Main Types of Soil: Classification of Soil
Rock versus Soil
Main Types of Soil: Civil Engineers divide the materials that exist in the crust of the earth into soil and rock.
Soil definition vs rock
Soil is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. On the other side, we define rocks as a natural aggregate of minerals connected by cohesive forces that are strong and permanent.
Since different persons have different interpretations of the meaning of “strong” and “permanent”, the boundary between soil and rock is arbitrary. There is also a fact that there are multiple aggregates of mineral particles that we can not easily put in classification as soil or rock. That’s why we use the term “soil” to the materials that 100% satisfy the previous definition.
I would like to mention that previous definitions are used by civil engineers; other specializations such as “geologists” used other definitions. For example, geologists understand rock as all the material that constitutes the earth’s crust, without attention to the degree to which the mineral particles are bound together, and they apply the term “soil” to only that portion of the earth’s crust that is capable of supporting vegetation.
This imposes a responsibility on the civil engineer to understand other terms used by other specializations in situations they have to deal together.
What are the Main Types of Soil?
Soils may be divided into two main categories according to their constituents (i.e two main types of soil):
- Soils produced due to chemical and physical rock weathering
- Soils that have organic origin
When the products of rock weathering remain at the same place where they formed, they make a “residual soil”. If the produced soil was transported by any means, we call it as “transported soil”
Residual Soils
The majority of residual soils that have grown in semiarid or temperate climates are stiff, solid, and do not penetrate very far into the ground. However, residual soils may extend to depths of hundreds of meters, especially in warm, humid settings where exposure time was long. They could be solid and robust, but they might also be made of materials that are easily compressed surrounding less weathered rock. They could cause issues with foundations and other sorts of construction under these conditions.
Transported Soils
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.
Organic Soils
Soils of organic origin are formed chiefly in situ, by means of two methods:
- Growth and subsequent decay of plants such aspeat mosses or
- Accumulation of fragments of theinorganic skeletons or shells of organisms.
Thus, soils that have organic origin may be either organic or inorganic. “Organic soil” mainly refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.
Soil Classification for Construction
When it comes to construction, the soil at the location of the structure is explored using borings or test shafts. Then samples of soil are examined for the purpose of classification. The inspector prepares a boring log or shaft record containing the name of each soil and the elevation of its boundaries.
Some adjectives are given to the soil name to indicate properties such as stiffness, color, and other attributes.
The following list of soil types includes the names commonly used for field classification:
Sand and gravel
They are cohesionless (do not stick together) aggregates of rounded subangular or angular fragments of unaltered rocks or minerals. Particles with a size up to 2 mm are sand, and those with a particle size from 2 mm to 200 mm are gravel. Boulders is the name given for pieces with a diameter larger than 200 mm.
Hard Pan
The hardpan soil has an incredibly high resistance to the penetration of drilling tools. The majority of hardpans are composed of highly cohesive, well-graded, and dense aggregates of mineral particles.
Inorganic Silt
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 silt. Because of its smooth texture, inorganic silt is often mistaken for clay, but it may be readily distinguished from clay without laboratory testing. If a pat of wet inorganic silt was shaken in the palm of the hand releases enough water to give the surface a glossy appearance. The pat turns dull again when it is twisted between the fingers. The pat becomes brittle after drying and dust may be removed by rubbing it with a finger. Silt is generally impenetrable, but if it is loose, it can ascend into a shaft or drill hole like a viscous, thick fluid.
Organic Silt
Organic silt is a fine-grained, mostly plastic soil that contains an admixture of finely divided organic matter particles. There may also be shells and visible bits of partially decomposed vegetative stuff. The soil is a light to extremely dark gray in color, and it probably contains a significant amount of H2S, CO2, and other gaseous by-products of the breakdown of organic matter that give it a distinctive smell. Organic silt has a very low permeability and a very high compressibility.
Clay
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. The surface of dried clay pieces is extremely hard and cannot be scratched with the fingertips. Clay has a remarkably low level of permeability. Clays that stand out in the plastic state by having a soapy or waxy look and by being extremely tough are referred to be gumbo. When the water content is greater, they become noticeably sticky. A clay called organic clay contains finely split biological materials, which gives the clay some of its distinctive physical characteristics. Organic clay has a high strength while dry but is likely to be quite collapsible when moist. It is often black or dark gray, and it could smell strongly.
Peat
Peat is a fibrous mixture of tiny and macroscopic pieces of decomposing plant materials. Its color ranges from pale brown to black. Peat is nearly always inappropriate for supporting foundations because it is so compressible. While several methods have been devised to transport earth embankments over peat deposits without the risk of them breaking through the surface, the settling of these embankments is expected to be significant and to last for many years at a diminishing pace.
