Shear strength of soil is the resistance to deformation by continuous shear displacement of soil particles by the action of shear stresses.

Shear stresses > Shear Strength, failure takes place

• Failure may be sinking of footing or movement of a wedge of soil behind a retaining wall forcing it to move or slide.
• Shear strength is due to friction between particles, interlocking, and cohesion.
• Principal plane – shear stress = 0
• Normal stresses acting on Principal Planes are called Principal Stresses.
• Critical stress values and Obliquities generally occur on major and minor principal planes (two-dimensional solution)

Mohr-Coulomb failure theory:

Coulomb – 1776

Mohr – 1900 (extended Coulomb’s theory)

The theory states that:

1. Materials essentially fail in shear. the critical shear stress causing shear failure depends upon the properties of the material as well as normal stress on the failure plane.
2. The shear strength is equal to shear stress at failure on a potential plane.
3. In a material subjected to three-dimensional principal stress (Ϭ1, Ϭ2, and Ϭ3), the intermediate principal stress Ϭ2 doesn’t have any influence on the strength of the material.

Mathematically,

τ = f(Ϭ)

τ = shear stress

Ϭ= normal stress

MOHR’S STRESS CIRCLE 

Ϭ – Normal stress

τ -Shear stress

Ϭ1- major principal plane

Ϭ3- minor principal plane

α = Inclination to major plane

Resolving the forces horizontally;

Ϭ3*BC = Ϭ*AC Sinα – τ *AC Cosα

Vertically;

Ϭ1*AB = Ϭ*AC Cosα + τ *AC Sinα

Dividing both sides by AC;

Ϭ3*Sinα = Ϭ*Sinα – τ *Cosα – (i)

And,

Ϭ1*Cosα = Ϭ* Cosα + τ *Sinα- (ii)

Multiplying (i) by Cosα and (ii) by Sinα and subtracting (i) from (ii)

Cosα*Sinα (Ϭ1-Ϭ3) = τ (Sin²α + Cos²α)

Therefore,

τ = {(Ϭ1-Ϭ2)Sinα}/2 – (iii)

Substituting τ in eqⁿ (i)

Ϭ3 Sinα = Ϭ Sinα – {(Ϭ1-Ϭ3)*Sin2α*Cosα}/2

Or, Ϭ3 = Ϭ – {(Ϭ1-Ϭ3)*2Cos²α}/2

Or, Ϭ3 = Ϭ – (Ϭ1-Ϭ3)*Cos²α

Or, Ϭ = Ϭ3 + (Ϭ1-Ϭ3)* Cos²α

Or, Ϭ = Ϭ3 + (Ϭ1-Ϭ3)* {(1+Cos2α)/2}

           = Ϭ3 + {(Ϭ1-Ϭ3)/2} + {(Ϭ1-Ϭ3)/2} *Cos2α

Therefore,

Ϭ = {(Ϭ1+Ϭ3)/2} + {(Ϭ1-Ϭ3)/2} *Cos2α – (iv)

MOHR’S CIRCLE

Values of Ϭ, τ for α are plotted, locus of all the points gives a circle known as Mohr Circle

OE – Normal Stress (inclined at α)

ED – Shear Stress (inclined at α)

A – Minor Principal Stress

B – Major Principal Stress

These plane intersects at A (pole)

Principle Planes inclined to the Co-ordinate Axis :

Draw MN//BP (P = Pole) (Major Plane)

                   Similarly NO//AP           (Minor Plane)

                   OM//P’P  (P’ = Pole of inclined Plane)

                   P’X = τ; OX = Ϭ_n   (for inclined)

Angle of Obliquity:

Relation between Ф and α :

Coulomb failure criterion:

Τ_f = C + Ϭ_nf * tanФ (linear function)

Mohr failure criterion :

Τ_f = f (Ϭ_nf)  (Unique function)

Mohr-Coulomb failure Criterion :

θ= 45 + Ф/2

References:

1. Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
2. Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution

The post Shear Strength of Soils appeared first on OnlineEngineeringNotes.

Physical Properties: Identified by visual observation & Hand tools

Chemical Properties: Identified based on the effect of different chemicals

Thermal Properties: Effect of Heat

Types of Building Materials

Stone

Stone is the naturally available construction material and is obtained from a quarry, process of extracting stones by excavating, wedging, heating & blasting is called quarrying and is composed of different minerals and with different processes.

Classification of stones:

Geological Classification:

Igneous Rocks

• Solidification of molten mass above or below the earth’s surface.

