Soil Mechanics & Foundation Engineering : Properties of Soils & Classification and Structure of Soil

By Sajal Gupta|Updated : July 1st, 2021

BYJU'S Exam Prep Brings you 60 Days Study Plan for the preparation of DFCCIL Civil Engineering. This Study Plan will be free and will be very beneficial for the students preparing and targeting the DFCCIL Exam. Save this article as it will get updated on a daily basis as scheduled. 

Properties of Soils, Classification and Structure of Soil

Properties of Soils

Phase Diagram

  • Soil mass is in general a three-phase system composed of solid, liquid and gaseous matter in a blended form with each other.

But in phase diagram – for understanding, these three matters are shown separately.

  • In supersaturated, state with change in water content volume of voids changes hence volume of soil changes.



The diagrammatic representation of the different phases in a soil mass is called the "phase diagram".




Water content


where, WW = Weight of power

WS = Weight of solids

There can be no upper limit to water content, i.e. w ≥ 0

Void ratio


where, Vv = Volume of voids

V = Total volume of soil

Porosity cannot exceed 100% i.e.,

0 < n < 100

Void ratio is more important engineering property.

Degree of Saturation


where, Vw = Volume of water

Vv = Volume of voids

0 ≤ S≤ 100

for perfectly dry soil : S = O

for Fully saturated soil : S = 100%


Air Content

image010 Va = Volume of air

Sr + ac = 1

% Air Void 


na =

Unit Weight

A. Bulk unit weight


Thus Bulk unit weight is total weight per unit volume.


B. Dry Unit Weight is the weight of soil solids per unit volume.


  • Dry unit weight is used as a measure of denseness of soil. More dry unit weight means more compacted soil.

C. Saturated unit weight: It is the ratio of total weight of fully saturated soil sample to its total volume.


Submerged unit weight or Buoyant unit weight (γ): It is the submerged weight of soil solids per unit volume.


image018 = unit wt. of saturated soil

γ = unit wt. of water

Unit wt. of solids:


γ is roughly 1/2 of saturated unit weight.

Specific Gravity

True/Absolute Special Gravity, G

  • Specific gravity of soil solids (G) is the ratio of the weight of a given volume of solids to the weight of an equivalent volume of water at 4℃.


G = 2.6 to 2.75 for inorganic solids

= 1.2 to 1.4 for organic solids

  • Apparent or mass specific gravity (Gm): Mass specific gravity is the specific gravity of the soil mass and is defined as the ratio of the total weight of a given mass of soil to the weight of an equivalent volume of water.


where, γ is bulk unit wt. of soil

γ = γsat for saturated soil mass

γ = γd for dry soil mass

Gm < G

In India, G is reported at 27℃,


Relative density (ID)

To compare the degree of denseness of two soils.





A. when particles are arranged in cubical array

emax = 91%, nmax = 47.6%

B. When particles are arranged in prismoidal array (Rhomohedral Array)

emin = 35%, nmin = 25.9%

Relative Compaction

Indicate: Degree of denseness of cohesive + cohesionless soil


Relative Density

Indicate: Degree of denseness of natural cohesionless soil

Some Important Relationships

(i) Relation between image030

image031 , image032  &  image033

(ii) Relation between e and n

image034 or image035

(iii) Relation between e, w, G and S:

Se = w. G

(iv) Bulk unit weight (γ) in terms of G, e, w and γw γ, G, e, Sr, γw


image039 {Se = w. G}

(v) Saturated unit image040 weight in terms of G, e & γw

Sr = 1 image041

(vi) Dry unit weight γd in terms of G, e and γw

Sr = 0


(vii) Submerged unit weight (γ') in terms of G, e and γw


(ix) Relation between degree of saturation (s) w and G


Methods for determination of water content

(i) Oven Drying Method

  • Simplest and most accurate method
  • Soil sample is dried in a controlled temperature (105-110℃)
  • For organic soils, temperature is about 60℃. Soil having gyprum, temperature image048
  • Sample is dried for 24 hrs.
  • For sandy soils, complete drying can be achieved in 4 to 6 hrs.
  • Water content is calculated as:


where, W1 = weight of container

W2 = weight of container + moist sample

W3 = weight of container + dried sample

Weight of water = W2 – W3

Weight of solids = W3 – W1

(ii) Pycnometer Method

  • quick method
  • capacity of pycnometer = 900 m/.
  • this method is more suitable for cohesionless soils.
  • used when specific gravity of soil solids is known
  • Let W1 = Wt. of empty dried pycnometer bottle

