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Saturday, September 23, 2023 ,
The basic unit of this type of clay is formed by atomic bond of the unsatisfied face of silica sheet and either face of aluminum sheet as seen in Figure 1.

Figure 1. Kaolinite clay formation
The bond between two sheets is strong and, also, it is the primary bond. However, the stack of two sheets (with thickness 7.2 Ã… [Angstrom]) is not a form of clay yet. Many layers of this basic kaolinite unit make a kaolinite clay particle. Figure 3.3 shows an electron photomicrograph of well-crystallized kaolinite clay particles.

Figure 2. Electron photomicrograph of kaolinite clay
From the picture, it can be estimated that the diameter of a particle is about 5 μm, and the thickness of the particle is about one-tenth of that (i.e., 0.5 μm). Thus, it is required to have about 700 layers of the basic unit to make a kaolinite clay particle in the picture. The bond between each basic silica and aluminum sheet unit is the one between exposed OH- and satisfied O2- and is called a hydrogen bond. This bond is not as strong as the previous atomic bond (primary bond) but much stronger than the bond between exposed O2- and O2- in case of montmorillonite clay, which will be discussed later. Hydrogen bond is categorized as a primary bond in many literatures, but it shall be noted that this is a marginally strong bond. Because of its nature of bonds within the kaolinite particle, this clay is rather stable, has less swelling and shrinking characteristics, and is less problematic.
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Saturday, September 16, 2023 ,
The water content w, also called the moisture content, is defined as the ratio of weight of water Ww to the weight of solids (Ws or Wd) in a given mass of soil.

Equation 1. Natural water content expressed by weights
The water content is generally expressed as a percentage. However, when used in the formulae giving relationship between certain quantities, it may also be expressed as a fraction. Rewriting Eq. 1, we have Eq. 2.

Equation 2. Water content as a fraction
The usual procedure to find the natural water content is to take a mass of about 20 g to 30 g of soil sample in a container and determine its mass M very accurately. The soil sample is then kept in an oven (105°C–110°C) for about 24 hours so that it becomes perfectly dry. Its dry mass Md is then determined and the water content is calculated from the relation,

Equation 3. Natural water content expressed by masses
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Saturday, September 16, 2023 ,
Clay needs special attention because of its small particle size. As discussed in the grain size distribution section, soils with their particle diameters less than 5 μm (2 μm in some classification systems) are classified as clay or clay-size particles. In such a small size, electrical interactive forces become more significant as compared to the physical frictional interactive forces in the case of larger grain soils (sand and gravel).

To understand various unique engineering behaviors of clay, it is most beneficial to study microstructures of clay particles first. The microstructural observation greatly helps to understand macrobehavior.

Figure 1.  Silica and aluminum sheets
In nature, basically there are three types of clay minerals-namely, kaolinite clay, illite clay, and montmorillonite clay. These clays have different atomic structures and behave differently and are all made of two basic atomic sheets- namely, silica tetrahedral sheets and aluminum octahedron sheets, as seen in Figure 1. Naturally abundant atom silica (Si) and aluminum atom (Al) occupy the center positions of the sheets, and oxygen atom (O2-) and hydroxyl (OH-) are strongly bonded to those core atoms, respectively. These bonds are either ionic or covalent, and actual bonds in silica and aluminum sheets are combinations of these two types of bonds.

Note that the ionic bond is due to exchange of orbiting electrons of two atoms such as Na+ (sodium ion) and Cl- (chlorine ion) to make NaCl (sodium chloride = salt), and the covalent bond is due to sharing electrons in their orbits such as two H+ (hydrogen ions) to form H2 (hydrogen gas). These atomic bonds are very strong and can never be broken by ordinary physical forces. They are called the primary bonds.

A silica tetrahedral sheet is symbolized with a trapezoid, of which the shorter face holds electrically unsatisfied oxygen atoms and the longer face holds electrically satisfied oxygen atoms. An aluminum octahedron sheet is symbolized with a rectangle with top and bottom faces having the same characteristics of exposed hydroxyl (OH-).

In most instances in nature, sheets are further bonded together, basically due to the unsatisfied face of a silica sheet to form various clay minerals.
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Saturday, September 16, 2023 ,
Specific gravity G (or Gs) is defined as the ratio of the weight of a given volume of soil solids at a given temperature to the weight of an equal volume of distilled water at that temperature, both weights being taken in air. In other words, it is the ratio of the unit weight of soil solids to that of water.

