the trenching head kept 6 in. away from the pavement while excavating the trench soil, as indicated by Note 1 in Figure 45. For rubblization and crack-and-seat projects, edge drains are installed prior to pavement cracking. On widening projects, edge drains are installed prior to excavating the widening trench for the pavement. Where edge drains are placed adjacent to new HMA pavements, the edge drains are installed after placing the non-wearing courses but before placing the wearing courses. Interceptor drains located at the ends of PCC joints and mid-panel cracks are installed before placing PASSRC or geotextile stress relief layers. Source: MnDOT Std. Drwgs. 5-297.432. Figure 45. MnDOT standard drawings for subsurface drain trench.

Case Examples 51   Subsurface Drainage System Testing and Maintenance As previously discussed, MnDOT policy is to use drainage systems only where commit- ments are made by district staff to regularly inspect and maintain the system components. Quality assurance testing during construction uses a probe mounted on the end of a flexible fiberglass rod with a diameter of 1 nominal pipe size smaller than the drainpipe being inspected. Inspections are conducted through the discharge pipe, radius connection, and at least 3 ft into the main discharge line. Crushed or deformed pipes or connections must be replaced at no additional cost to the DOT. Visual inspections, cleaning, and repairing of outlets and headwalls are conducted on an annual basis. Probing of drainage pipes and camera inspections are conducted as needed, typi- cally at intervals of 5 or more years. Vegetation growth, roadside slope debris, and topsoil are removed as necessary to maintain system integrity. Case Example 3: Missouri Department of Transportation Introduction The Missouri Department of Transportation (MoDOT) recognizes the importance of incor- porating good drainage and uses pavement underdrains in new construction and select reha- bilitation projects to minimize the detrimental effects of water within or beneath the pavement structure. Pavement underdrains are installed for base drainage, removing excess ground- water, and intercepting water in unstable soil to prevent slides. The primary source of water addressed by MoDOT is surface water entering through joints and cracks. Working MoDOT documents and design manuals that were used in the development of this case example include the following: • MoDOT Standard Specifications for Highway Construction, 4th Edition, April 2021 (MoDOT Std. Spec.); • MoDOT Engineering Policy Guide, 2021 Web Posting (MoDOT EPG); • MoDOT Standard Detail Drawings (MoDOT Std. Drwgs.); and • MoDOT Plans for Proposed I-49 (MoDOT I-49). Continuous pavement edge drains are required for new rigid or flexible pavement structures on medium- and heavy-duty routes, and permeable base courses are provided on all heavy-duty routes. Heavy-duty routes include interstates and other roadways built to interstate standards, whereas medium-duty routes include other principal arterials. Permeable base courses and continuous edge drains may be excluded where a minimum of 12 to 18 in. of daylighted rock base can be furnished on top of the subgrade. For pavement rehabilitation projects with existing moisture-related distress, discrete aggregate underdrains are incorporated to remove excess water from problem areas. Figures 46 through 49 provide examples of standard detail drawings for the types of pavement underdrainage systems that MoDOT constructs. Design Factors For heavy-duty pavements, MoDOT has traditionally used 4-in. permeable base layers, stabilized with either asphalt or cement materials, at the option of the contractor, placed over a 4-in. layer of dense-graded aggregate. The aggregate gradation for the stabilized permeable base is designated as Grade 4, with MoDOT specification limits provided in Table 7. Asphalt-stabilized permeable base mixtures include 2.5% PG 64-22, PG 70-22, or PG 76-22 asphalt binders by weight. Cement-stabilized base mixtures contain 2.5 sacks of cement per cubic yard of mix.

52 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Source: MoDOT Std. Drwgs 605.101, Sheet 1. Source: MoDOT Std. Drwgs 605.101, Sheet 1 Figure 46. MoDOT underdrainage standard drawings for heavy-duty pavements. Figure 47. MoDOT underdrainage standard drawings for medium-duty pavements.

Source: MoDOT I-49, Sheet 8 Figure 48. MoDOT typical section for daylighted rock base along I-49 project.

Source: MoDOT Std. Drwgs. 605.101, Sheet 4. Figure 49. MoDOT standard drawings for discrete aggregate underdrains.

