Effects of Subsurface Drainage on Pavement Performance: Analysis of the SPS-1 and SPS-2 Field Sections (2007)

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1Background Subsurface drainage systems are commonly believed to be beneficial to the performance of both asphalt concrete (AC) and portland cement concrete (PCC) pavements. Over at least the past 80 years, pavement engineers have observed that excessive water in pavement structures can accelerate rutting, fatigue cracking, and roughness in AC pavements and fault- ing, fatigue cracking, D-cracking, reactive aggregate distress, and roughness in PCC pavements. The use of subsurface drainage systems has been widely advocated as a way to com- bat the detrimental effects of water in pavement structures. Such drainage systems include not just granular bases, but also open-graded granular or treated layers and longitudinal edgedrains and outlets. Guidance on the design and construc- tion of subsurface drainage systems is readily available (1–10). Information that convincingly demonstrates the benefits of subsurface drainage systems on pavement performance and pavement life is, however, less readily available. Elfino et al. point out that some studies have shown better perfor- mance in drained pavements than in undrained pavements, while other studies have found no difference in performance (9). The Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures sums up the situation in this way: The current state of the art is such that conclusive remarks regarding the effectiveness of pavement subsurface drainage or the need for subsurface drainage are not possible. (10) The reasons often mentioned for why subsurface pave- ment drainage systems do not always yield improvements in performance include inadequate design, improper construc- tion, 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 pave- ments do not consistently 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 suf- ficient 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. In recent years, many state highway agencies have become less enthused about subsurface pavement drainage because of concerns about construction difficulties, the need to conduct frequent maintenance of edgedrains and their outlets, and scant evidence of performance benefits that justify the costs of drainage system installation and maintenance. NCHRP 1-34 Drainage Studies Between 1995 and 1998, researchers involved in NCHRP Project 1-34, Performance of Subsurface Pavement Drainage, evaluated the effectiveness of subsurface drainage systems in AC and PCC pavements (11). The findings from that study were based on relatively small sets of paired pavement sections with and without subsurface drainage. Pavement sections from the Long-Term Pavement Performance (LTPP) Studies’ SPS-1 (Strategic Study of Structural Factors for Flexible Pavements) and SPS-2 (Strategic Study of Struc- tural Factors for Rigid Pavements) experiments were not in- cluded in the NCHRP study because they were not of suffi- cient age at the time. In addition, the field data collection effort for Project 1-34 did not include an assessment of which subdrainage systems were functioning and which were not. C H A P T E R 1 Introduction