Equation 1: G = γs/γw

γw is the unit weight of water and is 9.81 kN/m3 or 62.4 lb/ft3. The Geotechnical Standard specifies 20°C as the standard temperature for reporting the specific gravity.

Some qualifying words like: true, absolute, apparent, bulk or mass, etc., are sometimes added to the term ‘specific gravity’. These qualifying words modify the sense of specific gravity as to whether it refers to soil particles or to soil mass. The soil solids have permeable and impermeable voids inside them, the permeable voids being capable of getting filled with water. If all the internal voids of soil particles (permeable and impermeable) are excluded for determining the true volume of solids, the specific gravity obtained is called absolute or true specific gravity. The apparent or mass or bulk specific gravity Gm denotes the specific gravity of soil mass and is given by

Equation 2: Gm = γ/γw

Unless otherwise specified, we shall denote the Specific Gravity G (or Gs) (defined by Eq. 1) as the specific gravity of soil solids. Table 1 gives the values of specific gravity of some important soil constituents.

Table 1. Specific gravity of soil constituents
Most soils have a rather narrow range of Gs values: 2.65 to 2.70. This implies that solid particle is about 2.65 to 2.70 times heavier than the weight of water for the same volume. If a specific gravity test was not performed during the initial evaluation of geotechnical engineering problems, assuming Gs as a value between 2.65 or 2.70 would not produce a major error in the results.
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Sunday, September 10, 2023 ,

Rounding off a number is necessary so that the accuracy of the result will be the same as that of the problem data. As a general rule, any numerical figure ending in a number greater than five is rounded up and a number less than five is not rounded up. The rules for rounding off numbers are best illustrated by examples. 

Suppose the number 3.5587 is to be rounded off to three significant figures. Because the fourth digit (8) is greater than 5, the third number is rounded up to 3.56. Likewise 0.5896 becomes 0.590 and 9.3866 becomes 9.39. If we round off 1.341 to three significant figures, because the fourth digit (1) is less than 5, then we get 1.34. Likewise 0.3762 becomes 0.376 and 9.871 becomes 9.87. 

There is a special case for any number that ends in a 5. As a general rule, if the digit preceding the 5 is an even number, then this digit is not rounded up. If the digit preceding the 5 is an odd number, then it is rounded up. For example, 75.25 rounded off to three significant digits becomes 75.2, 0.1275 becomes 0.128, and 0.2555 becomes 0.256.

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Sunday, September 10, 2023
The four basic quantities -length, time, mass, and force- are not all independent from one another; in fact, they are related by Newton’s second law of motion, F = ma. Because of this, the units used to measure these quantities cannot all be selected arbitrarily. The equality F = ma is maintained only if three of the four units, called base units, are defined and the fourth unit is then derived from the equation.

The International System of units, abbreviated SI after the French Système International d’Unités, is a modern version of the metric system which has received worldwide recognition. As shown in Table 1, the system defines length in meters (m), time in seconds (s), and mass in kilograms (kg). The unit of force, called a newton (N), is derived from F = ma. Thus, 1 newton is equal to a force required to give 1 kilogram of mass an acceleration of 1 m/s2 (N = kg.m/s2). Think of this force as the weight of a small apple.

If the weight of a body located at the “standard location” is to be determined in newtons, then W = mg must be applied. Here measurements give g = 9.80665 m/s2; however, for calculations, the value g = 9.81 m/s2 will be used. Therefore, a body of mass 1 kg has a weight of 9.81 N, a 2-kg body weighs 19.62 N, and so on, Fig. 1.

Table 1. International System of Units
Prefixes: When a numerical quantity is either very large or very small, the SI units used to define its size may be modified by using a prefix. Some of these prefixes used are shown in Table 2. Each represents a multiple or submultiple of a unit which, if applied successively, moves the decimal point of a numerical quantity to every third place. For example, 4 000 000 N = 4 000 kN (kilo-newton) = 4 MN (mega-newton), or 0.005 m = 5 mm (milli-meter). Notice that the SI system does not include the multiple deca (10) or the submultiple centi (0.01), which form part of the metric system. Except for some volume and area measurements, the use of these prefixes is generally avoided in science and engineering.