Case Examples 55   Sieve Size Percent Passing by Weight Gradation A Gradation B 1 ½ in. 100 1 in. 95 – 100 100 ¾ in. 90 – 100 ½ in. 25 – 60 20 – 55 No. 4 0 – 10 0 – 10 No. 8 0 – 5 0 – 5 in. Source: MoDOT Std. Spec., Section 1009.3.4. Table 7. MoDOT gradation specifications for Grade 4 aggregates. For medium-duty pavements, MoDOT uses a 4- to 6-in. layer of a Type 5 aggregate base to provide some degree of drainage for the pavement. Longitudinal edge drains are included only along the outside shoulder, with transverse outlets spaced every 250 to 500 ft. Table 8 pro- vides the MoDOT gradation specifications for Type 5 aggregates. Rock base layers, where used, are composed of locally available durable stone or broken concrete that has been approved for use by the engineer and contains no more than 10% soil, sand, shale, or non-durable rock. There are no specific gradation requirements other than that the maximum particle size is limited to 2/3 of the layer thickness and that the material must be well graded from coarse to fine. Rock base layers are typically covered with a 2-in. layer of finer materials to allow for a smooth surface on which to place the pavement. Rock base layers are the preferred option for drainage layers when suitable materials are locally available, mainly due to their simplicity of construction and their proven field perfor- mance. Observations of pavements approaching 30 years of service indicate that daylighted rock layers experience minimal vegetation growth along the exposed shoulder portions and retain adequate porosity for effective drainage. Edge drain systems use 4-in. perforated polyvinyl chloride (PVC) or high-density polyethylene (HDPE) pipes placed in AASHTO Class 2 geotextile-wrapped trenches and typically back- filled with MoDOT Grade 5 drainage aggregates (porous backfill). Table 9 provides the gradation requirements for Grade 5 aggregates. Edge drainpipes are provided with non-perforated lateral outlets spaced between 250 and 500 ft, depending on the longitudinal grade. Figure 50 provides standard detail drawings for outlets on gradients and at sag locations. Each outlet has a cast-in-place splash pad, as previously shown in Figures 46 and 47, and a press-formed rodent screen. Sieve Size Percent Passing by Weight 1 in. 100 ½ in. 60–90 No. 4 35–60 No. 30 10–35 No. 200 0–15 Source: MoDOT Std. Spec., Section 1007.3.2. Table 8. MoDOT gradation specifications for Type 5 aggregates.

56 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Sieve Size Percent Passing by Weight 1 ½ in. 100 1 in. 95–100 ½ in. 60–80 No. 4 40–55 No. 8 5–25 No. 16 0–8 No. 50 0–5 Source: MoDOT Std. Spec., Section 1009.3.5. Table 9. MoDOT gradation specifications for Grade 5 aggregates. Treated Permeable Base Drain Design Process Treated permeable bases are designed using the AASHTOWare Pavement ME Design procedure, and are designed to be structural, highly permeable drainage layers. The rock base and stabilized permeable base layers are modeled as high-modulus (50 ksi) unbound granular bases of variable thickness. The required concrete and asphalt surface layer thicknesses are determined based on the selected base thickness, design traffic, and subgrade soil char- acterization. Various design alternatives are used in alternate pavement bidding scenarios to determine which type of drainable base is ultimately constructed. Treated Permeable Base Course Construction Asphalt-stabilized permeable base layers are constructed using a PG 64-22, PG 70-22, or PG 76-22 binder at a rate of 2.5% by weight of the mixture. The final mixture, when discharged from the pug mill or drum, must be at a temperature in the range of 250–300°F. The mixture is placed in lifts not exceeding 4 in. in thickness after compaction. Rolling is performed as soon as the mat has cooled sufficiently to avoid shoving or lateral movement under the weight of a 5- to 10-ton steel-wheeled roller. A minimum of three passes of the roller must be made, and compaction should continue until no further displacement is noted. Compaction must be completed before the temperature of the mixture drops below 100°F. Cement-stabilized permeable base layers are constructed using Type I cement at a rate of 2.5 sacks per cubic yard. Fly ash and ground granulated blast furnace slag are restricted from use. Normal concrete consolidation equipment, such as vibrators or vibrating pans, are considered to be adequate, provided that the mixture can be satisfactorily compacted. The compacted mixture must be cured for a minimum of 48 hours. A fine water mist may be applied several times each day to maintain adequate internal moisture. Subsurface Drainage System Testing and Maintenance MoDOT conducts video camera inspections of subsurface drainage components after all paving is completed. The engineer may randomly select at least 10% of the lateral outlet pipes for inspection and may extend the inspection to 500 ft of the mainline pipe. The video camera is centered in the 4-in. outlet and longitudinal pipes and must be capable of negotiating the 90o long sweep elbow. If the inspection reveals crushed or compressed pipe, separated joints, obstructions within the pipe that prohibit the passage of the camera head, rips or cracks in

