12-10-2012, 11:54 AM
FLEXIBLE PAVEMENT DESIGN MANUAL
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INTRODUCTION
PURPOSE
The objective of this manual is to provide a Pavement Design Engineer with sufficient information so that the necessary input data can be developed and proper engineering principles applied to design a new flexible pavement, or develop a properly engineered rehabilitation project. This design manual addresses methods to properly develop a rehabilitation project, pavement milling, and the computations necessary for the pavement design process. It is the responsibility of the Pavement Design Engineer to insure that the designs produced conform to Department policies, procedures, standards, guidelines, and good engineering practices.
GENERAL
Chapter 334 of the Florida Statutes, known as the Florida Transportation Code, establishes the responsibilities of the state, counties, and municipalities for the planning and development of the transportation systems serving the people of the State of Florida, with the objective of assuring development of an integrated, balanced statewide system. The Code's purpose is to protect the safety and general welfare of the people of the State and to preserve and improve all transportation facilities in Florida. Under Section 334.048(3) Code sets forth the powers and duties of the Department of Transportation to develop and adopt uniform minimum standards and criteria for the design, construction, maintenance, and operation of public roads. The standards in this manual represent minimum requirements, which must be met for flexible pavement design for new construction and pavement rehabilitation of Florida Department of Transportation projects. Any variances should be documented in project files.
FLORIDA CONDITIONS
A number of coefficients and variables are specified in this manual. They should be considered as standard values for typical Florida projects. There may be instances where a variance from the values would be appropriate. In these instances, the Pavement Design Engineer will stay within the bounds established by the basic AASHTO Design Guide, justify the variance, and document the actions in the Pavement Design File.
PAVEMENT SYSTEM
The following define the general pavement layers in a flexible pavement system. Some of the most important layers are shown in Figure 2.1. The definitions are presented "top-down" through the pavement structure with the stronger layers on top of the weaker layers. The concept of stronger layers on top of weaker layers, as load stresses are spread out and down through the pavement, is further supported by the horizontal extension of weaker layers beyond stronger layers in a pyramidal effect (See Figure 2.1). Standard department practice is to extend the base 4"beyond the edge of the structural course. This is very important when dealing with granular materials. Without this support, vehicle loads would cause failure along the pavement edge. The pavement structure or system as it is sometimes referred to, is the pavement layers designed to support traffic loads and distribute them to the roadbed soil or select embankment material.
Reliability (%R)
The use of Reliability (%R) permits the Pavement Design Engineer to tailor the design to more closely match the needs of the project. It is the probability of achieving the design life that the Department desires for that facility. The Pavement Design Engineer is cautioned, however, that a high reliability value may increase the asphalt thickness substantially. The models are based on serviceability and not a specific failure mechanism, such as rutting. Recommended values range from 75% to 99% and can be found in Table 5.2. It is important to note that this is not a direct input into the AASHTO Design Equation. The use of a converted value known as the Standard Normal Deviate (ZR) is input into the equation. The reliability value replaces the safety factor that was previously imbedded in the Soil Support Value.