W2 = Wt. of pycnometer + Soil

W3 = Wt. of pycnometer + Soil + Water

W4 = Wt. of pycnometer + Water.


image051 are in anticlockwise order)

(iii) Calcium Carbide Method/Rapid moisture Meter Method Field Method

  • Quick method (requires 5 to 7 minutes); but may not give accurate results.
  • The reaction involved is 


  • Soil sample weights 4-6 gms.
  • The gauge reads water content with respect to total mass of soil. i.e., image053

(In this equipment pressure calibrated against water content with respect to total mass)

  • Actual water content image054


wr is moisture content recorded, expressed as fraction of moist wt. of solid.

w is actual water content.

(iv) Sand Bath Method (Field Method)

  • quick, field method
  • used when electric oven is not available.
  • soil sample is put in a container & dried by placing it in a sand bath, which is heated on the kerosene store.
  • water content is determined by using same formula as in oven drying method.

(v) Torsion Balance Moisture Meter Method

  • quick method for use in laboratory.
  • Infrared radiations are used for drying sample.
  • Principle: The torsion wire is prestressed accurately to an extent equal to 100% of the scale reading. Then the sample is evenly distributed on the balance pan to counteract the prestressed torsion and the scale is brought back to zero. As the sample dries, the loss in weight is continuously balanced by the rotation of a drum calibrated directly to read moisture% on wet basis.

(vi) Alcohol Method

  • It is a quick method adopted in field.
  • Should not be used for organic soil and soils containing calcium compound.

Determination of specific gravity of soil solids

  • Pycnometer method is used.
  • Instead of pycnometer, Density bottle (50 ml) OR Flask (500 ml) can also be used.

Let, W1 = Weight of empty pycnometer

W2 = Weight of pycnometer + soil sample (oven dried)

W3 = Weight of pycnometer + soil soilds + water

W4 = Weight of pycnometer + water



Methods for the determination of insity unit weight

(A) Core-Cutter Method

  • Used in case of cohesive soils.
  • Cannot be used in case of hard and gravelly soils.


  • The method consists of driving a core-cutter (Volume = 1000 cc) into the soil and removing it, the cutter filled with soil is weighted. Volume of cutter is known from its dimensions and in situ unit weight is obtained by dividing soil weight by volume of cutter.
  • If water content is known in the laboratory, the dry unit weight can also be computed.


(b) Sand Replacement Method

  • Used in case of hard and gravelly soils.
  • A hole in ground is made. The excavated soil is weighted. The volume of hole is determined by replacing it with sand. Insitu unit weight is obtained by dividing weight of excavated soil with volume of hole.


(c) Water Displacement Method

  • Suitable for cohesive soils only, where it is possible to have a lump sample.
  • A regular shape, well trimmed sample is weighted. (W1). It is coated with paraffin wax & again weighted (W2). The sample is now placed in a metal container filled with water upto the brim. Let the volume of displaced water be Vm. Then volume of uncoated specimen is calculated as,


where, = unit wt. of paraffin wax

Thus, bulk unit wt. of soil

  • Sands + Gravels: Bulky grains

Bulk grains classified as – angular, Subangular, Sub rounded, rounded, well rounded

Higher angularity ∝ Higher Shear Strength

  • Clay Minerals: Flaky grains

Grain size distribution


  • Sieve Analysis: (For Coarse Grained Soils)

The fraction retained on 4.75 mm sieve is called the gravel fraction which is subjected to coarse sieve analysis.

The material passing 4.75 mm sieve is further subjected to fine sieve analysis if it is sand or to a combined sieve and sedimentation analysis if silt and clay sizes are also present.

  • Coarse Sieves: 4.75 mm, 10 mm, 20 mm, 80 mm.
  • Fine Sieves: 75 μ, 150 μ, 212 μ, 425 μ, 600 μ, 1 mm, 2 mm.
  • Concept of "Percentage finer"

% retained on a particular sieve


Cumulative % retained = sum of % retained on all sieves of larger sizes and the % retained on that particular sieve.

"Percentage finer" than the sieve under reference = 100% - Cumulative % retained.

