Soil Mechanics Unit 1 Notes

SOIL MECHANICS - UNIT I SOIL MECHANICS OBJECTIVE

After undergoing this course, the student gains adequate knowledge on engineering properties of soil. 1. INTRODUCTION 10 Nature of Soil - Problems with soil soil - phase relation - sieve analysis sedimentation analysis analysis – – Atterberg Atterberg limits - classification for engineering purposes - BIS Classification system - Soil compaction - factors affecting compaction – compaction – field field compaction methods and monitoring.

water – Various Various forms – forms – Influence Influence of clay 2. SOIL WATER AND WATER FLOW 8 Soil water – minerals – minerals – Capillary Capillary rise – rise – Suction Suction - Effec E ffective tive stress concepts in soil so il – – Total, Total, neutral and effective stress distribution in soil - Permeability P ermeability – – Darcy’s Darcy’s LawLaw- Permeability measurement in the laboratory – laboratory – quick quick sand condition - Seepage – Seepage – Laplace Laplace Equation - Introduction Introdu ction to flow nets – nets – properties and uses - Application to simple problems. problems. 3. STRESS DISTRIBUTION, COMPRESSIBILITY AND SETTLEMENT 10 Stress distribution in soil media – media – Boussinesque Boussinesque formula – formula – stress stress due to line load and Circular and rectangular loaded area - approximate methods - Use of influence charts – charts – Westergaard Westergaard equation for point load - Components of settlement - Immediate and consolidation settlement - Terzaghi's one dimensional consolidation theory theo ry – – governing governing differential equation – equation – laboratory laboratory consolidation test – test – Field Field consolidation curve – curve – NC NC and OC clays - problems on final and time rate of consolidation 4. SHEAR STRENGTH 9 Shear strength of cohesive and cohesionless soils - Mohr - Coulomb failure theory – theory – Saturated Saturated soil - Strength parameters - Measurement of o f shear strength, direct shear, Triaxial compression, UCC and Vane shear t ests – ests – Types Types of shear tests based on drainage and their applicability - Drained and undrained behaviour of clay and sand – sand – Stress Stress path for conventional triaxial test. 5. SLOPE STABILITY 8 Slope failure mechanisms - Modes - Infinite slopes - Finite slopes – slopes – Total and effective stress analysis - Stability analysis for purely cohesive and C- C- soil soilss - Method of slices – slices – Modified Modified Bishop’s method - Friction circle method - stability number – number – problems – problems – Slope protection measures. TOTAL: 45 PERIODS TEXT BOOKS:

1. Punmia P.C., “Soil Mechanics and Foundations”, Laximi Publications Pvt. Ltd., New Delhi, Delhi, 1995. 2. Gopal Ranjan and Rao A.S.R., “Basic and applied soil mechanics”, New Age International Publishers, New Delhi, 2000. SAKTHI ENGINEERING COLLEGE

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SOIL MECHANICS - UNIT I 3. Venkatramaiah, C. “Geotechnical Engineering”, New Age International Publishers, New Delhi, 1995 4. Khan I.H., “A text book o f Geotechnical Engineering”, Prentice Hall of India, New Delhi, 1999. REFERENCES

1. Coduto, D.P., “Geotechnical Engineering Principles and Practices”, Prentice Hall of India Private Limited, New Delhi, 2002. 2. McCarthy D.F., “Essentials of Soil Mechanics and Foundations Basic Geotechniques”, Sixth S ixth Edition, Prentice-Hall, New Jersey, 2002. 3. Das, B.M, “Principles of Geotechnical Engineering”, (fifth edition), Thomas Boo ks/ cole, 2002 4. Muni Budhu, “Soil Mechanics and Foundations”, John Willey & Sons, Sons, Inc, New York, 2000.

SAKTHI ENGINEERING COLLEGE

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SOIL MECHANICS - UNIT I

Introduction to Soil Mechanics The term "soil" can have different meanings, depending upon the field in which it is considered. To a geologist, it is the material in the relative thin zone of the Earth's surface within which roots occur, and which are formed as the products of past surface processes. The rest of the crust is grouped under the term "rock". To a pedologist, it is the substance existing on the surface, which supports plant life. To an engineer, it is a material that can be:    

built on: foundations of buildings, bridges built in: basements, culverts, tunnels built with: embankments, roads, dams supported: retaining walls

Soil Soil M echani cs is a discipline of Civil Engineering involving the study of soil, its behaviour and

application as an engineering material. Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles, which are produced by the mechanical and chemical disintegration of rocks, regardless of o f whether or not they contain an admixture of organic constituents. Soil consists of a multiphase aggregation of solid particles, water, and air. This fundamental composition gives rise to unique engineering properties, and the description of its mechanical behavior requires some of the most classic classic principles of engineering mechanics. Engineers are concerned with soil's mechanical properties: permeability, stiffness, and strength. These depend primarily on the nature of the soil grains, the current stress, the water content and unit weight. In the Earth's surface, rocks extend upto as much as 20 km depth. The major rock types are categorized as igneous, sedimentary, and metamorphic.   