Table 2. Prefixies
Rules for Use: Here are a few of the important rules that describe the proper use of the various SI symbols:

• Quantities defined by several units which are multiples of one another are separated by a dot to avoid confusion with prefix notation, as indicated by N = kg.m/s2 = kg.m.s-2. Also, m.s (meter-second), whereas ms (milli-second).

• The exponential power on a unit having a prefix refers to both the unit and its prefix. For example, mN2= (mN)2 = mN.mN. Likewise, mm2 represents (mm)2 = mm.mm.

• With the exception of the base unit the kilogram, in general avoid the use of a prefix in the denominator of composite units. For example, do not write N/mm, but rather kN/m; also, m/mg should be written as Mm/kg.

• When performing calculations, represent the numbers in terms of their base or derived units by converting all prefixes to powers of 10. The final result should then be expressed using a single prefix.

Also, after calculation, it is best to keep numerical values between 0.1 and 1000; otherwise, a suitable prefix should be chosen. For example, 

(50 kN).(60 nm) = [50.(103) N] [60.(10-9) m] = 3000.(10-6) N.m = 3.(10-3) N.m = 3 mN.m

Notes:
•Historically, the meter was defined as 1/10,000,000 the distance from the Equator to the North Pole, and the kilogram is 1/1000 of a cubic meter of water.
•The kilogram is the only base unit that is defined with a prefix.
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Friday, September 08, 2023 ,
Soil is a complex physical system. A mass of soil includes accumulated solid particles or soil grains and the void spaces that exist between the particles. The void spaces may be partially or completely filled with water or some other liquid. Void spaces not occupied by water or any other liquid are filled with air or some other gas.

Figure 1. Soil as a three phase system
‘Phase’ means any homogeneous part of the system different from other parts of the system and separated from them by abrupt transition. In other words, each physically or chemically different, homogeneous, and mechanically separable part of a system constitutes a distinct phase. Literally speaking, phase simply means appearance and is derived from Greek. A system consisting of more than one phase is said to be heterogeneous.

Since the volume occupied by a soil mass may generally be expected to include material in all the three states of matter-solid, liquid and gas, soil is, in general, referred to as a “three-phase system”. (Fig. 1)

Figure 2. (a) Saturated soil (b) Dry soil represented as two-phase systems
A soil mass as it exists in nature is a more or less random accumulation of soil particles, water and air-filled spaces as shown in Fig. 2 (a). For purposes of analysis it is convenient to represent this soil mass by a block diagram, called ‘Phase-diagram’, as shown in Fig. 2 (b). It may be noted that the separation of solids from voids can only be imagined. The phase-diagram provides a convenient means of developing the weight-volume relationship for a soil.

When the soil voids are completely filled with water, the gaseous phase being absent, it is said to be ‘fully saturated’ or merely ‘saturated’. When there is no water at all in the voids, the voids will be full of air, the liquid phase being absent ; the soil is said to be dry. (It may be noted that the dry condition is rare in nature and may be achieved in the laboratory through oven-drying). In both these cases, the soil system reduces to a ‘two-phase’ one as shown in Fig. 2 (a) and (b). These are merely special cases of the three-phase system.
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Friday, September 08, 2023 ,
In this website, SI units have been used throughout. However, prior to this, metric units were in use, not only in India, but also in several parts of the world. Most of the equipment in these laboratories have the use of metric units. These equipment use grams and kilograms. Hence, mass units of grams and kilograms (formerly called ‘weight’) will continue to be used. Also, the term ‘mass’ and ‘weight’ are commonly used interchangeably. This may cause some problem with ‘unit weight’ terms. In this text, ‘mass’ will be represented in terms of grams and kilograms while ‘weight’ will be represented in terms of Newtons and kilo Newtons. The term density is defined as mass per unit volume; hence it will be represented in terms of g/cm3 or kg/m3 units. On the contrary, the term ‘unit weight’ is the weight per unit volume. Hence, it will be represented in terms of kN/m3.

The value of gravitational constant g will be taken as 981 cm/s2 (9.81 m/s2), for computational purposes.

