Flexible Pavement Empirical Design Example

Design a new flexible pavement for a major interstate highway using the following conditions (four lanes each direction):

Solution

By looking at several different design periods and reliability levels this example gives an idea of the relative influence of these inputs. Work can be verified by using the Flexible Pavement Structural Design Utility.

ESALs per year

This step involves converting the daily traffic volume into an annual ESAL amount.  Pavements are typically designed for the critical lane or “design lane”, which accounts for traffic distribution.

ESALs per year = (Vehicles/day) (Lane Distribution Factor) (days/yr) (ESALs/Vehicle)

Design ESALs

The standard multiplier to calculate compound growth is:

Calculate the Effective Subgrade Support

Note that in this example there are two different values of subgrade resilient modulus given: one for dry months and one for wet months.  Realistically, subgrade support varies even more than this simplistic assumption, however, the same method for estimating a design subgrade resilient modulus can be used for more precise assumptions.  The standard method in the 1993 AASHTO Guide for Design of Pavement Structures involves calculating a weighted average subgrade resilient modulus based on the relative pavement damage.  Because lower values of subgrade resilient modulus result in more pavement damage, lower values of subgrade resilient modulus are weighted more heavily. The relative damage equation used in the 1993 AASHTO Guide for Design of Pavement Structures is:

Therefore, over an entire year the calculations would be:

Use the 1993 AASHTO Empirical Equation

Using the previously calculated ESAL results and the 1993 flexible pavement structural design equiation the following pavement thickness designs can be calculated:

In this particular example, which only shows one set of possible solutions, the HMA surface course and base course layer depths were kept constant and the HMA binder course depth was varied depending upon requirements.  Notice also that a change in reliability level from 90% to 99% results in a SN increase of about 1.0 and a resultant increase in HMA thickness of about 55 mm (2 inches).  It is interesting to note that in most empirical design procedures HMA, no matter what the specific mix designation or size, is treated equally.  Here, the 12.5 mm (0.5 inch) Superpave surface course and the 25 mm (1 inch) dense-graded binder course are structurally equivalent.