Source: MoDOT Std. Drwgs 605.101, Sheet 1. Figure 50. MoDOT standard drawing for pipe aggregate drain outlets.

58 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance the pipe wall, or longitudinal sags that allow silt to collect or water to stand in more than half of the pipe depth, repair or replacement of the deficient portions of the outlet and/or longitudi- nal pipe must be performed at the contractor’s expense. Case Example 4: Pennsylvania Department of Transportation Introduction The Pennsylvania Department of Transportation (PennDOT) recognizes the importance of good drainage and incorporates subsurface drainage into all new construction and recon- struction projects along interstate and limited-access freeways, as well as collector and arterial routes. Working PennDOT documents and design manuals that were used in the development of the case example include the following: • Pavement Policy Manual, Publication 242, Change 5, December 1, 2019 (PennDOT 242); • Design Manual Part 2 – Highway Design, Publication 13M, April 2021 (PennDOT 13M); • PennDOT Drainage Manual, Publication 584, March 2015 (PennDOT 584); • Construction Specifications, Publication 408, April 2021 (PennDOT 408); and • Standards for Roadway Construction, Publication 72M, February 2021 (PennDOT 72M). According to Publication 242, sec. 2, p. 8, Many factors associated with good drainage are costly and oftentimes some compromise is made to reduce costs. Poor ride quality, more frequent maintenance during the life of the pavement and shorter service life are the predictable results of such compromise. These one-time costs should be considered investments in the pavement system. More often than not, these initial costs are more economically feasible than the costs associated with the results of compromise. Treated permeable base courses (TPBC) and continuous pavement base drains are used to enhance pavement performance, minimize the detrimental effects of frost heave, and reduce pavement maintenance requirements. Installation locations include the outside edge of the pavement and along the low side of superelevated pavement sections, the median side of the pavement in areas where subsurface water is a particular problem, and on rehabilitation projects where the existing roadway does not already have functioning edge drains. Lateral drains are installed at transitions from cut to fill and at other needed locations as identified by the local maintenance representative. Subgrade drains are also considered where the existing roadway shows evidence of water damage and in sag areas. Figure 51 provides standard details for TPBC and prefabricated edge drains (PennDOT 72M). Design Factors PennDOT currently uses asphalt-treated permeable base course (ATPBC) and cement-treated permeable base course (CTPBC) materials under concrete driving surfaces. Pavement structures are designed using the 1993 AASHTO Design Guide procedures. For concrete pavement design, pavement subbases include a TPBC over a dense-graded 2A subbase. ATPBC and CTPBC are bid as alternates. Table 10 provides a tabulation of maximum and minimum pavement layer thicknesses for concrete pavements. The influence of TPBC is incorporated by a modification to the loss of support factor and the drainage coefficient. Table 11 provides a tabulation of the loss of support factors currently used for concrete pavements. For new construction or reconstruction projects where a subbase consisting of treated permeable base course over unbound aggregate is