• Crystalline glossy or fussed texture

• Ex: – Granite, Basalt, Diorite, Trap dolerite, Syenite, Pegmatite, Gabbro etc.

Sedimentary Rocks

• Gradually deposition of sand, clay, debris, etc. by the action of rain, wind, sun etc.
• Stratified
• Ex: – limestone and sandstone, conglomerate, gypsum, dolomite, magnesite, chalk, shale, kankar,Tripoli, diatomite, etc.

Metamorphic Rocks

• Change in texture or mineral composition of rocks under heat and excessive pressure
• Ex: – marble, Gneiss, Quartzite, Slate, Schist, etc.

Physical classification:

Stratified Rocks:

• Distinct layers which can be separated
• E.g.: limestone, slate, and sandstone.

Unstratified Rocks:

• No sign of strata
• E.g.: granite, marble, etc.

Chemical classification:

Silicious Rocks

• Silica as the main constituents
• E.g.: granite, quartzite, gneiss etc.

Argillaceous Rocks

• Clay or Alumina as main constituents
• E.g. of silicious rock: slate, laterite, kaolin, etc.

Calcareous Rocks

• Lime or calcium
• E.g.: limestone, marble, etc.

Important building stones:

1. Granite

• mainly composed of quartz, felspar, mica

What is Feldspar?

“Feldspar is no other than silicate of aluminum with varying amounts of potash, soda, or lime.

• Sp. Gravity -= 2.64
• Compressive strength = 70 – 130 MN/m²
• color depends on felspar – brown, grey, green, pink
• Offer high resistance to weathering
• Easily polished and worked
• Used for the external facing of buildings

2. Slate

• Argillaceous rock
• Alumina + sand /carbonate of lime
• Sp. Gravity = 2.8
• Compressive strength = 60-7- MN/m²
• Grey/dark blue color
• Hard, tough, fine-grained
• Used in cisterns
• Slate as tiles, an excellent roof covering material.

3. Gneiss

• Gneiss:
• A silicious rock
• Composed of quartz and felspar
• More easily worked than granite
• Used for street paving

4. Sandstone

• Silicious sedimentary rock.
• Composed of quartz, lime, and silica
• Sp. Gravity = 2.65- 2.9
• Compressive strength = 35 -40 MN/m²
• White, grey, brown, pink, etc.
• Strong and durable
• Used for ashlar works, moldings, carvings, etc.

5. Limestone

• Sedimentary rock of calcareous variety
• Sp. Gravity = 2.6
• Brown, yellow, dark grey colors
• Used large quantities in blast furnaces
• Used as stone masonry for walls.

6. Marble

• Metamorphic rock of calcareous variety
• Sp. Gravity = 2.7
• Very hard, ornamental work, takes a fine polish
• Carving, decoration
• Kankar
• Impure limestone containing 30 % alumina & silica
• Used for foundations of buildings

7. Laterite

• Laterite contains a high percentage of iron oxide.
• Porous and cellular structure
• Sp. Gravity = 2 -2.2
• Laterite blocks are suitable for light roads, and inferior buildings.
•  a very good road metal

8. Moorum

• Decomposed laterite
• Deep brown or red color
• Used in  garden walks & paths

9. Quartzite

• Siliceous sandstones
• Strong, durable, used as road metal/ railway ballast

Bricks

Bricks are construction materials made by moulding the tempered clay to a suitable shape and size which is in a plastic condition, dried in sun, and burnt in a kiln or clamp.

Other constituents are lime, magnesia, sodium, potassium, manganese, and Iron oxide.

Excess of Alumina,Silica & Lime

Alumina – brick crack/warp on drying

Silica – brick brittle and weak

Lime –  brick to melt and distort during burning

Alkaline salt – efflorescence

Manufacture of Bricks:

Preparation of Brick clay

• Earth left for atmospheric action for a few weeks after digging called weathering.

• 1.5 – 2.5 m³ of brick soil – 1000 bricks

• Tempered in pug mills

• Process of mixing clay, water, and other ingredients is known as Kneading.

Moulding bricks:

• Handmade bricks superior to machine-made bricks

• Drying of Bricks:
• Bricks are arranged in rows on their edges on a slightly raised ground called hacks.

