PROFESSIONAL DEVELOPMENT ENGINEERING A PERMANENT ROAD FOUNDATION SERIES BY DAVID B. ANDREWS, P.E., AND MARK MARIENFELD, P.E. AIA Provider No.

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1 SPONSORED BY ENGINEERING A PERMANENT ROAD FOUNDATION BY DAVID B. ANDREWS, P.E., AND MARK MARIENFELD, P.E. PROFESSIONAL DEVELOPMENT SERIES AIA Provider No

2 Sponsored By: ENGINEERING A PERMANENT ROAD FOUNDATION PDH2 CONTINUING EDUCATION The Professional is a unique opportunity to earn continuing education credit by reading specially focused, sponsored articles in Informed Infrastructure. This article also is available online at INSTRUCTIONS This Professional Development Section provides an opportunity to earn continuing-education credit without cost. Take a look at the learning objectives, read this sponsored article, and complete the quiz at www. v1-education.com. Answers to the quiz will automatically be calculated. If eight or more of the questions are answered correctly, participants can immediately download a certificate of completion and be awarded 1.0 professional-development hours (equivalent to 1.0 continuing-education units in most states). V1 Media is an approved provider of the American Institute of Architects Continuing Education System (AIA/ CES). It is up to each licensee to determine if this credit meets their governing board s registration requirements. LEARNING OBJECTIVES At the conclusion of this article, the reader should be able to understand: Why roads fail prematurely and how to assess the damage of unbound aggregate layers within their jurisdiction. The detrimental effects of road base layers becoming contaminated with subgrade soils. How to create a cost-effective Permanent Road Foundation (PRF) using a separation/stabilization geotextile. Why a PRF is the most durable and cost-effective road support layer compared to alternative strategies. How critical friction, durability and hydraulic properties uniquely position nonwoven geotextiles as the preferred geosynthetic to enable a PRF. How the preservation of unbound aggregate layers can dramatically lower the Life Cycle Cost (LCC) of transportation infrastructure. Unbound aggregate materials are the largest transportation infrastructure asset for most state and local Departments of Transportation (DOTs). This asset is used in road bases and subbases as well as for surfacing of unpaved roads. Unbound aggregate layers also are the most vulnerable transportation infrastructure, as these layers are subject to premature failure due to deterioration, contamination and adverse hydraulic conditions. These out of sight, out of mind, underappreciated and often under-engineered layers are the subject of this PDH article. The loss of support of unbound aggregate results in full-depth reconstruction, a total loss of the aggregate assets, and road downtime with traffic congestion and construction-related traffic accidents. Readers will be shown improved best-management design practices and how to preserve unbound aggregate assets to enhance the performance of these layers. Use of the correct aggregate and a separation/stabilization geotextile can construct a Permanent Road Foundation (PRF) so full-depth reclamation and total reconstruction may be avoided. The quality of aggregate enabled by the selection of the proper geotextile will maximize the cost effectiveness of these now-permanent road layers. Demonstration is provided for how the structural and hydraulic benefits of nonwoven geotextiles make them the most cost-effective geosynthetic to be used in the establishment of a PRF. Comparison will be made between unbound aggregate PRF and other types of road support layers. The PRF will significantly lower the Life Cycle Cost (LCC) of the road. Overall Life Cycle Assessment (LCA) considerations also will be discussed. Unbound Aggregate Infrastructure Asset In most areas, unbound aggregate is the most costeffective means of achieving the structural support required to either directly support traffic in an unpaved road or to support the paved layers in pavements. Local aggregates are not always ideal, so DOTs commonly import higher-quality aggregates when the greater performance and durability justifies the cost. Higherquality aggregates exhibit the properties of hardness, durability, angularity, and a structurally efficient and permeable grain-size distribution. Although surficial pavement layers deteriorate under traffic and by being exposed to the elements, unbound aggregate base support layers can and should survive several cycles of pavement rehabilitation. Some local aggregate sources can deteriorate by abrasion and

