Gravel Roads - Design and Maintenance Original Presentation for National ASCE Webinar

Gravel Roads - Gravel Roads - Design and Design and

MaintenanceMaintenanceOriginal Presentation for National ASCE Original Presentation for National ASCE

WebinarWebinarPresented byPresented by::

Ken Skorseth, Program ManagerKen Skorseth, Program Manager

SD Local Transportations Assistance Program SD Local Transportations Assistance Program (SDLTAP)(SDLTAP)

South Dakota State UniversitySouth Dakota State University

Overview of Presentation

•Gravel Road Design Issues

Geometrics.

Surface Aggregate Specification.

Thickness Design Basics.

Stabilizing Gravel

Gravel Rds Maint & DesignGravel 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”.


2001 Green Book and “Little Green Book”


Basic GeometricsBasic Geometrics

Designspeed(km/h)

Majoraccess

Minor access

Recreational & scenic

Industrial/commercial access

Resource recovery

AgriculturalAccess

15 - 18.0 18.0 20.0 20.0 22.0

20 - 18.0 18.0 20.0 20.0 24.0

25 18.0 18.0 18.0 21.0 21.0 24.0

30 18.0 18.0 18.0 22.5 22.5 24.0

35 18.0 18.0 18.0 22.5 22.5 24.0

40 18.0 18.0 20.0 22.5 - 24.0

45 20.0 20.0 20.0 23.0 - 26.0

50 20.0 20.0 20.0 24.5 - -

55 22.0 - 22.0 - - -

60 22.0 - - - - -

Note: Total roadway width includes the width of both traveled way and shoulders.

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)

Major Access road classification requires only 18 ft. (5.5 m) roadway width at 40 mph (64 km) design speed

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.


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.


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 %.


Shoulder Maintenance is Important



Outstanding Shoulder Maintenance

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!


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.


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.


Design Issue – Surface Gravel (con’t)

Corrugation or “washboarding” which is surface distress that is directly related to surface aggregate

specification.


Design Issue – Surface Gravel (con’t)Similar ADT, Similar

geometrics, but different surface materials!


Design Issue – Surface Gravel (con’t)Sample specifications comparison:


Better when modified to 8 - 15

25mm

19mm

12mm

4.75mm 2.36mm

425 µm

75 µm

Gravel Rds Maint & DesignGravel Rds Maint & DesignValue of Quality Surface Value of Quality Surface GravelGravel Less corrugation (washboard)Less corrugation (washboard)

Less loose aggregate Less loose aggregate accumulating between wheel accumulating between wheel paths and on shoulderpaths and on shoulder

Less blade maintenance needed to Less blade maintenance needed to keep road in good conditionkeep road in good condition

Less dustLess dust

Less public complaintLess public complaint

Gravel Rds Maint & DesignGravel Rds Maint & DesignA Current SDDOT Research A Current SDDOT Research Project Comparing Surface Project Comparing Surface Gravel QualityGravel Quality Three test sections have been Three test sections have been

constructed.constructed. Each test section has three surface Each test section has three surface

materialsmaterials1.1. Substandard, but commonly usedSubstandard, but commonly used

2.2. Meets SD Standard Specification for Meets SD Standard Specification for Gravel Surfacing – low percentage passing Gravel Surfacing – low percentage passing #200 (#200 (75 µm) sieve and low plasticitysieve and low plasticity

3.3. Modified Specification having minimum Modified Specification having minimum 10% passing the #200 (10% passing the #200 (75 µm) sieve and sieve and minimum plasticity index of 7minimum plasticity index of 7


Construction Construction done in Oct, done in Oct, 20112011

Observations Observations as of last week as of last week 7-27-20127-27-2012


Performing loose aggregate or “float” test


72 lbs (32.7 kg) removed from ten inch (250mm) cross section on surface


Only 12 lbs (5.4 kg) removed from same cross section on modified section!


-No Corrugation on modified section since construction-Blade maintenance has been averaging 4:1 on substandard vs modified sections


When gravel road geometry When gravel road geometry and material quality are and material quality are understood and well managed, understood and well managed, stabilization may be the next stabilization may be the next step when paving is not step when paving is not feasible.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!


Some Basics on Thickness Some Basics on Thickness DesignDesign

From SDDOT Rural Road Design, Maint, & Rehab Guide

14.5 inches (370 Millimeters) of gravel needed to carry 25 to

50 trucks per day over weak subgrade!

What if you can’t build What if you can’t build it?it?


Wind power Wind power generation generation

tower tower construction construction


Medium Medium sized sized

crane used crane used in tower in tower const.const.


How do How do you move you move it legally it legally on a state on a state or federal or federal

hwy?hwy?

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”


You may contact me at:

Ken SkorsethSDSU/SDLTAPBox 2220-Harding HallBrookings, SD 57007-0199 phone 800-422-0129fax 605-688-5880email: [email protected]


Documents

Gravel Roads - Design and Maintenance Original Presentation for National ASCE Webinar