Researcher's Guide to The LTPP Layer Thickness Data

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1. Background
2. Purpose and Scope of the Guide
3. Guide Organization

Background

Pavement layer material type and thickness data are very important for many types of analyses, including back calculation of pavement moduli, mechanistic analysis of pavement structures, and performance modeling. The accuracy of layer thickness data has a great impact on the outcome of practically all analyses of pavement performance. As part of the Long Term Pavement Performance (LTPP) program data collection effort, a large amount of data related to layer material type and thickness has been collected from several sources, including the following:

• Inventory and design records.
• Core measurements from materials sampling and testing.
• Field logs of bore holes.
• Shoulder auger probe logs.
• Test pit logs.
• Field elevation measurements before and after layer placement for Specific Pavement Studies (SPS) sections.
• Ground Penetrating Radar (GPR) measurements (planned to be collected).

The pavement layer thickness data from these sources (except GPR) exist in many different LTPP tables. For example, tables TST_AC01, TST_AC01_LAYER, and TST_PC06 contain core measurement data. Tables SPS*_LAYER and SPS*_LAYER_THICKNESS contain field elevation data. (Note: The name SPS*_LAYER used herein refers to the SPS1_LAYER, SPS2_LAYER, SPS5_LAYER, SPS6_LAYER, SPS7_LAYER, SPS8_LAYER, and SPS9_LAYER tables. The name SPS*_LAYER_THICKNESS refers to the SPS1_LAYER_THICKNESS, SPS2_LAYER_THICKNESS, SPS5_LAYER_THICKNESS, SPS6_LAYER_THICKNESS, SPS7_LAYER_THICKNESS, and SPS8_LAYER_THICKNESS tables.) The inventory or planned layer thickness data are stored in other tables (e.g., INV_LAYER and RHB_LAYER). Field identified material type and depths to strata top and strata bottom are stored in table TST_SAMPLE_LOG. The design layer thickness data can be found in the experimental designs for newly constructed SPS sections.

Using the above information, the LTPP Regional Support Contractors (RSC's) complete tables TST_L05, TST_L05A, and TST_L05B. Table TST_L05 stores project-level material type information for SPS experiments with multiple sections constructed at the same site. Table TST_L05A contains measured layer material type and thickness data at the beginning, within, and at the end of a section, based on the core measurements and field test pit information. TST_L05B provides the representative thickness for the section. These representative layer materials and thicknesses at the section level are the best available estimates of the layer thicknesses in the LTPP database.

Purpose and Scope of the Guide

The main purposes of this guide for LTPP thickness data are:

• To explain to the LTPP data users what and where different types of layer material type and thickness data reside in the LTPP database.
• To present LTPP thickness within-section variability.
• To summarize as-designed versus as-constructed layer thickness comparisons.
• To provide guidelines to search for the most appropriate thickness information for different research purposes.

Field sampling, materials testing, and other layer thickness data collection activities utilized by LTPP are discussed briefly, along with the characterization of the within-section thickness variation and designed versus constructed thickness data variations for the LTPP sections.

Guide Organization

The guide consists of six chapters. This chapter provides background information, purpose and scope, and the organization of the guide. Chapter 2 presents LTPP layer material type and thickness data collection and storage in the LTPP database. It also provides the LTPP data users useful information as to what and where different thickness-related data reside in the database, as well as the limitations of the data. Chapter 3 discusses the variations in the layer thickness at different locations within an LTPP section. Typical variations in layer thickness for different layer and material types are discussed. Assessments were also made to whether distributions of these thickness measurements follow a normal distribution. Chapter 4 compares the as-designed layer thicknesses with the as-constructed or measured thicknesses. Statistical evaluation results are presented regarding differences between the designed thickness and the as-constructed thickness. Chapter 5 presents recommendations regarding what data to use under different scenarios. Chapter 6 provides an additional note about the LTPP data release used for this study and other information.

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