Igneous rocks: formed from crystalline bodies of cooled magma. Sedimentary rocks: formed from layers of cemented sediments. Metamorphic rocks: formed by the alteration of existing rocks due to heat from igneous intrusions intrusions or pressure due to crustal movement.

Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical ph ysical and chemical weathering. The T he nature and structure of a given g iven soil depends depend s on the processes and conditions that formed it: 

Breakdown of parent rock: weathering, decomposition, erosion.


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SOIL MECHANICS - UNIT I  



Transportation to site of final deposition: gravity, flowing water, ice, wind. Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine. Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.

All soils originate, directly or indirectly, from different rock types. Physical Physical weatherin weatherin g reduces the size of the parent rock material, without any change in the

original composition of the parent rock. Physical or mechanical processes taking place on the earth's surface include the actions of water, frost, temperature changes, wind and ice. They cause disintegration and the products are mainly coarse soils. The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is climatic change. In exfoliation, the outer shell separates from the t he main rock. Heavy rain and wind cause erosion of the rock surface. Adverse temperarture changes produce fragments due to different thermal coefficients of rock minerals. The effect is more for freezethaw cycles. Chemical weatherin weatherin g not only breaks up the material into smaller particles but alters the nature not

of the original parent rock itself. The main processes responsible are hydration, oxidation, and carbonation. New compounds are formed due to the chemical alterations. Rain water that comes in contact with the rock surface reacts to form hydrated oxides, carbonates and sulphates. If there is a volume increase, the disintegration continues. Due to leaching, watersoluble materials are washed away and rocks ro cks lose their cementing properties. Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or alteration. The results of chemical weathering are generally fine soils with altered mineral grains.

Phase Relations of Soils Soil is not a coherent solid material like steel and concrete, but is a particulate material. Soils, as they exist in nature, consist of solid particles (mineral grains, rock fragments) with water and air in the voids between the particles. The water and air contents are readily changed by changes in ambient conditions and location. As the relative proportions of the three phases vary in any soil deposit, it is useful to consider a soil model which will represent these phases distinctly and properly quantify the amount of each phase. A schematic diagram d iagram of o f the three-phase system is shown in terms t erms of weight and volume symbols respectively for soil solids, water, and air. The weight of air can be neg lected.


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The soil model is given dimensional values for the solid, water and air components. Total volume, V = Vs + Vw + Vv

Three-phase System Soils can be partially saturated (with both air and water present), or be fully saturated (no air content) or be perfectly dry (no water content). In a saturated soil or a dry soil so il,, the three-phase t hree-phase system thus reduces to two phases o nly, as shown.


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SOIL MECHANICS - UNIT I For the purpose of engineering analysis and d esign, it is necessary to express relations between the weights and the volumes of the three phases. The various relations can be grouped into:   

Volume relations Weight relations Inter-relations

Volume Relations As the amounts of both water and air are variable, the volume of soli so lids ds is taken as the t he reference quantity. Thus, several relational volumetric quantities may be defined. The following following are the basic volume relations: o f soil solids (Vs), and is 1. Void ratio (e) is the ratio of the vo lume of voids (Vv) to the volume of expressed as a decimal.

2. Porosity (n) is the ratio of the volume of voids vo ids to the total volume of soil (V ), and is

expressed as a percentage. Void ratio and porosity are inter-related to each other as follows:

and 3. The volume of water (Vw) in a soil can vary between zero (i.e. a dry soil) and the volume of voids. This can be expressed as the degree of saturation (S) in percentage.

For a dry soil, S = 0%, and for a fully saturated soil, S = 100%. 100% .


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SOIL MECHANICS - UNIT I 4. Air content (a c) is the ratio of the volume of air (Va) to the volume of voids.

5. Percentage air voids (n a) is the ratio of the volume of air to the total volume.

Weight Relations Density is a measure of the quantity of mass in a unit volume of material mater ial.. Unit weight is a measure of the weight of a unit volume vo lume of material. Both can be used interchangeably. The units of density are ton/m³, kg/m³ or g/cm³. The following are the basic weight relations: 1. The ratio of the mass of water present to the mass of solid particles is called the water content (w), or sometimes the moisture content.