• Burning of bricks:
• Clamp burning = 60 % out-turn
• Kiln burning = 80 – 90 % out-turn
• Takes 24 hrs. at 1000 – 1200 ⁰c.
• Cool for 12 days

Classification of Bricks:

1. First Class:

• Well burnt, smooth, even surface, rectangular, uniform reddish.
• Water absorption (≤ 20 %) at 24 hrs.
• Min. crushing strength = 10.5 MN/m²

2. Second Class:

• Slightly overburnt, rough, no perfect rectangle
• Water absorption (≤ 22 %)

3. Third Class:

• Under burnt, soft, easily broken
• Water absorption (≤ 25 %)

4. Jhama bricks

• Over burnt, irregular, dark bluish color

What is the size of the Brick?

•  Size of Brick = (19 cm * 9 cm * 4 cm)

                          Or (19 cm * 9 cm * 9 cm)

• Sp. Gravity = 2

• For 1 m³ of brick masonry = 550 machine-made bricks

                                                  500 handmade bricks

Special Bricks:

Squint bricks: construction of acute and obtuse squint quoins

Paving bricks: used for street pavements

Round bricks: circular pillars

Perforated & Hollow: Partitions walls

Refractory Bricks: withstanding temperature, low Coeff. of expansion & contraction.

• Acid Bricks: (fire Bricks & silica bricks)
• Basic Bricks: ( Dolomite bricks, Magnesite bricks and bauxite bricks)
• Neutral Bricks: (Chrome bricks, chrome-magnesite Bricks, spinel Bricks)

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The deformation of soil is a natural and physical phenomenon. Civil Engineering structures built on soils such as buildings, dams, bridges, roads, rails, etc. transmit load over a large area of soil through foundations or substructures. Soil tends to deform with the settlement or fail in shear. Therefore, the resistance to deformation of the soil is its bearing capacity. This resistance to deformation depends on various factors such as water content, bulk density, angle of friction, and the manner of application of loads.

Bearing capacity

The supporting power of soil is called the bearing capacity of the soil. In other words, the load or pressure developed under the foundation without introducing any damaging movement in the foundation and in the supporting structure is called the bearing capacity of the soil. It depends upon the grain size of the soil, the size, and the shape of the footing. The bearing capacity of soil increases with the decrease in the area of the footing.

Types of bearing capacity

1. Ultimate bearing capacity

It is defined as the gross pressure intensity at the base of the foundation at which the soil fails in shear. It is denoted by q_f. Gross pressure intensity at the bed of footing is due to weight of the superstructure including self-weight of footing and overburden pressure due to earth fill and is denoted by q.

• Net ultimate bearing capacity

It is the minimum net pressure intensity at the base of the foundation causing shear failure of soil. It is denoted by q_nf.

q_nf=q_f–gD

Where, D= depth of soil

Net pressure intensity is the difference between gross pressure intensity and original over burden pressure.It is denoted by q_n.It is given by the equation.

q_n=q-gD

gD= overburden pressure

• Safe bearing capacity

The maximum pressure which soil can carry safely without the risk of shear failure. It is obtained by adding net safe bearing capacity plus original overburden pressure. It is denoted by q_s.

q_s= q_nf/F + gD

• Net safe bearing capacity

It is the net ultimate bearing capacity divided by factor of safety.

q_ns = q_nf/F

• Allowable bearing capacity

Allowable bearing capacity is the loading intensity at which neither the soil fails by shear nor excessive settlement. It is used for designing of any structure.

Factor Influencing Bearing Capacity

There are various factors influencing the bearing capacity of the soil. The bearing capacity varies over a range for cohesive and cohesion-less soil. The physical features of foundation such as type of foundation, size of foundation, depth of foundation and shape of foundation significantly affect the bearing capacity. The amount of total and differential settlement is one of the main controlling factors for the bearing capacity of soil. The relative density in the case of granular soil and consistency in the case of cohesive soil play a deceive role in influencing the bearing capacity. The physical as well as engineering properties of soils such as density, cohesion and friction, position of water table and original stresses are the main factors governing the soil bearing capacity.

Factors influencing bearing capacity of soils

• Types of soil (coarse grained soil has greater bearing capacity than fine grained soil)
• Physical features of the foundations (types, size, shape, depth, rigidity)
• The amount of total and differential settlement
• Physical properties of soil (grain size)
• Position of water level (WT)
• Fluctuation in the level of ground water table
• Structural arrangement of soil

References

1. Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
2. Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution

The post Bearing capacity of soils; types of bearing capacity & factors influencing bearing capacity appeared first on OnlineEngineeringNotes.

• The geometrical arrangement of soil particle in a soil mass is known as soil structure.
• The ingredient necessary to give soil deposit cohesion is called clay material.
• Over 50% of soil deposits whose particle diameter is 0.002 mm or less is called clay.
• Clay mineral are composed of tiny crystalline substance of one or more members of a small group of mineral.
• Their particle are very small in size, very flasky in shape and have considrable surface area.
• Thery can only be viewed form electronic microscopic.
• The clay minerals can be divided on the basis of crystalline arrangement into three main group.

