Soil Mechanics - I

Soil Mechanics - I

Prepared by:Engr Mamoon Kareem

Department of Civil EngineeringSwedish CollegeOf Engg & Tech Wah Cantt.

Lecture # 3,4Chapter # 1. Introduction to Soil Mechanics

(Part 2)

Introduction to Soil Mechanics

Weathering of Rocks

Soil and its Types

Physical Properties of Soil

Chapter Outlines

Physical Properties of Soil• Color• Soil Structure• Particle Shape and Size• Specific Gravity• Soil Phases• Porosity• Void Ratio

• Moisture Content• Degree of Saturation• Air Content• Consistency Limit• Particle Size Distribution• Relative Density

1. Color Significance: Identification Purposes Colour depends upon:

Type of soil mineral Organic content Amount of coloring oxides Degree of oxidation

Examples: Black color Manganese Compound Green or Blue Ferrous Compounds Red, Brown or Yellow Iron Grey Organic matter

2. Soil Structure Soil Structure is defined as the grouping or

arrangement of soil particles with respect to one another.

Factors that affect the structure are: Shape and Size Mineralogical Composition Nature and Composition of Water

Structures in Cohesionless Soil Single Grained

Soil particles are in stable position The shape and size distribution of soil particles and their

relative positions influence the denseness of packing. Irregularity in the particle shapes generally yields an

increase in the void ratio Honeycombed

Relatively small sand and silt form small arches with chains of particles.

They can carry an ordinary static load because of large inter-particle spaces.

2. Soil Structure

Structures in Cohesive Soil Flocculent Structure:

The clay minerals are extremely flaky in shape and have a large surface area-to-mass ratio.

Flocculated structure is developed when the edge of one clay particle is attracted to the flat face of another

Dispersed Structure: Develops when the edges and faces of the clay particles

have similar electrical charge Also develops as a result of remolding by the transportation

process (man-made earth fills )

2. Soil Structure

2. Soil Structure

3. Particle Shape and Size Different shapes:

3. Particle Shape and Size Nomenclature of material (soil type) and range of

sizes

4. Specific Gravity The ratio of the unit weight of a substance, to the

unit weight of water at 4oC How many times a substance (or material) is

heavier than water

4. Specific Gravity Significance: Used for determination and calculation of

many other soil properties ,as

Particle size analysis by hydrometer test Porosity and void ratio Unit weight Critical hydraulic gradient Degree of saturation or zero air void

4. Specific Gravity Specific Gravity of some Minerals and Soil

types

5. Soil Phases Any homogeneous part of a soil mass

different from other parts in the mass and clearly separated from them is called a phase.

Fundamental phases:

1. Solid phase, 2. Liquid phase 3. Gaseous or vapour phase.4. Ice phase (in cold regions)

Schematic diagram indicating different soil

phases

6. Porosity The ratio of volume of all the voids “Vv” to

the total volume of the soil mass “V” is known as the porosity.

Where V = Vs + Vv V = Total volume of soil mass Vs = Volume of solid particles of soilVv = Volume of voids, which may be filled with air or water or both

Porosity falls in the range of 0 n 100

How to calculate Porosity?

7. Void Ratio The ratio of volume of voids present in a

soil mass to the volume of solid particles. It is denoted by “e”.

The void ratio is expressed as a number and the limiting values can be within the range.

s

v

VV

soilinslidsofvolumesoilinvoidsofvolumee

How to calculate Void Ratio?

8. Air Content The ratio of the volume of air present in

the voids to the total volume of a soil mass.

Since; Vv = Va + Vw

Air content or Air Void Ratio fall within the range of

sv

wvav VV

VVVV

AorA

.1000 percentA

9. Degree of Saturation The condition when voids are partially

filled with water is expressed by the degree of saturation or relative moisture content. It is the ratio of actual volume of water in voids “Vw” to the total volume of voids “Vv”.

satv

w

v

w

mm

WW

VVS

Ww – is the weight of water actually present in the voids.Wv – is wt of water that can fill all the voids. m – actual moisture content.msat – moisture content when all voids are totally filled with water.

The range of “S” 0 S 100.

10. Moisture Content The amount of water present in the voids of a

soil in its natural state.

The common range of moisture content for most soil is 20-40 percent.

Oven dried soil has zero percent moisture and the soils which appear dry (i.e., air dried soil) often have 2 to 4 percent moisture content.