Case Examples 59   Source: PennDOT 72M. Figure 51. PennDOT prefabricated base drains. Courses Minimum Depths Maximum Depths Interstates and Other Limited-Access Freeways Arterials Collectors Local Roads Alternative* A B JPCP 9 in. 9 in. 9 in. 8 in. 7 in. 20 in. TPBC 4 in. 3 in. 4 in. 4 in. 4 in. ** 4 in. 2A 4 in. 6 in. 4 in. 4 in. 4 in. 6 in. JRCP 8 in. 8 in. 6 in. 6 in. 6 in. — TPBC 4 in. 3 in. 4 in. 4 in. 4 in. ** 4 in. 2A 4 in. 6 in. 4 in. 4 in. 4 in. 6 in. *Either Alternative A or B is acceptable. **TPBC may be eliminated on low-volume/local roads, but if so, increase depth of 2A to 6 in. Source: PennDOT 242, Section 8.11. Table 10. PennDOT layer depths for concrete pavements. Type of Material Loss of Support Factor Open-Graded Subbase (OGS) 1.0 2A Subbase 1.0 Asphalt Base Course/SuperPave Base Course 0.5 Rubblized PCC Base Course 0.5 Cracked and Seated PCC Base Course 0.5 Treated Permeable Base Course (ATPBC or CTPBC) 0.5 Source: PennDOT 242, Section 8.6. Table 11. PennDOT loss of support factors for concrete pavements.

60 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance used and an underdrain system with positive subdrainage is constructed, a drainage coefficient of Cd = 1.1 is used. For rehabilitation projects where past pavement performance has indicated substandard drainage performance of the existing pavement and significant drainage improve- ments are not proposed, a drainage coefficient of Cd = 1.0 is used. For asphalt pavement design, pavement subbases include a dense-graded 2A subbase. Table 12 provides a tabulation of structural coefficients used for base and subbase materials in flexible pavements. Pavement Base Drain Design Process Pavement base drains are built using perforated or porous pipes installed parallel to the high- way and are used to lower groundwater levels, to drain slopes, and to drain the pavement structure. Circular diameters between 4 in. and 8 in. are typically used and are considered ade- quate for normal use. Pipes are placed in trenches and surrounded by coarse aggregate that is both pervious to water and capable of protecting the pipe from infiltration by the surrounding soil. The proper spacing of outlets is important for a functional pavement base drain system, espe- cially on relatively flat grades. The outlet spacing is determined based on the surface infiltration rate, pavement width, pipe type, pipe diameter, and longitudinal pipe gradient. The following example illustrates this process for a two-lane asphalt pavement located in Delaware County (in southeastern Pennsylvania) using the following design parameters: • 2-lane, 7.3-m (24-ft) wide pavement drained on each side; • 1-yr, 1-hr design rainfall; • Smooth-walled PVC pipe, 4-in. diameter; and • Longitudinal pipe gradient of 0.5% (0.005 in./in.). Step 1 – Determine Infiltration Rate The design infiltration rate is determined by first establishing the design storm total and then multiplying it by an adjustment coefficient based on the pavement type (0.5 for asphalt and 0.67 for concrete). Design infiltration rates are based on storm duration, frequency, and location, with the aid of tables and figures provided in Publications 584 and 13M. As shown in Table 13, Map A would be used to determine the region number for a design storm with a 1-year frequency and 1-hr intensity. Figure 52 provides the Map A regional rainfall contours, which indicate that Delaware County (in southeastern Pennsylvania) is located in Rainfall Region 5. Type of Material Structural Coefficient Base Course–New Construction or Reconstruction Crushed Aggregate (CABC) 0.14 Crushed Aggregate, Type DG (CABCDG) 0.18 Subbase–New Construction, Reconstruction, or Existing to Be Overlaid Open-Graded Subbase 0.11 No. 2A Subbase 0.11 Asphalt-Treated Permeable Base Course (ATPBC) 0.20 Cement-Treated Permeable Base Course (CTPBC) 0.20 Rubblized Cement Concrete 0.20 Source: PennDOT 242, Section 9.8. Table 12. Select PennDOT structural layer coefficients for flexible pavements.