1.2 Types of Clay Minerals:

a. Kaoline Mineral:

• Kaoline structural unit consists of an alumina sheet combined with a silica sheet.
• The total thickness of structural unit is about 7 Angstrom (7Å).

Where,

1 Å = (10^-10m) or (10^-7mm)

• Kaoline mineral is formed by stacking one over the other.
• Kaoline mineral are mostly in hexagonal shape.
• The structure unit is joined together by hydrogen bond which is formed between the oxygen of silica sheet and hydroxyls of alumina sheet.
• Most common example of kaoline mineral is china clay.
• Kaoline mineral have thickness is about 0.05 micron.
• The specific surface is 15 m²/gm.

b. Montmorillonite:

• Montmorillonite structural unit consist of alumina sheet sandwiched between two silica sheet.
• The thickness of each structural unit is about 10 Å.
• The two structural unit are joined together by a link between oxygen ions of the two silica sheet. The link is form due to natural attraction of cations in intervening space and due to Vander waal force.
• The negatively charged surface of silica sheet attracts water in the space between two structural unit which causes expansion of material.
• The soil containing large amount of montmorillonite mineral exhibits shrinkage and high swelling characteristics.
• Montmorillonite minerals have thickness of 0.001 micron to 0.005 micron.
• The specific surface is about 800 m²/gm.

c. Illite:

• The structural unit of illite is same as montmorillonite but there is some isomorphous substitution of aluminium for silicon, in the silica sheet and the resultant charge deficiency is balanced by potassium ion which bond the layers in stack.
• Illite crysal does not swell so much in presence of water as montmorillonite.
• The thickness of illite is between 0.005 micron to 0.05 micron.
• The specific surface is about 80 m²/gm.

1.3 Clay Particle Interaction:

• The forces between soil particle is of two types. They are as follows:
  • Gravitational force
  • Surface force
• Surface for is more dominant over the gravitational force in the case of clay particle which behave like colloids. Colloid is a particle with  high specific surface which behavior is influenced by surface energy than mass energy.
• Gravitational force being proportional to mass are important in case of coarse grained soil only.
• Surface for may be attractive or repulsive.

a. Attractive force:

1. Vander Waal force
2. Hydrogen bond
3. Ionic bond

b. Repulsive force:

1. Due to similar charge
2. Cation repulsion

• For a given type of clay in suspension the net force between the adjacent particles at a given distance is the algebraic sum of the repulsive and attractive force acting at the distance.
• Potential force or inter-particle force decreases with increase in distance.
• If total potential energy between two particle decreases the particle will experience attraction and will flocculate but if there is increase in total potential energy the particle will experience repulsion and will disperse. The various factor affecting flocculation or dispersion are electrolyte concentration, temperature, ion valence, pH value, dielectric constant and anion adsorption.

Diffuse Double Layer:

• Normally clay soil are associated with water and its properties are significantly influence with the presence of water.
• Water molecule is dipole and molecules act as a bar magnet and the face of clay mineral carry negative charge except at edge.
• The layer extending from clay particle surface to limit of attraction is known as diffuse double layer.
• The water field in diffuse double layer is known as absorbed water or oriented water.
• Absorbed water imparts plasticity to clay.

1.4 Soil Structure:

The geometric arrangement of soil particle with respect to one another is known as Soil structure. The soil in nature have different strucutre depending upon the particle size and mode of formation.

The various soil mass structure found in natural soil deposit are decribed below:

a. Single – grained structure:

• This kind of structure are found in coarse grained soil.
• Grains which cohesion less and makeup soil like gravel and sand form such kind of structure.
• These grains are large enough and gravitational force is more dominant on them. When these particle deposited they acquire an equilibrium position by each particle being in contact with other surrounding particles.
• Closely the particle are packed together leaving very less void space between them and the structure is denser. When soil particle are loosely packed their is large void space between them and makeup loose soil structure.

b. Flocculated structure:

• Clay particle are very small, flaky in shape and they have large surface area.
• Surface force is more dominant than gravitational force.
• Flocculated structure particle have negative charge on the surface and positive charge on edges. They combine with each other by joining negative surface of particle to positive edge of the other.
• Soil with flocculated structure have large amount of void. So their void ratio is high. These soil are less sensitive to vibration as they formed a strong electrical bond.

c. Honeycomb structure:

