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And good drainage is even more critical for dirt
roads than for gravel, if such a thing is possible. “When you get graded
earth without any aggregate surfacing, adequate drainage is more
critical than at any other time,” he said.
He adds that the driving lanes aren’t the only
consideration. “Along with the crown, shoulder and ditch drainage is
even more critical with gravel roads than paved roads, although
obviously pavements will fail if you can’t keep water away from them,”
Skorseth said.
The way noncohesive gravel moves about the
surface of an unpaved road also will slow drainage from the road.
“Gravel roads will develop a berm or windrow of loose aggregate along
the edge of the traveled way and that will restrict drainage,” Skorseth
said. “This comes from two sources, either poor or careless use of a
motor grader in maintenance, in which aggregate is lost off the toe of
the moldboard, or we have poor surface aggregate in place and it keeps
shifting to the sides of the roadway, and can’t be recovered. Both are
huge problems for drainage of unpaved roads, and will lead to what
engineers call a secondary ditch along the sides of the road.”
V is for victory
The life of a gravel road will be greatly enhanced
even by the most modest drainage channel. “If an owner can simply maintain a
minimal V-ditch at the edge of the roadway, it will pay big dividends,”
Skorseth said. “This includes curves, where the road must be shaped into a
super-elevated cross-section. A minimal ditch will be needed on the inside
radius of a curve to carry water away. That’s the most effective way road
life can be prolonged.”
Construction of the V-ditch is simple enough. “The
motor grader alone will do quite an adequate job of simply creating and
maintaining a V-ditch,” Skorseth said. “However, if we have adequate
right-of-way width, I like to see a full ditch section, with a 4- to 8-foot,
flat-bottomed ditch, if we can. But road agencies deal with width
constraints, especially east of the Mississippi. There, the rights-of-way
can be so old, sometimes evolved from wagon paths, and agencies don’t have
the option of purchasing additional right-of-way. So at least, the agency
should try to get a simple V-ditch at the edge.”
Most of the western United States, Skorseth said,
does have the benefit of a much wider right-of-way. In South Dakota, by law,
there is a 66-foot right-of-way, which is derived from the old “four rod”
measure. “That’s an incredible luxury and enables you to build that
flat-bottomed ditch,” he said. “Excavation for the ditch gives the road a
better separation from natural ground, with the opportunity for better
cross-section.”
Good vegetation control on shoulders is also
essential. Quite often, that’s just a matter of mowing, but plant growth
regulators can also be effective when used correctly. Of course, the latter
can be controversial if the local public is hostile or not otherwise well
informed as to their safety and efficacy.
The motor grader will not work well doing blade
maintenance when it begins to pick up vegetation along the margin of the
road. “Vegetation does not roll across the moldboard the way aggregate does,
and it can aggravate the operator,” Skorseth said.
Motor grader operator training also must be
considered. “Training of motor grader operators tends to be inadequate in
most of the places I go,” Skorseth said. “Where I have seen
less-than-best-practice being used, quite often it’s not the fault of the
operator,” he said. “Instead they have just been placed on a machine with
minimal direction or training, and they develop bad habits which become
ingrained, and it becomes very hard for them to change even when exposed to
the best practices. It’s truly as much art as science, but it requires a lot
of skill with some good advice and training from the start. We can get a
much higher level of skill from our operators, but it just doesn’t always
happen.”
Surface aggregate
High-quality aggregate is essential for the surface
of an unpaved road, Skorseth told Better Roads. The selection of
good-quality surface aggregate may cost more up front, he warns, but if a
maintenance cycle of five to seven years is calculated before placement of
another aggregate surface, agencies will find their maintenance costs go
down.
“A good surface aggregate will have a smaller
top-size stone than base aggregates, for example,” Skorseth said. “Too
often, base aggregates are used for surfacing. A good surface aggregate mix
also will have a higher percentage of plastic fines in the gradation, which
will give the aggregate surface a bound characteristic. Drainage is always
first, but thereafter the quality of the surface aggregate affects not only
the cost of maintenance, but performance.”
The bound surface will mitigate surface corrugation
or washboarding, which the public always will see as the worst surface
defect of a gravel road, Skorseth told Better Roads. “But much of
washboarding is material-related,” he said. “There are three primary causes:
People’s driving habits in which they will drive excessively or brake very
hard; prolonged dry weather, because it is hard to keep any aggregate in a
bound state when you get into a prolonged dry spell unless you are using
some form of stabilization; and poor aggregate, with a low percentage of
fine material coupled with low or no plasticity.”
