1. Transportation Engineering
Approaches to Pavement Design

2. General Approaches to Pavement Design
In its simplest form, the design approach consists of the following steps:
Define the loading and environmental conditions;
Select materials with appropriate properties for use in the pavement;
Select the thickness of pavement, based on empirical rules or on a stress – strain analysis of the pavement structure; and
Adjust the initial design using different materials until a satisfactory design emerges.

3. Approaches to Pavement Design
There are two approaches to the pavement design.
Empirical Approach
Mechanistic-Empirical Approach

4. Empirical Approach
“An empirical approach is one which is based on the results of experiments or experience.”
Generally, it requires a number of observations to be made in order to ascertain the relationships between input variables and outcomes.
It is not necessary to firmly establish the scientific basis for the relationships between variables and outcomes as long as the limitations with such approach are reorganized.

5. Benefits
It uses material properties that relates 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

6. Empirical Approach
An empirical approach is one which is based on the results of experiments or experience.
Generally, it requires a number of observations to be made in order to ascertain the relationships between input variables and outcomes.
It is not necessary to firmly establish the scientific basis for the relationships between variables and outcomes as long as the limitations with such approach are reorganized.

7. Many pavement design procedures use an empirical approach
Many pavement design procedures use an empirical approach. This means that the relationship between design inputs and pavement failure were arrived at through experience, experimentation or combination of both.
Empirical design methods can range from extremely simple to quite complex.

8. Empirical Equations used in design of pavement.
Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance).  There are many different types of empirical equations available today but this section will present the 1993 AASHTO Guide basic design equation for flexible pavements as an example. 

9. This equation is widely used and has the following form:
Log10(W18)=Zr x So x log10(SN + 1)-0.20+(log10((ΔPSI)/( )) /(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07
where:
W18=predicted number of 80 kN (18,000 lb.) ESALs
ZR=standard normal deviation
So=combined standard error of the traffic prediction and performance prediction

10. SN=Structural Number (an index that is indicative of the total pavement thickness required)
SN =a1D1 + a2D2m2 + a3D3m3+...
ai =ith layer coefficient
di =ith layer thickness (inches)
mi =ith layer drainage coefficient
Δ PSI=difference between the initial design serviceability index, po, and the design terminal serviceability index, pt
MR=sub-grade resilient modulus (in psi)

11. Empirical Methods in Practice
Empirical methods in use are;
Group Index Method
CBR Method

12. Mechanistic-Empirical Approach
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.

13. 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. 

14. Benefits
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 relates 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

15. Mechanistic-Empirical Methods
Mechanistic-Empirical methods in use are
AASHTO
Portland Cement Association Method
British Method