3 crushing under traffic, but many areas have the option to use a competent aggregate such as crushed granite or quartzite. Unbound base aggregate costs range from about $1 to $2 per inch thickness per square yard. This cost generally is the most cost-efficient pavement support material. Cement-, lime- or asphalt-treated soil or aggregate are other ways to attain the necessary structural support. These chemically treated materials have inherent weaknesses, such as much higher cost, pavement cracking due to shrinkage, and chemical or hydraulic breakdown over time. Therefore, unbound aggregate is the most widely used transportation asset, as it is used beneath unpaved roads as well as beneath asphalt concrete and Portland cement concrete pavements. Figure 1. Subgrade Pumping Root Cause for Premature Road Failures Excuses often are made for the failure of a pavement to reach its design life, including excess traffic loading, climatic effects and poor pavement material performance. If a detailed forensic analysis is conducted of why the road prematurely failed in local spots or throughout, deterioration often is found within the unbound aggregate support layers. Unbound aggregate base layers progressively deteriorate, most often due to increasing contamination by subgrade soil migrating upward. Even with high California Bearing Ratio (CBR) soils up to CBR 8, hydraulic pumping of the subgrade fines will drive soil fines upward, contaminating the subbase and base course aggregate layers. As little as 15 percent total fines content in an aggregate base layer can cut its bearing capacity by 50 percent (Jorenby and Hicks), and only 8 percent fines can clog the permeability of the aggregate layer leading to base saturation and total loss of strength due to pore pressure buildup (Cedergren). A common misconception is to think base contamination can be prevented by using a tight, wellgraded aggregate base; these bases still are progressively contaminated, especially in the presence of moisture (Al Qadi), since such a gradation is poorly draining. Capillary effects and the absence of a driving head of water often result in dense-graded base acting like a sponge at low hydraulic gradients. This results in trapped water in the pavement section and very poor drainage (FHWA Geotechnical Aspects of Pavements Reference Manual). The strength of these tight bases also must be discounted due to their inability to rapidly drain. By AASHTO 1993 Pavement Design, an aggregate base with good drainage properties can provide up to double the strength per inch thickness as a poorly draining aggregate base. Effects of moisture, freeze/thaw and base-course contamination degrade the stiffness and strength of the base layer. Premature pavement failure can occur from excess subgrade rutting and pumping, aggregate contamination or degradation, loss of fines, poor drainage, frost action and swelling soils (FHWA Geotechnical Aspects of Pavements Reference Manual). Forensic evaluations of prematurely failed pavements are best practices but are not usually performed. The thickness of the remaining clean, unbound aggregate support layer should be measured and compared to the as-built structure. As this support layer has diminished, pavements often require more pavement thickness to make up for this loss of structural support, sometimes to the point of becoming a full-depth pavement, as shown in Figure 2. It s easy to see the subgrade contamination of surficial aggregate placed on an unpaved road. The intermixing of the aggregate and subgrade leads to regular maintenance and frequent new stone replacement. This same clean aggregate is being lost under pavements; it s just not as noticeable. Establishment of a Permanent Road Foundation (PRF) Using the proper geosynthetic and a competent base aggregate, engineers can create a PRF minimizing the possibility for future full-depth reclamation/reconstruction. PRF preserves the road foundation so maintenance activities on the pavement may focus on surficial repairs. Without needing reconstruction, new asset expenditures as well as road downtime with traffic congestion and construction-related accidents and deaths are minimized. Separation of the base course layer and subgrade is the key to pavement longevity using any design methodology, including AASHTO s 1993 Guide for Design of Pavement Structures and AASHTOWare Pavement ME. Polypropylene geotextiles have been placed at the subgrade/ aggregate base interface for more than 50 years in pavement applications. Exhumed geotextiles up to 40 years old have been tested, showing their nearly original properties, effectiveness to separate dissimilar soils, and an ability to provide filtration of water while retaining soil. Figure 2. Total Loss of Aggregate Base Due to Contamination PDH3