Thus, in order to convert the density (expressed in terms of g/cm3) into unit weight, multiply the former by 9.81. For example, if a soil mass has a mass of 216 g and volume of 120 cm3.
  • Density = (Mass/Vol.) = 216/120 = 1.8 g/cm3
  • Unit Weight = (Weight/Vol.) = (216/120) x 9.81 = 17.66 kN/m3
Pressure, which is defined to be a force per unit area, will be expressed in terms of kN/m2 or in term of kilopascals (kPa).
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Friday, September 08, 2023 ,
The field of soil mechanics is very vast. The civil engineer has many diverse and important encounters with soil. Apart from the testing and classification of various types of soils in order to determine its physical properties, the knowledge of soil mechanics is particularly helpful in the following problems in civil engineering.

1. Foundation design and construction: Foundation is an important element of all civil engineering structures. Every structure — building, bridge, highway, tunnel, canal or dam — is founded in or on the surface of the earth. It is, therefore, necessary to know the bearing capacity of the soil, the pattern of stress distribution in the soil beneath the loaded area, the probable settlement ofthe foundation, effect of groundwater and the effect of vibrations, etc. The suitability of various types of foundations — i.e., spread foundation, pile foundation, well foundation, etc. — depend upon the type of soil strata, the magnitude of loads and groundwater conditions. A knowledge of shrinkage and swelling characteristics of soil beneath the foundation is also very essential.

2. Pavement design: A pavement can either be flexible or rigid, and its performance depends upon the subsoil on which it rests. The thickness of a pavement and its component parts, depends upon some certain characteristics of the subsoil, which should be determined before the design is made. On busy pavements, where the intensity of traffic is very high, the effect of repetition of loading and the consequent fatigue failure has to be taken into account. Apart from these, other problems of pavement design are : frost, heave and thaw with their associated problems of frost damage to pavements ; frost penetration depth ; remedial measures to prevent frost damage ; problems of ‘pumping’ of clay subsoils and suitability of a soil as a construction material for building highways or railways, earth fills or cuts, etc. A knowledge of the techniques for the improvement of the soil properties such as strength and stability is very much helpful in constructing pavements on poor soils by stabilising them.

3. Design of underground structures and earth retaining structures: The design and construction of underground (subterranean) and earth retaining structures constitute an important phase of engineering. The examples of underground structures include tunnels, underground buildings, drainage structures and pipelines. The examples of earth retaining structures are : gravity retaining wall, anchored bulk heads and cofferdams. A knowledge of soil structure interaction is essential to design properly such structures subjected to soil loadings.

4. Design of embankments and excavations: When the surface of the soil structure is not horizontal, the component of gravity tends to move the soil downward, and may disturb the stability of the earth structure. A thorough knowledge of shear-strength and related properties of soil is essential to design the slope and height (or depth) of the embankment (or excavation). The possibility of the seeping groundwater reducing the soil strength while excavating must also be taken into account. It may sometimes be essential to drain the subsoil water, to increase the soil strength and to reduce the seepage forces. Deep excavations require lateral braces and sheet walls to prevent caving in.

5. Design of earth dams: The construction of an earth dam requires a very thorough knowledge of whole of the Soil Mechanics. Since soil is used as the only construction material in an earth dam, which may either be homogeneous or of composite section, its design involves the determination of the following physical properties of soil : index properties such as density, plasticity characteristics and specific gravity, particle size distribution and gradation of the soil; permeability, consolidation and compaction characteristics, and shear strength parameters under various drainage conditions. Since huge earth mass is involved in its construction, suitable soil survey to the nearby area may be essential for the borrow-pit area. The determination of the optimum water content at which maximum density will be obtained on compaction, is probably the most essential aspect of the design. Apart from the seepage, characteristics of the dam section must be thoroughly investigated since these have the greatest impact on the stability of the slopes as well as the foundations of the dam. The consolidation characteristics help in predicting the long range behaviour of the dam toward settlement and the consequent reduction in the pore pressure. Lastly, the possible effect of vibrations during an earthquake should also be taken into account while designing. The performance of the soil in the designs cited above depends upon the characteristics of soil. Therefore, the testing of soil with relation to the determination of its physical properties, and the evaluation of effects of certain other factors such as seepage conditions, etc. forms the most essential part of the development of soil engineering. It is through research only that design and construction methods are modified to give maximum safety and/or economy, and new methods are evolved. The knowledge of theoretical soil mechanics, assuming the soil to be an ideal elastic isotropic and homogeneous material, helps in predicting the behaviour of the soil in the field.
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Friday, September 08, 2023 ,
The ‘structure’ of a soil may be defined as the manner of arrangement and state of aggregation of soil grains. In a broader sense, consideration of mineralogical composition, electrical properties, orientation and shape of soil grains, nature and properties of soil water and the interaction of soil water and soil grains, also may be included in the study of soil structure, which is typical for transported or sediments soils. Structural composition of sedimented soils influences, many of their important engineering properties such as permeability, compressibility and shear strength. Hence, a study of the structure of soils is important.