Of those three causes, the road agency has control
over only one: the aggregate mix. “You can’t make it rain and you can’t
change people,” Skorseth said. “If you really want to deal with that
problem, you have to take a very hard look at your material specification.”
Conserving aggregate
Skorseth says gravel roads have a hidden attribute:
they are very forgiving. “I’ve seen them go into a failed condition during
prolonged wet weather, but given some sunshine and wind, and a reasonably
skilled motor grader operator, they can be put back into a passable
condition for a fraction of the cost of failed paved roads,” he observed.
If unpaved roads must serve year-round traffic under
all weather conditions, then aggregate surfacing is the way to go, he said.
The SD LTAP recommends a minimum of 3 inches of aggregate. “Three inches
does not guarantee any appreciable increase in structural strength, but it
gives you enough aggregate for blade maintenance purposes that you can
continue to shape and work the aggregate without getting into the earth
subgrade,” said Skorseth.
Over years of use and maintenance, the aggregate
works its way into the dirt and begins to perform as a base. This condition
likely will be accompanied by loose aggregate on the surface, and that’s not
good.
“Loose aggregate is a distress on gravel road
surfaces, but it’s also a given,” Skorseth said. “It’s a matter of how much
you will have. I’ve seen so much that it’s become a hazard, but I’ve also
seen a minimal amount which is acceptable.”
Aggregate loss from traffic is caused by dusting
away of fines if it’s an untreated surface, and some from whip-off of
aggregate from passing vehicles. There also will be loss through maintenance
operations, particularly snow plowing, which will cast the aggregate far
from the highway and make it unrecoverable. But the most significant loss is
from aggregate sinking into the subgrade, or the subgrade soils pumping or
migrating up into the aggregate, particularly under heavy traffic.
“As with our paved roads, heavier and heavier truck
traffic is causing a lot more difficulty in gravel road maintenance,”
Skorseth told Better Roads. “That’s particularly true in the Great Plains
region, where heavy agricultural traffic loads cause gravel roads to
deflect. And if you have weak subgrades, there will be migration of fines
into the surface.”
Stabilization options
Unpaved road stabilization will solve many ills of
these roads, but it is not a universal need. “It varies so much across the
country,” Skorseth said. “When we’re building in the high plains, the
mountain west, and the desert southwest, we are in semi-arid regions where
moisture is not a huge problem, and use of a geotextile might be superfluous
in those areas.
“But in regions of high moisture, coupled with weak
soils, then it becomes very cost-effective to use a geosynthetic like
textiles or grids,” Skorseth said. In those conditions, Skorseth says
building an aggregate base might be cost-effective, though budgets seldom
allow for base construction with stabilized subgrade and base aggregate,
followed by surface aggregate.
What may be more cost-effective is to do spot
stabilization, he said. “Generally, experienced managers know where they get
frequent failure, and spot repair is extremely effective,” Skorseth said.
Dirt or aggregate?
Dirt roads without aggregate surface have value and
have their place.
“A dirt road is simply graded earth in which the
native soil is shaped into a geometric cross section that will allow for a
crown driving surface, and a minimal ditch,” Skorseth said. A dirt road can
handle only the very lowest traffic volumes, and again, best practice
drainage is the key to better performance. “I have seen surface maintenance
performed on graded earth roads over and over on the traveled way,” Skorseth
said, “when in reality, if they had gotten off the traveled way and
corrected some of their drainage problems, much of the surface problems
would have gone away.”
The decision to go from graded earth to aggregate
surface too often is an economic choice, not a functional choice. “I work in
some regions in which they have no usable surface aggregate within a
reasonable haul distance,” Skorseth said. “For a state or federal highway,
or most county roads, we would go ahead and spend the money. But on
low-volume roads, the money is not there.
“However, traffic and road classification should be
figured into the mix. If the access road is just for agricultural or
forestry use, or recreation, it can remain a graded earth road with no
surfacing, and users will have to understand that a period of prolonged wet
weather or spring thaw will make the road impassable. If that’s acceptable,
and aggregate is not available at a reasonable haul distance and affordable
price, then those roads will perform at their lower cost.”
Ills of unpaved roads
Unpaved roads suffer from a variety of ills, nearly
all water-related, and similar to paved road maladies in appearance only.
