skorseth - gravel road design track 1 4-18-11

8/12/2019 Skorseth - Gravel Road Design Track 1 4-18-11

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2011 NACE Conference

Minneapolis, MN

byKen Skorseth

SDLTAPProgram Manager

Gravel Road Design & Maintenance


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Critical IssuesRoadway Shape

Drainage

Surface Gravel Selection

Preservation


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Design Issue Basic Geometrics:

Be familiar with the AASHTO publication:

Geometric Design of Very Low-Volume LocalRoads (ADT < 400)

Commonly called the Little Green Book.


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Green Book and Little Green Book

Gravel Rds Maint & Design


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Nearly 80% of the roads in the UShave traffic volumes of 400 vehicles

per day or less.!! (quote from Little

Green Book)

It becomes very difficult to

construct and maintain these very

low-volume roads to a high

geometric standard.

Gravel Rds Maint & Design


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Design

speed

(km/h)

Major

access

Minor

access

Recreational &

scenic

Industrial/

commercial

access

Resource

recovery

Agricultural

Access

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 Constructionof Very Low-Volume Local Roads in Rural Areas

From: AASHTO Guidelines for Geometric Design of VeryLow-Volume Local Roads (ADT


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Design Issue Crown:

An important design issue is crown on gravel roadsurfaces. Generally recommended crown for gravel

surfaces is 4% which is double the crown used in

pavements.


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Design Issue Crown: (cont)

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.


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There are conflicting views on

crown:

1/3 to 1/2 in. per ft. recommended by NACEmanual Blading Aggregate Surfaces 1986 edition.

2 to 6% for low-type pavements recommended by

AASHTO Green Book pg 387 2001 edition.


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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 dont use less than 3%.


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Design Issue Surface Gravel

The issue of good surface gravel

(aggregate) cannot be emphasized

enough!!

Good aggregate surfacing differs

from base and other constructionaggregates.

When its right, problems diminish!


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Design Issue Surface Gravel

(cont)Material Specifications Discussion:

Many state DOTs do not have asurface aggregate spec.

Many specifications that do existare quite loose and do not allow

close enough control of gradation.


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Design Issue Surface Gravel (cont)

Too often, surface aggregate isperceived as not important, hence

quality suffers.In study completed in Canada in2003 samples were taken fromseveral aggregate suppliersstockpiles being marketed assurface aggregate--- (cont next slide)


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Only 14% of the samples met thecompanies own specifications

when tested by independent labs!Quality control was almost

nonexistent.

Information from Material and PerformanceSpecifications for Wearing Course Aggregates on

Forest Roads by G. Legere & S. Mercier.


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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 willremain embedded in the surface.


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Similar ADT, Similar geometrics, but different

surface materials!


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Corrugation orwashboarding

which is

surface distress

that is directly

related tosurface

aggregate specification.


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Sample specifications comparison:


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Another sample spec:WisDOT CRUSHED AGGREGATE SHOULDER COURSE

Gradation No. 3SIEVE SIZE CRUSHED GRAVEL CRUSHED STONE

1 inch 100 100

3/4 inch 95 -100 95-100

3/8 inch 50 90 50-90

No. 4 35 70 35-70

No. 10 20 55 15-55

No. 40 10 - 35 -

No. 200 9 - 15 5 - 15


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Design Issue Stage Construction

In my experience, if a road/streetis to be maintained three years or

more as an unpaved road, use

material and crown specification forgravel roads.

If less than three years, use baseaggregate and crown specificationfor pavement. (It will be hard to

maintain in the interim).


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Design Issue Stage Construction(cont)

Little Crown

and poor

surfaceaggregate,

but road is to

paved in three

years.


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Prepared subgrade

Coarse aggregate

base, few fines, 6-12

inches thick

Surface layer with inch

topsize stone and good

percentage of plastic fines

minimum 3 inches thick

Not the real, but the ideal!

Design Issue Perfect Gravel Road


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Gravel Roads Managing

Maintenance

There are two primary issues:

Managing frequency of blademaintenance.

Keeping control of shape ontraveled way and shoulder.


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Crown

One of the biggest challenges in gravel road

maintenance.


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Some roads have too little crown, some have

too much.


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Imagine a cattle trailer on this road.


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The road has 13 inches of crown on a 20 top


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Two Track oad


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Crown Gauge is Helpful


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Manageable Road Width


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Crown should be near inch per ft (or 4%)

Example: 24 ft roadway should have approx. 6

inches of crown.


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The Next Challenge High Shoulders


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Outstanding example


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Shouldering

Operations


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Similar ADT, Similar roadway shape, but

different surface materials!

Dramatic Contrast


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Six years of performance


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A Remarkable ContrastA Chloride Stabilized Road Since 1994!


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Without Adequate Funds for BaseImprovement and Paving StabilizedGravel is an Alternative.


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Heavy Residential Traffic & ContinuedDevelopment


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Get ready to face the challenges ofmaintaining gravel roads in thefuture

Thanks!

skorseth - gravel road design track 1 4-18-11

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