Its value is 0% for dry soil and its magnitude can exceed 100%. 2. The mass of solid particles is usually expressed in terms of their particle unit weight or specific gravity (Gs) of the soil grain solids .

wher wheree

= Uni Unit wei weigh ghtt of wate waterr

For most inorganic soils, the value of Gs lies between 2.60 and 2.80. The presence of organic material reduces the value of Gs. 3. Dry unit weight

is a measure of the amount of solid particles per unit vo lume.


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4. Bulk unit weight volume.

5. Saturated unit weight water.

is a measure of the amount of o f solid particles plus water per unit

is equal to the bulk density when whe n the total voids is filled up with

6. Buoyant unit weight or submerged unit weight is the effective mass per unit volume when the soil is submerged below standing water or below the ground water table.

I nter nter -Relation -Relation s It is important to quantify the state of a soil immediately after receiving in the laboratory and prior to commencing other tests. The water content and unit unit weight are particularly important, since they may change during transportation and storage. Some physical state properties are calculated following the practical measurement of others. For example, dry unit weight can be determined from bulk unit weight and water content. The following are some inter-relations:

1. 2. 3. 4.


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5. 6. 7.

Example 1: A soil has void ratio = 0.72, moisture content = 12% and Gs= 2.72. Determine its (a) Dry unit weight (b) Moist unit weight, and the 3 (c) Amount of water to be added per m to make it saturated.

Use Solution:

3

= 15.51 kN/m

(a) (b)

3

=

= 17.38 kN/m =

(c)

3

=

= 19.62 kN/m 3

Water to be added per m to make the soil saturated =

= 19.62 – 19.62 – 17.38 17.38 = 2.24 kN/m3 3

Example 2: The dry density of a sand w ith porosity of 0.387 is 1600 kg/m . Find the void ratio

of the soil and the specific gravity of the soil solids. [Take

]

n = 0.387 3

= 1600 kg/m


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Solution:

(a) e =

(b)

=

= 0.631

=

Gs =

Soil Classification It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be meaningful and concise in an engineering context, so that engineers will be able to understand and interpret. It is important to distinguish between description and classification: Description of soil is a statement that describes the physical nature and state of the soil. It can be a description of a sample, or a soil in situ. It is arrived at by using visual examination, simple tests, observation of site conditions, geological history, etc. Classification of soil is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. A classification for engineering purposes should be based mainly on mechanical properties: permeability, stiffness, strength. The class to which a soil belongs can be used in its description.

The aim of a classification system is to establish a set of conditions which will allow useful comparisons to be made between different soils. The T he system must be simple. s imple. The relevant criteria for classifying soils are the size distri of of particles and the plasticity of of the soil. distri buti on

Particle Size Distribution For measuring the distribution of particle sizes in a soil sample, it is necessary to conduct different particle-size tests . Wet sieving is carried out for separating fine grains from coarse grains by washing the soil specimen on a 75 micron sieve mesh.


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SOIL MECHANICS - UNIT I Dry sieve analysis is carried out on o n particles coarser than 75 micron. Samples (with fines removed) are dried and shaken through a set of sieves of descending size. The weight retained in each sieve is measured. The cumulative percentage quantities finer than the sieve sizes (passing each given sieve size) are then determined.

The resulting data is presented as a distribution curve with grain size along x-axis (log scale) and percentage passing along y-axis (arithmetic scale). Sedimentation analysis is used only for the soil fraction finer than 75 microns. Soil particles are allowed to settle from a suspension. The decreasing density of the suspension is measured at various time intervals. The procedure is based on the principle that in a suspension, the terminal velocity of a spherical particle is governed by the diameter of the particle and the properties of the suspension.

In this method, the soil is placed as a suspension in a jar filled with distilled water to which a deflocculating agent is added. The soil particles are then allowed to settle down. The concentration of particles remaining in the suspension at a particular level can be determined by using a hydrometer. Specific gravity readings of the solution at that same level at different time intervals provide information about the size of particles that have settled down and the mass of soil remaining in solution. The results are then plotted between % finer (passing) and log size.

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


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SOIL MECHANICS - UNIT I From the complete grain-size distribution curve, useful information can be o btained such as: 1. Grading characteristics , which indicate the uniformity and range in grain-size distribution. 2. Percentages (or fractions) fractions) of gravel, sand, silt and clay-size. Grading Characteristics

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 po ints are located located first on the grading curve. D60 = size at 60% finer finer by weight D30 = size at 30% finer finer by weight D10 = size at 10% finer finer by weight The grading characteristics are then determined as follows: 1.Effective size = D10 2. Uniformity coefficient , 3. Curvature coefficient ,

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


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SOIL MECHANICS - UNIT I 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 soil, i.e. a soil which has a very narrow particle size range. Consistency of Soils

The consistency of a fine-grained soil refers to its firmness, firmness, and it varies with the water content of the soil. A gradual increase in water content causes the soil to change from solid to semi- solid to plastic to liquid states. The water contents at which the consistency changes from one state to the other are called consistency limits (or Atterberg Atterberg limits ). The three limits are known as the shrinkage limit (WS), plastic limit ( WP), and liquid limit (WL) as shown. The values of these limits limits can be obtained from laboratory tests.

