PRINCIPAL TYPES OF SOILS

The materials that constitute the earth's crust are rather arbitrarily divided by the civil engineer into the two categories, soil and rock. Soil is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. Rock, on the other hand, is a natural aggregate of minerals connected by strong and permanent cohesive forces. Since the terms "strong" and "permanent" are subject to different interpretations, the boundary between soil and rock is arbitrary one. As a matter of fact, there are many natural aggregates of mineral particles that are difficult to classify either as soil or as rock. In this text, however, the term soil will be applied only to materials that unquestionably satisfy the preceding definition. 

Although the terminology described in the preceding paragraph is generally understood by civil engineers, it is not in universal use. To the geologist, for example, the term rock implies all the material that constitutes the earth's crust, regardless of the degree to which the mineral particles are bound together, whereas the term soil is applied only to that portion of the earth's crust that is capable of supporting vegetation. Therefore, the civil engineer who makes use of information prepared by workers in other field must understand the sense in which the terms soils and rock are used.

On the basis of the origin of their constituents, soils can be divided into two large groups, those that consist chiefly of the results of chemical and physical rock weathering, and those that are chiefly of organic origin. If the products of rock weathering are still located at the place where they originated, they constitute a residual soil. Otherwise they constitute a transported soil, regardless of the agent that performed the transportation. 

Residual soils that have developed in semiarid or temperate climates are usually stiff and stable and do not extend to great depth. However, particularly in warm humid climates where the time of exposure has been long, residual soils may extend to depths of hundreds of meters.They may be strong and stable, but they may also consist of highly compressible materials surrounding blocks of less weathered rock. Under these circumstances they may give rise to difficulties with foundations and other types of construction. Many deposits of transported soils are soft and loose to a depth of more than a hundred meters and may also lead to serious problems.

Soils of organic origin are formed chiefly in situ, either by the growth and subsequent decay of plants such as peat mosses or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.

The soil conditions at the site of proposed structure are commonly explored by means of test borings or test shafts. The inspector on the job examines samples of the soil as they are obtained, classifies them in accordance with local usage, and prepares a boring log or shaft record containing the name of each soil and the elevation of its boundaries. The name of the soil is modified by adjectives indicating the stiffness, color, and other attributes. At a later date the record may be supplemented by an abstract of the results of tests made on the samples in the laboratory.

The following lists of soil types includes the names commonly used for field classification.

• Sand and gravel are cohesionless aggregates of rounded subangular or angular fragments of more or less unaltered rocks or minerals. Particles with a size up to 2 mm are referred to as sand, and those with a size from 2 mm to  200 mm as gravel. Fragments with a diameter of more than 200 mm are known as boulders.

Figure 1:AIMS Angularity Index

• Hardpan is a soil that has an exceptionally great resistance to the penetration of drilling tools, usually found below the uppermost topsoil layer. Most hardpans are extremely dense, well-graded, and somewhat cohesive aggregates of mineral particles.

• Inorganic silt is a fine-grained soil with little or no plasticity. The least plastic varieties generally consist of more or less equidimensional grains of quartz and are sometimes called rock flour, whereas the most plastic types contain an appreciable percentage of flake-shaped particles and are referred to as plastic silts, Because of its smooth texture, inorganic silt is often mistaken for clay, but it may be readily distinguished from clay without laboratory testing. If shaken in the palm of the hand, a pat of saturated inorganic silt expels enough water to make its surface appear glossy. If the pat is bent between the fingers, its surface again becomes dull. This procedure is known as the shaking test. After the pat has dried, it is brittle and dust can be detached by rubbing it with the finger. Silt is relatively impervious, but if it is in loose state it may rise into a drill hole or shaft like a thick viscous fluid. The most unstable soils of this category are known locally under different names, such as bull's liver.

• Organic silt is a fine-grained more or less plastic soil with an admixture of finely divided particles of organic matter. Shells and visible fragments of partly decayed vegetable matter may also be present. The soil ranges in color from light to very dark grey, and it is likely to contain a considerable quantity of H2S, CO2, and various other gaseous products of the decay of organic matter which give it a characteristic odor. The permeability of organic silt is very low and its compressibility very high.

• Clay is an aggregate of microscopic and submicroscopic particles derived from the chemical decomposition of rock constituents. It is plastic within a moderate to wide range of water content. Dry specimens are very hard, and no powder can be detached by rubbing the surface of dried pats with the fingers. The permeability of clay is extremely low. The term gumbo is applied, particularly in the western United States, to clays that are distinguished in the plastic state by a soapy or waxy appearance and by great toughness. At higher water contents they are conspicuously sticky.

• Organic clay is a clay that owes some of its significant physical properties to the presence of finely divided organic matter. When saturated, organic clay is likely to be very compressible, but when dry its strength is very high. It is usually dark gray or black and it may have a conspicuous odor.

• Peat is a somewhat fibrous aggregate of macroscopic and microscopic fragments of decayed vegetable matter. Its color ranges between light brown and black. Peat is so compressible that it is almost always unsuitable for supporting foundations. Various techniques have been developed for carrying earth embankments across peat deposits without the risk of breaking into the ground, but the settlement of these embankments is likely to be large and to continue at a decreasing rate for many years.

