Rocks and soils have several
other uses apart from the
ones we just finished discussing. Some of the ways we
use rock are for revetment and for granular surfacing
(gravel roads).
Revetment is material used
to protect soil from surface
erosion on structures in and around rivers and streams. As
you have learned earlier, the higher the volume and
velocity of water flow, the larger the size of material
that can be transported. Revetment is stone that
is large enough to not erode away. You might see
crushed stone material from about 3 to 9 inches in
diameter in ditch bottoms to prevent erosion.
The
Iowa Department
of Transportation uses
larger rock to protect
the bridge piers from
erosion. This material is
often called riprap; each
piece can range in size from
approximately 6 inches to
3 feet in diameter, and can
weigh up to 650 pounds.
Erosion stone is material
used in ditch bottoms to
prevent run off and erosion. You might see crushed
stone material from about 3 to 6 inches in diameter.
Roads with granular surfacing
is also known as gravel
roads. You have probably all ridden on or at least seen
a gravel road. These types of roads are commonly used
in the country where the amount of traffic is very low.
Crushed rock is spread on a compacted earth subgrade
to reduce the rutting effects of traffic and erosion due
to water runoff. Periodically, new crushed rock must be
added to replace what is lost to erosion and maintenance
activities like snow plowing.
Earlier in this section,
we talked about topsoil, but we
never said how or if we use it. So how do we use topsoil?
Topsoil is not used in the pavement system because it is
not a strong, durable material, and is not highly permeable.
Topsoil is also not used in embankments because the
organic material in the topsoil can decompose. If it
decomposed, the reduced volume would lead to
pavement settlement.
Remember when we said that
Iowa topsoil is excellent
for growing things? Topsoil is spread over embankments
and other disturbed areas to allow grass and other
plants to grow. The plants act as a kind of revetment,
preventing surface water from eroding away the soil.
Often the first step in construction
is to use
bulldozers to scrape off the topsoil into a big pile.
The topsoil is spread out after everything is
constructed. You could say that topsoil is the
first and last step in highway construction.
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