# Mechanistic-Empirical Pavement Design

Mechanics is the science of motion and the action of forces on bodies.  Thus, a mechanistic approach seeks to explain phenomena only by reference to physical causes.  In pavement design, the phenomena are the stresses, strains and deflections within a pavement structure, and the physical causes are the loads and material properties of the pavement structure.  The relationship between these phenomena and their physical causes is typically described using a mathematical model.  Various mathematical models can be (and are) used; the most common is a layered elastic model.

Along with this mechanistic approach, empirical elements are used when defining what value of the calculated stresses, strains and deflections result in pavement failure.  The relationship between physical phenomena and pavement failure is described by empirically derived equations that compute the number of loading cycles to failure.

The basic advantages of a mechanistic-empirical pavement design method over a purely empirical one are:

• It can be used for both existing pavement rehabilitation and new pavement construction
• It accommodates changing load types
• It can better characterize materials allowing for:
• Better utilization of available materials
• Accommodation of new materials
• An improved definition of existing layer properties
• It uses material properties that relate better to actual pavement performance
• It provides more reliable performance predictions
• It better defines the role of construction
• It accommodates environmental and aging effects on materials

The benefit of a mechanistic-empirical approach is its ability to accurately characterize in situ material (including subgrade and existing pavement structures).  This is typically done by using a portable device (like a FWD) to make actual field deflection measurements on a pavement structure to be overlaid.  These measurements can then be input into equations to determine existing pavement structural support (often called “backcalculation”) and the approximate remaining pavement life. This allows for a more realistic design for the given conditions.