• Sometimes smaller soil particle of silt sized when depositing join with one another and form a bridge like structure. They contain large void between those bridge and make the soil very loose in nature. Such type of structure is called honeycomb structure.
• Since honeycomb structure are loose they can support load only static condition. When they are subjected to vibration or shock, the structure collapses and large deformation takes place and soil achieves relatively dense state.

d. Dispersed structure:

• When clay soil are remolded their flocculated structure change. Also, particle change their orientation form edge to face orientation to face to face orientation. Such kind of structure is called dispersed structure.
• Dispersed structure soil have relatively lower volume of void and low void ratio.
• Dispersed structure soil have low shear strength, high compressibility and low permeability.
• When both coarse grained and fine grained particle are present in the soil they make up to two kind of soil structure.
  • Coarse grained skeleton
  • Cohesive matrix structure

e. Coarse grained skeleton:

• In coarse grained skeleton coarse grained particle are in large amount than fine grained particle and coarse particle remain in direct contact with other coarse particle forming a framework or a skeleton.
• The space between these large grain is occupied by fine particle.
• In coarse grained skeleton the soil are stable and less compressible and can take heavy load without much deformation.

f. Clay matrix structure or Cohesive matrix structure:

• In clay matrix structure fine grained particle are in large amount than coarse grained particles.
• Coarse grain appear embedded in fine grain and there is no direct particle to particle contact between them.
• Clay matrix structure soil behavior is similar to ordinary clay deposit.

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Introduction to Clay Minerals: Clay Particle interaction and Soil Structure appeared first on OnlineEngineeringNotes.

Soil classification is the arrangement of soils into different groups such that the soil in a particular group have similar behavior.

Requirement for soil classification

• It should have a limited number of groups.
• It should be based on engineering properties.
• It should be simple.

1.2 Field identification of soil

1. Gravel from Sand:

• Particle larger than 4.75 mm and smaller than 80 mm are gravel.
• Particle from 4.75 mm to 0.075 mm are sand.

2. Sand from Silt:

•   Find sand cannot be easily distinguished from silt by simple visual examination.
• It is possible to differentiate by dispersion test.

Dispersion test:

The test consists of pouring spoonful of sample into jar of water. If the material settles down in one to two minute it is sand. If the material takes about 15 minute to 1 hour then it is silt.

3. Silt from clay:

Microscopic examination of particle are possible only in lab. In the absence few simple test are:

a. Shaking test (Dilatancy test):

In this test a part of soil mixed with water to a very soft consistency is shaken after placing in the palm of the hand. If the soil is silt water will rise quickly to the surface and give it a shiny glistening appearance. If it is clay the water cannot move easily and hence it continues to look dark. An estimate of relative proportions of silt and clay in an unknown soil mixture can be made by nothing whether the reaction is rapid, slow or nonexistent.

b. Dry strength:

The strength of a soil in a dry state is and indication of its cohesion and hence its nature. It can be estimated by crushing a 3 mm size dried fragment between thumb and forefinger. A clay fragment can be broken only with great effort where a silt fragment crushes easily.

c. Rolling test (Toughness test):

A thread is attempted to be made out of a moist soil sample with a diameter of about 3 mm. If the material is silt it is not possible to make such a thread without disintegration and crumbling. If it is clay such a thread can be made even to a length of about 30 cm and supported by its own weight when held at ends.

1.3 Soil classification system- Particle size (MIT classification), Textural, ISCS, USCS and AASHTHO soil classification system:

1. MIT System of Classification:

MIT system of classification of soil was developed by Prof. G. Gilboy at Massachusetts Institute of Technology in USA. In this system the soil is divided into four groups:

• Gravel: Particle size greater than 2 mm.
• Sand: Particle size between 0.06 mm to 2 mm.
• Silt: Particle size between 0.002 mm to 0.06 mm.
• Clay: Particle size smaller than 0.002 mm.

2. Textural Classification:

• Used to determine the percentage of sand, silt and clay size.
• This method doesn’t reveal any properties of soil.
• Suitable for soil type having particle size less than sand.
• It doesn’t provide plasticity characteristics which is essential for classification of soil.

3. AASTHO Classification System:

(American Association of State Highway and Transportation official)

• It is particularly useful for classifying soil for highway.
• This system classifies both coarse grain and fine grained soil using particle size analysis and plasticity characteristics of soil.
• In this system soils are divided into seven types designated as A-1 to A-7 based on their relative expected quality for a road embankment.
• Some are divided into subgroups. Soils within each groups are evaluated according to the group index calculated from empirical formula.

