What is Soil?

A surficial material formed by chemical, physical, and biological weathering.

What Variables Control the Soil in an Area?

Climate and weather

Topography

Time

Parent material

Vegetation (dependent on the climate, weather and water)

Biological and chemical agents

How Do Soils Vary?

Grain size and hardness -

(There are 3 basic particle sizes; these create the 3 basic soil types: sand, silt, and clay.)

Color

Grain size and shape

Chemical composition

Amount of pore spaces - open spaces filled with air

Amount of moisture

Permeability

Why is Soil Important to Consider in an Earthquake?

Although structures are supported on soil, most of us rarely consider soil, its differences, and its subsequent effect on structures in an earthquake. Some soil is hard, like rock, and can support over 40 tons per square foot (Levy & Salvadori, 1992), while other soil is weak, like loose sand. Different soil properties can affect seismic waves as they pass through a soil layer. In some areas, there may be many different types of soils layered one upon another before hard rock is encountered. Sometimes, ground shaking will be amplified. This will influence what needs to be done to structures to help them fare better in an earthquake. Also, a phenomenon known as liquefaction or ground failure can occur in moderate to major earthquakes.

What is Liquefaction?

When there is ground water less than 30 feet from the surface in soils that contain layers of sand, the pressures generated by repetitive squeezing of the earth by several seconds of seismic wave vibrations will cause the ground water to flow up and out. When this occurs, the sand grains, which have no strength except when touching each other, are forced apart. The ground the takes on the properties of a semi-solid. When it happens over a large area, houses and buildings with inadequate foundations may actually sink slightly. When liquefaction happens in a small area, liquefied sand can be ejected to the surface through fissures in the overlying layers. Soil failure, as described earlier, will have a larger impact on pipelines and pile foundations, and other structures below the surface of the earth.

Does Liquefaction Always Occur During an Earthquake?

No. Liquefaction occurs only under ideal conditions as a result of an earthshaking event and is controlled by the following variables:

Grain size of the soil

Duration of the earthquake and amplitude and frequency of shaking

Distance from the epicenter

Location of the water table

Cohesiveness of the soil

Permeability of the layer

Information provided by: http://mceer.buffalo.edu