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CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH INTRODUCTION The primary objective of this research project was to identify improved methods for draining rainwater from the surface of multi-lane pavements and to develop guidelines for their implementation. Improved methods for drawing water from the surface of multi-lane pavements are needed because of the import role that drainage plays ~ the mitigation of hydroplaning and splash and spray. The tendency for hydroplaning and splash and spray depends on the thickness of Me film of water on the pavement. Therefore, ~ order to quantitatively evaluate the effectiveness of the different methods, a mode! for predicting the depth of flow, or water film thickness, resulting from rainfall on multi-lane pavements was developed and Incorporated Into computer-based design guidelines. In the process of completing this study, a number of specific tasks were addressed. These included: . . A literature review to establish the state of practice regarding analytical models for predicting rainfall water depths and to establish current design practices for removing rainfall runoff from multilane pavements. A review of current design methods and analytical procedures for estimating sheet flow across highway pavements. 1
. . . . The development of improved models that describe the water film thickness resulting from sheet flow on Impervious and pervious pavement surfaces. Laboratory rainfall runoff data for determining the roughness coefficient (Mann~ng's n) for pavement surfaces for which data was not available ~ ache literature. Skid resistance measurements to supplement current hydroplaning data in the literature and to better quantify the onset of hydroplaning as a function of the depth of the water film flowing over the pavement surface. Desian procedures and criteria that can be used bv user agencies In the selection of the most cost-effective means for controlling surface drainage and incorporate the procedures and criteria into a computer program. RESEARCH APPROACH The need for improved drainage is prompted by the hydroplaning and excessive splash and spray that can result when thick films of water develop on the pavement surface. The tendency for hydroplaning and splash and spray are micronized if the thickness of the water film on the pavement surface is minimized. Therefore, the mam focus of this study concentrated on methods for predicting and controlling the flow and flow path length of rain water flowing across the pavement surface. 2
Water FiEn Thickness Figure ~ provides a definition of Me water film thickness as it flows across the pavement surface. The thickness of the water film that contributes to hydroplaning is the mean texture depth (MTD) plus the thickness of the water film above the tops of the surface asperities. The MTD depends on the macrotexture of the pavement surface. The macrotexture is the texture or roughness of the pavement surface that is caused primarily by the coarse aggregate. Techniques for measuring the macrotexture are described later in this report. The water below the MTD is trapped in the surface and does not contribute to the drainage of the pavement. Drainage or flow occurs in the total flow layer, y, which is the water film thickness (WFT) plus the mean texture depth. Increasing the macrotexture or depth is important because it allows a reservoir for water (depth below the MTD) and enhances drainage (depth above the MTD). The flow path for a particle of water falling on a pavement surface is simply defined as the line determined by the slope along the pavement surface. Thus, the maximum flow path for a pavement section is the longest flow path for the section-the maximum distance that a rainfall droplet can flow between the point of contact with the water film and its point of exit from the pavement, as presented in figure 2. For a given quantity of rainfall per unit area of pavement, reducing the flow path will result in a more shallow depth of flow and a concomitant reduction in the propensity for hydroplaning or excessive splash and spray. 3
W$: in '\W 1 1 \ ~ Ago: ._ In 1 a)<,,l W~ 3 o en - o Ct - - x - Ct #` U' U] · _ I_ - sit _, Ct Cot o \ - gO ~ _ s /> V' ~ /' by/ my\ '} ~ a) In - ~Q it: o ·_ - ._ t;; a . - US ·_ Figure 1. Definition of water film thickness, mean texture depth, and total flow. 4
50 45 40 a' 30 ce Q i_ o - ~ 20 a) 25 15 10 5 O Depth of | 35 - Water Film / Thickness ~ (Wow 1 ~ al IT Mean Texture depth, 1.27 mm ~ , ~ . it_ _ 0 1 0 20 30 40 Water him thickness, mm 50 45 40 0 35 a' 30 w ~0 2~; - =' 20 15 10 / |Design plane | 1~ . =. 0 1 2 3 4 Water exits design plane '~low 1 Rainfall drop enters pavement surface at comer Distance across plane, m Note: The figures above show a plan view of a design plane (nght) and a profile of the water film thickness (WFT) as water flows along flow path (left). Water falling on the pavement first fills the macrotexture flower left, here 1.27 mm deep) at which point it reaches tops of the asperities of the coarse aggregate particles. At this point, the depth of the water film increases (from zero) until the water exits either an edge of the pavement plane (case here) or a drainage appurtenance. A plane is defined as a section of pavement that has the same geometric charactenstics. In the drainage mode! used in this study, the drainage across the pavement is modeled by linking adjacent design planes. Figure 2. Definition of flow path and design plane.