The following types of structure are commonly studied: (a) Single-grained structure (b) Honey-comb structure (c) Flocculent structure.

Figure 1. Single-grained structure
Single-grained Structure: Single-grained structure is characteristic of coarsegrained soils, with a particle size greater than 0.02 mm. Gravitational forces predominate the surface forces and hence grain to grain contact results. The deposition may occur in a loose state, with large voids or in a sense state, with less of voids. (Fig. 1)

Figure 2. Honey-comb structure
Honey-comb Structure: This structure can occur only in fine-grained soils, especially in silt and rock flour. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces also play an important role in the process of settling down. Miniature arches are formed, which bridge over relatively large void spaces. This results in the formation of a honey-comb structure, each cell of a honey-comb being made up of numerous individual soil grains. The structure has a large void space and may carry high loads without a significant volume change. The structure can be broken down by external disturbances. (Fig. 2)
 
Figure 3. Flocculent structure
Flocculent Structure: This structure is characteristic of fine-grained soils such as clays. Inter-particle forces play a predominant role in the deposition. Mutual repulsion of the particles may be eliminated by means of an appropriate chemical; this will result in grains coming closer together to form a ‘floc’. Formation of flocs is ‘flocculation’. But the flocs tend to settle in a honeycomb structure, in which in place of each grain, a floc occurs.

Thus, grains grouping around void spaces larger than the grain-size are flocs and flocs grouping around void spaces larger than even the flocs result in the formation of a ‘flocculent’ structure. (Fig. 3)

Figure 4. Card-house structure of flaky particles
Very fine particles or particles of colloidal size (< 0.001 mm) may be in a flocculated or dispersed state. The flaky particles are oriented edge-to-edge or edge-to-face with respect to one another in the case of a flocculated structure. Flaky particles of clay minerals tend to from a card house structure (Reference: Lambe, 1953), when flocculated. This is shown in Fig. 4.

Figure 5. Dispersed structure
When inter-particle repulsive forces are brought back into play either by remoulding or by the transportation process, a more parallel arrangement or reorientation of the particles occurs, as shown in Fig. 5. This means more face-to-face contacts occur for the flaky particles when these are in a dispersed state. In practice, mixed structures occur, especially in typical marine soils.

The term ‘Texture’ refers to the appearance of the surface of a material, such as a fabric. It is used in a similar sense with regard to soils. Texture of a soil is reflected largely by the particle size, shape, and gradation. The concept of texture of a soil has found some use in the classification of soils to be dealt with later.
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Friday, September 08, 2023 ,
Soil is formed by the process of ‘Weathering’ of rocks, that is, disintegration and decomposition of rocks and minerals at or near the earth’s surface through the actions of natural or mechanical and chemical agents into smaller and smaller grains.

The factors of weathering may be atmospheric, such as changes in temperature and pressure; erosion and transportation by wind, water and glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water, especially rainwater, with time.

Obviously, soils formed by mechanical weathering (that is, disintegration of rocks by the action of wind, water and glaciers) bear a similarity in certain properties to the minerals in the parent rock, since chemical changes which could destroy their identity do not take place.

It is to be noted that 95% of the earth’s crust consists of igneous rocks, and only the remaining 5% consists of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth’s surface area. Feldspars are the minerals abundantly present (60%) in igneous rocks. Amphiboles and pyroxenes, quartz and micas come next in that order.

Rocks are altered more by the process of chemical weathering than by mechanical weathering. In chemical weathering some minerals disappear partially or fully, and new compounds are formed. The intensity of weathering depends upon the presence of water and temperature and the dissolved materials in water. Carbonic acid and oxygen are the most effective dissolved materials found in water which cause the weathering of rocks. Chemical weathering has the maximum intensity in humid and tropical climates.