Soils that exhibit one or more distinctive or uncommon characteristics, such as a root-hole structure or a noticeable and regular stratification, develop under certain geological circumstances. These characteristics make such soils simple to identify in the field, hence they have been given unique names that are widely used to refer to them. Some of these items are defined and described in the sentences that follow.
Till
Till is an unstratified glacial deposit of clay, silt, sand, gravel, and boulders.
Tuff
Tuff is a fine-grained aggregation of extremely tiny mineral or rock pieces released by volcanoes during explosions. It is laid down by water or wind.
Loess and Modified Loess
The wind-blown silt known as loess is homogeneous, cohesive, and often light brown in color. Most of the particles have sizes that fall between the precise range of 0.01 and 0.05 mm. The cohesiveness is caused by the presence of a binder, which might be mostly clayey or calcareous in nature. The permeability in vertical direction is often significantly larger than in horizontal directions due to the ubiquitous existence of continuous vertical root holes, and the material may also stand on almost vertical slopes. Deposits of true loess have never been soaked. The link between the particles weakens with saturation, and the deposit’s surface may settle.
Loess that has undergone secondary processes, such as transient immersion, erosion and subsequent deposition, chemical changes that have destroyed the link between the particles, or chemical breakdown of the more perishable ingredients like feldspar, has lost some of its usual properties. Loess loam is created by thorough chemical breakdown and differs from other types of modified loess in that it is more plastic.
Loess and Modified Loess
The term “diatomaceous earth” (kieselguhr) refers to a deposit of fine, often white, saliceous powder made up primarily or entirely of diatom remnants. A class of tiny, unicellular, freshwater or marine algae with silicified cell walls is referred to as a diatom.
Lake Marl
A white, fine-grained, powdery calcareous deposit called lake marl is produced by pond plants. Peat beds are typically linked to it.
Marl
For a variety of reasonably stiff or extremely stiff marine calcareous clays that are greenish in color, the name “marl” is relatively loosely employed.
Shale
A clastic sedimentary rock called shale is mostly made up of silt- and clay-sized particles. The majority of shales exhibit fissility and are laminated; the rock tends to break along relatively flat, smooth surfaces that are parallel to the bedding. Mudstone or clay rock is the name given to the clastic sedimentary deposit when fissility is totally absent. The compressive strength of shales can range from less than 2.5 MPa to more than 100 MPa, depending on the clay mineralogy, void ratio, and level of diagenetic bonding or weathering.
Adobe
Adobe is a term applied to a great variety of light-colored soils ranging from sandy silts to very plastic clays.
Caliche
The term “calcite” describes soil strata in which evaporation-deposited carbonates have bonded the soil grains together. These layers typically form several meters below the surface, and they can be as thick as a few meters. Their creation requires a semiarid environment.
Varved Clay
Medium-gray inorganic silt and darker silty clay are found in alternating layers in varved clay. Rarely do the layers’ thicknesses surpass 10 mm, although occasionally even thicker varves are seen. At the end of the Ice Age, melt water carried the components into freshwater lakes. Varved clays are likely to include both the undesirable attributes of silts and those of soft clays.
Bentonite
A clay having a high montmorillonite concentration. The majority of bentonites were created by chemically changing volcanic ash. Dried bentonite expands more in the presence of water than other dried clays, while saturated bentonite contracts more as it dries.
Combined Soils
When two different soil types are combined to create a soil, the more significant component is stated as a noun, while the less significant component is conveyed as a modifying adjective. For instance, the term “silty sand” describes a soil that is mostly made up of sand but also contains a tiny quantity of silt. A sandy clay is a soil that possesses clay’s characteristics but also contains a large quantity of sand.
The phrases loose, medium, and dense are used to qualitatively characterize the aggregate qualities of sand and gravel, whereas the categories hard, stiff, medium, and soft are used to describe the aggregate properties of clays. The boring foreman or inspector would often assess these terms based on a number of variables, such as the uniformity of the samples and how easy or difficult it was to advance the drilling and sampling instruments. When the mechanical characteristics are anticipated to have a significant impact on design, the qualitative descriptions should be supported with quantitative data. This is because this type of evaluation might result in a very inaccurate conception of the general character of the soil deposit. The quantitative information is commonly obtained by means of laboratory tests on relatively undisturbed samples, or by suitable in situ tests.
The possibility of mistakes while correlating the boring logs is decreased by keeping track of the colors of the various strata found in nearby borings. The color might also represent a genuine variation in the characteristics of the soil. For instance, if the top layer of a submerged clay strata is stiffer and more yellowish or brown than the clay below, it was likely briefly exposed to desiccation as well as weathering. When multiple colors appear in the same soil strata, terms like mottled, marbled, spotted, or speckled are used to describe the situation. Organic soils are frequently linked with dark or dull colors.
Main Types of Soil
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