Here are the most important ones:
Rutting. To fight rutting, improve drainage,
remove the weak subgrade soils, and replace them with a select material of
greater strength, possibly also adding a geosynthetic. Persistent rutting
can only be fixed through subgrade improvement. Here geotextiles and
geoforms, or chemical or mechanical stabilization come into play. Improved
drainage always will help, because rutting always is associated with weak
subgrades that are made plastic by undrained water. If the rutting is
seasonal and happens under heavy harvest hauls but not under normal
stresses, the owning agency may just live with it, Skorseth said. “They
reshape the road after harvest and for the remaining 10 to 11 months out of
the year the road performs well.”
Corrugation or washboarding. Corrugation can be
caused by lack of moisture, and by bad driving habits.
A car or truck’s shock absorbers bounce in response
to surface irregularities. On an unpaved road, at each downward response,
dry, incohesive surface materials are forced downward or displaced as the
shock absorber or suspension rebounds upward. As a series of ruts and dips
appears, subsequent vehicles begin to bounce or oscillate at the same spots
and the early corrugation reinforces itself.
“Even with the best of aggregates, the road will not
get enough moisture so the fine material can work into a cohesive surface,”
Skorseth said. “You can do very aggressive reshaping during a moist period,
and temporarily remove the corrugation, but the longer-term fix always is
through altering the material, through better percentages of fine material
in the aggregate, or going further with stabilization of the aggregate,
treating good aggregate in place with liquid magnesium chloride, or calcium
chloride in liquid or flake form.”
He recommends the top inch of aggregate be loosened
and uniformly shaped ahead of application of liquid chloride compounds, so
the compounds can get penetration up to 1.5-inches deep. These materials are
hydrophilic, and draw moisture from the atmosphere, keeping the surface damp
and bound, so to speak.
Potholing. Potholing on gravel roads is the result
of poor drainage, often due to the lack of a crown. There’s a difference
between potholing and rutting. Potholing invariably is the result of a
failure to drain a surface properly. Wherever the water collects, the
surface will be softened, and tires will pound the material out and the
pothole will grow from small to severe. The roadway surface must be reshaped
and good crown established if it wasn’t there in the first place.
Soft spots. Soft spots come from weak subgrade, or
nonuniform aggregate as placed. “A small section with too much clay or
fines, perhaps through aggregate segregation, can result in pockets of too
much plastic, fine material,” Skorseth said. It must be dug out and
replaced.
Depressions. Depressions can be created in an
unpaved road surface as the result of imperfect grading. “Nine times out of
10 the grader and blade maintenance will be operated too fast, and the
grader begins to lope or bounce,” Skorseth said. “Or, the motor grader loses
its stability and may rock from side to side. Depressions will be cut into
the roadway, but they can be eliminated by reducing motor grader speed or
adjusting the angle and pitch of the moldboard, sometimes through
articulation of the motor grader to a different angle to maintain stability.
Motor grader tire pressures will affect stability as well.”
Environmental problems
Despite their bucolic appearance, unpaved roads can
degrade the local environment.
“To residents living along unpaved roads, the
traffic-generated dust penetrates their homes causing a nuisance and health
problems such as hay fever and allergies,” say Colorado’s Addo, Sanders, and
Chenard in their May 2004 report, Road Dust Suppression: Effect on
Maintenance Stability, Safety and the Environment Phases 1-3.
“Crops and vegetation near unpaved roads can be
covered with the airborne dust stunting their growth due to the shading
effect and clogging of the plant’s pores,” they say. “Fine particles
resulting from traffic actions can also be washed off during precipitation
events and carried into nearby creeks, streams, and lakes, increasing their
respective particulate loading. For motorists using the unpaved roads the
traffic-generated dust can reduce visibility and cause driving hazards.”
Fight fugitive dust pollution by stabilizing
surfaces for unpaved roads in heavily populated areas where particulate
matter in the atmosphere is a real issue, Skorseth said. This includes
semi-arid regions with frequent dusting, high traffic volume areas, and
roads with poor natural aggregates that won’t remain bound.
“Environmental concerns are a byproduct of other
aggravated issues, such as corrugation or loose aggregate on the road,” he
said. “At that point it will be cost-effective to use a stabilizer,
particularly on higher volume roads, where they will reduce overall
maintenance costs. Dust control there is a bonus, but the tradeoff to the
local agency is that you have dramatically reduced the loss of fine
material, you don’t have to do blade maintenance as frequently, and you
don’t have to replace surface gravel as frequently. It works for everyone.
But it’s not cost-effective on a very low volume road.”