Group Index (GI):

GI = (F – 35) [0.2 + 0.005 (LL – 40)] + 0.01 (F – 15) (PI – 10)

• Greater the GI value lesser desirable a soil for highway construction.
• A soil with lower number A-1 is more suitable for highway material than A-4.
• In AASTHO system initially there was now place for organic soil. So additional group A-8 was introduced for peat.

4. Unified Soil Classification System:

    (USCS)

It is the most popular system for use in all types of engineering problems involving soil.

The various symbol used are:

a. Coarse grained soil:

If more than 50% of soil mass contains particles which are larger than 0.075 mm which could not pass 0.075 mm sieve.

If fines ≤ 5%

• Soil is well graded and gravel. Then it is represented by GW.
• Soil is poorly graded and gravel. Then it is represented by GP.
• If soil is sand.

SW: Well Grained Sand

SP: Poorly Grained Sand

                 If fines ≥ 12%

• Soil is gravel and contains silt as its fines represented as GM.Similarly for sand.

SM: Silty Sand

• Soil is gravel contails clay as its fines as GC.Similarly for sand.

SC: Clayey Sand

                 If fines 5 – 12%

• Soil is well graded gravel with 8% of -silt.

GW – GM

• Soil is poorly graded gravel with 8% of silt.

GP – GM

• Soil is well graded gravel with 9% of clay.

GW – GC

• Soil is poorly graded gravel with 9% of clay.

GP – GC

b. Fine – grained soil:

If more than 50% of soil passes from 0.075 mm sieve.

• LL after oven dried – LL before oven dried = ≥ 30% it is organic soil ( OH & OL).
• LL after oven dried – LL before oven dried = < 30% it is inorganic soil ( ML & MH).

c. Highly organic soil:

These soil are identified by visual inspection.

5. Indian Standard Soil Classification System:

    (ISSCS)

ISSCS is similar to USCS. There is one difference in the classification of fine grained soil. The fine grained is sub- divided into three category low, medium and high.

a. Coarse-grained: Soils are same as USCS.

b. Fine grained soil:

• Low: Have less liquid limit than 35%.
• Medium: Liquid limit greater than 35% but less than 50%.
• High: Liquid limit greater than 50%.

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Soil Identification and it’s Classification: Particle size, Textural, ISCS, USCS, and AASHTHO soil classification system appeared first on OnlineEngineeringNotes.

Soil mass consists of solid soil particles containing void space between them. Space between the soil particles is known as void and these void is filled with either air or water or both.

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

When soil masses consist of air, water and solid particles it is known as three phase system.

When soil masses consist of water and solid particle it is known as two phase system for saturated soil.

When soil masses consist of air and solid particle it is known as two phase system for dry soil.

Equation of three phase system:

Let,

V = Total volume of soil

V_s = Volume of solid particle

V_a = Volume of air

V_w = Volume of water

V_v = Volume of void

So, from the definition of

Total volume of soil is equal to the sum of volume of air, water and solid particle respectively.

i.e. V = V_w + V_a + V_s

or, V = V_s + V_v [∴V_w + V_a ]

Similarly,

For weight

Total weight (W) = Weight of solid (W_s) + Weight of water (W_w) + Weight of air (W_a)

i.e W = W_s + W_a + W_w

Also,

Weight of void (W_v) = W_a + W_w

or, W_v = W_w [∴ Weight of air is negligible, so W_a =0 ]

∴ W_v = W_w

Total weight of soil mass,

W = W_s + W_w

Similarly, in terms of mass

M = M_s + M_w

1.2 Index properties and their determination for coarse and fine grained soil

The properties of soil which helps to know engineering behaviors of soil and also helps to determine the classification of soil accurately is called index properties of soil.

The list of index properties:

1. Water content
2. Specific gravity of soil
3. Field ( In – situ) density
4. Particle size distribution
5. Consistency limit and indices
6. Density index

1.2.1 Determination of Various Index Properties:

1. Determination of water content

a. Oven drying method:

Process:

• Take a clean container and weight the container.
• Take about 100 gm wet sample and weight it again.
• Keep the soil with container in oven for overnight by maintain 104^o C temperature.
• After drying find the weight of dry soil and container.

Calculation:

Weight of container = W₁

Weight of container + Wet soil = W₂

Weight of container + Dry soil = W₃

Water content (w) = (W_w / W_s) * 100%

or, w = {(W₂ – W₁) / (W₃ – W₁)} * 100%

2. Determination of specific gravity

a. Pycnometer method:

Process:

• Take empty pycnometer.
• Take about 200 gm of dry sample in pycanometer and weight it.
• Add water in pycnometer and weight it.
• Take empty pycnometer and fill with water and weight it.