If a soil is made up of a combination of two different soil types, the predominant ingredient is expressed as a noun, and the less prominent ingredient as a modifying adjective. For example, silty sand indicates a soil that is predominantly sand but contains a small amount of silt. A sandy clay is a soil that exhibits the properties of a clay but contains an appreciable amount of sand.

The aggregate properties of sand and gravel are described qualitatively by terms loose, medium, and dense, whereas those of clays are described by hard, stiff, medium, and soft. These terms are usually evaluated by the boring foreman or inspector on the basis of several factors, including the relative ease or difficulty of advancing the drilling and sampling tools and the consistency of the samples. However, since this method of evaluation may lead to a very erroneous conception of the general character of the soil deposit, the qualitative descriptions should be supplemented by quantitative information whenever the mechanical properties are likely to have an important influence on design. The quantitative information is commonly obtained by means of laboratory tests on relatively undisturbed samples, or by suitable in situ tests.

A record of the color of the different strata encountered in adjacent borings reduces the risk of errors in correlating the boring logs. Color may also be an indication of a real difference in the character of the soil. For example, if the top layer of a submerged clay stratum is yellowish or brown and stiffer than the underlying clay, it was probably exposed temporarily to desiccation combined with weathering. Terms such as mottled, marbled, spotted, or speckled are used when different colors occur in the same stratum of soil. Dark or drab colors are commonly associated with organic soils.

Under certain geological conditions soils from that are characterized by one or more striking or unusual features such as a root-hole structure or a conspicuous and regular stratification. Because of these features, such soils can easily be recognized in the field and, consequently, they have been given special names by which they are commonly known. The following paragraphs contain definitions and descriptions of some of these materials.

• Till is an unstratified glacial deposit of clay, sand, gravel, and boulders.

• Tuff is a fine-grained water - or wind-laid aggregate of very small mineral or rock fragments ejected from volcanoes during explosions.

• Loess is a uniform, cohesive, wind-blown sediment, and is commonly light brown. The size of most of the particles ranges between the narrow limits of 0.01 and 0.05 mm. The cohesion is due to the presence of a binder that may be predominantly calcareous or clayey. Because of the universal presence of continuous vertical root holes, the permeability in vertical direction is usually much greater than in horizontal directions; moreover, the material has the ability to stand on nearly vertical slopes. True loess deposits have never been saturated. On saturation the bond between particles is weakened and the surface of the deposit may settle.

Figure 2: Loess

• Modified loess is a loess that has lost its typical characteristics by secondary processes, including temporary immersion, erosion and subsequent deposition, chemical changes involving the destruction of the bond between the particles, or chemical decomposition of the more perishable constituents such as feldspar. Thorough chemical decomposition produces loess loam, characterized by greater plasticity than other forms of modified loess.

• Diatomaceous earth (kieselguhr) is a deposit of fine, generally white, saliceous powder composed chiefly or wholly of the remains of diatoms. The term diatom applies to a group of microscopic unicellular marine or fresh-water algae characterized by silicified cell walls.

• Lake marl or boglime is a white fine-grained powdery calcareous deposit precipitated by plants in ponds. It is commonly associated with beds of peat.

• Marl is a rather loosely used term for various fairly stiff or very stiff marine calcareous clays of greenish color.

• Shale is a clastic sedimentary rock mainly composed of silt-size and clay-size particles. Most shales are laminated and display fissility; the rock has a tendency to split along relatively smooth and flat surfaces parallel to the bedding. When fissility is completely absent, the clastic sedimentary deposit is called mudstone or clay rock. Depending on clay mineralogy, void ratio, and degree of diagenetic bonding or weathering, compressive strength of shales may range from less than 2.5 MPa to more than 100 MPa.

• Adobe is a term applied in the southwestern United States and other semiarid regions to a great variety of light-colored soils ranging from sandy silts to very plastic clays.

• Caliche refers to layers of soil in which the grains are cemented together by carbonates deposited as a result of evaporation. These layers commonly occur at a depth of several meters below the surface, and their thickness may range up to a few meters, A semiarid climate is necessary for their formation.

• Varved clay consists of alternating layers of medium gray inorganic silt and darker silty clay. The thickness of the layers rarely exceeds 10 mm, but occasionally very much thicker varves are encountered. This constituents were transported into freshwater lakes by melt water at the close of the Ice Age. Varved clays are likely to combine the undesirable properties of both silts and soft clay.

• Bentonite is a clay with a high content of montmorillonite. Most bentonites were formed by chemical alternation of volcanic ash. In contact with water, dried bentonite swells more than other dried clays, and saturated bentonite shrinks more on drying.

Each term used in the field classification of soils includes a great variety of different materials. Furthermore, the choice of terms relating to stiffness and density depends to a considerable extent on the person who examines the soil. Consequently, the field classification of soils is always more or less uncertain and inaccurate. More specific information can be obtained only by physical tests that furnish numerical values representative of the properties of the soil.

Source: Soil Mechanics in Engineering Practice by Terzaghi et al