The range of water content is:

100soildryofweight

waterofweightm

Different forms of moisture The moisture/water in the voids of a soil

mass can occur in a variety of forms. Depending upon the form of occurrence they are given different names e.g.,

Hygroscopic Moisture Film Moisture Capillary Moisture Chemically Bound Moisture

Different forms of moisture1. Hygroscopic Moisture:

Also known as adsorbed moisture, contact moisture or surface bound moisture.

This form of soil moisture exists as a very thin film of moisture surrounding the surfaces of individual soil particles and is held by the forces of adhesion.

It depends upon temperature and humidity. It is not affected by gravitational forces, capillary forces and air

drying at ordinary ordinary temperature. The approximate values of hygroscopic moisture for various soils

are as under:

1- Sand 1-2 %2- Silt 7-9 %3- Clay 17-20 %

Different forms of moisture2. Film Moisture:

The moisture film attached to the soil particles, above the layer of hygroscopic moisture film, is known is film moisture.

It is held by the molecular forces and is not affected by gravity.

The amount of film moisture depends on the specific surface i.e., higher the specific surface higher will be the film moisture and vice versa.

Different forms of moisture3. Capillary Moisture:

The moisture which in held within the voids of capillary size. The capillary moisture is continuously connected to the groundwater table.

Capillary water can be removed from the soil by drainage

Different forms of moisture4. Chemically Bound Moisture:

Moisture contained chemically within the mineral particles and can be removed only by chemical processes of the substance when the crystalline structure of the mineral breaks.

Chemically bound moisture is not important for common soil engineering problems and therefore is not determined.

11. Particle Size Distribution

The percentage of various particle sizes present in a soil is known as particle size distribution or gradation.

Particle size analysis is made by sieving or by sedimentation. Sieving method – when particle size > .074 mm Sedimentation method – when particle size

< .074mm


The sieves normally required are as follows:


Significance:

Engineering classification of soils. Selection of the most suitable soil for

construction of roads, airfields, levees, dams and other embankments.

To predict the seepage through soil (although permeability tests are more generally used)

To predict the susceptibility to frost action. Selection of most suitable filter material.


The gradation curve: A gradation curve is drawn by plotting the

percentage finer (%age passing) on ordinate against the particle sizes on abscissa.

The gradation curves indicate the type of soil, and provide very important information related to the properties and behavior of soil


The gradation curves have great importance in civil engineering and are extensively used for the following purposes. Determination of Effective Grain (Particle) Size. Determination of Uniformity co-efficient. Determination of co-efficient of Curvature. Determination of percentage of different soil types in

a soil sample e.g., sand, silt, clay. Determination of percentage larger or finer than a

given size. Classification of soil. Design of filters. Concrete mix design.


Well-Graded Soil: A soil containing an assortment of particles with

a wide range of sizes. A well-graded soil has following merits:

1. Higher shear strength 2. Higher density 3. Reduced Compressibility 4. Higher stability 5. Higher Bearing Capacity 6. Low permeability

well graded uniformly graded Ideal packing, due to particles Loose packing, as smallerranging from large to small particles to fill voids are sizes missing


Uniformly-Graded Soil: A uniformly graded soil is defined as a soil

containing particles having a limited range of sizes (Almost the same sizes)

Poorly-Graded Soil: A poorly graded soil is defined as a soil

containing particles of varying sizes with intermediate particle sizes missing.

Such soils give lower density and lower strength. The gradation curve of a poorly graded soil show

steps indicating an excess of certain particle sizes, and a deficiency of others


The gradation curves:a) well graded soil b) uniformly

graded soil b) poorly graded soil.


Co-efficient of uniformity: When the value of Cu is less than 4, the soil is

generally considered as uniformly graded.

A higher value of Cu represents a wide range of particle sizes and the soil is termed as well graded.

)32.1(10

60

DDCu


Co-efficient of curvature: It is also known as coefficient of gradation (Cg) or

Co-efficient of Concavity.

Cc = 1, represents that all the soil particles have the same size, and the soil is uniformly graded.

Cc between 0.2 and 2.0 indicate well graded or poorly graded soil.

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)(

1060

230

DDD

Cc

12. Relative Density (Dr) The term relative density (also called

density index, ID) is used to express the state of compactness of a granular soil.

The following relationship between the void ratio values is termed as the relative density.

12. Relative Density (Dr) The range of values for relative densities

(Dr) and the commonly referred state of compaction for granular soil.

13. Atterberg or Consistency Limits

The consistency of a soil means its physical state with respect to the moisture content present that time.

Consistency states are:1. Solid state 2. Semi solid state3. Plastic state 4. Liquid state.


Boundaries of the above four states are:

Shrinkage Limit: It is the moisture content at which a soil changes from solid state to semi-solid state.