What constitutes a low-volume threshold depends on
the local climate. In a moist climate, as many as 100 cars per day won’t
stir up dust, but in an arid or semi-arid climate, 25 cars can cause a dust
problem.
“When the cost of stabilization is less than the
greater need to add aggregate or blade maintenance, then do it,” Skorseth
told Better Roads. “We don’t see a lot of test sections on unpaved roads,
but I believe in them when using a unique product. Always do a test section
and observe it for a year. It could be 300 to a 1,000 feet in length. You
will see if the stabilizer will indeed work with your soils or aggregates. I
have seen too many local governments buy into a particular product because
they’ve seen good performance in a neighboring agency, or have been promised
good performance by a vendor. In the worst case scenario I saw 40 miles done
with poor performance, and everybody gets upset.”
The need for shared research is borne out in recent
research by North Dakota State University’s Upper Great Plains
Transportation Institute. “Research identified that some North Dakota
counties are facing a shortage in quality gravel,” UGPTI researchers said.
“Other counties may experience a shortage in the near future. The use of
chemical additives, such as soil stabilizers and dust suppressants, may help
to reduce the need and demand for gravel ... [h]owever, not all of these
products will work on every soil type. The objective of this study was to
survey county road officials about their use of chemical additives to
stabilize the soil and reduce dust.”
For this work, questionnaires were mailed to the
county engineers and road supervisors in Colorado, Montana, North Dakota,
South Dakota, Utah, and Wyoming. According to the survey results, 60
agencies used 90 chemical additives for dust control/soil stabilization. Of
the categories identified, the chloride additives were the most widely used
(64%). Also used were clay additives (18%), bituminous binders (8%), and
adhesives (6%). “Most of the agencies that used chemical additives stated
that they had success with the products,” UGPTI said. More information is
available at their Web site at www.ugpti.org.
Sedimentation
Sedimentation can be controlled by careful
consideration of roadway shape and by improving drainage. “Sometimes we see
excessive crown in an unpaved road,” Skorseth said. “Heavy rainfall will
flow off so fast that it will wash fine material with it.”
But the great enemy of streams is silt-laden water
that comes directly from the road into the waterway. In a proper drainage
scheme, it drains to a shoulder line and right over the hinge point to the
ditch, where silts can be filtered out by the vegetation. When berms or
windrows of gravel form along the travel path, they can create a ditch on
the road; during heavy rains, water will flow down that secondary ditch,
carrying silt and moving small aggregate, eventually spilling over into a
swale. The problem occurs particularly in rolling terrain,
Sedimentation can lead to critical situations. For
example, in 2002, watershed biologist Reuven Walder, Salmon Protection and
Watershed Network in Forest Knolls, California, used a grant from the
National Fish and Wildlife Foundation to assess unpaved roads suspected to
be contributing excessive amounts of sediment to the stream system.
Crews surveyed a total of 11,241 feet (2.13 miles)
of dirt and gravel non-maintained county road and private road, and 22
stream crossings in the San Geronimo sub-Watershed between March and June
2002. Based on these surveys, an estimated 21,097 cubic yards of sediment
were delivered to the creek from these roads. Calculations estimate that 758
cubic yards will be delivered to the creek system in the next 10 years,”
Walder said. “It is critical to address these roads and prevent further
unnecessary degradation to habitat of threatened salmonid populations.”
When to pave
Paving an unpaved road can elevate the road to
higher service levels and satisfy local voters. But because the costs of
maintaining unpaved roads are so low, and because they can serve a limited
number of vehicles so well, agencies should think twice before they commit
to paving an unpaved road.
“Too often we bow to public pressure to pave an
unpaved road, without a real commitment to design of a good pavement, and
without an adequate budget,” Skorseth said. “Quite honestly, all we do is
trade gravel road maintenance problems for asphalt road maintenance
problems. And then, asphalt road maintenance problems are so much more
costly to repair, especially as you get deeper into the life cycle.”
The University of Kentucky’s Kentucky Transportation
Center adapted an essay from the Vermont Local Roads Program of St.
Michael’s College, which states the issues succinctly. Ten questions must be
answered, the center says, stating unpaved roads should be paved only:
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After developing a road management program.
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When the local agency is committed to effective
management.
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When traffic demands it.
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After standards have been adopted.
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After considering safety and design.
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After the base and drainage are improved.
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After determining the costs of road preparation.
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After comparing pavement costs, pavement life, and
maintenance costs.
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After comparing user costs.
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After weighing public opinion.