Case Examples 61   Duration Frequency 1 year 2 year 5 year 10 year 25 year 50 year 100 year 500 year 5 min C C C C B B B - 10 min C C C C C C C - 15 min A A A A A C C - 30 min A A A A A C C - 60 min A A A A A C C - 2 hr E E E E E E E - 3 hr E E E E E E E - 6 hr D D D D D D D - 12 hr F F F F F F F - 24 hr F F F F F F F F Source: PennDOT 584, Section 7.A.1. Table 13. PennDOT appropriate rainfall region maps. Using tabulated values in Table 14, the 1-yr, 1-hr storm total is determined to be 1.17 in. Multiplying this total by the asphalt pavement adjustment coefficient of 0.5 yields a design total of 0.585 in./hr. Step 2 – Determine Outlet Spacing The various types of base drainpipes can be made hydraulically equivalent by the proper selection of outlet spacing for each type. Circular diameters between 4 in. and 8 in. are typi- cally used and are considered adequate for normal use. The maximum functional outlet spacing for various conditions may be readily determined from the design nomograph shown in Fig- ure 53 using inputs for design infiltration rate, drainage width, pipe gradient, pipe size, and roughness coefficient. Roughness coefficients for the various types of pipes used are provided in Table 15, which indicates that for this case example, a value of 0.10 would be selected for the smooth-walled PVC pipe. As shown in Figure 54, the maximum outlet spacing for this case example is determined by entering the right side of the nomograph with the pipe gradient value of 0.005, and then drawing a line from that pipe gradient through the pipe diameter of 4 in. and n-coefficient of 0.10 until it intercepts Pivot Line (1). Enter the left side of the nomograph with the calculated design infil- tration rate of 0.585 in./hr, and then draw a line through the drainage width of 12 ft to Pivot Line (2). This can be simplified by passing this line through the appropriate number of lanes sloped toward each pavement base drain. Now, connecting the points of intersection on Pivot Lines (1) and (2) with a straight line, the maximum functional distance (L) between outlets can be determined as approximately 1,000 ft. If the resultant maximum outlet spacing for the given set of conditions is too small to be practically applied on a particular project, the pipe diameter and/or pipe gradient may be increased, or a pipe with a lower n-coefficient selected, each of which would result in a lengthened outlet spacing. Permeable Base Course Construction Cement-treated permeable base course (CTPBC) is constructed using an AASHTO No. 57 or No. 67 gradation with a maximum of 2% passing the No. 200 sieve. Type I or II cement may be used at a rate of 200 lbs per cubic yard, with a maximum water–cement ratio of 0.40. Up to 10% of the required cement may be substituted with fly ash at a 1.1:1 substitution ratio. The CTPBC may be placed with an approved asphalt paver or mechanical spreader, equipped with screed, plate vibrator, and fully automated sensors to control profile and transverse grade. The CTPBC is compacted using steel-wheel power rollers with a manufacturer’s certified metal

Source: PennDOT 584, Section 7.A.1. Figure 52. PennDOT Map A rainfall regions.

Case Examples 63   Source: PennDOT 13M, Section 10.3. Figure 53. PennDOT nomograph for determining maximum outlet spacing. Duration Region 5 Rainfall Totals (in.) 1-yr Storm 2-yr Storm 5-yr Storm 10-yr Storm 25-yr Storm 50-yr Storm 100-yr Storm 500-yr Storm 5 min 0.37 0.45 0.52 0.58 0.68 0.75 0.83 - 10 min 0.58 0.69 0.81 0.90 1.04 1.15 1.26 - 15 min 0.71 0.85 1.00 1.11 1.29 1.42 1.56 - 30 min 0.94 1.14 1.37 1.56 1.82 2.04 2.27 - 60 min 1.17 1.42 1.76 2.03 2.39 2.69 3.04 - 2 hr 1.39 1.69 2.12 2.46 2.93 3.34 3.90 - 3 hr 1.53 1.86 2.33 2.71 2.25 3.75 4.34 - 6 hr 1.91 2.31 2.91 3.40 4.12 4.70 5.34 - 12 hr 2.37 2.86 3.56 4.20 5.15 5.96 6.86 - 24 hr 2.83 3.40 4.22 4.95 6.10 7.16 8.43 12.40F Source: PennDOT 584, Section 7.A.1. Table 14. PennDOT Region 5 rainfall totals. Manning’s “n” Values Types of Pipe 0.010 Polyvinyl Chloride (PVC) with Smooth Inner Walls 0.012 Porous Cement Concrete Pipe; Helically Corrugated Circular Metal Pipe (4 in. through 8 in.); Corrugated High-Density Polyethylene (HDPE) with Smooth Inner Walls 0.015 Corrugated High-Density Polyethylene (HDPE) with Corrugated Inner Walls; Helically Corrugated Circular Metal Pipe (250 mm [10 in.]) Source: PennDOT 13M, Section 10.3 Table 15. PennDOT values for Manning’s roughness coefficients.