Calculation:

M₁ = Mass of empty pycnometer

M₂ = Mass of dry sample fill in pycnometer

M₃ = Mass of wet sample ( saturated sample)

M₄ = Mass of water in pycnometer

M_s= Mass of dry soil

Now,

    M₄ = M₃ – M_s + M_w

or, M₄ = M₃ – M_s + V_s *ρ_w

or, M₄ = M₃ – M_s + M_s * (1 / G) ————(1)

[∴ G = ρ_s / ρ_w]

Also,

M_s = M₂ – M₁

Putting the value in equation (1).

   M₄ = M₃ – M₂+ M₁ + (M₂ – M₁) / G

or, M₄ – M₃ + M₂ – M₁ = (M₂ – M₁) / G

or, G = {( M₂ – M₁) / (M₄ – M₃ + M₂ – M₁)}

or, G =  ( M₂ – M₁) / (M₄ + M₂) – (M₁ + M₃)

       ∴ G =  ( M₂ – M₁) / (M₄ + M₂) – (M₁ + M₃)

3. Determination of field density

a. Core cutter method

Process:

• Take a clean core cutter.
• Clean and level the ground where density is to be measured.
• Press cylinder cutter into the ground to its full depth with the help of rammer.
• Remove the core cutter from the soil.
• Take the mass of cutter and soil.

Calculation:

M₁ = Mass of clean core cutter

M₂ = Mass of cutter with soil

H = Height of cutter

d = Diameter

A = Area of cutter

Now,

Mass of soil (M) = M₂ – M₁

Volume of soil = A * H = (πd²/4)* H

Where,

H = 130 mm

d = 100 mm

Then,

Density (ρ ) = M/V

b. Sand replacement method:

Process:

• The site is cleaned and a square tray with a central hole is place on the surface.
• The hole of diameter is equal to the diameter of hole in tray.
• The excavated soil is collected in tray and weighted and water content is determined.
• The pouring cylinder is fitted with sand to about ¾ capacity and weight is taken and also its density is found out.
• The cylinder is then place over the hole and tap is opened and sand is allowed to run to fill the excavated whole and conical end.
• When no further flow of sand takes place, the tap is closed and bottle with remaining sand is weighted. The weight of sand filling the cone of bottle is taken separately.  

Calculation:

Weight of soil in the hole = W₁

Weight of pouring cylinder + sand before pouring = W₂

Weight of cylinder + sand after pouring = W₃

Weight of sand filling conical funnel = W₄

Weight of sand filling hole = W₂ – W₃ – W₄

Unit weight of sand = γ

Now,

Volume of sand = (W₂ – W₃ – W₄) / γ

                          = Volume of hole = V

Bulk density (γ) = W₁ / (1 + w)

Dry density (γ_d) = γ / (1+w)

4. Particle Size Distribution:

a. Sieve analysis:

• Sieve is utensil made up of spun brass having size of square opening in mm or microns (80mm – 75µ) and diameter of sieve is 15-20 cm and used for separating coarse and fine grained soil.
• Sieve analysis is done for coarse grained soil.
  • Gravel: Size greater than 4.75mm.
  • Sand: Size in between 75 µ to 4.75mm.
•  The sieves are stacked one over the other with decreasing size from top to bottom. The sieve of largest opening is kept at the top to bottom. The sieve of largest opening is kept at the top. A lid or cover is place at the top of largest sieve. A receiver known as pan which has no opening is placed at the bottom of smallest sieve.
• There are two type of sieve analysis:
  • Dry sieve analysis
  • Wet sieve analysis

b. Sedimentation analysis:

Sedimentation analysis or wet mechanical analysis is conducted on soul fraction finer than 75micron which is kept in suspension in a liquid medium usually water.

Sedimentation analysis is based in Stoke’s Law according to which gives the terminal velocity of small sphere setting in a fluid of infinite extent.

Now,

Terminal velocity (v) = {gD² * (G – 1) ρ_w} / 18η ———(1)

If a particle fall through height H_e in “t” minutes,

v = H_e / 60t ————-(2)

Combining equation (1) and (2)

D = M (H_e / t)^1/2

Where,

M = {30η / g(G – 1) ρ_w }

Uses of Particle size distribution curve:

• Classification of coarse grained soils.
• Determination of coefficient of permeability.
• To know susceptibility of soil to frost action.
• Design of drainage filter.
• Determination of shear strength of soil.
• Compressibility of soil.
• Used for soil stabilization.
• Design of pavement.
• Indicate mode of deposition of soil.
• Age of soil deposit.