Plastic Limit: It is the moisture content at which a soil changes from semi-solid state to plastic state.

Liquid Limit: It is the moisture content at which a soil changes from plastic state to liquid state.


1. Shrinkage Limit It is that moisture content at which a reduction in

moisture will not cause a decrease in the total volume of soil mass, but an increase in moisture will result in an increase in volume of soil mass.

At Shrinkage Limit The Degree Of Saturation is 100%. At certain point during drying process, air begins to enter

the soil mass and the volume decrease becomes appreciably less than the volume of water lost.

The shrinkage limit is not given much importance since it is not used in soil classification.


1. Shrinkage Limit Concept of surface tension forces and induced

compressive stresses(a) Particle separated due to thick moisture film(b) Meniscus contracting due to drying process(c) Meniscus tending to tear off (d) Meniscus fully torn off allowing air entry


Relationship between volume and moisture content:

The soils which show higher shrinkage upon drying also swell more upon wetting and are known as expansive soils. Expansive soils are very dense and hard in dry state due to

very high shrinkage stresses

Shrinkage cracks at Rawal lake which dried due to drought


2. Plastic Limit The moisture content at which a soil can be

rolled into threads of 1/8” (3.2mm) diameter without cracking and crumbling.

Threads thinner than 1/8” (3.2 mm) diameter are possible, if the moisture is higher than the plastic limit.

And if the moisture is less than plastic limit the thread will crumble before reaching the required diameter of 1/8” (3.2 mm).


2. Plastic Limit


3. Liquid Limit

The moisture content at which 25 blows of Cassagrande apparatus closes a standard groove cut in the soil paste along a distance of 12.7 mm (0.5 in).

The moisture content which gives a penetration depth of 20mm of the standard cone (fall cone test) into the soil, when the cone is released for 5 seconds.


3. Liquid Limit


Plasticity Index

Plasticity Index indicates the range of moisture through which a cohesive soil behaves as a plastic material

It is the numerical difference between liquid and plastic limits. It is expressed as:


Range of Plasticity Index

P.I. = 0 The soil is non-plastic and non-cohesive. P.I. < 7 The soil is low plastic and partly cohesive. P.I. 7 - 17 The soil is medium plastic and cohesive. P.I. > 17 The soil is highly plastic and very cohesive.


Change of liquid, plastic and shrinkage limits with plastic properties (not to scale, just to show comparison).


Liquidity Index The ratio of difference between the moisture

content and plastic limit to the plasticity index.

L.I < 0, (i.e. negative value) the field moisture content is less than the plastic limit, and hence the soil is in a semi-solid state.

IPLPm

LPLLLPmIL

..

....


Consistency of a soil at its natural moisture content:

▪ L.I < 0,the soil is in a semi-solid or solid state (hard)

▪ 0.00 < L.I ≤ 0.25, the consistency is stiff or hard

▪ 0.25 < L.I ≤ 0.50, the consistency is medium

▪ 0.5 < L.I ≤ 0.75, the consistency is soft

▪ 0.75 < L.I ≤ 1, the consistency is very soft

▪ L.I > 1, the soil is in a liquid state


Flow Index The slope of the flow curve (graph between log N and moisture

content drawn for the determination of liquid limit) is known as the flow index and is equal to:

Any two soils, although having the same plasticity indices and/or the liquid limits may have different values of flow index, and hence may possess varying degree of cohesiveness and shear strength.

F.I =

F.I =


Flow Index The slope of the flow curve (graph between log N and moisture

content drawn for the determination of liquid limit) is known as the flow index and is equal to:

Any two soils, although having the same plasticity indices and/or the liquid limits may have different values of flow index, and hence may possess varying degree of cohesiveness and shear strength.

F.I =

F.I =


Case-I: Two soils having the same values of plasticity index

No. of blows are indicative of the resistance to deformation or shear strength. For the same drop of moisture ∆m, the No. of blows for flat curve increase very much, indicating higher shear strength. Therefore, the soils with same plasticity index may posses different shear strength.


Case-I: Two soils having the same values of plasticity index No. of blows are

indicative of the resistance to deformation or shear strength. For the same drop of moisture ∆m, the No. of blows for flat curve increase very much, indicating higher shear strength. Therefore, the soils with same liquid limit may posses different shear strength.


Toughness Index

Soils having same values of plasticity indices may vary in toughness. This property of a soil is expressed by the toughness index.

Toughness and dry strength increases with increase in toughness index.

)31.1(.......IFIPIT

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Soil Mechanics - I