Iowa DOT's
policy is to be
sensitive to the environment
and foster and enhance
environmental stewardship.
IOWA DEPARTMENT OF TRANSPORTATION
Presents
ROCKS AND ROADS
Unit 2: What materials
are used in road
construction?
B. Base
Onto the middle layer of the cake - the base.
As we mentioned earlier, this layer consists of rocks. You may be wondering, how do we get the rocks that we use in paving? The rocks come from quarries, underground mines, and gravel pits. Do you know where the closest quarry is to your school? Well, look on the Iowa Quarry Map and see if you can find it.
Rocks from quarries and mines go through several processing steps before they are ready to be added to the concrete mix.
1. Holes are drilled either into the quarry wall (face), or into the quarry floor.
2. The holes are filled with dynamite and they blast the face. The rock comes out in large boulders. They call the large boulders "shot rock" because it has just been "shot" from the face of the quarry.
3. The shot rock gets scooped into large end loaders. The endloaders put the rock into hauling trucks. The hauling trucks take the rock over to a conveyor belt. The rock is unloaded and placed on a conveyor belt. The conveyor belt carries the rock to the crushing machine.
4. The crusher breaks it down into many different sized pieces. For instance, a one-ton boulder can be reduced to a pile of ¾-inch sized pieces after going through the crusher. If the "crushed" rock is going to be used in portland cement concrete (also known as PCC), the crushed rock must be washed.
5. Rocks are put into stockpiles according to their size.
6. Iowa Department of Transportation materials technicians take samples from these stockpiles. The technicians test the material to see if it meets the requirements for the desired usage.
7. Once the rocks have passed the testing phase, the technician certifies them. The rocks can then be loaded onto the contractor’s trucks and hauled to the job site or paving plant.
So as you can see, there are many steps to getting the rocks needed for a paving job.
Rocks from gravel pits or pit lakes go through fewer steps than quarried or mined rocks because they do not have to go through the blasting stage. The two basic methods for getting rocks from a gravel pit or pit lake are by dredging or dragline.
Dredging is a process by which the rocks are pumped out using a very large suction hose. The hose is placed in the pit and they pump the rocks out of the "pit lake" and into a washing box. Once they wash them, the rocks are placed on a conveyor that passes over a series of screens that divide them by size. They pile each of the sizes separately because different sizes go into different products.
Dragline is a process by which a giant scoop is lowered on the end of a long line into the "pit lake." The scoop is then dragged along the bottom over to the edge of the lake. The material in the scoop goes through the same washing and sizing process used in the dredging process. Iowa DOT materials technicians also test rocks from gravel pits or pit lakes to see if the rocks meet the requirements for the desired usage.
For dry gravel pits, the topsoil
is scraped off the ground. The deposit is scooped out by an end loader and
loaded onto trucks. The trucks take the rocks over to conveyors.
The rock is unloaded onto the conveyors. The conveyors carry the
rock through the same washing and sizing process described for wet
pits.
TO: Unit
2 C. Rocks are tested, too?