5. Consistency of soil:

• The physical state of fine grained soil at particular water content is known as consistency.
• Mostly used for fine grained.
• The limiting water content at which a soil passes from one state of consistency to another state is called consistency limit.

Determination of consistency limit:

a. Liquid limit (LL):

At liquid limit, the soil possesses a small value of shear strength. The liquid limit is the minimum water content at which the soil is still in liquid state but has a small shearing strengh agaist flowing.

b. Plastic Limit (PL):

In plastic limit soil can be moulded into any shape.

w_p = W_w / W_s = (W₁ – W₂) / W₂

where,

W₁ = Weight of soil before drying

W₂ = Weight of soil after drying

c. Shrinkage limit:

In shrinkage limit water content is lowest.

w_s = {(M₁ – M_d) – (V₁ – γ_d) γ_w} / M_d

Where,

M₁ = Mass of wet soil pat

V₁ = Volume of wet soil pat

M_d = Mass of fry soil pat

V_d = Volume of dry soil pat

Or,

w_s = {(V_d γ_w) / M_d} – 1/G

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Solid-Water-Air Relation and Index properties of soil and their determination for coarse and fine grained soil appeared first on OnlineEngineeringNotes.

Soil are the uncemented aggregate of mineral and decayed organic matter containing liquid and gas in the empty space of solid particles.

Types of soil :

• Residual soil:

A soil is said to be residual if it is formed by disintegration of rocks without transportation.

• Transported soil:

A soil is said to be transported by disintegration of rocks with the action of transfortation medium like air, water etc.

Rock is the natural occuring solid substance that are formed by minerals.

1.2 Formation process of soil :

Rock-soil cycle or Rock-soil interaction:

• It is an interaction between rock and soil.
• Soil is formed due to disintegration of rock.

Disintegration of rocks:

There  are two types of disintegration of rocks:

1. Physical Disintegration :

Formation of cohesion less soil and sand.

• Temperature change:

Change in temperature causes expansion and contraction of rocks and hence cracks occurs.

• Wedging action of rocks:

The wedging action of ice causes expansion of rocks and hence disintegration occurs.

• Spreading of roots of plants:

It causes cracking in rocks and hence disintegration occur.

• Abrasion:

Due to air, water and glacier wear and tear of rock occurs and hence disintegration occur.

2. Chemical decomposition:

Formation of cohesive soil and clay.

• Hydration:

Rock in contact with water causes hydrolysis and hence change in volume occur.

• Carbonation:

CO₂ + H₂O → H₂CO₃ ( Carbonic acid)

Carbonic acid in contact with rock causes disintegration.

• Oxidation:

Oxygen ion in contact with rock minerals causes decomposition.

• Solution:

The submerged rock causes expansion and hence decomposition.

1.3 Definition of soil mechanics and its importance in civil engineering:

Soil mechanics is the branch of science that deals with the physical properties of soil and the behaviour of soil masses subjected to various types of forces.

Importance of soil mechanics in civil engineering:

• Foundation:

Foundations are required to transmit load of the structure to soil, safely and efficiently without the shear failure and excessive settlement of soil mass.

• Retaining structures:

When sufficient space is not available for the mass of soil to spread and to form a safe structure it is required to retain the soil. Soil engineering gives the theories of earth pressure on retaining structure.

• Stability of slopes:

If soil surface is not horizontal then there is a component of weight of soil which tends to move downward and causes instability of slope. Hence, soil engineering provides the methods for checking stability of slopes.

• Underground structure:

It helps to design and construct the underground structures such as tunnels, shafts, conduits etc.

• Pavement design:

A pavement is a hard crust provied on soil for the purpose of a smooth and strong surface on which vechicles can move. Soil mechanics gives the idea about design of pavement.

• Earth dam:

Earth dam are huge structure in which soil is used as a construction material. The earth dam are built for creating water reservoirs. Extrem care must be taken while constructing earth dam because the failure of eath dam may lead huge disaster.

• Miscellaneous soil problem:

Miscellaneous soil problem related to soil such as soil heave, soil subsidence, frost heave, shrinkage and swelling of soil etc. has to be tackled by geotechnical engineer during constrution phase. Soil mechanis gives the idea about these problems.

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Definition of soil mechanics and its importance in civil engineering: Formation process of soil and its major types appeared first on OnlineEngineeringNotes.