You can review this entire essay at
www.ltapt2.org/gravel/gravelroads11.htm#Introduction.
“Traffic volume and weight directly affect road
longevity,” says Marisa DiBiaso of the New Hampshire Technology Transfer
Center. “The New Hampshire DOT recommends that roads with less than 50
average daily traffic be unpaved. For ADT over 200, the DOT recommends an
asphalt-paved surface. For roads between 50 and 200 ADT, road managers
should consider vehicle weights and past performance. If the unpaved road is
performing well, with reasonable maintenance costs, paving is rarely
justified. They should, however, consider applying a dust suppressant, which
will also stabilize the road surface.”
Measuring the Roughness of Unpaved Roads
A new, vehicle-mounted, digital road roughness
sensing and logging system can help road managers manage their unpaved and
paved roads, and better plan maintenance.
The Roughometer II system from Humboldt
Manufacturing provides a practical, portable method of quickly assessing
road quality for both paved and unpaved roads, permitting road agencies to
manage grading schedules and evaluate materials for unpaved roads, and plan
pavement preservation operations and evaluate materials for paved roads.
The unit can be installed in almost any vehicle,
making it easy to collect road surface quality data and downloading it to a
PC. Roughness values are displayed in International Roughness Index or
NAASRA Roughness Measurement in table form or as a graphical output showing
how roughness values vary along the surveyed route. More information is
available at http://64.177.189.124/ email_02.html.
Bridging Low-Volume Roads Creatively
To provide Iowa county engineers with low-cost
bridges, the Bridge Engineering Center at Iowa State University investigated
the feasibility of using railroad flatcars as the superstructure for bridges
on low-volume roads.
Several characteristics make flatcars desirable for
bridge superstructures: flatcars are easy to install; can be used on current
or new abutments; they are available in various lengths; and they are
relatively inexpensive. A feasibility study indicated that properly designed
flatcar bridges are capable of supporting Iowa legal loads. Two flatcar
demonstration bridges confirmed the constructability, adequacy, and relative
economy of the concept.
As a result of this research, the use of railroad
flatcars has helped Iowa counties economically address particular
transportation requirements. Counties can purchase railcars for a fraction
of the cost of steel beams and decking materials. Iowa’s Winnebago County
constructed a 89-foot by 27-foot railroad flatcar bridge at a cost of less
than $30 per square foot — a substantial savings from the nearly $70 per
square foot for a standard concrete slab bridge in Iowa. The savings for the
six flatcar bridges constructed to date in Iowa exceed the cost of the
research.
More information is available at
http://trb.org/
publications/trnews/trnews234flatcarbridges.pdf.
Glossary of Gravel Road Terms
Following is a glossary of terms used in gravel
roads maintenance and design, adapted from Gravel Roads Maintenance and
Design Manual, by Skorseth and Selim, South Dakota LTAP. See For More
Information sidebar for downloading information.
Articulation. Relating to gravel roads, it refers
to a machine with a jointed main frame. This assists in steering the
machine, allowing it to work in an angled configuration, yet move forward in
a straight line.
Ballast. Extra weight added to a machine such as
iron weights mounted to the wheels or frame. Liquid material such as a
water/calcium chloride solution placed in the tires can also serve as
ballast.
Density. The weight of material in pounds or
kilograms per unit of volume (cubic feet or meters).
Grader. Any device either self-propelled or
mounted on another machine used for final shaping and maintenance of earth
or aggregate surfaces. Occasionally, a simple, towed drag-type device is
referred to as a grader.
Gravel. A mix of stone, sand, and fine-sized
particles used as sub-base, base, or surfacing on a road. In some regions,
it may be defined as aggregate.
Moisture Content. That portion of the total weight
of material that exists as water.
Moldboard. The part of the grader that is actually
used to cut, mix, windrow, and spread material.
Motor Grader. Any self-propelled machine designed
primarily for the final mixing and shaping of dirt or surfacing material.
Sometimes referred to as a maintainer, patrol, or simply a “blade”.
Optimum Moisture. The percentage of water (by
weight) in material that allows it to be compacted to achieve greatest
density.
Paved Road. Any road that has a semi-permanent
surface placed on it such as asphalt or concrete. Gravel surfaced roads are
virtually always referred to as unpaved roads.
Segregation. A problem that arises when the coarse
and fine material separates and no longer forms a uniform blend of material.
Windrow. A ridge or long, narrow pile of material
placed by a grader while performing construction or maintenance operations.
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