64 Subsurface Drainage Practices in Pavement Design, Construction, and Maintenance Source: PennDOT 13M, Section 10.3. Figure 54. Determination of maximum outlet spacing for PennDOT case example. weight from 8 to 10 tons. The use of the vibratory paver screed alone to achieve compaction is not allowed. A minimum of one pass with the roller is used to achieve compaction, where one pass is defined as one trip of the roller in one direction over any one spot. Care is taken to ensure that the CTPBC is not compacted to the point that the layer is not free draining or the aggregate is crushed. No more than 1 hour is allowed from the time water is added to the aggregate and cement to the time CTPBC compaction is completed. The contractor may use a retarding admixture to increase the time to 11/2 hours. Construction traffic is not allowed on the CTPBC for at least 3 days after the CTPBC has been placed. Overlying pavement on the CTPBC is not allowed during this time period. After the 3-day period, construction traffic is restricted to trucks and equipment required to place the next layer or any adjacent CTPBC lift or pavement course. Any areas that are damaged or contaminated must be replaced at no cost to PennDOT. Asphalt-treated permeable base course (ATPBC) is constructed using a PG 64S-22 binder and the mix composition indicated in Table 16. The ATPBC may be placed with an approved slip-form paver or mechanical spreader equipped with fully automated sensors to control profile and transverse grade. The mixture is placed in lifts not exceeding 4 in. in thickness after compac- tion. The mixture must be allowed to cool to 100°F before placement of subsequent layers or pavement courses. Rolling is performed as soon as the mat has cooled sufficiently to avoid shoving or lateral movement of the ATPBC. The ATPBC is seated using an 8- to 10-ton steel-wheeled roller or a vibratory roller operated in static mode only and is compacted by applying four roller passes. One roller pass is defined as one trip of the roller in one direction over any one spot. Additional passes are allowed only to eliminate any surface irregularities or creases. The material must not be compacted to the point that it is not free draining or the aggregate is crushed. Traffic is not permitted on the ATPBC except for trucks and equipment required to place the next layer. Areas

Case Examples 65   Sieve Size Percent Passing by Weight 1 ½ in. 100 1 in. 95–100 ½ in. 35–65 No. 4 12–24 No. 16 6–16 No. 200 0–5 PG 64S-22 Asphalt Content 2.0%–3.0% Source: PennDOT408, Section 360.2 (e). Table 16. PennDOT gradation requirements for ATPBC. damaged or contaminated must be replaced as directed and at no cost to PennDOT. If necessary, the ATPBC may be recompacted before the start of subsequent paving. Subsurface Drainage System Testing and Maintenance PennDOT conducts inspections of subsurface drainage components on a 4- to 5-year cycle. Outlet locations are marked by delineator posts to aid in field identification. Visual inspections of the outlets and headwalls are conducted to determine the integrity of these components and to check for current or past water flow from the outlets. Probing of the drainage pipes is also conducted to confirm that no blockage exists. Subsurface drainage component problems noted in field reports include collapsed longitudinal drainage or transverse outlet pipes, buried outlets/headwalls, damaged outlet pipes/headwalls, and missing rodent screens. Summary This chapter presents case examples of current DOT practice for subsurface drainage design in Alabama, Minnesota, Missouri, and Pennsylvania, representing designs appropriate for wet- freeze, seasonally wet-freeze, wet-freeze-thaw, and wet-non-freeze climatic zones. Highlights of the case examples presented include the use of cast-in-place headwalls and water flow testing during construction (Alabama), the commitment to subsurface drainage system maintenance during the life of the pavement (Minnesota), the use of daylighted permeable base layers composed of large-sized, crushed rock materials (Missouri), and the use of statewide maps for infiltration assessment and design nomographs for maximum outlet spacing (Pennsylvania).