Aggregates
Aggregate
is the formal name for crushed rock, for rock broken up before
use. Limestone or dolomite are the most common kinds of rock
crushed for aggregate. One very visible use of aggregate is
for "gravel" roads, roads where layers of crushed rock provide
a surface superior to that provided by soil or earth.
Each piece of aggregate comes from the rock crusher as an angular
fragment. These rock fragments never quite fit together again,
leaving many small gaps, or pores, between solid bits of rock.
Water can drain easily through these pores, but they remain
open, even when compressed by a heavy load, because of contacts
between strong, difficult-to-compress, pieces of aggregate.
Limestone and dolomite make the best aggregate because they
are relatively soft. Sharp edges break off, leaving rounded
edges in contact with your 80,000 mile tires. Soft rocks are
also easier on rock crushers than hard rocks would be.
Good think limestone is appropriate for aggregate. Crushed quartzite
is used for road metal. Quartzite is harder than steel, but
this quartzite is brittle and it shatters into splinters in
the crusher. Roads surfaced in quartzite aggregate are long
lasting but hard on tires. Edges remain sharp for years and
a fragment can penetrate a tire if wedged into the tread.
Russia also has few good sources of limestone or dolomite for
road metal. For two centuries, this proved an advantage. Armies
of Napoleon and Hitler got bogged down in muddy Russian roads.
Supply lines were unreliable, cavalry and tanks immobilized,
artillery left deployed in a most inefficient manner. While
the U.S.A. built a network of strategic defense highways (the
Interstates) and a farm-to-market system of paved roads, Russia
viewed highways as potential invasion routes and allowed its
surface transportation system to remain dominated by canals,
rivers and railroads. Today, this lack of surface transportation
infrastructure poses a serious challenge to agricultural efficiency
in the former Soviet Union. Subsistence farmers might survive
without good roads, but unreliable or costly transportation
raise to cost and threaten the quality of food supplies.
Aggregate is even more important for paved highways than it
is for gravel roads. Water is a highway's enemy. The first attempts
to construct a log road through the Great Black Swamp of northwest
Ohio resulted in a turnpike that continuously sank into the
mud. Water-saturated soil (mud) flows under pressure. It moves
to the side, not simply downward. In some places along Ohio's
log road, construction crews lost count of how many logs had
sunk out of sight into this apparently bottomless swamp. Freezing
water is also destructive. Water expands as it freezes, making
small holes larger and breaking apart the pavement.
A well-engineered highway includes ditches and a bed of aggregate
to drain away the water. Pavement is supported by a thick bed
of aggregate, compacted by heavy rollers so that it will not
deform further by traffic, but retaining many pores through
which water can escape into drainage ditches. Aggregate is also
used to isolate foundations from damaging effects of expansive
soils.
The main factor that determines the price of aggregate is the
cost of transportation from quarry to customer. A quarry 25
miles from a job might ship 8 loads per truck per day to that
job, while a quarry 50 miles away is limited to 4 loads per
truck per day. Most aggregate is used within 50 miles of the
quarry from which it is extracted. Loading and unloading railroad
cars or barges with aggregate raise costs.
Limestone quarries impact the environment in a variety of ways.
Truck traffic (noise, exhaust, dust, traffic accidents, roads
damaged by heavy loads) is the most common complaint. Quarry
operators usually purchase buffer strips that keep dust and
noise from the quarry contained, but rock is frequently loosened
by blasting. Quarry blasts, even those too light to damage nearby
structures, disturb the neighbors. It is not uncommon for a
quarry operator to install a temporary vibration monitor to
prove that ground motions from blasts fall within permit limitations.
Shots while the monitor is running tend to be only fraction
the size of normal shots, but lawyers for the quarry use this
technical information to silence complaints. Once the vibration
monitoring contractor leaves, blasts return to their normal
levels. This is difficult to prove unless a permanent seismograph
station is in operation within 10 or 20 miles of the quarry.
Some limestone quarries extend below the water table. When this
occurs, pumps are needed to keep equipment dry. In some cases,
the limestone is low permeability and water wells are not seriously
drawn down. However, some quarries have drained the water from
aquifers a mile or more from the quarry. Where laws regarding
groundwater ownership and theft are vague in this matter, property
owners seeking restoration of their water supply face an uphill
fight.
The fact that many quarries fill with water after they close
shows that they are connected to groundwater aquifers (most
geologists already know this, but it is frequently useful to
point to evidence more obvious to the average citizen).
Limestone forms on the floor of warm tropical seas. Unlike many
chemicals, calcium carbonate is less soluble in warm water than
in cold water. Many mollusks and coral colonies grow shells
of calcium carbonate in a crystalline form called aragonite.
After these animals die, seawater dissolves some of this chemical.
When CaCO3-saturated water moves from cold depths into warm
shallow waters, it precipitates out of solution but in the more
difficult to dissolve crystalline structure of calcite. Limestone
we mine today represents deposition on the floors of prehistoric
oceans. Today, thick beds of limestone and dolomite (MgCO3)
are accumulating in The Bahamas and in shallow seas of the western
Pacific.
