1. Gravel Roads - Design and Maintenance Original Presentation for National ASCE Webinar
Presented by:
Ken Skorseth, Program Manager
SD Local Transportations Assistance Program (SDLTAP)
South Dakota State University

2. Gravel Rds Maint & Design
Overview of Presentation
Gravel Road Design Issues
Geometrics.
Surface Aggregate Specification.
Thickness Design Basics.
Stabilizing Gravel

3. Gravel Rds Maint & Design
Design Issue – Basic Geometrics:
Be familiar with the AASHTO publication: Geometric Design of Very Low-Volume Local Roads (ADT < 400)
Commonly called the “Little Green Book”.

4. Gravel Rds Maint & Design 2001 Green Book and “Little Green Book”

5. Total roadway width (ft) by functional subclass
US CUSTOMARY
Total roadway width (ft) by functional subclass
Basic Geometrics
Guidelines for Total Roadway Width for New Construction of Very Low-Volume Local Roads in Rural Areas
From: AASHTO – Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT <400)
Design
speed
(km/h)
Major
access
Minor access
Recreational & scenic
Industrial/
commercial access
Resource recovery
Agricultural
Access 15 -
18.0
20.0
22.0 20
24.0 25
21.0 30
22.5 35 40 45
23.0
26.0 50
24.5 55 60
Note: Total roadway width includes the width of both traveled way and shoulders.
Major Access road classification requires only 18 ft. (5.5 m) roadway width at 40 mph (64 km) design speed

6. Gravel Rds Maint & Design
Design Issue – Crown:
An important design issue is crown on gravel road surfaces. Generally recommended crown for gravel surfaces is 4% which is double the crown used in pavements.

7. Gravel Rds Maint & Design
Design Issue – Crown: (con’t)
Clear illustration of 2% crown on road to the left and 4% on the road to the right. Water will not drain off an aggregate surface with only 2% crown. This must be addressed in design and during construction.

8. Gravel Rds Maint & Design
Design Issue – Crown: (con’t)
There are conflicting views on crown:
2 to 6% for “low-type pavements” recommended by AASHTO Green Book pg 387 – 2001 edition.
The FHWA Gravel Roads Manual recommends crown at or near 4%.
Note: in arid and semi-arid regions, gravel roads may perform with less crown, but don’t use less than 3 %.

9. Gravel Rds Maint & Design Shoulder Maintenance is Important

10. Outstanding Shoulder Maintenance
Gravel Rds Maint & Design
Outstanding Shoulder Maintenance

11. Gravel Rds Maint & Design
Design Issue – Surface Gravel
The issue of good surface gravel (aggregate) cannot be emphasized enough!!
Good aggregate surfacing differs from base and other construction aggregates.
When it’s right, problems diminish!

12. Gravel Rds Maint & Design
Design Issue – Surface Gravel (con’t)
Material Specifications Discussion:
Many state DOTs do not have a surface aggregate spec.
Many specifications that do exist are quite loose and do not allow close enough control of gradation.

13. Gravel Rds Maint & Design
Design Issue – Surface Gravel (con’t)
Surface aggregate differs from base aggregate in two fundamental ways:
The need for more plastic fines to serve as binder.
Smaller top-sized stone that will remain embedded in the surface.

14. Gravel Rds Maint & Design
Design Issue – Surface Gravel (con’t)
Corrugation or “washboarding” which is surface distress that is directly related to surface aggregate
specification.

15. Gravel Rds Maint & Design
Design Issue – Surface Gravel (con’t)
Similar ADT, Similar geometrics, but different surface materials!

16. Gravel Rds Maint & Design
Design Issue – Surface Gravel (con’t)
Sample specifications comparison:
25mm
19mm
12mm
4.75mm
2.36mm
425 µm
75 µm
Better when modified to

17. Value of Quality Surface Gravel
Gravel Rds Maint & Design
Value of Quality Surface Gravel
Less corrugation (washboard)
Less loose aggregate accumulating between wheel paths and on shoulder
Less blade maintenance needed to keep road in good condition
Less dust
Less public complaint

18. A Current SDDOT Research Project Comparing Surface Gravel Quality
Gravel Rds Maint & Design
A Current SDDOT Research Project Comparing Surface Gravel Quality
Three test sections have been constructed.
Each test section has three surface materials
Substandard, but commonly used
Meets SD Standard Specification for Gravel Surfacing – low percentage passing #200 (75 µm) sieve and low plasticity
Modified Specification having minimum 10% passing the #200 (75 µm) sieve and minimum plasticity index of 7

19. Construction done in Oct, 2011
Gravel Rds Maint & Design
Construction done in Oct, 2011
Observations as of last week

20. Performing loose aggregate or “float” test
Gravel Rds Maint & Design
Performing loose aggregate or “float” test

21. Gravel Rds Maint & Design
72 lbs (32.7 kg) removed from ten inch (250mm) cross section on surface

22. Gravel Rds Maint & Design
Only 12 lbs (5.4 kg) removed from same cross section on modified section!

23. Gravel Rds Maint & Design
-No Corrugation on modified section since construction -Blade maintenance has been averaging 4:1 on substandard vs modified sections

24. Gravel Rds Maint & Design
When gravel road geometry and material quality are understood and well managed, stabilization may be the next step when paving is not feasible.
A gravel road stabilized with liquid MgCl2 in Meade County, SD. No blade maintenance was been done between construction in summer of 2011 and retreatment in July 2012!

25. Some Basics on Thickness Design
Gravel Rds Maint & Design
From SDDOT Rural Road Design, Maint, & Rehab Guide
Some Basics on Thickness Design
14.5 inches (370 Millimeters) of gravel needed to carry 25 to 50 trucks per day over weak subgrade!

26. What if you can’t build it?
Gravel Rds Maint & Design
What if you can’t build it?

27. Wind power generation tower construction
Gravel Rds Maint & Design
Wind power generation tower construction

28. Medium sized crane used in tower const.
Gravel Rds Maint & Design
Medium sized crane used in tower const.
How do you move it legally on a state or federal hwy?

29. Gravel Rds Maint & Design
General principles for managing gravel roads can be found in the FHWA Gravel Roads Manual. It can be found online by searching for “Gravel Roads Manual”

30. Gravel Rds Maint & Design
You may contact me at:
Ken Skorseth
SDSU/SDLTAP
Box 2220-Harding Hall
Brookings, SD
phone
fax