Pavement Design - Regional Factor, Frost Depth and Serviceability 2

[CivelRocks]

I have two Engineer's pavement calculations in front of me and they use different methods to calculate pavement cross sections. I also have two flexible pavement design books which are explaining a slightly different method then the two engineers use.

Some issues that I have are as follows:

1) Regional Factor - What is this and how does it effect your SN required? It is not in the 1993 AASHTO Empirical Formula.

2) In the one report Frost heave decreases the Serviceability Index from 2.2 to 1.3 (initial is 4.2, final is 2.0)

3) Does Frost Depth effect pavement cross sections and how? NH's frost depth is 4 feet while Virginia's is 2.5. I can have 6 inches of stone in Virginia while I have to have 8-12 in NH.

 

[dicksewerrat]

Minnesota has 12-18. Frost depth 6-8 feet

 

[emmgjld]

Ahh, the joys of Flexible Pavement Design. There are as many modifications of design methods as there are authors. Your question addresses the AASHTO design methods and I do mean methods.

1) Regional Factor - What is this and how does it
effect your SN required? It is not in the 1993
AASHTO Empirical Formula.

It is important to know that the AASHTO 86 incorporated a significant change in their analysis method. The Regional Factor had been used to account for the effects of differing climate and to provide a means of adjustment.

The AASHTO 86 method recognized that subgrade moisture (time of subgrade saturation) appeared to be more significant than a simplistic Regional Factor and could be represented as a reasonable design input. The Regional Factor was out and the Drainage Coefficient is now in.

The Drainage Coefficient assumes reduced stability of the subgrade soils and the Aggregate Base (ABC) when saturated for extended periods. I have seen some discussions which discount this reduced stability when saturated, but my own experience confirms the assumptions of AASHTO 86/93.

My experience of using the Regional Factor was generally fair to poor, for most soils in Colorado. I greatly approve of using the Drainage Factor, along with a judicious use of Geotextiles, for separation and minor reinforcement. Using Geotextiles, the amount of Aggregate Base can be reduced and the longterm intrusion of the subgrade soil into the ABC can be effectively eliminated.

2) In the one report Frost heave decreases the
Serviceability Index from 2.2 to 1.3 (initial
is 4.2, final is 2.0)

3) Does Frost Depth effect pavement cross sections
and how? NH's frost depth is 4 feet while
Virginia's is 2.5. I can have 6 inches of
stone in Virginia while I have to have 8-12 in NH.

Frost Heave can severely decrease the performance of the pavement structure. The presence of water in the subgrade is required for Frost Heaving. No water, No Frost Heave. Somewhat similar to the idea of Drainage Coefficient. BUT it is important to realize that the subgrade softening due to saturation is NOT the same as deterioration of subgrade density due to Frost Heaving. Different localities have different methods for addressing this issue. Local experience may not always have the best method of analysis but usually represents what is perceived to work.