Top Soil Mechanics Short Question Answer

All important soil mechanics short questions and answers are summarized below.

Q.1) How Soil is Formed?

Soil is formed by either,

(i) physical disintegration or

(ii) chemical decomposition of rocks

a) Physical disintegration

Physical disintegration or mechanical weathering of rocks to smaller particles is due to the action of agents such as the expansive forces of freezing water in fissures, sudden changes of temperature or the abrasion of rock by moving water or glaciers.

Temperature changes which cause volume change set up tensile and shear stresses in the rock alternatively leading to the fracture or even large rocks.

Erosion by wind and rain is a very important factor and a continuing event. Cracking forces by growing plants and roots in voids and creases of rock can force fragments apart.

Coarse-grained soils, such as gravel and sand are formed by the process of physical disintegration.

b) Chemical decomposition

When chemical decomposition or chemical weathering of rocks takes place, original rock minerals are transformed into new minerals by chemical reactions.

Chemical decomposition can transform hard rock minerals into soft, easily erodible matter. The principal types of decomposition are hydration, oxidation, carbonation and leaching. The chemical decomposition of rocks results in the formation of clayey soils.

Q.2) What are the types of soil?

On the basis of the origin of their constituent (formation of soil), soil can be divided into two large groups:

  1. Residual soil

Residual soils are those that remain in the place of their formation as a result of the weathering of parent rocks. The depth of residual soil depends primarily on climatic conditions and the time of exposure. An important characteristic of residual soil is that the sizes of grains are indefinite.

  1. Transported soil

Transported soils are the soil that is found at locations for removes from their place of formation. The transporting agencies of such soils are glaciers, wind and water. Transported soils are further classified according to the transporting agency and method of deposition.

Q.3) Define three-phase system of soil

Soil mass is a three-phase system. it consists of solid particles, liquid and gas. For all practical purposes, liquid may be considered to be water and gas as air. The three-phase system is also known as the block system.

The void space between the solid grains is filled partially with water and partially with air only. But in the case of perfectly saturated soil, the voids are fully filled with water.

In this case (fully saturated or fully dried) showing only two materials either soil and water or soil and air, is called a two-phase system. Another case, which represents the three materials soil, water and air, is called the three-phase system. Fully saturated and consolidated soil is called a one-phase system.

Q.4) What are the basic assumption in Boussinesq’s theory of stress distribution in soil?

Boussinesq gives the solution to the problem of stress distribution in soil due to point load acting at the soil surface, with the aid of the theory of elasticity. The following assumption is made by Boussinesq in obtaining the solutions;

  1. The soil mass is in the elastic medium for which elasticity E is constant.
  2. The soil mass is homogeneous i.e. it has identical properties at different points or the same soil properties with depth.
  3. The soil mass is isotropic i.e. it has identical properties in all directions.
  4. The soil mass is semi-infinite i.e. it extends infinitely in all directions below a level surface.
  5. The self-weight of soil is ignored.
  6. The soil is initially unstressed.

Q.5) What is the compressibility of soil?

The nature of the deformation of soil under compressible loads may be elastic, plastic or compressive, or a combination of these.

Elastic deformation causes lateral bulging with little change of porosity, and the material recovers fully upon removal of load. Plastic deformation is due to the lateral flow of soil under pressure with a negligible rebound after the removal of load.

Compressive deformation occurs when the particles are brought closer together by pressure causing volume change in the soil. The property of soil by virtue of which volume decrease occurs under applied pressure is termed as its compressibility.

  • Compression of soil particles and water in the voids.
  • Compression and expulsion of air in the voids.
  • Expulsion of water in the voids.

Q.6) What is consolidation in soil mechanics?

The process in which gradual reduction in the volume of soil mass occurs under sustained loading and is primarily due to expulsion of pore water is known as consolidation.

It is a time-dependent phenomenon, especially in clays and it is continuous for a long time. It is the property of fine-grained soil because the fine-grained soil mass has a large void ratio and higher water content.

