Chapter 12.04 - Article 6 - Pavement Structure Design

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6.1 General Policy

The procedure for the design of pavement structure sections shall be based upon the most recent edition of the Colorado Department of Transportation’s Roadway Design Manual and the AASHTO Guide for Design of Pavement Structures. Minor modifications to the CDOT manual procedure and specific minimum values of constants or factors are specified herein.

6.1.1 Road Surfaces

It is the policy of Pueblo County that the surface of all new roads constructed within a public right-of-way shall be hot bituminous pavement (asphalt) or Portland cement concrete pavement. The sole exception to this policy is any road classified as a Local Minor Residential road, which may be constructed with a final gravel surface--subject to the approval of the Director of Public Works--provided:
a. The projected traffic volume 20 years in the future does not exceed 100 vehicles per day.
b. The proposed road profile and other design details combined with the surrounding topography and other characteristics will not create the potential for erosion and loss of gravel due to storm run-off.
c. No special maintenance problems or difficulties due to the gravel surface are anticipated by the Public Works Department considering the location and proposed use of the road.

6.1.2 Pavement Thickness

The required thickness of pavement shall be based upon the provisions of this Article 6. In no case, however, shall the thickness of hot bituminous pavement for a road to be maintained by the County be less than three inches.

6.2 Pavement Structure Design Report

A pavement design report shall be submitted prior to the approval of any roadway construction plans involving new pavement, overlay of existing pavement or widening of existing pavement. The report shall be prepared by a professional engineer licensed to practice in the State of Colorado.
The pavement design report shall include the following minimum information:
a. Soil logs along the proposed roadway alignment at a maximum of 500 foot intervals. Logs shall include a soil profile for a minimum depth of four feet below the proposed sub-grade elevation.
b. Each representative sample shall be classified according to the AASHTO Unified Soil Classification Table, and shall have an Atterberg Limits Test and sieve analysis performed.
c. The pavement design procedure is based upon the resilient modulus, MR, of the sub-grade soils. The value of MR may be obtained from a HVEEM Stabilometer test ‘R" value by the following formulae:
to convert HVEEM "R" to soil support, S1
S1 = [("R" - 5)/11.29] + 3
to convert S1 to MR
MR = 10 (S1+ 18.72 )/6.24
d. Proposed average daily traffic volumes (ADT) for each road based on 100% of full development plus a 10% adjustment for construction traffic. Traffic analysis for the purpose of pavement design shall be as specified in Article 2.
e. Recommended structural sections, based on the design considerations, proposed typical sections, and sections of roadway which may require additional stabilization or treatment.

6.3 Design Considerations

The following elements are to be used in the design procedure:
a. The design procedure is based on the number of 18,000 pound single axle equivalent load applications (18K EDLA) per traveled lane. In no case shall the design 18K EDLA be less than the following values:

Local Minor Residential
Local Access
Local Commercial
Local Industrial
Minor Collector
Major Collector
Minor Arterial
Principal Arterial/expressway
b. The serviceability loss, ?PSI, for local roads and minor collectors shall be 2.5; for major collectors and arterials it shall be 2.0.
c. The reliability factors used shall be the following:

Local Minor Residential
Local Access
Local Commercial
Local Industrial
Minor Collector
Major Collector
Minor Arterial
Principal Arterial/expressway
d. Overall standard deviation, SO, shall be 0.44 for all designs.
e. An adjustment to the structural layer coefficients for drainage conditions will not normally be needed for new construction. (Less than the fair drainage assumed under the CDOT method would not be allowed under new construction). In no case will structural layer coefficients be modified by a factor greater than 1.0. In those instances where, due to existing constraints or physical conditions, occasional moisture levels within the base or sub-base may approach saturation, the following factors will apply:

quality of drainage
percent of time pavement is exposed to moisture levels approaching saturation
< 1%
1 - 5%
5 - 25%
very poor
f. Strength coefficients per one inch layer of a given material for layered pavement design purposes shall be as follows:

hot bituminous pavement
class 6 base course
class 2 sub-base course
Strength coefficients for specially treated base materials for design purposes must be approved by the Director of Public Works.

6.4 Full Depth Asphalt

Full depth asphalt designs will only be allowed with the approval of the Director of Public Works, and will not be allowed on clay sub-grade soils.

6.5 Layered Design Analysis

Layered structures (asphalt on aggregate base course) shall be designed in accordance with the principles shown in Figure 16 in Appendix 1. First, the structural number required over the sub- grade soil is computed. In the same way, the structural number required over the sub-base layer should also be computed, using the applicable strength values for each. By working with differences between the computed structural numbers required over each layer, the maximum allowable thickness of any layer can be computed. For example, the maximum allowable structural number for the sub-base material would be equal to the structural number required over the sub-base subtracted from the structural number required over the sub-grade soil. In a like manner, the structural numbers of the other layers may be computed. The thickness for the respective layers may then be determined as indicated in Appendix 1, Figure 15.
Required nomographs and tables are contained in Appendix 1, Figures 16 and 17. An example pavement structure design can be found in Appendix 3.

6.6 Gravel Road Design

The procedure accepted by Pueblo County for selection of base and sub-base thickness for gravel roads is based upon the AASHTO Guide for Design of Pavement Structures. Portions of this section including the tables and nomographs in Appendix 1, Figures 18, 19 and 20 are taken from that publication.

6.6.1 Factors

The allowable values for factors used in gravel road design are as follows:
a. Allowable rutting, RD = 2 inches
b. Gravel loss, GL = 2 inches
c. Serviceability loss, ?PSI = 3.0
d. Quality of roadbed soils for use in the table in 6.6.1 f shall be based upon HVEEM Stabilometer "R" value as follows:

R < 15
Very Poor

R = 15 - 25
R = 25 - 35

R = 35 - 55
R > 55
Very Good
e. Season lengths, in number of months, shall be based upon U. S. climatic region VI, and are as follows:

Winter (roadbed frozen)
3.0 months
Spring thaw (roadbed saturated)
1.5 months
Summer (roadbed dry)
4.5 months
Spring/fall (roadbed wet)
3.0 months
f. Seasonal roadbed soil resilient moduli, MR (psi), as a function of the relative quality of the road bed material shall be as follows:

Quality of roadbed soil
Season (roadbed soil moisture condition)
very good
very poor

6.6.2 Gravel Design Example


An example of a gravel roadway design is contained within Appendix 3, including text descriptions of the design steps in the AASHTO method.