Two of these are utilised in the classification of fine soils: Liquid limit (WL) - change of consistency from plastic to liquid state Plastic limit (WP) - change of consistency from brittle/crumbly to plastic state

The difference between the liquid limit limit and the plasti p lastic c limit is known as the plasticity index (IP), and it is in this range of water content that the soil has a plastic consistency. The consistency of most soils in the field will be plastic or semi-solid.

Indian Standard Soil Classification System SAKTHI ENGINEERING COLLEGE

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SOIL MECHANICS - UNIT I Classification Based on Grain Size The range of particle sizes encountered in soils is very large: from boulders with dimension of over 300 mm down to clay particles that are less than 0.002 mm. Some clays contain particles less than 0.001 mm in size which behave as colloids, i.e. do not settle in water.

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 bas is for grouping soil particles into boulder, cobble, gravel, sand, silt or clay. Very coarse soils

Coarse soils

Boulder size

> 300 mm

Cobble size

80 - 300 mm

Gravel size (G)

Sand size (S)

Fine soils

Coarse

20 - 80 mm

Fine

4.75 - 20 mm

Coarse

2 - 4.75 mm

Medium

0.425 - 2 mm

Fine

0.075 - 0.425 mm

Silt size (M)

0.002 - 0.075 mm

Clay size (C)

< 0.002 mm

Gravel, sand, silt, and clay are represented by group symbols G, S, M, and C respectively. Physical weathering produces very coarse and co arse soils. Chemical weathering produce generally fine soils. Coarse-grained soils are those for which more than 50% o f the soil material by weight has particle sizes greater than 0.075 mm. They are basically divided into either gravels (G) or sands (S).

According to gradation , they are further grouped as well-graded (W) or poorly graded (P). If fine soils are present, they are g rouped as containing co ntaining silt fines (M) or as containing clay fines (C). For example, the combined symbol SW refers to well-graded sand with no fines. Both the position and the shape o f the grading curve for a soil can aid in establishing its identity and description. Some typical grading curves are shown.


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poo rly-graded medium SAND Curve A - a poorly-graded Curve B - a well-graded GRAVEL-SAND (i.e. having equa l amounts of gravel and sand) Curve C - a gap-graded COBBLES-SAND Curve D - a sandy SILT Curve E - a silty CLAY (i.e. having little amount of sand)

tho se for which more than 50% of the material has particle sizes less than Fine-grained soils are those 0.075 mm. Clay particles have a flaky shape to which water adheres, thus imparting the property of plasticity . A plasticity chart , based on the values of liquid limit (WL) and plasticity index ( IP), is provided in ISSCS to aid classification. The 'A' line in this chart is expressed as IP = 0.73 (WL - 20).


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Depending on the point in the chart, fine soils are divided into clays (C), silts (M), or organic p ercentage of the mass of organic matter in a soils (O) . The organic content is expressed as a percentage given mass of soil to the mass of the dry dr y soil solids.Three divisions divisions of plasticity are also a lso defined as follows.

Low plasticity Intermediate plasticity High plasticity

WL< 35% 35% < WL< 50% WL> 50%

The 'A' line and vertical lines at WL equal to 35% and 50% separate the soils into various classes. For example, the combined symbol CH refers to clay of high plasticity. Soil classification using group symbols is as follows: Group Symbol

Classification


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SOIL MECHANICS - UNIT I Coarse soil soil s

GW GP GM GC

Well-graded GRAVEL Poorly-graded GRAVEL Silty GRAVEL Clayey GRAVEL

SW SP SM SC

Well-graded SAND Poorly-graded SAND Silty SAND Clayey SAND

F in e soils

ML MI MH

SILT of low plasticity SILT of intermediate plasticity SILT of high plasticity

CL CI CH

CLAY of low plasticity CLAY of intermediate plasticity CLAY of high plasticity

OL OI OH

Organic soil of low plasticity Organic soil of intermediate plasticity Organic soil of high plasticity

Peat Pt Activity "Clayey soils" necessarily do not consist of 100% clay size part icles. The proportion of clay mineral flakes (< 0.002 mm size) in a fine soil so il increases its its tendency to t o swell and shrink with changes in water content. co ntent. This is called the activity of the clayey soil, and it represents the degree of plasticity related to the clay content. Activity = (PIasticity index) /(% clay particles by weight)

Classification as per activity is: Activity

Classification

< 0.75

Inactive

0.75 - 1.25

Normal

> 1.25

Active


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Liquidity Index In fine soils, especially with clay size content, the existing state is dependent on the current water content (w) with respect to the consistency limits (or Atterberg limits). The liquidity index (LI) provides a quantitative measure of the present state.

