User:Mkhan32/Mechanistic Empirical Design of Highways

Introduction
Mechanistic-empirical design is a method of designing highway pavements. It combines empirical relationships obtained from the field data with theoretical predictions based on the mechanics of materials. This method relates inputs such as traffic, loadings, soil strength, climate, etc. to the actual pavement response. Currently, the pavement design method being used by most highway authorities is Empirical Design. In Empirical Design, the pavement is designed based on data obtained from the field only. Mechanistic-empirical method is more accurate than the empirical method because the empirical method only relies on the field performance, while the mechanistic-empirical method combines both the field performance and theoretical prediction models. It is important to adopt Mechanistic-based pavement design procedure because it employs more efficient and sophisticated analytical tools and structural analysis models.

By using theoretical information safety factors can be reduced and the vast majority of the time roadway mat thicknesses can be reduced while improving overall performance (longevity) of the roadway. The amount of money saved in terms of material on the roadway through the additional research far outweighs the additional money spent on that research.

Mechanistic-Empirical Design
Procedure 

Mechanistic-Empirical (M-E) design procedure involves three main steps. In the first step, pavement response values are obtained in the field or in a laboratory test. In the second step, theoretical values of pavement response are calculated using mechanics. Then finally, the data obtained from the field and laboratory tests is related to the predicted pavement response by using advanced structural models. The structural models are based on an analysis method caled finite element analysis. If the pavement response predicted by models does not match the observed pavement distress, shift factors are used to adjust predicted pavement performance to  match the field-observed pavement  performance. Thus, confidence and improved reliability are developed in the M-E procedure.

Why Mechanistic-Empirical Approach is Needed

It is important to adopt Mechanistic-based pavement design procedure because it employs more efficient and sophisticated analytical tools and structural analysis models. It has been concluded that the currently available flexible pavement structural models and computer codes for mechanistic analysis are adequate for Mechanistic-Empirical thickness design procedures. Dependence on observed performance is necessary because theory alone has not proven to be sufficient to design pavements realistically. Mechanistic methods rely on the scientist’s ability to translate the analytical calculations obtained by structural analysis of pavement response to performance data. This results in a more relaible pavement design and improves the long-term performance of highways.

It is especially important to implement it at this time due to the price escalation of crude. M-E can greatly decrease the amount of asphalt required by both reducing the safety factor since we know more about the performance of the material, and utilizing the tension inherent in the polymerized product.

Issues and Challenges
There are still a few challenges in adopting the mechanistic-based approach. Distress models are the weak point in the M-E design, but are compensated for by not reducing the safety factor fully. Extensive field calibration and verification are required to establish reliable distress prediction models, something that is not easy to achieve. A major credibility gap in M-E design procedures is the large magnitude of the shift factors between predicted and observed pavement distress and performance. Another complicating factor is the adequacy and accuracy of the input traffic data. The procedures used to estimate historic traffic data do not reflect a consistent methodology or procedure. Also, one of the major issues is the need for financial resources to fully implement the M-E design procedure. Most state highway authorities at this time do not have the necessary personnel, time, and financial resources to support a large-scale M-E implementation program. The state highway authorities, at the same time, do not have adequate available flexible pavement data and information to support M-E verification and calibration activities. Most state authorities think that the current M-E procedures are very sophisticated and complex and that they would rather have a more simplified procedure for the more common design condtions. However, any changes to this method based on what information is available are, especially now, cost effective.