4 When selecting the proper geosynthetic to enable a permanent road foundation, six factors are critical to maximizing service life and performance of road base layers: 1) strength, 2) filtration, 3) construction survivability, 4) water flow through the geosynthetic, 5) transmissivity within the plane of the geosynthetic, and 6) efficient frictional interaction. First, a geotextile must be used, because a geogrid offers no effective separation. If a geogrid is used, a nonwoven geotextile must be placed below it for separation. Measured by the functions above, needle-punched nonwoven geotextiles are the best-performing, most cost-effective geotextile for separation/stabilization. The geotextile may be justified as a design safety factor, ensuring that the original design thickness of the pavement support layers is maintained throughout the life of the pavement. As an inexpensive safety factor, the nonwoven geotextile can compensate for an area of weak subgrade soil or an area of the road that experiences excess moisture in service. Geotextiles offer a much more cost-effective solution than the traditional use of additional subbase or sacrificial stone as a safety factor. A geotextile also enables the use of a more free-draining and structurally efficient aggregate base, without the fear of subgrade intrusion/ contamination. Design Considerations of a PRF The AASHTO-recommended nonwoven geotextile, installed, typically costs less than 1 inch of aggregate base and offers benefits equal to several inches of additional base aggregate thickness. The effective separation function of nonwoven geotextiles at the subgrade soil/aggregate base interface prevents the typical loss of 4 inches or more of aggregate base due to subgrade soil upward intrusion and contamination (Jorenby and Hicks). The stabilization function of these geotextiles has been widely proven to increase the bearing capacity of the subgrade soil by 80 percent and to confine and strengthen the aggregate base layer. These benefits provide superior capital and lifecycle cost savings. The candidate geosynthetic should be assessed for hydraulic and filtration compatibility, construction and long-term durability, frictional interaction, long-term separation, and cost. Commonly, claims are made about highstrength and/or high-modulus products being preferred. This is incorrect, because in more than 90 percent of road projects, the geosynthetic isn t strained enough to exercise significant membrane strength of the geosynthetic. For example, to achieve 5 percent strain on the geosynthetic, there needs to be 1.6 feet of rutting on a 10-foot-wide road, which is unlikely in a designed road section. Geotextile Selection Considerations Strength, Filtration, Soil Retention and Construction Survivability AASHTO M288 Standard for Geosynthetic Specifications for Highway Applications defines filtration and soil-retention properties for geotextile separation/ stabilization applications. Nonwoven geotextiles have the highest permittivity and water flow rate, up to 25 times the flow rate of woven geotextiles. Nonwoven geotextiles also have a much higher porosity, allowing high water flow while providing the smallest opening sizes to retain subgrade soil particles. M288 recognizes the superior durability of nonwoven geotextiles by requiring much higher strength in woven geotextiles to perform on par with lower-strength nonwoven geotextiles. Needle-punched nonwoven geotextiles are strong, yet they have the ability to locally elongate, when necessary, to avoid puncture and tearing. The local elongation of nonwoven geotextiles also helps them conform to the bottom of the aggregate layer as the bottom stones seat into the subgrade soil for excellent aggregate lock-up and confinement. Class1 Class 2 Class 3 Elongation Elongation Elongation Woven Nonwoven Woven Nonwoven Woven Nonwoven < 50% 50% < 50% 50% < 50% 50% 315 lbs 203 lbs 248 lbs 158 lbs 180 lbs 113 lbs 55% more strength required -- 57% more strength required Figure 3. Rock Interlock on Subgrade Excellent Aggregate Confinement with Nonwoven Geotextile Table 1. AASHTO M Geotextile Grab Strength (ASTM D4632) Requirements -- 59% more strength required -- PDH4

5 Table 2. Comparison of AASHTO M288 Geotextile Water Flow Rates Geosynthetic Structure Nonwoven Woven Units M288 Class A Permittivity Sec -1 Water Flow Rate gpm/sf Nominal Weight oz/sy This makes them more appropriate for placement over severe subgrade conditions, with little surface preparation. Woven geotextiles and geogrids have fixed, low-elongation strands, which are susceptible to rupture and zipper-type tearing when placed over sharp objects or under excess traffic-induced strain. Nonwoven geotextiles also are much more resistant to abrasion. For durability, railroads use a heavy nonwoven geotextile beneath new railroad ballast stone, which is usually placed over some existing stone. Nonwoven geotextiles also are recommended beneath large