Classification as per liquidity index is: Liquidity index

Classification

>1

Liquid

0.75 - 1.00

Very soft

0.50 - 0.75

Soft

0.25 - 0. 50

Medium stiff

0 - 0.25

Stiff

<0

Semi-solid

Visual Classification Soils possess a number of physical characteristics which can be used as aids to identification in the field. A handful of soil rubbed through t hrough the fingers can yield the following: SAND (and coarser) particles are visible to the naked eye. SILT particles become dusty when dry and are easily brushed off hands. CLAY particles are sticky when wet and hard when dry, and have to be scraped or washed off hands.

Worked Example The following test results were obtained for a fine-grained soil: WL= 48% ; WP = 26% Clay content = 55% SAKTHI ENGINEERING COLLEGE

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SOIL MECHANICS - UNIT I Silt Silt content = 35% Sand content = 10% In situ moisture content = 39% = w Classify the soil, and determine its activity and liquidity index Solution:

Plasticity index, IP = WL – W WP = 48 – 48 – 26 26 = 22% Liquid limit lies between 35% and 50%. According to the Plasticity Chart, the soil is classified as CI, i.e. clay o f intermediate plasticity. plasticity.

Liquidity index ,

=

= 0.59

The clay is of normal activity and is of soft consistency.

Compaction of Soi Soi ls Compaction is the application of mechanical energy to a soil so as to rearrange its particles and reduce the void ratio. It is applied to improve the properties of an existing so il or in the process of placing fill such suc h as in the construction of embankments, road bases, runways, earth dams, and reinforced earth walls. Compaction is also used to prepare a level surface during construction of buildings. There is usually no change in the water content and in the size of the individual soil particles. The objectives of compaction are:   

To increase soil shear strength and therefore its bearing capacity. To reduce subsequent settlement under working loads. To reduce soil permeability making it more difficult difficult for water to flow through.

Laboratory Compaction The variation in compaction with water co ntent and compactive effort is first determined in the laboratory. There are several tests with standard procedures such as:  

Indian Standard Light Compaction Test (similar to Standard Proctor Test) Indian Standard Heavy Compaction Co mpaction Test (similar (similar to Modifi Mo dified ed Proctor Test) T est)

I ndi an Standard Standard L igh t Compaction Compaction Tes Test 3

Soil is compacted into a 1000 cm mould in 3 equal layers, each layer receiving 25 blows of a 2.6 SAKTHI ENGINEERING COLLEGE

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SOIL MECHANICS - UNIT I kg rammer dropped from a height of 310 mm above the soil. The compaction is repeated at various moisture contents. I ndi an Standard H eavy Compac Compacti ti on T est

It was found that the Light Compaction Co mpaction Test (Standard Test) could not reproduce the densities measured in the field under heavier loading conditions, and this led to the development d evelopment of the Heavy Compaction Test (Modified Test). The equ ipment and procedure are essentially the same as that used for the Standard Test except that the soil is compacted in 5 layers, each layer also receiving 25 blows. The same mould mou ld is also used. To provide the increased compactive effort, a heavier rammer of 4.9 kg and a greater drop height of 450 mm are used. Dry Density - Water Content Relationship

To assess the degree of compaction, it is necessary to use the dry unit weight, which is an indicator of compactness of solid soil particles in a given vo lume. The laboratory testing is meant to establish the maximum dry density that can be a ttained for a given soil with a standard amount of compactive effort. In the test, the dry density cannot be determined directly, directly, and as such suc h the bulk density and the t he moisture content are obtained first to calculate the dry density as density, and w = water content.

, where

= bulk

A series of samples of the soil are compacted at different water contents, and a curve is drawn with axes of dry density and water content. The resulting plot usually has a distinct peak as cohesive soils (or soils with fines), and are shown. Such inverted “V” curves are o btained for cohesive known as compaction curves.


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SOIL MECHANICS - UNIT I Dry density can be related to water content and degree of saturation (S) as

Thus, it can be visualized that an increase of dry density means a decrease of voids ratio and a more compact soil. Similarly, dry density can be related to percentage air voids (na) as

The relation between moisture content and dry unit weight for a saturated soil is the zero airco mpletely by compaction, no matter how much voids line. It is not feasible to expel air completely compactive effort is used and in whatever manner.