  • Sedimentation Analysis

According to stokes law, the terminal velocity is given by,


image066 = density of grains (g/cm3)

image066 = density of water (g/cm3)

μ = viscosity of water

g = acceleration due to gravity (cm/s2)

D = Diameter of grain (cm)

If 'h' the height through which particle falls in time't', then


  • Pipette Method

In this method, the weight of solids per cc of suspension is determined directly by collecting 10 cc of soil suspension from a specified sampling depth.

If md = dry mass (obtained after drying the sample) then, mass present in unit vol. of pipette


If Md = total mass of soil dissolved in total volume of water (V) then mass/unit volume

Therefore, % finer is given by = %


In m is the mass of dispersing agent dissolved in the total volume V, then actual % finer,


  • Hydrometer Method

In this method the weight of solids present at any time is calculated indirectly by reading the density of soil suspension.

  • Calibration of Hydrometer

Establishing a relation between the hydrometer reading RH and effective depth (He).

The effective depth is the distance from the surface of the soil suspension to the level at which the density of soil suspension is being measured.


Effective depth is calculated as


where, H1 = distance (cm) between any hydrometer reading and neck.

h = length of hydrometer bulb

VH = volume of hydrometer bulb

AJ = area of the cross section of the jar.

Reading of Hydrometer is related to sp. gr. or density of soil suspension as:


Thus a reading of RH = 25 means, Gss = 1.025 and a reading of RH = -25 means, Gss = 0.975% finer is given as:


where, G = sp. gr. of soil solids

RH = final corrected value of hydrometer

V = Total volume of soil suspension

W = weight of soil mass dissolved.

  • Corrections to Hydrometer Reading

(i) Meniscus correction: (Cm)

Hydrometer reading is always corresponding to the upper level of meniscus.

Therefore, meniscus correction is always positive (+Cm).

(ii) Temperature correction: (Ct)

Hydrometers are generally calibrated at 27℃. If the test temperature is above the standard (27℃) the correction is added and, if below, it is subtracted.

(iii) Dispersing/Defloculating agent correction: (Cd)

The correction due to rise in specific gravity of the suspension on account of the addition of the defloculating agent is called Dispersing agent correction (Cd).

Cd is always negative.

The corrected hydrometer reading is given by


  • Grain Size Distribution Curves


Curve-1: Well graded soil: good representation of grain sizes over a wide range and its gradation curve is smooth.

Curve-2: Poorly graded soil/ Uniform gradation:

It is either an excess or a deficiency of certain particle sizes or has most of the particles about the same size.

Curve-3: Gap graded soil: In this case some of the particle sizes are missing.

Curve-4: Predominantly coarse soil.

Curve-5: Predominantly fine soil.

The diameter D10 corresponds to 10% of the sample finer in weight on the Grain size distribution curve. This diameter D10 is called effective size.

Similarly, D30 and D60 are grain dia. (mm) corresponding to 30% fine and 60% finer.

The shape parameters related to these are:

(A) Coefficient of Uniformity image080

(B) Coefficient of Curvature image081

  • for a soil to be well graded:
    [1 < Cc < 3] and [Cu > 4] for gravels:
    [Cu > 6] for sands.
  • Cu = 1 for uniform soils/poorly graded soils.

Consistency of clays: Atterberg limits


LL = WI = liquid limit

PL = Wp = plastic limit

SL = Ws = Shrinkage limit

V1 = Volume of soil mass at LL

Vp = Volume of soil mass at PL

Vd = Volume of soil mass at SL

Vs = Volume of solids

Plasticity Index (Ip): It is the range of moisture content over which a soil exhibits plasticity.

Ip = WL - Wp

WL = water content at LL

Wp = water content at PL

If PL ≥ LL, Ip is reported as zero.

Soil classification related to plasticity index:


Relative Consistency or Consistency – index (Ic): to study behaviour saturated fine grained soil at its natural water content



If IC < 0, the natural water content of soil (wN) is greater than wL and the soil mass behaves like a liquid, but only upon disturbance.

If IC > 1, soil is in semi solid state and will be very hard or stiff.

  • Liquidity Index (IL)


For a soil in plastic state IL varies from 0 to 1.


  • Flow Index (If)



  • Toughness Index (It)


For most of the soils: 0 < IT < 3

When IT < 1, the soil is friable (easily crushed) at the plastic limit.

  • Shrinkage Ratio (SR)


where, V1 = Volume of soil mass at water content w1%.

V2 = volume of soil mass at water content w2%.