The total compression of saturated clay strata under excessive effective pressure may be considered as the sum of;

  1. Immediate compression

The portion of the settlement of a structure that occurs more or less simultaneously with applied loads is referred to as initial or immediate compression.

  1. Primary consolidation

If the rate of compression of the soil layer is controlled solely by the resistance of the flow of water under the induced hydraulic gradients, the process is referred to as primary consolidation. 

  1. Secondary consolidation

Once the excess pore water becomes zero that is excess pore pressure gets fully dissipated, the primary compression under the applied stress ends. It is found in practice that some compression takes place even after the primary compression has stopped.

Q.7) What is consolidation in soil mechanics?

The general solution of the Laplace equation yields two sets of curves orthogonal to each other. One set of curves is known as the flow lines and the other is set as equipotential lines.

The net formed by intersecting the possible flow lines and equipotential lines is known as flow net. Flow net is a graphical representation of seepage flow through a soil mass.

A flow line represents the path traced by an individual water particle. An equipotential line is a contour or line joining points of equal potential or head. The flow lines and equipotential lines cut each other at right angles i.e.they are mutually orthogonal. The space between any two adjacent flow lines is called flow channel.

Properties of the flow net

Following properties of flow, net are considered before preceding for construction and application of flow nets.

  1. Flow lines and equipotential lines cut each other at right angles i.e. they are mutually orthogonal.
  2. Each field is an approximate square and in a well-constructed flow net, one should be able to draw a circle touching all four sides.
  3. In a homogeneous soil, every transition in the shape of two types of curves will smooth, being either elliptical or parabolic in shape.
  4. The rate of flow through each flow channel is the same.
  5. The same potential drop occurs between two successive equipotential lines.

Q.8) What is compaction in soil mechanics?

Compaction of soil may be defined as the process of packing the soil particles by reducing the air voids in the soil, by mechanical means.

The degree of compaction of soil is characterized by its dry density. In the construction of highway embankments, earth dams, and many other engineering projects, loose soil must be compacted to increase its unit weight.

Objective of compaction

  1. Compaction increases the dry density of soil, thus increasing shear strength and bearing capacity through an increase in frictional characteristics.
  2. Decreasing the undesirable settlement of structures.
  3. Control undesirable volume changes.
  4. Compaction brings about a low permeability of the soil.
  5. Increasing the stability of slopes.

The factors which affect compaction are:

  1. Water content
  2. Amount of compaction
  3. Types of soil
  4. Method of compaction and 
  5. Admixtures

a) Water content:- As it evident from laboratory compaction tests, the dry density increases with water content, attains a maximum at O.M.C. and thereafter decreases with further increase in water content.

b) Amount of compaction:- The amount of compaction affects both maximum dry density and optimum water content. An increase of compactive effort, for a given soil, causes an increase in dry density and decrease in optimum water content.

c) Types of soil:- For a given compaction effort, the maximum dry density achieved depends to a large extent upon the soil type. In general, well-graded coarse-grained soils attain much higher maximum density at lower maximum content when compared with fine-grained soils.

d) Method of compaction:-  The dry density achieved depends not only upon the amount of compactive effort but also on the method of compaction. For the same amount of compactive effort, the dry density will depend upon whether the method of compaction utilizes kneading action or rolling action, dynamic action, or static action.

e) Admixtures:- The addition of certain admixtures to soils results in modification of their compaction properties. Calcium chlorid3e has been a widely used chemical additive.

What is a compaction curve? Describe its salient features.

A curve that is plotted between the water content as abscissa and the corresponding dry density as ordinate is the compaction curve.

The salient feature of compaction curves are:

  1. The curve is usually of a hyperbolic form when the points obtained from the tests are smoothly joined.
  2. The peak point of the compaction curve is the point with the maximum dry density, corresponding to maximum dry density is water content is known as optimum water content.
  3. At the water, content lower the O.M.C. the soil is rather stiff and has a lot of void spaces, and therefore dry density is low.
  4. Water content beyond the O.M.C. reduces the dry density because the extra water starts occupying the space which otherwise could have been occupied by soil grains under compaction.

Q.9) What is the application of the soil classification system?

