Pavement Design
Current
Tech Memos pertaining to pavement design:
Office of Technical Support
Technical
Memorandum No. 97-20-CME-04
Technical
Memorandum No. 98-02-MRR-01
R-Value Design
As written in Chapter 5 of the Mn/DOT Geotechnical and Pavement Manual,
design is based on the soil R-value, and is summarized below:
Pavement design involves the determination of the most cost-effective combination of pavement type and structure for a roadway, which will be functionally and structurally adequate during the design life of the pavement. The following inputs are required:
- Traffic
- Environmental conditions
- Material properties
The design consists of determining the total thickness of pavement required above the subgrade, as well as the various thicknesses of each of the pavement components, for traffic and subgrade conditions.
Structural designs are based upon the cumulative damaging effect of traffic over a 20-year period.
Traffic
A request for a traffic analysis should be submitted to the District
Traffic Forecaster. The Asphalt Paving Guide provides an outline
of traffic calculations for reference.
Soil
R-value
The District Materials and/or Soils Engineer must determine a design
R value for the subgrade soil. An independent testing lab can also provide
this information. The R value selected for design is generally based on
the average value minus one standard deviation of the test results obtained
on samples taken during the geotechnical investigation. The Mn/DOT Geotechnical
and Pavement Manual contains helpful information about estimated R values
for given AASHTO soil types.
Asphalt
Pavement with Aggregate Base
The design of asphalt pavement with aggregate base is based on the concept
of the Granular Equivalent (GE). The granular equivalent thickness of
a pavement is determined by assigning granular equivalent values to pavement
materials on the basis of their contribution to the pavement strength
in comparison to the strength offered by a layer of Mn/DOT class 5 or
6 aggregate base. Figure 1 is taken from the Mn/DOT Geotechnical and Pavement
Manual, and is used to determine the total granular equivalent required
for the pavement, and a minimum base granular equivalent value.
Figure 1. Pavement design chart for asphalt pavement with aggregate base. Get PDF
Figure 1 is used to determine the required GE, expressed in inches, for the design-lane, cumulative, 18-kip ESALs and subgrade R values. After the required GE is determined, it is converted into the appropriate bituminous wearing course, bituminous binder course, bituminous base, and aggregate base thicknesses for the pavement using the values given below. Once layer thicknesses have been established, total pavement thickness and layer composition is determined.
Granular Equivalent (GE) Values for Typical Pavement Materials
| Material | Specification | GE Factor |
| Plant-mixed bituminous pavement | Wearing course | 2.25 |
| Plant-mixed bituminous pavement | Base course | 2.00 |
| Road-mix surface | 2321 | 1.50 |
| Road-mix base | 2321 | 1.50 |
| Bituminous treated base | (Rich) 2204 | 1.50 |
| Bituminous treated base | (Lean) 2204 | 1.25 |
| Aggregate base | Class 5,6 | 1.00 |
| Aggregate base | Class 3,4 | 0.75 |
| Selected granular material | 0.50* |
* May be used in design when so approved by the Pavement Design Engineer.
For example, if calculation of traffic and an estimated R value for an area resulted in a required GE of 16 for a pavement, the designer could develop the following designs:
Design 1|
Material
|
Thickness (inches)
|
GE
|
Total GE
|
| Class 5 aggregate base |
8.0
|
1.00
|
8.00
|
| Plant-mixed bituminous base course |
2.0
|
2.00
|
4.00
|
| Plant-mixed bituminous wear course |
2.0
|
2.25
|
4.50
|
|
Total GE
|
16.50
|
||
|
Material
|
Thickness (inches)
|
GE
|
Total GE
|
| Class 5 aggregate base |
6.0
|
1.00
|
6.00
|
| Plant-mixed bituminous base course |
3.5
|
2.00
|
7.00
|
| Plant-mixed bituminous wear course |
1.5
|
2.25
|
3.375
|
|
Total GE
|
16.375
|
||
Full
Depth Asphalt Pavement
Full depth asphalt pavement is defined as a pavement structure in which
every layer above the subgrade or improved subgrade is asphalt. Figure
2 is used to determine the bituminous pavement thickness for full-depth
pavement.
Figure 2. Pavement
design chart for full-depth bituminous pavement. Get
PDF
Other design methods are available, including:
1. The Mechanistic-Empirical
Design (MnPAVE) methodology
2. The Mn/DOT
Soil Factor Design
