COMPOSITION OF SOILS
Soils are formed from the physical and chemical weathering of rocks. Physical weathering involves reduction of size without any change in the original composition of the parent rock. The main agents responsible for this process are exfoliation, unloading, erosion, freezing, and thawing. Chemical weathering
causes both reductions in size and chemical alteration of the original parent rock. The main agents responsible for chemical weathering are hydration, carbonation, and oxidation.
Often, chemical and physical weathering take place in concert. Soils that remain at the site of weathering are called residual soils. These soils retain many of the elements that comprise the parent rock. Alluvial soils, also called fl uvial soils, are soils that were transported by rivers and streams. The composition of these soils depends on the environment under which they were transported and is often different from the parent rock. The profi le of alluvial soils usually consists of layers of different soils. Much of our construction activity has been and is occurring in and on alluvial soils. Glacial soils are soils that were transported and deposited by glaciers. Marine soils are soils deposited in a marine environment.
Common descriptive terms such as gravels, sands, silts, and clays are used to identify specifi c textures in soils. We will refer to these soil textures as soil types; that is, sand is one soil type, clay is another. Texture refers to the appearance or feel of a soil. Sands and gravels are grouped together as coarse-grained soils. Clays and silts are fi ne-grained soils. Coarse-grained soils feel gritty and hard. Fine-grained soils feel smooth. The coarseness of soils is determined from knowing the distribution of particle sizes,
which is the primary means of classifying coarse-grained soils. To characterize fi ne-grained soils, we need further information on the types of minerals present and their contents. The response of fi ne-grained soils to loads, known as the mechanical behavior, depends on the type of predominant minerals present.
Currently, many soil descriptions and soil types are in usage. A few of these are listed below:
- Alluvial soils are fi ne sediments that have been eroded from rock and transported by water, and have settled on river and stream beds.
- Calcareous soil contains calcium carbonate and effervesces when treated with hydrochloric acid.
- Caliche consists of gravel, sand, and clay cemented together by calcium carbonate.
- Collovial soils (collovium) are soils found at the base of mountains that have been eroded by the combination of water and gravity.
- Eolian soils are sand-sized particles deposited by wind.
- Expansive soils are clays that undergo large volume changes from cycles of wetting and drying.
- Glacial soils are mixed soils consisting of rock debris, sand, silt, clays, and boulders.
- Glacial till is a soil that consists mainly of coarse particles.
- Glacial clays are soils that were deposited in ancient lakes and subsequently frozen. The thawing of these lakes revealed soil profi les of neatly stratifi ed silt and clay, sometimes called varved clay. The silt layer is light in color and was deposited during summer periods, while the thinner, dark clay layer was deposited during winter periods.
- Gypsum is calcium sulfate formed under heat and pressure from sediments in ocean brine.
- Lacustrine soils are mostly silts and clays deposited in glacial lake waters.
- Lateritic soils are residual soils that are cemented with iron oxides and are found in tropical regions.
- Loam is a mixture of sand, silt, and clay that may contain organic material.
- Loess is a wind-blown, uniform, fi ne-grained soil.
- Marine soils are sand, silts, and clays deposited in salt or brackish water.
- Marl (marlstone) is a mud (see defi nition of mud below) cemented by calcium carbonate or lime.
- Mud is clay and silt mixed with water into a viscous fluid.
Minerals are crystalline materials and make up the solids constituent of a soil. The mineral particles of fi ne-grained soils are platy. Minerals are classifi ed according to chemical composition and structure. Most minerals of interest to geotechnical engineers are composed of oxygen and silicon—two of the most abundant elements on earth. Silicates are a group of minerals with a structural unit called the silica tetrahedron. A central silica cation (positively charged ion) is surrounded by four oxygen anions negatively charged ions), one at each corner of the tetrahedron (Figure ). The charge on a single tetrahedron is 24, and to achieve a neutral charge cations must be added or single tetrahedrons must be linked to each other sharing oxygen ions. Silicate minerals are formed by the addition of cations and interactions of tetrahedrons. Silica tetrahedrons combine to form sheets, called silicate sheets or laminae, which are thin layers of silica tetrahedrons in which three oxygen ions are shared between adjacent tetrahedrons (Figure ). Silicate sheets may contain other structural units such as alumina sheets.
Alumina sheets are formed by combination of alumina minerals, which consists of an aluminum ion surrounded by six oxygen or hydroxyl atoms in an octahedron (Figure ). The main groups of crystalline materials that make up clays are the minerals kaolinite, illite, and montmorillonite. Kaolinite has a structure that consists of one silica sheet and one alumina sheet bonded together into a layer about 0.72 nm thick and stacked repeatedly (Figure ). The layers are held together by hydrogen bonds. Tightly stacked layers result from numerous hydrogen bonds. Kaolinite is common in clays in humid tropical regions. Illite consists of repeated layers of one alumina sheet sandwiched by two silicate sheets (Figure ). The layers, each of thickness 0.96 nm, are held together by potassium ions.