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Aggregate Surfacing Design

Standards and Guidance

This document describes the catalog method for aggregate surfacing thickness design. This method can be applied on FLH projects employing aggregate surfacing.

Exhibit 1 shows a design catalog of recommended surface aggregate layer thicknesses. Use this catalog to determine the aggregate layer thickness required for a project when more detailed information about the project is not available. The thicknesses shown are based on specific ranges of 18-kip ESAL applications at traffic levels: 

Traffic Level18-kip ESAL
High 60,000 to 100,000
Medium30,000 to 60,000
Low10,000 to 30,000

Two other assumptions inherent in this thickness design are that the effective resilient modulus of the aggregate material is 30,000 psi, regardless of the quality of the roadbed soil; and there is a 0.5-inch aggregate loss per year. Exhibit 1 is from the AGDPS.

Determine the in-situ soil properties and then use Exhibit 2 and Exhibit 3 to determine the Relative Quality of Roadbed Soil.

For Soil Classification, use: ASTM Classification of Soils for Engineering Purposes (ASTM D 2487), or AASHTO Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes (AASHTO M 145)

For Strength of Roadbed Soil, use: Resistance Value (AASHTO T 190); California Bearing Ratio (AASHTO T 193); and/or the Resilient Modulus (AASHTO T 307)

When test results from subgrade soil are unavailable, visually estimate the Unified Soil Classification using ASTM D 2488 or other means. Then use the AASHTO and ASTM Soil Classification chart in Exhibit 2 and Roadbed Soil Strength chart in Exhibit 3 to determine the Relative Quality of Roadbed Soil.

Determine the U.S. climatic region designation from Exhibit 4. This exhibit is from the AGDPS. 

Using Exhibit 1 with the appropriate Relative Quality of Roadbed Soil, traffic level, and U.S. climatic region designation, determine the minimum thickness of aggregate surfacing in inches. When a higher type of pavement design is recommended from Exhibit 1, contact the Division Pavement Engineer.

Exhibit 1 – Aggregate Surface Thickness Design Catalog

Recommended aggregate thickness (in inches) for the six U.S. climatic regions, five relative qualities of roadbed soil and three levels of traffic.
Relative Quality
of Roadbed Soil		 U.S. Climatic Region
Traffic LevelIIIIIIIVVVI
Very Good

High

Medium

Low

8"

6"

4"

10"

8"

4"

15"

11"

6"

7"

5"

4"

9"

7"

4"

15"

11"

6"

Good

High

Medium

Low

11"

8"

4"

12"

9"

5"

17"

12"

7"

10"

7"

4"

11"

9"

5"

17"

12"

7"

Fair

High

Medium

Low

13"

11"

6"

14"

11"

6"

17"

12"

7"

12"

10"

5"

13"

10"

5"

17"

12"

7"

Poor

High

Medium

Low

^

^

9"

^

^

10"

^

^

9"

^

15"

8"

^

15"

8"

^

^

9"

Very Poor

High

Medium

Low

^

^

11"

^

^

11"

^

^

10"

^

^

8"

^

^

8"

^

^

9"

^ Higher type pavement design recommended.

Exhibit 2 – AASHTO and ASTM Soil Classification

Design chart showing the AASHTO and ASTM Soil Classification

Exhibit 3 – Roadbed Soil Strength

Design chart showing Roadbed Soil Strength

Exhibit 4 – The Six Climatic Regions in the United States

Map of US with the six climatic zones drawn
The six climatic regions in the United States
RegionCharacteristics
IWet, no freeze
IIWet, freeze – thaw cycling
IIIWet, hard freeze, spring thaw
IVDry, no freeze
VDry, freeze – thaw cycling
VIDry, hard freeze, spring thaw
Last updated: Wednesday, September 23, 2026