Certain important engineering properties of soils belong to different soil groups. The relative suitability of different soil groups for various engineering works can be related to their properties.

The soil engineer however should be cautio9us in the use of soil classification. Design should never be based on soil classification alone. Soil classification plays an important role in decision-making and indicates the direction in which detailed investigation must be carried out.

Q.10) What is the difference between AASHTO and USCS soil classification?

AASHTO system is for finding out the suitability or otherwise of soils as sub-grade for highways only. USCS is for determining the suitability of soils for general use.

Both systems, however, has the same basis. They classify soils according to the particle size analysis and the plasticity characteristic.

  1. According to the AASHTO system, the soil is termed fine-grained, if more than 35% passes No. 200(0.075 mm)sieve, whereas in the ISCS if more than 50% passes that sieve. In that respect, the AASHTO system is somewhat better because the soil behaves as fine-grained when the percentage of fine is 35% and the limit of 50% in USCS is somewhat higher.
  2. In the AASHTO system, sieve No. 10(2 mm size) is used to divide the soil into gravel and sand, whereas in USCS, sieve no. 4(4.75 mm size) is used.
  3. In USCS, the gravelly and sandy soils are clearly separated, whereas, in the AASHTO system, a clear demarcation is not done.
  4. Symbols used in USCS are more descriptive and are more easily remembered than those in the AASHTO system.
  5. Organic soils are classified as OL and OH as peat (Pt) if highly organic in USCS. In the AASHTO system, there is no place for organic soil.
  6. USCS is more convenient to use than the AASHTO system. In the AASHTO system, the process of elimination is required which is time-consuming.

Q.11) What is the AASHTO classification of soil?

American Association of State Highway And Transportation Officials (AASHTO) classification system is useful for classifying soil for highways.

This classification is based on both particle size ranges and plasticity characteristics. The system comprises seven groups of inorganic soils, A-1 to A-7 with 12 sub-groups in all. The system is based on the following soil properties:

(i) particle size distribution

(ii) liquid limit

(iii) plasticity index.

A group index is introduced to further differentiate soils, containing appreciable fine-grained materials.

Group index is given by the following equation:

Group index (G.I.) = 0.2a + 0.005ac + 0.01bd

Where, a is that portion of percentage passing 75 sizes sieve, greater than 35 and not exceeding 75 expressed as a whole number (0 to 40) = (F-35)

b is that portion of percentage passing 75 sizes sieve, greater than 15 and not exceeding 35 expressed as a whole number(0 to 40) = (F-15)

c is that portion of the liquid limit greater than 40 = (WL-40), (0 to 20)

d is that portion of the plasticity index greater than 10 = (Ip-10), (0 to 20)

F is the percent passing No. 200 sieve (0.0075 mm) if F<35, use f-35 =0

Q.12) What is the USCS classification of soil?

The unified soil classification system (USCS) was first developed by Casagrande in 1948, and late in 1952, was modified by the Bureau of Reclamation and the corps of engineers of the United States of America. The various symbols used are given below.

1) PrimarySymbolsDescription
 WWell Graded
 PPoorly graded
2) SecondaryMNon-plastic fines
 CPlastic fines
 LLow plasticity
 HHigh plasticity

Q.13) What is the textural classification of soil?

A triangular classification chart has been developed by making use of grain size limits for sand, silt, and clay. The first step in this classification is to determine the percentage of sand, silt, and clay in a given sample by mechanical analysis.

A point is then located on the chart. The designation given on the chart for the4 areas in which the point falls is used as the classification of the sample. This method of classification does not reveal any properties of the soil other than grain size distribution.

 Soil fractions as per U.S. Department of Agriculture:

Soil fractionDiameter in mm

Q.14) Who is the father of soil mechanics?

Q.15) What is cohesion in soil mechanics?

Q.16) What is void ratio in soil mechanics?

Q.17) What is surcharge in soil mechanics?

Q.18) What is seepage in soil mechanics?

Q.19) What is pore water pressure in soil mechanics?

Q.20) What is soil mechanics?

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