Prepared by Engr Mamoon Kareem Department of Civil Engineering Swedish College Of Engg amp Tech Wah Cantt Lecture 34 Chapter 1 Introduction to Soil Mechanics Part 2 ID: 260882
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Slide1
Soil Mechanics - I
Prepared by:Engr Mamoon Kareem
Department of Civil Engineering
Swedish CollegeOf Engg & Tech Wah Cantt.
Lecture
# 3,4
Chapter # 1. Introduction to Soil
Mechanics
(Part 2)Slide2
Introduction to
Soil MechanicsWeathering of Rocks
Soil and its Types
Physical Properties of Soil
Chapter OutlinesSlide3
Physical Properties of Soil
ColorSoil StructureParticle Shape and Size
Specific GravitySoil Phases
PorosityVoid Ratio
Moisture Content
Degree of Saturation
Air ContentConsistency LimitParticle Size DistributionRelative DensitySlide4
1. Color
Significance: Identification PurposesColour depends upon:Type of soil mineral
Organic contentAmount of coloring oxidesDegree of
oxidationExamples:Black color Manganese Compound
Green or Blue Ferrous Compounds
Red, Brown or Yellow Iron
Grey Organic matterSlide5
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 SizeMineralogical CompositionNature and Composition of WaterSlide6
Structures in Cohesionless Soil
Single GrainedSoil particles are in stable positionThe 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 StructureSlide7
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 StructureSlide8
2. Soil StructureSlide9
3. Particle Shape and Size
Different shapes:Slide10
3. Particle Shape and Size
Nomenclature of material (soil type) and range of sizes Slide11
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 waterSlide12
4. Specific Gravity
Significance:Used for determination and calculation of many other soil properties ,as
Particle size analysis by hydrometer testPorosity and void ratioUnit weight
Critical hydraulic gradient Degree of saturation or zero air voidSlide13
4. Specific Gravity
Specific Gravity of some Minerals and Soil types Slide14
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:
Solid phase,
Liquid phase Gaseous or vapour phase.
Ice
phase
(in cold regions)Slide15
Schematic diagram indicating different soil phases Slide16
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 + V
v V = Total volume of soil mass Vs
= Volume of solid particles of soil
Vv = Volume of voids, which may be filled with air or water or both
Porosity falls in the range of
0
n
100Slide17
How to calculate Porosity?Slide18
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
.Slide19
How to calculate Void Ratio?Slide20
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 Slide21
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
”.
W
w
– is the weight of water actually present in the voids.
W
v
– is
wt
of water that can fill all the voids.
m – actual moisture content.
m
sat
– moisture content when all voids are totally filled with water.
The range of
“S”
0
S
100.Slide22
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: Slide23
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 MoistureCapillary
MoistureChemically Bound MoistureSlide24
Different forms of moisture
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 %Slide25
Different forms of moisture
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.Slide26
Different forms of moisture
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 Slide27
Different forms of moisture
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.Slide28
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 Slide29
11. Particle Size Distribution
The sieves normally required are as follows:Slide30
11. Particle Size Distribution
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.Slide31
11. Particle Size Distribution
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 Slide32
11. Particle Size Distribution
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.Slide33
11. Particle Size Distribution
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 smaller
ranging from large to small particles to fill voids are
sizes
missingSlide34
11. Particle Size Distribution
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 Slide35
11. Particle Size Distribution
The gradation curves:well graded soil
b) uniformly graded soil poorly
graded soil.Slide36
11. Particle Size Distribution
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.Slide37
11. Particle Size Distribution
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.Slide38
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.Slide39
12. Relative Density (Dr)
The range of values for relative densities (Dr) and the commonly referred state of compaction for granular
soil.Slide40
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:
Solid state Semi solid state
Plastic state Liquid state.Slide41
13. Atterberg or Consistency Limits
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.Slide42
13. Atterberg or Consistency Limits
Shrinkage LimitIt 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.Slide43
13. Atterberg or Consistency Limits
Shrinkage LimitConcept 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 entrySlide44
13. Atterberg or Consistency Limits
Relationship between volume and moisture content:Slide45
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 Slide46
13. Atterberg or Consistency Limits
Plastic LimitThe 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).Slide47
13. Atterberg or Consistency Limits
Plastic LimitSlide48
13. Atterberg or Consistency Limits
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. Slide49
13. Atterberg or Consistency Limits
Liquid LimitSlide50
13. Atterberg or Consistency Limits
Plasticity IndexPlasticity 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:Slide51
13. Atterberg or Consistency Limits
Range of Plasticity IndexP.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.Slide52
13. Atterberg or Consistency Limits
Change of liquid, plastic and shrinkage limits with plastic properties (not to scale, just to show comparison).Slide53
13. Atterberg or Consistency Limits
Liquidity IndexThe 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.Slide54
13. Atterberg or Consistency Limits
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 medium0.5 < 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 stateSlide55
13. Atterberg or Consistency Limits
Flow IndexThe 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 = Slide56
13. Atterberg or Consistency Limits
Flow IndexThe 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 = Slide57
13. Atterberg or Consistency Limits
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.Slide58
13. Atterberg or Consistency Limits
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.Slide59
13. Atterberg or Consistency Limits
Toughness IndexSoils 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.Slide60
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