Effect of Increasing Water Content As water is added to a soil at low moisture contents, it becomes easier for the particles to move past one another during the application of compacting force. The particles come closer, the voids are reduced and t his causes the dry density to increase. As the water content increases, the soil particles develop larger water films films around them. This increase in dry density continues till a stage is reached where water starts occupying the space that could have bee n occupied by the so il grains. Thus the water at this stage hinders the closer packing of grains and reduces the t he dry unit weight. The maximum dry density (MDD) occurs at an optimum water content (OMC), and their values can be obtained from the plot.

Effect of Increasing Compactive Effort The effect of increasing compactive effort is shown. Different curves are obtained for different compactive efforts. A greater compactive effort reduces the o ptimum moisture moisture content co ntent and increases the maximum dry density.


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An increase in compactive effort produces a very large increase in dry density for soil when it is compacted at water contents drier than the o ptimum moisture moisture content.It co ntent.It should be noted that for moisture contents greater than the optimum, the use o f heavier compaction effort will have only a small effect on increasing dry unit weights. It can be seen that the t he compaction curve is not a unique soil characteristic. It depends on the compaction effort. For this reason, it is important to specify the co mpaction procedure (light (light or heavy) when giving values of MDD and OMC.

Factors Affecting Compaction The factors that influence the achieved degree of compaction in the laboratory are:   

Plasticity Plasticity of the soil Water content Compactive effort

Compaction of Cohesion Cohesion l ess Soils Soil s

For cohesionless soils (or soils without without any fines), the standard compaction tests are difficult to perform. For compaction, application of vibrations is the most most effective method. Watering is another method. The seepage force of water percolating through a cohesionless soil makes the soil grains occupy a more stable positi po sition. on. However a large quantity of water is required in this method. To achieve maximum dry density, they can be compacted co mpacted either in a dry state or in a saturated state. For these soil types, it is usual to specify a magnitude of relative density (I D) that must be achieved. If e is the current void ratio or d is the current dry density, the relative re lative density density is usually defined in percentage as


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SOIL MECHANICS - UNIT I or

where e are the maximum and minimum void vo id ratios that can be determined from max max and e mi mi n standard tests in the laboratory, and dmin anddmax are the respective minimum and maximum dry densities On the basis of relative density, sands and gravels can be grouped into different categories: Relative density (%)

Classification

< 15

Very loose

15-35

Loose

35-65

Medium

65-85

Dense

> 85

Very dense

It is not possible to determine the dry density from the value o f the relative density. The reason is that the values of the maximum and minimum dry densities (or void ratios) depend o n the gradation and angularity of the soil grains.

En gineer gineer in g B ehaviour of Compact Compacte ed Soil Soil s The water content of a compacted soil is expressed with reference to the OMC. Thus, soils are said to be compacteddry of optimum or wet of optimum (i.e. on the dry side or wet wet side of OMC). The structure of a compacted soil is not similar on both sides even when the dry density is the same, and this difference has a strong influence on the engineering characteristics. Soil Structure For a given compactive e ffort, soils have a flocculated structure on the dry side (i.e. soil particles are oriented randomly), whereas they have a d ispersed structure on the wet side (i.e. particles are more oriented in a parallel arrangement perpendicular to t he direction of applied stress). This is due to the well we ll-developed -developed adsorbed water wat er layer (water film) surrounding each particle on the wet side.


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Swelling Due to a higher water deficiency and partially developed water films in the dry side, when given access to water, the so il will soak in much more water and then swell more. Shrinkage During drying, soils compacted in the wet side tend to show more shrinkage than those compacted in the dry side. In the wet side, the more orderly orientation of particles allows them to pack more efficiently. Construction Pore Water Pressure The compaction of man-made depo sits proceeds layer by layer, and pore water pressures are induced in the previous layers. So ils compacted wet of optimum will have higher pore water pressures compared to soils compacted dry of optimum, optimum, which have initially initially negative pore water pressure. Permeability The randomly oriented soil in the dry dr y side exhibits the same permeability in all directions, whereas the dispersed soil in the wet side is more per meable along particle orientation than across particle orientation. Compressibility At low applied stresses, the dry compacted soil is less compressible on account of its truss-like arrangement of particles whereas the wet compacted soil is more co mpressible. mpressible.


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SOIL MECHANICS - UNIT I The stress-strain curve of the dry compacted soil rises to a peak and drops down when the t he flocculated structure collapses. At high applied stresses, the initially flocculated and the initially initially dispersed soil samples will have similar structures, and they exhibit ex hibit similar compressibility compressibility and strength.