riprap stone due to their resistance to damage. Toughness measures both the strength and the ability to strain without rupture. Frictional Interaction for Stabilization The most important structural benefit translated from a geosynthetic to an unbound aggregate base is its frictional confinement of the aggregate. Membrane strength and reinforcement by a geotextile are not critical performance properties except when used over extremely weak subgrade soil (<1 CBR), where the higher modulus may aid in constructability. The interface friction properties of geogrids as well as woven and nonwoven geotextiles were tested in real road simulation direct-shear testing by ASTM D As Figure 4 demonstrates, both the geogrids and the nonwoven geotextile have frictional properties in line with the internal friction angle of typical base course aggregates. This means you re not introducing a slip plane into the road support system, and the aggregate is confined by the Figure 4. Geosynthetic Interface Friction Relative to Road Base Aggregate geosynthetics. Even coarse-surface texture-woven geotextiles achieve only about 70 percent interface efficiency, not effectively translating their strength to the system and not resisting lateral displacement of the aggregate, creating a weak-link slip plane. An important component of the nonwoven geotextile interface friction is how, when compacted, the bottom aggregate seats into the geotextile and the subgrade soil as an interlocking surface formed to the bottom of the aggregate layer. When the layered system is damp or wet, the nonwoven geotextile frictional advantage is increased. The application of a separation/stabilization geotextile with appropriate frictional properties beneath an unpaved road helps hold the unbound surface aggregate in place. Unpaved roads have higher shear stresses within the aggregate, since the stone layer isn t confined by a pavement surface. It s important that the geosynthetic lock into the base course and subgrade, and not create a failure shear plane. Confined, In-Service Advantages of Nonwoven Geotextiles Laboratory, open-air, index-strength testing of nonwoven geotextiles grossly underestimates their confined, in-service strength. Because of the way the nonwoven geotextiles are constructed, any confinement resists their tensile failure mode, which is the unraveling of the fibers rather than the breaking of the fibers. When confined, simulating in-service conditions, needle-punched nonwoven geotextiles exhibit a dramatic increase in their tensile strength, with lower strain and a higher modulus. Increases in stiffness of nonwoven geotextiles under soil confinement have been reported by McGown, Holtz, et al. (1982); Palmeira et al. (1996); and Yuan et al. (1998), showing that the stiffness of a needle-punched nonwoven polypropylene geotextile increases up to 300 percent under a confining pressure simulating in-service loading. Woven geotextiles and geogrids exhibit the same tensile strength properties whether they are confined in service or tested open in the laboratory. Pore Water Pressure Relief of Base Course Aggregate Nonwoven geotextiles have high porosity and the ability to transmit high volumes of water laterally within the plane of the geotextile. This lateral flow is proportional to the geotextile thickness and normal load on the geotextile, but even at high normal loads, like 200 kpa (29 psi), the confined nonwoven geotextile retains significant lateral PDH5

6 Figure 5. Geotextile Horizontal Wicking (Distance vs. Time) Based on ASTM C155 water transmission capabilities. This important lateral drainage function, not possible within a woven geotextile or a geogrid, is easily employed beneath pavement structures when the nonwoven geotextile is allowed an exit point for water to drain. Daylighting the nonwoven geotextile at the shoulder of the road, tying it into an edge drain or tying it to a drainable layer in the shoulder are three methods to promote lateral drainage. This ability to rapidly evacuate water can relieve pore pressure that would otherwise build up in subgrade or aggregate bases under truck traffic loading. Damaging pore pressure can override the bearing capacity of these support layers, leading to premature pavement failure. A nonwoven geotextile with weight of 8-16 ounces/square yard should be used to prevent pore water pressure build up and to laterally transmit water out of the pavement section beneath poorly draining aggregate layers, especially in non-arid climates. Added moisture in unbound aggregate base and subbase is anticipated to result in a loss of stiffness on the order of 50 percent or more Saturated fine-grain roadbed soil could experience modulus reductions of more than 50 percent (FHWA Geotechnical Aspects of Pavements Reference Manual). Road