Vd = volume of dry soil mass

Now, at SL, w2 = ws and V2 = Vd

∴ image092

If w1 & w2 are expressed as ratio,




Stress-strain curve for different consistency states




  • Unconfined Compressive Strength (qu)

Defined as the load per unit area at which an unconfined prismatic or cylindrical specimen of standard dimensions of a soil fails in a simple compression test.

qu = 2 x shear strength of a clay soil (under undrained condition).

qu is related to consistency of clays as:


  • Sensitivity (St): It is defined as the ratio of the unconfined compressive strength of an undisturbed specimen of the soil to the unconfined compressive strength of a specimen of the same soil after remolding at unaltered water content.

St ≤ 1: in case of stiff clay having cracks and fissures.

Soil classification based on sensitivity:


  • Thixotropy: It is the property of certain clays by virtue of which they regain, if left alone for a time, a part of the strength lost due to remoulding, at unaltered moisture content.

Higher the sensitivity, larger the thixotropic hardening.

  • Activ image101

Activity based classification of clays


Volume change during swelling or shrinkage  = (Ip and % clay) of Activity

Classification and Structure of Soil

Classification of Soils


It is adopted by IS code. It was given by A-Casagrande. It uses particle size distribution for coarse soils and plasticity for fine soils.

Classification of Soils


Sorting soils into groups showing similar behaviour based on index property, Generally used property are

Grand Size Distribution (ii) Plasticity

Depending upon intended use different classification systems have evolved:

1. Unified Soil Classification System (USCS)

Given by Casagrande

Intended for use in Airfield, Construction


Note: ISCS is a modified USCS system.

2. AASHTO Classification System

For Highway Construction

  • Soil Classified into 8 groups divided into subgroups based on group index. GI.

GI value ranges between

  • O(Good Subgrade Material) to 20 (Poor Subgrade Material)

3. Indian Standard Soil Classification System (ISSCS) % Fineness:

In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub-groups.

The grain-size range is used as the basis for grouping soil particles into boulder, cobble, gravel, sand, silt or clay.


Gravel, sand, silt, and clay are represented by group symbols G, S, M, and C respectively.

Physical weathering produces very coarse and coarse soils. Chemical weathering produce generally fine soils. 

  • % of soil passing through the 75μ sieve.
  1. % Fineness < 50 % = Soil contain mainly
    Coarse Grained fraction otherwise Fine grained fraction
  2. Fraction retained over the 75μ is undergone with plasticity studies, i.e. WL + IP identifies.





  • On the basis of fineness, coarse grain soils are further classified

Case-I: Well fineness is < 5%

  1. GW – Well graded gravel
    Cu . 4
    1 < Cc < 3
    Fineness < 5%
  2. GP – Poorly graded gravel
    Above values of Cu and Cc are not satisfied.
  3. SW – Well graded sand
    Cu > 6
    1 < Cc <3
  4. SP – Poorly graded sand/uniformly graded sand Cu and Cc are not in range.

Case-II: If fineness is 5% to 12% the dual symbol are used.

  1. GW – GC well graded gravel containing clay.
    Fineness – 5 to 12%
    Clay > Silt
    Gravel > Sand
    Cu > 4; 1 ≤ Cc ≤ 3
  2. GW – GM Well graded gravel containing silt
    Cu > 4
    1 ≤ Cu ≤ 3
    Silt > Clay
    Gravel > Sand
  3. SW – SC Well graded sand containing clay
    Sand > Gravel
    Clay > Silt
    Cu > 6
    1 ≤ Cc ≤ 12%
  4. SW – SM Well graded sand containing silt
    Sand > Gravel
    Silt > Silt
    Cu > Clay
    1 ≤ Cc ≤ 3
    Fineness 5 to 12%

For poorly graded soils like GP-GC, GP, GM, SP-SC SP-SM the values of Cu and Cc are not satisfied.

Case-III: When fineness is more than 12%

GC: Clayey gravel

Gravel > Sand

Clay > Silt Ip > 7%

GM: Silty gravel

Sand < Gravel

Clay < Silt Ip < 4%

SC: Clayey silt

Sand > Gravel

Silti < Clay Ip > 7%

SM: Silty sand

Sand > Gravel

Silt > Clay Ip < 4%

Note: For IP between 4 and 7, Dual Symbols are used.