F i el d Compaction and Specif pecif i cations To control soil properties in the field during earthwork construction, it is usual to specify the degree of compaction (also known as the relative compaction ). This specification is usually that a certain percentage o f the maximum dry density, as found from a laboratory test (Light or Heavy Compaction), must be achieved. For example, it could be specified that t hat field dry densities must be greater than 95% of the maximum dry density (MDD) as determined from a laboratory test. Target values for the range of water content near the optimum moisture content (OMC) to be adopted at the site can then be decided, as shown in the figure.

For this reason, it is important to have a g ood control over moisture content during compaction co mpaction of soil layers in the field. It is then up to the field contractor to select the thickness of each soil lift (layer of soil added) and the type of o f field equipment in order to achieve the specified amount of compaction. The standard of field compaction is usually controlled through either end-product specifications or method specifications. En d-Product Specif Specif icati ons

In end-product specifications, the required field dry density is specified as a percentage of the laboratory maximum dry density, usually 90% to 95%. T he target parameters are specified based on laboratory test results.


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SOIL MECHANICS - UNIT I The field water content working range is usually within ± 2% of the laboratory optimum moisture content. It is necessary to control the moisture moisture content so that it is near the chosen value. From Fro m the borrow pit, if the soil is dry, water is sprinkled and mixed thoroughly before compacting. If the soil is is too wet, it is excavated in advance and dried. In the field, compaction is done in successive horizontal layers. After each layer has been compacted, the water content and the in-situ density are determined at several rando m locations. These are then compared with the laboratory OMC and MDD using either of these two methods: the sand replacement method, or the core cutter method.

M ethod Specif pecif ication s

A procedure for the site is specified giving:   

Type and weight of compaction equipment Maximum soil layer thickness Number of passes for each layer

They are useful for large projects. This T his requires a prior knowledge of working with the borrow soils to be used.

Field Compaction Equipment There is a wide range of compaction equipment. The compaction achieved will depend on the thickness of lift (or layer), the type of roller, the no . of passes of the roller, and the t he intensity intensity of of pressure on the soil. The selection of equipment depends on the soil type as indicated. Equipment

Most suitable soils

Least suitable soils

Smooth steel dru m rollers(static or vibratory) Pneumatic tyred rollers

Well-graded sand-gravel, crushed rock, asphalt Most coarse and fine soils

Uniform sands, silty sands, soft clays Very soft clays

Sheepsfoot rollers

Vibrating plates

Fine grained soils, sands and gravels Uniform gravels, very with > 20% fines coarse soils Weathered rock, well-graded coarse Uniform materials, silty soils clays, clays Coarse soils with 4 to 8% fines

Tampers and rammers

All soil types

Grid rollers


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QUESTION BANK

UNIT 1- INTRODUCTION PART - A (2 marks) 1. Distinguish between Residual and Transported soil. s oil. (AUC May/June 2012)

2. Give the relation between γsat, G, γw and e. (AUC May/June 2012) 3. A compacted sample of soil with a bulk unit weight of 19.62 kN/m3 has a water content of 15 per cent. What are its dry density, degree of o f saturation and air content? Assume G = 2.65. (AUC Apr/May 2010) 4. What are all the Atterberg limits for soil and why it is necessary? (AUC Nov/Dec 2012)

5. Define sieve analysis and a nd sedimentation analysis and what is the necessity of these two analysis? a nalysis? (AUC Nov/Dec 2012) 6. Two clays A and B have the following properties: Atterberg limits Clay A Clay B Liquid limit 44 % 55% Plastic limit 29% 35% Natural water content content 30% 50% Which of the clays A or B would experience larger settlement under identical loads? Why? (AUC Apr/May 2010) 7. Determine the maximum possible voids ratio for a uniformly graded sand of perfectly spherical spherical grains. (AUC Nov/Dec 2011) 8. What is a zero air voids line? Draw a compaction curve and show the zer o air voids line. (AUC Nov/Dec 2011) 9. What is porosity of a given soil sample? (AUC Apr / May 2011) 10. What is water content in given mass of soil? (AUC Apr / May 2011) 11. Define : (a) Porosity (b) Void ratio. (AUC Nov/Dec 2010)


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12. Define effective size of particle in sieve s ieve analysis. (AUC Nov/Dec 2010) 13. Write any two engineering classification system of soil. (AUC Apr / May 2009) 14. List any one expression for finding dry density of soils. (AUC Apr / May 2009) 15. Define water content and compaction. 16. What are the laboratory methods of determination of water content? 17. Define degree of saturation and shrinkage ratio. 18. Define specific gravity and density index. 19. What do understand from grain size distribution? 20. What are consistency limits of soil? so il? 21. Define plasticity index, flow index and liquidity index. 22. What are the methods available for determination of in-s itu density? 23. What is the function of A-line Chart in soil classification? 24. Write the major soil classifications c lassifications as per Indian Standard Classification System. 25. Differentiate standard proctor from modified proctor test. PART - B (16 marks)

