DEFINITION OF CLAY

A fine-grained sediment with particles less than

0.000015 inch (1/256 mm) in diameter. Clays display colloidal

behavior as they are easily suspended in a fluid and

may not precipitate until different particles stick together to

form larger particles. Clay minerals include a number of

loosely defined amorphous or finely crystalline hydrous aluclay

minosilicates that form monoclinic lattices with two to three

repeating layers. Most clay minerals form by the chemical

alteration or weathering of silicate minerals such as feldspar,

pyroxene, and amphibole, and they may also be abundant in

the alteration zones around mineral deposits. Clay minerals

typically accumulate in soils, clay deposits, and shales. The

most common clay minerals include kaolinite, montmorillonite,

and illite.

Clay has been used for making pottery and other artifacts

since Neolithic times, because it is easily shaped when

wet, and baking clay items in a furnace transforms them into

rigid, impermeable vessels. However, some clays are hazardous

in certain circumstances. For instance, a group of

clays known as expansive clays expand by up to 400 percent

when water is added to their environment. The expansion of

these clays is powerful and has the force to crumble bridges,

foundations, and tall buildings slowly. Damage from

expanding clays is one of the most costly of all natural hazards

in the United States, causing billions of dollars in damage

every year.

In areas with expansive clays it is important to know the

shrink/swell potential of a soil before construction begins in

an area. The shrink/swell potential is a measure of a soil’s

ability to add or lose water at a molecular level. Expansive

clays add layers of water molecules between the plates of clay

minerals (made of silica, aluminum, and oxygen), loosely

bonding the water in the mineral structure. Most expansive

clays are rich in montmorillonite, a clay mineral that can

expand up to 15 times its normal dry size. Most soils do not

expand more than 25 to 50 percent of their dry volume, but

an expansion of 3 percent is considered hazardous.

Damage from shrinking/swelling clays is mostly to

bridges, foundations, and roadways, all of which may crack

and move during expansion. Regions with pronounced wet

and dry seasons tend to have a greater problem with expansive

clays than regions with more uniform precipitation distributed

throughout the year. This is because the adhered

water content of the clays changes less in regions where the

soil moisture remains more constant. Some damage from

shrinking and swelling soils can be limited, especially

around homes. Trees that are growing near foundations can

cause soil shrinkage during dry seasons and expansion in

wet seasons. These dangers of shrinking/swelling clays can

be avoided by not planting trees too closely to homes and

other structures. Local topography and drainage details also

influence the site-specific shrink/well potential. Buildings

should not be placed in areas with poor drainage as water

may accumulate there and lead to increased soil expansion.

Local drainage may be modified to allow runoff away from

building sites, reducing the hazards associated with soil

expansion.

In general, soils that are rich in clay minerals and organic

material tend to have low strength, low permeability, high

compressibility, and the greatest shrink/swell potential. These

types of soils should be avoided for construction projects,

where possible. If they cannot be avoided, steps must be

taken to accommodate these undesirable traits into the building

construction. Sand and gravel-rich soils pose much less

danger than clay and organic-rich soils. These soils are well

drained, strong, have low compressibility and sensitivity, and

low shrink/swell potential.

See also WEATHERING.

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