IOWA DEPARTMENT OF TRANSPORTATION
ROCKS AND ROADS
Think of the roadway as a layer cake.
The bottom layer is the subgrade. The subgrade is made of soils that have been specially prepared to meet the requirements to support the other two layers.
The middle layer is the base. The base is a mixture of crushed rock.
The top layer is the pavement. The pavement is the material on top of the roadway.
Letís begin with the soils in the subgrade. As you learned in Unit 1, earth materials can be described by their composition and deposition. The composition is what they are made of. Deposition is how they got there. The composition and depositional environments of earth materials control their engineering properties.
The engineering properties include:
Strength is the measure of the tendency to bend under pressure. Low-strength materials bend more than high-strength materials. On roadways the pressure comes from the weight of vehicles rolling over the pavement.
Permeability measures the ability of water to pass through the material. High permeability means water will pass through quickly, while low permeability means water will pass through very slowly.
Particle size is the measurement of the size of the individual particles that make up the material. They can take this measurement either on the natural material or after they have crushed the material. Particle size affects all of the other three properties mentioned.
One way to separate earth materials is by particle size. Letís discuss the engineering properties of the most common types of particles, based on size. Larger-size materials such as boulders and cobbles are not used for road or embankment construction. Smaller size materials such as sands and silts are often described as soil.
Boulders: Boulders are large pieces of rock. Boulders are often rounded and are larger than 12 inches in diameter.
Cobbles: Cobbles are similar to boulders. The sizes range from about 3 to 12 inches in diameter.
Gravel: Gravels are broken pieces of larger rocks. Gravels are often rounded. Gravels range in size from between 1/10 of an inch to 3 inches diameter. Gravels can vary in density, have high strength if confined in a small space, and have very high permeability.
Sand: Sands are granular earth materials. Sands can be rounded or angular. Sands range in diameter from about 1/10 to 1/300 of an inch. Sands usually have medium to high density, medium to high strength if confined in a small space, and relatively high permeability.
Silt: Silts are also granular earth materials and are similar to sand. Silts range in diameter from about 1/300 to1/13,000 of an inch. They usually have low to medium density and strength, and medium permeability.
Clay: Clays are very small, flat earth materials. Clays range in diameter from about 1/13,000 to 1/200,000 of an inch. Clays typically have low to medium strength and density, and low to very low permeability.
These materials can be found in the deposits left by glaciers, streams and rivers, and wind. These deposits often do not consist of just one type or size of particles. They are a combination of several particle sizes. Some of these types of deposits are:
Glacial till: Glacial till is a mixture of boulders, gravel, and, silt and clay deposited by glaciers. These deposits are usually very dense and compacted because at one time glaciers were sitting on top of them.
Loess: Loess is mainly composed of silt with some clay. Wind-deposited loess is very common in western Iowa along the Missouri River.
River silts and clays: These deposits are found along rivers. They are composed of silt and clay (as you might have guessed!). There are often sand layers and seams found in these deposits. These deposits form when rivers flood above their natural banks and then recede, leaving sediment behind in the flooded areas.
River and glacial sands: River and glacial sand deposits range from pure sand to sand with large amounts of interbedded silt, clay and gravel.
Topsoil: Topsoil is a mixture of clay, silt, sand and organic material. Topsoil forms as the exposed earth surface weathers and breaks down, incorporating decomposing plants.
TO: Unit 2 B. Base