1. Write down a neat procedure for determining water content and specific gravity of a given soil in the laboratory by using a pycnometer. (AUC Nov/Dec 2012) 2. Sandy soil in a borrow pit p it has unit weight of solids as 25.8 kN/m3, water content equal to 11% and bulk unit weight equal to 16.4 kN/m3. How many cubic meter of compacted fill could be constructed of 3500 m3 of sand excavated from borrow pit, if required value of porosity in the compacted fill is 30%. Also calculate the change in degree of saturation. s aturation. (AUC Nov/Dec 2012 )


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3. The following data on consistency limits are available for two soils A and B. SI.No. Index Soil A Soil B 1 Plastic limit 16% 19% 2 Liquid limit 30% 52% 3 Flow index 11 06 4 Natural 32% 40% water content Find which soil is (i) More plastic. (ii) Better foundation material on remoulding. (iii) Better shear strength as function of water content. (iv) Better shear strength at plastic limit. (AUC Apr/May 2010) Classify the soil as per IS classification system. Do those soils have organic matter? 4. By three phase soil system, prove that the degree of saturation S (as ratio) in terms of mass unit weight (γ), void ratio (e), specific gravity of soil grains (G) and unit weight of water (γw) is given by the e xpression

The mass of wet soil when compacted in a mould was 19.55 kN. The water content of the soil was 16%. If the volume of the mould was 0.95 m3. Determine (i) dry unit weight, (ii) Void ratio, (iii) degree of saturation and (iv) percent air voids. Take G = 2.68. (AUC May/June 2012) 6. In a hydrometer analysis, the corrected hydrometer reading in a 1000 ml uniform soil suspension at the start of sedimentation was 28. After a lapse of 30 minutes, the corrected hydrometer reading was 12 and the corresponding effective depth 10.5 cm. the specific gravity of the solids was 2.68. Assuming the viscosity and unit weight of water at the temperature of the test as 0.001 Ns/m2 and 9.81 kN/m3 respectively. Determine the weight of solids mixed in the suspension, the effective diameter corresponding to the 30 minutes reading and the percentage of particle finer than this size. (AUC May/June 2012) SAKTHI ENGINEERING COLLEGE

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7. An earthen embankment of 106 m3 volume is to be constructed with a soil having a void ratio of 0.80 after compaction. There are three borrow pits marked A, B and C having soils with voids ratios of 0.90, 0.50 and 1.80 respectively. The cost of excavation and transporting the soil is Rs 0.25, Rs 0.23 and Rs 0.18 per m3 respectively. Calculate the volume of soil to be excavated from each pit. Which borrow pit is the most economical? (Take G = 2.65). (AUC Nov/Dec 2011) 8. A laboratory compaction test on soil having specific gravity equal to 2.67 gave a maximum dry unit weight of 17.8 kN/m3 and a water content of 15%. Determine the degree of saturation, air content and percentage air voids at the maximum dry unit weight. What would be theoretical maximum dry unit weight corresponding to zero air voids at the optimum water content? (AUC Nov/Dec 2011) 9. A soil sample has a porosity of 40 per cent. The specific gravity of solids is 2.70. calculate i) Voids ratio ii) Dry density and iii) Unit weight if the soil is completely saturated. (AUC Apr / May 2011)

10. A soil has a bulk unit weight of 20.11 KN/m3 and water content of 15 percent. Calculate the water content of the soil partially dries to a unit weight of 19.42 KN/m3 and the voids ratio remains unchanged. (AUC Apr / May 2011)

11. Explain Standard Proctor Compaction test tes t with neat sketches. (AUC Nov/Dec 2010)

12. Soil is to be excavated from a barrow pit which has a density of 17.66kN/m3 and water content of 12%. The specific gravity of soil particle is2.7. The soil is compacted so that water content is 18% and dry density is16.2 kN/m3. For 1000 cum of soil in fill, estimate. (i) The quantity of soil to be excavated from the pit in cum and


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(ii) The amount of water to be added. Also determine the void ratios of the soil in borrow pit and fill. (AUC Nov/Dec 2010) 13. Explain all the consistency limits li mits and indices. (AUC Apr / May 2009) 14. Explain in detail the procedure for determination of grain size distribution of soil by sieve analysis. (8) (AUC Apr / May 2009) 15. An earth embankment is compacted at a water content of 18% to a bulk density of 1.92 g/cm3. If the specific gravity of the sand is 2.7, find the void ratio and degree of saturation of the compacted embankment. (8) (AUC Apr / May 2009) 16. Explain the procedure for determining the relationship between dry density and moisture content by proctor compaction test.


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Soil Mechanics Unit 1 Notes

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