‘Leaching’ is the process whereby water-soluble parts in the soil such as Calcium Carbonate, are dissolved and washed out from the soil by rainfall or percolating subsurface water. ‘Laterite’ soil, in which certain areas of Kerala abound, is formed by leaching.

Harder minerals will be more resistant to weathering action, for example, Quartz present in igneous rocks. But, prolonged chemical action may affect even such relatively stable minerals, resulting in the formation of secondary products of weatheing, such as clay minerals— illite, kaolinite and montmorillonite. ‘Clay Mineralogy’ has grown into a very complicated and broad subject  (Reference: ‘Clay Mineralogy’ by R.E. Grim).
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Tuesday, September 05, 2023 ,
Soils which are formed by weathering of rocks may remain in position at the place of region. In that case these are ‘Residual Soils’. These may get transported from the place of origin by various agencies such as wind, water, ice, gravity, etc. In this case these are termed ‘‘Transported soil’’. Residual soils differ very much from transported soils in their characteristics and engineering behaviour. The degree of disintegration may vary greatly throughout a residual soil mass and hence, only a gradual transition into rock is to be expected. An important characteristic of these soils is that the sizes of grains are not definite because of the partially disintegrated condition. The grains may break into smaller grains with the application of a little pressure.

The residual soil profile may be divided into three zones:

(i) the upper zone in which there is a high degree of weathering and removal of material; 
(ii) the intermediate zone in which there is some degree of weathering in the top portion and some deposition in the bottom portion;
(iii) the partially weathered zone where there is the transition from the weathered material to the unweathered parent rock. Residual soils tend to be more abundant in humid and warm zones where conditions are favourable to chemical weathering of rocks and have sufficient vegetation to keep the products of weathering from being easily transported as sediments. Residual soils have not received much attention from geotechnical engineers because these are located primarily in undeveloped areas. In some zones, sedimentary soil deposits range from 8 to 15 m in thickness.

Transported soils may also be referred to as ‘Sedimentary’ soils since the sediments, formed by weathering of rocks, will be transported by agencies such as wind and water to places far away from the place of origin and get deposited when favourable conditions like a decrease of velocity occur. A high degree of alteration of particle shape, size, and texture as also sorting of the grains occurs during transportation and deposition. A large range of grain sizes and a high degree of smoothness and fineness of individual grains are the typical characteristics of such soils.

Transported soils may be further subdivided, depending upon the transporting agency and the place of deposition, as under: 
  • Alluvial soils: Soils transported by rivers and streams: Sedimentary clays. 
  • Aeoline soils: Soils transported by wind: loess.
  • Glacial soils: Soils transported by glaciers: Glacial till.
  • Lacustrine soils: Soils deposited in lake beds: Lacustrine silts and lacustrine clays.
  • Marine soils: Soils deposited in sea beds: Marine silts and marine clays.
Broad classification of soils may be: 
  • Coarse-grained soils, with average grain-size greater than 0.075 mm, e.g., gravels and sands. 
  • Fine-grained soils, with average grain-size less than 0.075 mm, e.g., silts and clays.
These exhibit different properties and behaviour but certain general conclusions are possible even with this categorisation. For example, fine-grained soils exhibit the property of ‘cohesion’—bonding caused by inter-molecular attraction while coarse-grained soils do not; thus, the former may be said to be cohesive and the latter non-cohesive or cohesionless.
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Tuesday, September 05, 2023 ,

The following are some commonly used soil designations, their definitions and basic properties:

Bentonite: Decomposed volcanic ash containing a high percentage of clay mineral— montmorillonite. It exhibits high degree of shrinkage and swelling.

Black cotton soil: Black soil containing a high percentage of montmorillonite and colloidal material; exhibits high degree of shrinkage and swelling. The name is derived from the fact that cotton grows well in the black soil.

Boulder clay: Glacial clay containing all sizes of rock fragments from boulders down to finely pulverised clay materials. It is also known as ‘Glacial till’.

Caliche: Soil conglomerate of gravel, sand and clay cemented by calcium carbonate.

Hard pan: Densely cemented soil which remains hard when wet. Boulder clays or glacial tills may also be called hard-pan— very difficult to penetrate or excavate.