base saturation even 10 percent of the time can reduce pavement life by 50 percent (Cedergren, 1987). In interlayer trials at the University of Minnesota, the resiliency of a nonwoven geotextile with 14 ounces/ square yard weight was found to perform like a pump under repeated truck traffic loading, rapidly evacuating water from the pavement structure (Lederle, Hoegh, Burnham, Khazanovich, TRB 2013). Ground Water Drainage or Capping Due to Capillary Rise Along with the ability to laterally transmit water driven by gravity or pore pressure, nonwoven geotextiles have the ability to wick water and passively transmit water away from transportation structures. The presence of water can weaken the structure, lubricate failure planes and cause Figure 6. Function of a Capillary Break problems such as frost heave in cold climates. Woven geotextiles with special wicking yarns placed in the transverse roll direction can wick minor amounts of water, but nonwoven geotextiles have the ability to wick more water to greater distances to better evacuate damaging pore water. Heavyweight nonwoven geotextiles can provide a capillary break to stop or redirect groundwater, which can rise in fine-grained soils through the process of soil suction. If this rising pore water moves into the shallow-earth frost zones in cold climates, then damaging ice lenses and frost heave of roads can develop. Geotextile Cost Considerations Woven slit-film geotextiles (designated as <50 percent elongation) were originally derived from carpet backing and are currently allowed by AASHTO M288 for separation/stabilization applications but are not permitted for drainage applications. Woven slit-film geotextiles impose a barrier to drainage and have few larger openings, which can expand under strain to allow piping of fines. These geotextiles may introduce a low-friction slip plane. Higher-quality woven geotextiles such as AASHTO M288 Class 1A geotextiles are permissible when constructed with monofilament, multifilament or fibrillated yarns as well as when enhanced permeability and surface friction properties have been documented via testing. These specialized woven geotextiles can cost up to five times as much as a nonwoven geotextile that will perform as well. Geogrid solutions are not recommended because they should always be accompanied by a nonwoven geotextile, and the combination is an expensive way to do the same job the nonwoven alone will accomplish. Therefore, nonwoven geotextiles are the most cost-effective geosynthetic. For most separation/stabilization applications, an AASHTO M288 Class 1 nonwoven geotextile (>50 percent elongation) is a best practice, with the option of increasing to a 16 ounces/square yard nonwoven geotextile to accommodate the hydraulic conditions found at some sites. Lifecycle Cost Improvement Using PRF DOTs must carefully manage their infrastructure assets to maximize their service life and minimize their maintenance and reconstruction costs. One of the largest transportation assets for DOTs is their unbound aggregate base layers. Creating a PRF using a separation/stabilization nonwoven geotextile and a competent aggregate will preserve the integrity of this asset and reduce future full-depth reclamation/reconstruction. Avoiding the reconstruction balloon payment will dramatically lower the lifecycle cost of a road as well as prevent road downtime and related congestion and construction-related traffic deaths. PDH6

7 Figure 7. Traditional Cost of Road in Cumulative Dollars Figure 7 depicts a traditional road expenditure cycle in cumulative dollars over the lifecycle of a road. Figure 8 shows a typical road expenditure cycle with a nonwoven geotextile used to create a PRF. Total road reconstruction is avoided with PRF, decreasing the lifecycle cost of the road. Lifecycle Assessment Lifecycle assessment has become an extremely important factor in road design. The use of a nonwoven separation/stabilization geotextile avoids many of the environmental impacts consistent with traditional road construction and reconstruction, including the following: Conservation of aggregate resources. One truckload of geotextiles is equivalent to 850 truckloads of stone. Less aggregate is required, reducing the number of overall trucks needed during construction and lowering the project s carbon footprint. Less over-excavation is required for less construction time and emissions. Minimal road downtime for repair or reconstruction, which results in less traffic congestion and emissions. Less unsustainable waste material to be removed during reconstruction activities. Less dust generated on unpaved roads underlain with a nonwoven geotextile (Bowders). Summary There s no downside to placing an inexpensive nonwoven geotextile under both paved and unpaved roads, but