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Neal Bolton
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By
Neal Bolton
Flipping
on a light switch or turning on a faucet: These are
acts so common that we take them for granted. We never
give them a thought as long as they’re working.
And so it is with the roads at your landfill.
It’s
assumed that you’ll keep the roads in good shape.
And so when you do, there’s seldom a pat on the
back or a little something extra in your paycheck; it’s
just part of the job. And there lies the rub. Sure,
it’s part of the job, but a darn tough job it is,
even more so because it’s often a thankless job.
Wouldn’t
it be great to build roads like the Romans built? With
plenty of thought to site selection and careful construction
(we’re talking stones that were hand-placed), these
roads were built to last. In fact, many are still in
use today!
Think of
it: Roads that last not just through the wet season
but through the centuries. Now that’s durability.
Anything that can stand up to the test of time–for
that length of time–is likely to offer some good
lessons for today’s road builder. Let’s look
at some of the things that make for durable roads.
Design
Considerations
A good road
starts with a good design. But don’t get hung up
on the word "design." Depending on the application,
the design could be a detailed set of engineered blueprints
or something sketched on the whiteboard in the back
office. A design is simply the "think" before
you act or the "look" before you leap. Here
are some things to think about before you take the leap
of building a new road at your landfill.
1.
Type of Road
Good design
means matching the road to the application. Consider
the type of vehicles that will use the road. Must it
handle 40-ton transfer trucks … or light, periodic
traffic (as would be the case if the road leads only
to a groundwater monitoring well)?
Developing
a checklist can help you not miss some important aspect
of the road design. A basic example is shown below.
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Road
Design Checklist
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Item
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Purpose
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Criteria
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Typical
Choices
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Cross-Slope
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Cross-slope
helps direct water off the road.
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Too
little cross-slope will allow ponding, erosion,
and increased maintenance and can ultimately lead
to road failure. Too much cross-slope can make
the road unsafe.
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1-2%
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Super-Elevation
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Super-elevation,
the amount of banking on a turn, helps vehicles
safely maintain higher speeds on turns.
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The
amount of super-elevation (measured in % slope)
is based on vehicle speed and curve radius. Higher
speeds and sharper turns require more super-elevation.
Too much can cause vehicles to tip toward the
inside. Too little can cause vehicles to slide
outward.
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Varies,
but typical values are in the 1-5% range
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Longitudinal
Slope
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Longitudinal
slope refers to the slope of the road either uphill
or downhill.
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While
steeper roads can reduce travel distance, roads
that are too steep can cause vehicles to move
too slow (uphill) or too fast (downhill). The
risks can increase during periods of rain, snow,
or ice.
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Up
to 8%
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Width
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Road
width includes the driving lanes and shoulder.
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Roads
should be wide enough to allow for safe two-way
traffic. Also, roads should have regular turnouts
or enough shoulder width to get stalled vehicles
off the roadway.
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Varies,
but typical values range from 16 to 30 ft.
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Underlying
Base
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The
underlying base, be it waste or soil, provides
a certain amount of strength. Any additional strength
required must be met by the base/sub-base.
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The
underlying base is typically native soil or waste.
A road built on native soil is preferred to a
road built on waste. Roads built on garbage will
usually have settlement problems.
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Varies
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Base/
Sub-Base
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Base/sub-base
refers to the prepared layer(s) of base above
the waste or native soil. Sub-base is intended
to provide strength to the road.
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The
type and quantity of sub-base varies depending
on the underlying material. On native soil, 6-12
in. of gravel may suffice; however, roads built
on top of waste might require 2-3 ft. of compacted
prior to placement of gravel. Geotextiles or discarded
asphalt/concrete rubble may also be used to replace
and/or supplement a soil or gravel sub-base.
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Soil,
pit-run, gravel, geotextiles, asphalt/
concrete rubble, etc.
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Surface
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To
provide good traction and low maintenance at a
minimum cost.
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While
the criteria can vary from one landfill to another,
the goal is to find the most economical road surface
that provides good traction and durability. It’s
also important to take into consideration such
things as tire wear, windshield damage, and dust.
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Compacted
soil, gravel, asphalt, rubble, etc.
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Drainage
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To
remove water quickly and safely from the roadway
while minimizing erosion or sedimentation problems.
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It’s
important to consider traffic volume, rainfall,
and even the cost of road failure when designing
roadway drainage systems.
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Ditches,
culverts, downdrains, etc.
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2.
Service Life
Will it provide
short-term access to a remote corner of the landfill
… or will it be used as the main access road for
the next 10 years? Knowing the answers to these questions
is critical. When it comes to road construction, longevity
costs money.
The service
life of the road will impact not only the cost of the
road but also the type of road you want to build. You
might be surprised to find that the break-even point
(i.e., based on cost) between paved roads and gravel
roads occurs fairly soon–often in just two to three
years. The key lies in the reduced maintenance cost
offered by a paved road. Of course, if the road is on
top of waste, a paved road won’t hold up very well
because of settlement.
Because of
the many factors that can go into road construction,
you might wish to use a computer to model various types
of road designs.
3.
Location
The concept
of good road layout and placement is as important today
as it was in Roman times. Obviously if you’re hand-placing
every stone in the road, you’d want to make sure
the road is in the right place. Announcing to the crew,
"The last 2 miles of road–or rather the last
27 million hand-placed stones–is, uh, in the wrong
place," could mean a quick mutiny and a short trip
to the lion’s den for the project engineer. There
was pretty low tolerance for failure, if you know what
I mean.
Today, having
to abandon and relocate an improperly placed road might
not mean curtains for the planner, but it sure can be
expensive. In a landfill application, proper road placement
is a key part of sound planning. Although you might
not expect a service life of 2,000 years, the longer
your roads last, the more economical they become.
In an ideal
situation, you’d want to locate the road on original
soil in a well-drained area where it will be unaffected
by waste settlement or the landfill operation in general.
In this case, a standard asphalt road on gravel base
would likely suffice. When your choices for the road
location are limited, however, you might have to use
an alternative design. Here are some examples:
Wood
Chips. Where a short-term road must cross a
poorly drained area, or as a quick fix across a muddy
section of an existing road, consider using chipped
wood. A layer of chipped wood will absorb lots of moisture
and provide a fairly stable driving surface. Of course,
this assumes that your landfill is chipping wood/greenwaste.
For many landfills, chipped wood is much cheaper than
gravel, especially when uses for the chipped wood/greenwaste
are limited.
Manufactured
Products. For a long-term road in a marginal
location (e.g., over waste), many landfills use manmade
materials for roadbed stabilization. These can include
various types of geotextiles (grids, fabrics, and such)
and mats made of wood, steel, or other material.
One business,
the LOMA Company, manufactures a high-density polyethylene
mat called Dura-Base. These interlocking mats can provide
a stable, all-weather driving surface. And they can
be used, reused, or stored on-site.
Every landfill
is unique. And while I suppose that if you’re running
a landfill in Rome, you’d want to build roads as
the Romans did, if you’re anywhere else you’ll
need to decide for yourself what type of road is best.
Neal Bolton
is a consultant specializing in landfill operations
and management. He is principal of Blue Ridge Services
in Atascadero, CA.
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