In order to develop quantitative guidelines for increasing pavement surface drainage, it was necessary to develop models that can predict the thickness of the water film flowing over the pavement surface. This type of flow is called sheet flow. The aforementioned models, which are an essential part of the guidelines, depend on values of Manning's n (hydraulic roughness coefficient) for the pavement surface. This fact necessitated the measurements of Manning's n for some selected surfaces for which data was not available In the literature. Methods for Reducing Water Film Thickness There are five techniques that can be used to reduce water film thickness: alteration of surface geometry, installation of drainage appurtenances, use of permeable or porous asphalt paving mixtures, grooving (portland cement concrete), and enhancement of surface texture through mixture selection and design. Surface geometry factors, such as cross-slope and superelevation, have traditionally been employed to remove water from the pavement surface. However, pavement geometry must be designed in accordance with American Association of Highway and Transportation Officials (AASHTO) design guidelines (1), limiting the degree to which surface geometry can be used to ~ninimize water film thickness. Therefore, other approaches, in addition to the modification of surface geometry, are needed. Appurtenances, such as grate inlets and slotted drains, are a means for removing surface water from the pavement. Permeable asphalt concrete pavements, such as open-graded friction courses (OGAFC) used in the United States and porous asphalt as used in many parts of Europe, are another means for reducing the flow of surface water across the pavement. 6
These surfaces also provide a means for draining water from beneath the tire, thereby reducing hydroplaning potential. Finally, texture modification, as typified by the recent developments in the texturing of concrete pavements, and the grooving of asphalt and Portland cement concrete (PCC) pavements also provide a means for reducing water film thickness. Research Program The research program that was followed during this study was designed to provide the additional data needed to implement the methods that were identified for reducing water film thickness, to provide models that predict the depth of sheet flow when these techniques are used, and to provide guidelines so that the design engineer can facilitate their implementation. An overview of the research program is illustrated in figure 3. The primary focus of the research was placed on identifying the most promising techniques for predicting and controlling water film thickness as a means for minimizing the potential for hydroplaning. Limited attention was given during the research project to the mitigation of splash and spray. Research Products The two major products of this research were (~) a set of guidelines (2) Mat can be used by highway design engineers to consider alternate methods for improved surface drainage and (2) an interactive computer program (PAVDRN) for predicting the depth of sheet flow on pavement surfaces. In order to develop the guidelines, it was necessary to develop a hydraulic model for predicting the depth of the water flow on the pavement surface. This model is the
Task 1. Literature Review 1 Task 2. State of Practice 1 ' ~ Task 4. Interim Report Task 5. Evaluation and Model Refinement I I Task 6. Development of Guidelines ~ 1 Environmental Factors Pavement Geometry Appurtenances Material Properties Models: - Sheet flow - Skid resistance - Hydroplaning Task 3. Model Analysis } 1 1 Figure 3. Research tasks. 8 Task 7. Final Report
basis of an interactive computer program that can be used by a design engineer in the process of designing a highway pavement section. The model, which predicts the depth of sheet flow resulting from rainfall, is based on pavement geometry, pavement surface type and texture, the presence of appurtenances, and rainfall rate. By selecting maximum allowable water film thicknesses that can be allowed without the onset of hydroplaning, the design engineer can use the proposed design guidelines to select and specify the pavement geometry, surface characteristics and mixture type, and appurtenance design required to satisfy the hydroplaning criteria. The design guidelines and associated computer program allow the design engineer to select pavement geometries that minimize sheet flow; to select and locate drainage appurtenances; and to select various mixture types and surface textures that will also minimize water film thickness and the potential for hydroplaning. In order to develop the computer model and the design guidelines, it was necessary to conduct permeability studies on various open-graded or porous asphalt mixtures, to establish Manning's n for selected PCC and asphalt concrete surfaces, and to conduct full-scale skid testing on open-graded pavement surfaces. Findings from the literature review and based on assessment of the current state-of-the- art based are summarized in Chapter 2 along with an overview of the proposal methods for controlling surface drainage. The rationale for choosing the models that were used in developing the guidelines is provided in Chapter 3. The results of testing performed during this study are presented in Chapter 4, and a set of recommendations and conclusions are given in Chapter 5. An overview of the interactive computer or program, PAVDRN, which was 9
developed as part of this study, is given in Appendix A. Other supporting documentation is given in Appendices B through D. The "Proposal Design Guidelines for Improving Pavement Surface Drainage" and the PAVDRN program are available from NCHRP via the ~nternet and present detailed descriptions of several of the experiments to determine Man~g's n. 10