there are tremendous benefits. Agencies can no longer afford to build or maintain roads built to out-ofdate standards, so designers need to be proactive in using a nonwoven geotextile in the creation of a Permanent Road Foundation (PRF). Transportation infrastructure asset management must consider this technology when assessing this primary asset: unbound aggregate layers. As a smart strategy, local agencies with the ability to set building codes for residential construction are Figure 8. Typical Road Expenditure Cycle with Nonwoven Geotextile Used to Create a Permanent Road Foundation mandating the use of a nonwoven separation/stabilization geotextile by subdivision developers, so when the municipality inherits the road, there will be less maintenance costs. This also helps eliminate the cost of a future total reconstruction. The authors highly recommend agencies do their own investigations and perform forensic evaluation of aggregate support layers when there are pavement failures. Forensic evaluations often are done for bridge and retaining-wall failures, which have neither the financial nor safety impact of road-foundation failures. References Al-Qadi, I., Brandon, T.I., and Bhutta, S.A Geosynthetic stabilized flexible pavements, Proceedings of Geosynthetics 97, Long Beach, CA, pp Cedegren, H.R Drainage of Highway and Airfield Pavements, Robert E. Krieger Publishing Company Inc., pp Christopher, B.R., Schwartz, C., and Boudreau, R Geotechnical aspects of pavements, Reference Manual of the United States Federal Highway Administration, U.S. Department of Transportation Publication No. FHWA NHI Freeman, E., and Bowders, J.J Geotextile Separators for Dust Suppression in Unbound (Gravel) Roads, Transportation Research Journal: Journal of the Transportation Research Board, No. 1989, Vol. 2. Washington, D.C. Jorenby, N.B., and Hicks, R.G Base Course Contamination Limits, Transportation Research Record Lederle, R., Hoegh, K., Burnham, T. and Khazanovich, L Drainage Capabilities of a Nonwoven Fabric Interlayer in an Unbonded Concrete Overlay, Transportation Research Board 92nd Annual Meeting Compendium of Papers, Washington, D.C. McGown, Holtz, et al Palmeira et al Yuan et al PDH7

8 ENGINEERING A PERMANENT ROAD FOUNDATION SPONSORED BY: Quiz Questions This quiz can be completed online at Your score will be tabulated while you wait, and you will receive your certificate upon completion if you correctly answer eight or more questions. Registration on v1-education.com is required to access the quiz. Use the Sign Up link in the top right of v1-education.com to register. If you are already registered, simply enter your credentials to access the quiz. 1) The stiffness and strength of the base layer can be degraded due to the following: A. Moisture B. Freeze thaw C. Base course contamination D. All of the above 2) Nonwoven geotextiles have up to times the flow rate of woven geotextiles. A. 10 B. 15 C. 20 D. 25 3) Capillary effects and the absence of a driving head of water often result in dense-graded base acting like a sponge at low hydraulic gradients. This results in trapped water in the pavement section and very poor drainage. A. True B. False 4) Lateral water flow is proportional to the geosynthetic thickness and normal load on the geosynthetic. At high normal loads, such as 200 kpa (29psi), what geosynthetic retains the ability to have significant lateral water transmission capabilities? A. Geogrid B. Woven geotextile C. Nonwoven geotextile D. All of the above 5) Stiffness of a needle-punched nonwoven polypropylene geotextile increases up to percent under a confining pressure simulating in-service loading. A. 50 B. 100 C. 200 D ) Which type of geosynthetic can provide a capillary break to stop or redirect groundwater that can rise in finegrained soils through the process of soil suction? A. Geogrid B. AASHTO M288 woven geotextile C. Heavyweight nonwoven geotextile D. Lightweight nonwoven geotextile 7) AASHTO M288 recognizes the superior durability of nonwoven geotextiles by requiring much higher strength in woven geotextiles (>50% stronger) to perform on par with lower-strength nonwoven geotextiles. A. True B. False 8) As little as % fines contamination in an aggregate base layer can reduce its bearing capacity by 50%. A. 5 B. 10 C. 15 D. 20 9) The interface friction between a geosynthetic and soil does not need to be as high as the interface friction angle of the soil alone. A. True B. False 10) The effective separation function of nonwoven geotextiles at the subgrade soil/aggregate base interface prevents the typical loss of 4 inches or more of aggregate base due to subgrade soil upward intrusion and contamination. A. True B. False To complete the quiz and earn your PDH credit: PDH8

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