Laterite: Deep brown soil of cellular structure, easy to excavate but gets hardened on exposure to air owing to the formation of hydrated iron oxides.

Loam: Mixture of sand, silt and clay size particles approximately in equal proportions; sometimes contains organic matter.

Loess: Uniform wind-blown yellowish brown silt or silty clay; exhibits cohesion in the dry condition, which is lost on wetting. Near vertical cuts can be made in the dry condition.

Marl: Mixtures of clacareous sands or clays or loam; clay content not more than 75% and lime content not less than 15%.

Moorum: Gravel mixed with red clay.

Top-soil: Surface material which supports plant life.

Varved clay: Clay and silt of glacial origin, essentially a lacustrine deposit; varve is a term of Swedish origin meaning thin layer. Thicker silt varves of summer alternate with thinner clay varves of winter.
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Tuesday, September 05, 2023 ,

The knowledge of soil mechanics has application in many fields of Civil Engineering.

Foundations: The loads from any structure have to be ultimately transmitted to a soil through the foundation for the structure. Thus, the foundation is an important part of a structure, the type and details of which can be decided upon only with the knowledge and application of the principles of soil mechanics.

Underground and Earth-retaining Structures: Underground structures such as drainage structures, pipe lines, and tunnels and earth-retaining structures such as retaining walls and bulkheads can be designed and constructed only by using the principles of soil mechanics and the concept of ‘soil-structure interaction’.

Pavement Design: Pavement Design may consist of the design of flexible or rigid pavements. Flexible pavements depend more on the subgrade soil for transmitting the traffic loads. Problems peculiar to the design of pavements are the effect of repetitive loading, swelling and shrinkage of sub-soil and frost action. Consideration of these and other factors in the efficient design of a pavement is a must and one cannot do without the knowledge of soil mechanics.

Excavations, Embankments and Dams: Excavations require the knowledge of slope stability analysis; deep excavations may need temporary supports—‘timbering’ or ‘bracing’, the design of which requires knowledge of soil mechanics. Likewise the construction of embankments and earth dams where soil itself is used as the construction material, requires a thorough knowledge of the engineering behaviour of soil especially in the presence of water. Knowledge of slope stability, effects of seepage, consolidation and consequent settlement as well as compaction characteristics for achieving maximum unit weight of the soil in-situ, is absolutely essential for efficient design and construction of embankments and earth dams.

The knowledge of soil mechanics, assuming the soil to be an ideal material elastic, isotropic, and homogeneous material—coupled with the experimental determination of soil properties, is helpful in predicting the behaviour of soil in the field.

Soil being a particulate and hetergeneous material, does not lend itself to simple analysis. Further, the difficulty is enhanced by the fact that soil strata vary in extent as well as in depth even in a small area.

A through knowledge of soil mechanics is a prerequisite to be a successful foundation engineer. It is difficult to draw a distinguishing line between Soil Mechanics and Foundation Engineering; the later starts where the former ends.
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Monday, September 04, 2023 ,
The use of soil for engineering purposes dates back to prehistoric times. Soil was used not only for foundations but also as construction material for embankments. The knowledge was empirical in nature and was based on trial and error, and experience.

The hanging gardens of Babylon were supported by huge retaining walls, the construction of which should have required some knowledge, though empirical, of earth pressures. The large public buildings, harbours, aqueducts, bridges, roads and sanitary works of Romans certainly indicate some knowledge of the engineering behaviour of soil. This has been evident from the writings of Vitruvius, the Roman Engineer in the first century, B.C. Mansar and Viswakarma, in India, wrote books on ‘construction science’ during the medieval period. The Leaning Tower of Pisa, Italy, built between 1174 and 1350 A.D., is a glaring example of a lack of sufficient knowledge of the behaviour of compressible soil, in those days.

Coulomb, a French Engineer, published his wedge theory of earth pressure in 1776, which is the first major contribution to the scientific study of soil behaviour. He was the first to introduce the concept of shearing resistance of the soil as composed of the two components— cohesion and internal friction. Poncelet, Culmann and Rebhann were the other men who extended the work of Coulomb. D’ Arcy and Stokes were notable for their laws for the flow of water through soil and settlement of a solid particle in liquid medium, respectively. These laws are still valid and play an important role in soil mechanics. Rankine gave his theory of earth pressure in 1857; he did not consider cohesion, although he knew of its existence.