Classification of Fine Soils

  1. Silts (0.002 mm to 0.075 mm)
    • Coarse 0.02 to 0.075 mm
    • Medium 0.01 to 0.02 mm
    • Fine 0.002 to 0.01 mm
  2. Clay → <0.002 mm

(i) Low plastic soils (LL < 35%)

CL → Low plastic inorganic clay

ML → Low plastic silt

OL → Low plastic organic clay

(ii) Medium plastic soils (35% < 50%)

CI → Medium plastic inorganic clay

MI → Medium plastic silt

OI → Medium plastic organic clay

(iii) High plastic soils (LL > 50%)

CH → High plastic inorganic clay

MH → High plastic silt

OH → High plastic organic clay


Equation of A-line IP = 0.73 (WL – 20)

Equation of U-line IP = 0.9 (WL – 8)

Grain-Size Distribution Curve

The size distribution curves, as obtained from coarse and fine grained portions, can be combined to form one complete grain-size distribution curve (also known as grading curve). A typical grading curve is shown.


From the complete grain-size distribution curve, useful information can be obtained such as:

1. Grading characteristics, which indicate the uniformity and range in grain-size distribution.

2. Percentages (or fractions) of gravel, sand, silt and clay-size.

A grading curve is a useful aid to soil description. The geometric properties of a grading curve are called grading characteristics.


To obtain the grading characteristics, three points are located first on the grading curve.

D60 = size at 60% finer by weight 
D30 = size at 30% finer by weight 
D10 = size at 10% finer by weight

The grading characteristics are then determined as follows:


Both Cuand Cc will be 1 for a single-sized soil.

Cu > 5 indicates a well-graded soil, i.e. a soil which has a distribution of particles over a wide size range.

Cc between 1 and 3 also indicates a well-graded soil.

Cu < 3 indicates a uniform soili.e. a soil which has a very narrow particle size range. 

Structure of Soil

A soil particle may be a mineral or a rock fragment. A mineral is a chemical compound formed in nature during a geological process, whereas a rock fragment has a combination of one or more minerals. Based on the nature of atoms, minerals are classified as silicates, aluminates, oxides, carbonates and phosphates.

Out of these, silicate minerals are the most important as they influence the properties of clay soils. Different arrangements of atoms in the silicate minerals give rise to different silicate structures.

Basic Structural Units
Soil minerals are formed from two basic structural units: tetrahedral and octahedral. Considering the valencies of the atoms forming the units, it is clear that the units are not electrically neutral and as such do not exist as single units.

The basic units combine to form sheets in which the oxygen or hydroxyl ions are shared among adjacent units. Three types of sheets are thus formed, namely silica sheet, gibbsite sheet and brucite sheet.

Isomorphous substitution is the replacement of the central atom of the tetrahedral or octahedral unit by another atom during the formation of the sheets.

The sheets then combine to form various two-layer or three-layer sheet minerals. As the basic units of clay minerals are sheet-like structures, the particle formed from stacking of the basic units is also plate-like. As a result, the surface area per unit mass becomes very large.

  • A tetrahedral unit consists of a central silicon atom that is surrounded by four oxygen atoms located at the corners of a tetrahedron. A combination of tetrahedrons forms a silica sheet.


  • An octahedral unit consists of a central ion, either aluminium or magnesium, that is surrounded by six hydroxyl ions located at the corners of an octahedron. A combination of aluminium-hydroxyl octahedrons forms a gibbsite sheet, whereas a combination of magnesium-hydroxyl octahedrons forms a brucite sheet.


  • Montmorillonite Mineral 
    The bonding between the three-layer units is by van der Waals forces. This bonding is very weak and water can enter easily. Thus, this mineral can imbibe a large quantity of water causing swelling. During dry weather, there will be shrinkage.
  • Illite Mineral 
    Illite consists of the basic montmorillonite units but are bonded by secondary valence forces and potassium ions, as shown. There is about 20% replacement of aluminium with silicon in the gibbsite sheet due to isomorphous substitution. This mineral is very stable and does not swell or shrink.
  • Kaolinite Mineral
    A basic kaolinite unit is a two-layer unit that is formed by stacking a gibbsite sheet on a silica sheet. These basic units are then stacked one on top of the other to form a lattice of the mineral. The units are held together by hydrogen bonds. The strong bonding does not permit water to enter the lattice. Thus, kaolinite minerals are stable and do not expand under saturation.

    Kaolinite is the most abundant constituent of residual clay deposits.





write a comment

AE & JE Exams


Follow us for latest updates