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This report documents the research that incorporated reliability analysis into the existing mechanistic-empirical (M-E) flexible pavement design method for Minnesota. Reliability in pavement design increases the probability that a pavement structure will perform as intended for the duration of its design life. The report includes a comprehensive literature review of the state-of-the-art research. The Minnesota Road Research Project (Mn/ROAD) served as the primary source of data, in addition to the literature review. This research quantified the variability of each pavement design input and developed a rational method of incorporating reliability analysis into the M-E procedure through Monte Carlo simulation. Researchers adapted the existing computer program, ROADENT, to allow the designer to perform reliability analysis for fatigue and rutting. A sensitivity analysis, using ROADENT, identified the input parameters with the greatest influence on design reliability. Comparison designs were performed to check ROADENT against the 1993 AASHTO guide and the existing Minnesota granular equivalency methods. Those comparisons showed that ROADENT produced very similar design values for rutting. However, data suggests that the fatigue performance equation will require further modification to accurately predict fatigue reliability.

This report presents the results of a cooperative study on the field use of a permeameter, built by researchers at the Minnesota Department of Transportation (Mn/DOT) and the University of Minnesota, to estimate the saturated hydraulic conductivity of pavement base materials. Field measurements using the permeameter were taken on various highway construction projects, and researchers measured the saturated hydraulic conductivity of samples in the laboratory. Researchers also reviewed theories for converting a field-measured flow rate into a saturated hydraulic conductivity estimate. By numerical simulation and analysis of the field data, researchers determined an appropriate method for converting the Mn/DOT permeameter flow measurements into estimates of hydraulic conductivity. Variations between the field estimated and laboratory measured hydraulic conductivity are within one order of magnitude. Variations between the field estimate and numerical simulation, however, are much closer. The study found the Mn/DOT permeameter can be used to obtain a reliable estimate of the base hydraulic conductivity provided that the base layer is at least 15 cm (6 in.) deep. When the base is too thin, permeameter readings are restricted to early infiltration times.

Recent advances in flexible pavement design have prompted agencies to move toward the development and use of mechanistic-empirical (M-E) design procedures. This report analyzed seasonal trends in flexible pavement layer moduli to calibrate a M-E design procedure specific to Minnesota. Seasonal trends in pavement layer moduli were quantified using data from the Minnesota Road Research Project (Mn/ROAD) and Long Term Pavement Performance Seasonal Monitoring Program (LTPP SMP) sites located in Minnesota. The relationships investigated were between climate factors, subsurface environmental conditions, and pavement material mechanical properties. The results show that pavement layer stiffness is highly respondent to changes in the average daily temperature and available moisture. Five seasons were used to characterize the seasonal variations in pavement layer moduli for design purposes. Seasonal factors were used to quantify the cyclic variations in the pavement layer stiffness for a typical year. The maximum stiffness of the pavement layers is reached when temperatures are cooler. The hot mix asphalt layer moduli are at a minimum in the summer when temperatures are high. The granular base layer moduli are at a minimum during the early spring-thaw period when excess moisture is unable to drain. Finally, the fine-grained subgrade layer moduli are at a minimum in late spring and summer due to the low permeability and slow recovery of the material. The Integrated Climate Model (ICM) was used in this study to compare predicted data to actual data from Mn/ROAD. It was found that the ICM data compared favorably, however, the ICM was not able to predict the spring-thaw period.

This report focuses on an unsaturated flow model for research on the effects of moisture in pavement, which is a more comprehensive approach for determining roadway drainage. Results establish that SEEP/W software is a valuable tool for modeling unsaturated flow and that time to drain calculations based on unsaturated flow theory will generally be longer than time to drain evaluated under saturated flow assumptions.

This report summarizes the results of a literature research effort to assist the Minnesota Department of Transportation (Mn/DOT) in (1) evaluating methods and devices for reliably determining in-situ drainage characteristics of base and subgrade materials and (2) evaluating the specifics of the most promising options that are best suited to Mn/DOT's needs. This research effort is limited to existing information and studies. The primary goal of this effort is to identify a device or method that would be used by field inspectors to ensure that base and subgrade materials are capable of removing infiltrated water from pavement systems in order to prevent accelerated pavement deterioration. The device or method must be durable and easy to use by field personnel. The device or method must also be economical so that it can be distributed for state-wide use. The research effort is being performed in two phases. Phase I (summarized in this report) is a comprehensive survey and review of existing literature to identify methods and devices for measuring in-situ drainage characteristics of aggregate base and granular subgrade materials and to summarize salient characteristics of these methods and/or devices. Phase II will focus on a more detailed evaluation and· analysis of the most promising methods and/or devices identified in Phase I. The selection of the methods and/or devices for the Phase II study will be performed in conjunction with Mn/DOT's review of the Phase I findings.

This report documents the development of a mechanistic-empirical (M-E) flexible pavement thickness design method for use in Minnesota. The report includes a comprehensive literature review of the state of the practice. The Minnesota Road Research Project (Mn/ROAD) served as the primary source of data, in addition to information from the literature, during the development of the method. The mechanistic pavement model, WESLEA, provided reasonably accurate pavement response data and was chosen as the model for Minnesota. Pavement material properties and traffic loads were characterized using Mn/ROAD data. These characterizations were used both as input to WESLEA and to calibrate pavement performance equations to Minnesota conditions. The two modes of distress considered in this method are fatigue cracking and rutting. Fatigue performance is a function of tensile strain at the bottom of the asphalt concrete layer while the rutting is a function of compressive strain at the top of the sub grade. It was recommended that load spectra be used to characterize traffic. The computer program, ROADENT, resulted from this research. An interactive design tool, the program runs in a Windows environment. The report includes a user's guide.