By: Ken Skorseth, Contributing Author
Many people who live on gravel surfaced roads are simply
waiting for the day when those roads will be "oiled" or paved with an
asphalt surface. In regions where the population is growing, this will probably
be done. But, in much of the Great Plains as well as portions of the mountain
West and desert Southwest regions of the U.S., thousands of miles of gravel
roads will continue to serve the rural population. These roads will remain
gravel surfaced because of sparse population and low volumes of traffic. The
cost of paving can't be justified. Yet these roads are the only link to towns,
schools, hospitals and agricultural markets for many rural residents. It is
estimated that there are 1.6 million miles of unpaved roads in the U.S. alone
and millions of additional miles in the rest of the world. Unfortunately,
unpaved roads tend to be a forgotten portion of the transportation grid in the
U.S. There is little technical information or support to assist local agencies
in managing and maintaining these roads. Consequently, it becomes a
"seat-of-the-pants" system of management in many cases.
Stay in shape
Gravel road maintenance and management is changing as is
management on other surface types. For example, heavier trucks with higher tire
pressures and heavier and larger agricultural, mining and logging equipment
causes increased stress to gravel surfaces as well as pavement. While pavements
are being designed with greater thickness and better quality base aggregates
and surfacing, gravel roads often exist virtually the same as they were 30, 40
and even 50 years ago. This creates real management problems. It becomes more
important than ever to understand the two important components of managing
gravel roads--the use of the motor grader and selecting quality surface gravel.
Each component is equally important. However, the correct use of a motor grader
to establish and maintain correct roadway shape must be understood first. Thereafter,
good surface must be placed for a gravel road to perform well.
Maintaining the shape on the gravel road surface is a
constant challenge to the grader operator. While paved roads generally maintain
their shape for an indefinite period of time, gravel surfaces change
constantly.
Traffic continually displaces the surface gravel. Loose
material tends to collect between the wheel paths and also shifts to the
shoulder of the roadway. Regular blade maintenance is required to recover and
reshape the surface. The quality of the surface gravel, volume and type of
traffic and weather conditions will dictate how frequently blade maintenance
needs to be done, but deferred blade maintenance will cost in the long run.
The two greatest challenges to the grader operator are to
avoid creating a high shoulder or "secondary ditch" at the edge of
the roadway and to maintain the correct crown on the surface.
A high shoulder can develop in several ways. The most common
cause is gravel that is displaced by traffic to the shoulder and it lodges in
or near shoulder vegetation. This can be hard to recover with the grader. In
many regions, this is best done in the spring when shoulder vegetation is
dormant. This is referred to as "pulling the shoulder" and is a good
maintenance technique. Vegetative cover will quickly replace itself in most
regions. Care must be taken not to bring too much vegetation onto the roadway
thus creating a large windrow.
Another common cause of the high shoulder is too little
angle and too little pitch, or tilt, of the grader's moldboard. This allows
material to flow around the leading edge of the moldboard and build up a high
shoulder. Yet another cause of high shoulder is gouging with the moldboard at
the shoulder line and not matching the edge of the gravel surface to the
shoulder.
Perhaps the greatest controversy in maintaining and
reshaping gravel roads is establishing proper crown. While nearly all states
have standard specifications for pavement crown-- generally 2%--there is seldom
a specification for crown on gravel surfaces. A sound recommendation for gravel
roads is 4% crown. This can vary a bit, but too much or too little crown is
never good.
In addition to the percent of crown, another issue is the
surface profile. Many gravel surfaces have a parabolic (or rounded) crown. This
can come from the wrong sequence of blading passes by the grader; especially
making the final pass down the middle of the roadway.
However, a greater cause is the center wear that develops in
the moldboard's cutting edge. Center wear in the cutting edge will occur in any
gravel maintenance operation, but if not controlled, it will leave a rounded or
parabolic shape on the roadway. This will leave little or no crown in the
center, but excessive crown on the outer edges of the road surface.
This leads to two problems. Motorists will tend to drive
down the middle of the road, no matter how wide it is since that area will be
relatively flat. Potholes and excessive corrugation, or
"washboarding," will appear in the middle of the road as well since
drainage will be restricted. Good motor grader operators pay attention to this
problem and will make a great effort to keep their cutting edges straight. In
recent years, cutting edges with carbide inserts or carbide-tipped bits have
appeared on the market. This is a helpful tool because the carbide resists
abrasive wear and greatly reduces center wear in the cutting edge. Although
expensive, the extra cost is often offset by extra wear life.
The correct shape of the crowned surface is like the letter
"A" or the roof of a house with a straight line for drainage from the
centerline to shoulder. This is always a challenge--and skilled, knowledgeable
motor grader operators are a priceless asset to their company or agency.
Still in need of a quality mix
However, with the best use of a grader, gravel roads still
do not perform well unless good surface gravel is placed on them. What is good
surface gravel? The answer is a bit difficult. Because of low traffic and low budget
allocations, agencies generally have to work with materials that are affordable
and available locally.
Local materials vary greatly across the country--ranging
from high-quality quarry aggregates to seashells. Some states do not even have
a standard specification for surface gravel. Even if the state DOT has a
surface gravel (or aggregate) spec, a local agency is generally not required to
use it when purchasing material for routine road maintenance. Unfortunately,
this leads to many problems on the road, and many grader operators are blamed
for problems that are actually material-related.
The primary problems are excess "float," or loose
gravel on the surface, excess material loss to the shoulder and ditch and
excess corrugation. Despite budget constraints, managers should specify the
best material they can afford. It is generally good management to purchase a
lesser quantity of good quality gravel than vice versa.
Quality surface gravel is a good blend of stone sizes, sand
sizes and fine material. Too often, material that may be good base or cushion
gravel is assumed to be good for gravel surfacing. This is not true. Quality
base aggregate contains a higher percentage of crushed stone with larger top
size and generally few fines. This makes a strong, drainable foundation for
pavement.
Quality surface gravel has different characteristics. It
needs to have smaller top-size stone and a higher percentage of plastic fines
which will allow the gravel to take on a tight, or "bound,"
characteristic. The road surface should form a crust. This type of material
will shed water and drastically reduce corrugation and material loss from the
surface.
A real bonus is reduced frequency of blade maintenance which
offsets part of the material cost. Gravel that is processed by crushing is
almost always superior to screened material. If your state does not have a
gravel surfacing specification, a useful guide is the 2001 edition of the
AASHTO Standard Specifications for Transportation Materials and Methods of
Sampling and Testing, section 147.
The quality of surface gravel becomes especially critical
where dust control treatments are applied. Dust control products seldom perform
well when applied to poor surface gravel. In summary, it pays to study material
available in your region and learn to specify the best material you can afford.
Further help is available by requesting FHWA's Gravel
Roads--Maintenance and Design Manual.
It is a comprehensive, nontechnical resource and is available through
your state LTAP Center or by contacting the FHWA Reports Center, 9701
Philadelphia Court, Unit Q, Lanham, MD 20706. The manual can be viewed on the
web at www.ltapt2.org/gravelroad.htm.
About The Author: Skorseth is field services manager for the LTAP at South Dakota State University.