Under NCHRP Project 1-34B, completed in 1999, the Project 1-34 findings were reviewed, and a research plan was developed for further evaluating the effects of subsurface drainage on AC and PCC pavement performance, using data from the SPS-1 and SPS-2 experiments (12). A preliminary analysis of the data from the SPS-1 and SPS-2 experiments was conducted according to that research plan under NCHRP Project 1-34C, completed in 2003 (13). The SPS-1 experiment consists of AC pavement test sections with three undrained base types (dense-graded aggregate, asphalt-treated base, and asphalt-treated base over dense- graded aggregate) and two drained base types (permeable asphalt-treated base over aggregate and asphalt-treated base over permeable asphalt-treated base). The SPS-2 experiment consists of PCC pavement test sections with two undrained base types (dense-graded aggregate and lean concrete base) and one drained base type (permeable asphalt-treated base). In the SPS-1 and the SPS-2 experiments, the sections with a perme- able asphalt-treated base layer have longitudinal edgedrains and outlets. The main SPS-1 and SPS-2 experiments include no pavement sections with open-graded base layers that are “daylighted”—that is, that drain directly out to the fore slope. During the course of Project 1-34C, FHWA, with NCHRP support, contracted for the video inspection of edgedrains at the SPS-1 and SPS-2 sites, and the results of those inspections were used in Project 1-34C. Project 1-34C demonstrated the feasibility of applying appropriate statistical tests to the SPS-1 and SPS-2 performance data to assess the influence of such factors as the presence of subsurface drainage, despite some limitations in the SPS-1 and SPS-2 experiments that pose obstacles to their analysis. The findings from Project 1-34C were, however, considered preliminary because they were based on data collected only through mid-2001 and because the effects of truck traffic, climate, structural capacity, and subdrainage system functioning were not examined in depth. This project (1-34D) was conducted as a follow-up to Project 1-34C and makes use of • More recent performance data (LTPP data Release 19.0, January 2005), • Analysis of deflection data (to assess the relative structural contributions of different base types), and • Subdrainage system flow time measurements (to assess the functioning of the subsurface drainage systems). Research Objectives The objectives of this project were as follows: 1. Resolve the discrepancies between the as-designed and as- constructed drainage designs of the SPS-1 and SPS-2 test sections. 2. Resolve other discrepancies in the as-designed versus as- constructed conditions, such as whether or not fabric filters were used to separate permeable asphalt-treated base layers from subgrade materials. 3. Develop a method for quantitative testing of the func- tioning of subdrains in SPS-1 and SPS-2 test sections. 4. Conduct testing of subdrainage functioning in SPS-1 and SPS-2 test sections. 5. Use the results of the subdrainage testing together with the video inspection results obtained earlier to achieve a more complete and quantitative assessment of the functioning of subdrains in the test sections. 6. Analyze the deflection data from the SPS-1 and SPS-2 experiments for the purpose of quantifying differences in structural capacity among sections of different base types and thickness designs. 7. Incorporate data from the Minnesota Road Research Project (MnRoad) and Wisconsin Department of Trans- portation (DOT) drainage studies into the analysis. 8. Expand on the performance analysis conducted in NCHRP Project 1-34C, in light of the findings from the subdrainage testing and structural analysis, as well as other information. 9. Based on the results of the testing and analyses conducted, report on the quantifiable effects of subsurface drainage on the performance of AC and PCC pavements. Research Approach Observations from the video inspections of subdrains in the SPS-1 and SPS-2 sections with permeable asphalt-treated bases suggest that some discrepancies exist between the as-designed and as-constructed drainage systems. These discrepancies were reported to the LTPP program in a data analysis/operations feedback report, and the responses from the LTPP regional support centers were included in an earlier report (13). Information in the LTPP database suggests that inconsis- tencies exist among the different SPS-1 and SPS-2 sites with respect to the presence of filter fabric below the permeable asphalt-treated base layers. These inconsistencies were also reported to the LTPP program, and some clarifications were received from the LTPP regional support centers (13). For this study, a method for measuring the time of flow of water through the drained SPS-1 and SPS-2 test sections was developed. Pilot testing was conducted on a drained highway pavement in Wisconsin and at the Arkansas SPS-1 and SPS-2 sites. As a result of the pilot testing, some adjustments were made to the test procedure, and drainage flow time testing was subsequently conducted at all of the remaining SPS-1 and SPS-2 sites, as well as at the MnRoad site. In the SPS-1 and SPS-2 experiments, two of the experi- mental factors—base type and subdrainage—are confounded. 2

This was perhaps unavoidable, but it complicates the analysis of the performance of the SPS-1 and SPS-2 test sections. The confounding of these two experimental factors makes it difficult to ascertain how much any differences in perfor- mance between drained and undrained test sections are due to the presence or absence of a functioning subdrainage system, versus differences in base stiffness. To address this issue, this study included an analysis of the nondestructive deflection test data for every SPS-1 and SPS-2 test section for every date that the sections were tested. The primary goal of the deflection testing was to assess the relative structural contributions of different types of bases. The results of the deflection analyses are presented in this report. The effects of the base type/subdrainage factor on rough- ness, cracking, and rutting in the SPS-1 pavement sections and on roughness, cracking, and faulting in the SPS-2 pave- ment sections were analyzed. Details about the construction, materials testing, and data availability for the SPS-1 and SPS-2 experiments are available in the LTPP database (www.ltpp-products.com) and in the following reports: • Structural Factors for Flexible Pavements—Initial Evaluation of the SPS-1 Experiment (14), • Structural Factors for Jointed Plain Concrete Pavements: SPS-2 —Initial Evaluation and Analysis (15), and • LTPP Data Analysis: Influence of Design and Construction Features on the Response and Performance of New Flexible and Rigid Pavements (16). The first two of these reports also present some prelimi- nary findings concerning the performance of the SPS-1 and SPS-2 test sections, based on information available in Release 10 of the LTPP database (January 2000). The third report presents the findings of performance analyses conducted using information available in Release 17 of the LTPP data- base (January 2004). Organization of this Report The research conducted for this project is described in the following sequence: • Chapter 1—Introduction, • Chapter 2—Description of the SPS-1 and SPS-2 Experi- ments, • Chapter 3—Field Testing of Drainage Systems, • Chapter 4—Deflection Analysis of SPS-1 and SPS-2 Designs, • Chapter 5—Roughness and Distress in SPS-1 Flexible and SPS-2 Rigid Pavements, and • Chapter 6—Conclusions and Recommendations. 3