Boussinesq, in 1885, gave his theory of stress distribution in an elastic medium under a point load on the surface.

Mohr, in 1871, gave a graphical representation of the state of stress at a point, called ‘Mohr’s Circle of Stress’. This has an extensive application in the strength theories applicable to soil.

Atterberg, a Swedish soil scientist, gave in 1911 the concept of ‘consistency limits’ for a soil. This made possible the understanding of the physical properties of soil. The Swedish method of slices for slope stability analysis was developed by Fellenius in 1926. He was the chairman of the Swedish Geotechnical Commission.

Prandtl gave his theory of plastic equilibrium in 1920 which became the basis for the development of various theories of bearing capacity.

Terzaghi gave his theory of consolidation in 1923 which became an important development in soil mechanics. He also published, in 1925, the first treatise on Soil Mechanics, a term coined by him. (Erd bau mechanik, in German). Thus, he is regarded as the Father of modern soil mechanics’. Later on, R.R. Proctor and A. Casagrande and a host of others were responsible for the development of the subject as a full-fledged discipline.

Fifteen International Conferences have been held till now under the auspices of the international Society of Soil Mechanics and Foundation engineering at Harvard (Massachusetts, U.S.A.) 1936, Rotterdam (The Netherlands) 1948, Zurich (Switzerland) 1953, London (U.K.) 1957, Paris (France) 1961, Montreal (Canada) 1965, Mexico city (Mexico) 1969, Moscow (U.S.S.R) 1973, Tokyo (Japan) 1977, Stockholm (Sweden) 1981, San Francisco (U.S.A.) 1985, and Rio de Janeiro (Brazil) 1989. The thirteenth was held in New Delhi in 1994, the fourteenth in Hamburg, Germany, in 1997 , and the fifteenth in Istanbul, Turkey in 2001. The sixteenth is proposed to be held in Osaka, Japan, in 2005. These conferences have given a big boost to research in the field of Soil Mechanics and Foundation Engineering.
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Sunday, September 03, 2023 ,
The term ‘Soil’ has different meanings in different scientific fields. It has originated from the Latin word Solum. To an agricultural scientist, it means ‘‘the loose material on the earth’s crust consisting of disintegrated rock with an admixture of organic matter, which supports plant life’’. To a geologist, it means the disintegrated rock material which has not been transported from the place of origin. But, to a civil engineer, the term ‘soil’ means, the loose unconsolidated inorganic material on the earth’s crust produced by the disintegration of rocks, overlying hard rock with or without organic matter. Foundations of all structures have to be placed on or in such soil, which is the primary reason for our interest as Civil Engineers in its engineering behaviour.

Soil may remain at the place of its origin or it may be transported by various natural agencies. It is said to be ‘residual’ in the earlier situation and ‘transported’ in the latter. 

‘‘Soil mechanics’’ is the study of the engineering behaviour of soil when it is used either as a construction material or as a foundation material. This is a relatively young discipline of civil engineering, systematised in its modern form by Karl Von Terzaghi (1925), who is rightly regarded as the ‘‘Father of Modern Soil Mechanics’’.*

An understanding of the principles of mechanics is essential to the study of soil mechanics. A knowledge and application of the principles of other basic sciences such as physics and chemistry would also be helpful in the understanding of soil behaviour. Further, laboratory and field research have contributed in no small measure to the development of soil mechanics as a discipline.

The application of the principles of soil mechanics to the design and construction of foundations for various structures is known as ‘‘Foundation Engineering’’. ‘‘Geotechnical Engineering’’ may be considered to include both soil mechanics and foundation engineering.

In fact, according to Terzaghi, it is difficult to draw a distinct line of demarcation between soil mechanics and foundation engineering; the latter starts where the former ends. Until recently, a civil engineer has been using the term ‘soil’ in its broadest sense to include even the underlying bedrock in dealing with foundations. However, of late, it is wellrecognised that the sturdy of the engineering behaviour of rock material distinctly falls in the realm of ‘rock mechanics’, research into which is gaining impetus the world over.
* According to him, ‘‘Soil Mechanics is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of soil particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain an admixture of organic constiuents’’.
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