Final Draft Introduction to Soil and Dam Engg. 12-3-2009

7/25/2019 Final Draft Introduction to Soil and Dam Engg. 12-3-2009

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TABLE OF CONTENTS

CHAPTER-I SOIL AND DAMS 6

1.1 Introduction 6

1.2 Soil 6

1.3 Formation Of Soil 7

1.4 Types Of Soil 8

1.5 Soil Mechanics 13

1.6 Soil Mass Subjected To Different Types Of Forces 14

1.7 Salient Features Of Dams 22

1.8 Types Of Dams 35

1.9 Arch Dams 37

1.10 Embankment Dam 40

1.11 Buttress Dams 44

1.12 Gravity Dams 46

CHAPTER-II PHYSICAL PROPERTIES OF SOIL 49

2.1 Introduction 49

2.2 Colour 49

2.3 Soil Structure 49

2.4 Particle Shape And Size 52

2.5 Specific Gravity 54

2.6 Soil Phases 56

2.7 Porosity 58


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2.8 Void Ratio 59

2.9 Moisture Content 60

2.10 Degree Of Saturation 62

2.11 Air Void Ratio Or Air Content 63

2.12 Atterberg Or Consistency Limits 63

2.13 Particle Size Distribution 75

2.14 Relative Density (Dr) Or Density Index (Id) 80

2.15 Multiple Choice Questions (Mcq) 82

2.16 Examples: 84

2.17 Problems: 97

CHAPTER-III CAPILLARITY AND PERMEABILITY 99

3.1 Introduction 99

3.2 Capillarity 99

3.3 Capillary Affects In Soil 101

3.4 Capillary Movement In Soil 102

3.5 Importance Of Capillarity In Civil Engineering 102

3.6 Factors Affecting Capillarity 105

3.7 Permeability 107

3.9 Factors Affecting Permeability 109

3.11 Constant Head Permeameter 113

3.12 Variable Head Permeameter 114

3.13 Field Determination Of Permeability 116


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3.14 Theory Of Ordinary Perfect Wells (Dupuit Thiems Theory) 117

3.15 Derivation Of Equation For The Co-Efficient Of Permeabilityity K 118

3.16 Multiple Choice Questions (Mcq) 125

3.17 Examples: 127

3.18 Problems 132

CHAPTER-IV SEEPAGE AND FLOWNET 133

4.1 Introduction 133

4.2 Importance Of Seepage Studies 133

4.3 Laplace Equation For Two Dimensional Flow 133

4.4 Flow Nets 136

4.5 Properties Of Flow Net 136

4.6 Types Of Flow Nets 137

4.7 Boundary Conditions 144

4.8 Anisotropic Soil Conditions 149

4.8 Anisotropic Soil Conditions 150

4.9 Construction Of Flownet 153

4.10 Application Of Flownet 157

4.11 Examples 160

4.12 Problems 174

CHAPTER-V PIPING CONTROL AND FILTER DESIGN 176


5.2 Piping Failure 177


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5.3 Factor Of Safety Against Heave Piping 179

5.4 Prevention Of Piping 180

5.5 Protective Filters 183

5.6 Design Criteria For Protective Filters 183

5.7 Types Of Filters 183

5.8 Examples 187

5.9 Problems 193

CHAPTER-VI DAMS 195


6.3 Selection Dam Type 196

6.4 Selection Of Dam Site 197

6.5 Determination Of Dam Height 197

6.6 Instrumentation 198

6.7 Inspection Of Dam 198

6.8 Embankment Dams 199

6.9 Component Of Earth Dams 200

6.10 Design Criteria For Earth Dam 202

6.11 Technical Standards For Earthfill Dam 204

CHAPTER-VII HYDROPOWER PROJECTS 221


7.2 Components Of Hydelpower Scheme: 221

7.3 Factors Affecting Economy Of Plant 224


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7.4 General Types Of Plant Layout 226

7.5 Governing Of An Impulse Turbine (Pelton Wheel) 229

7.6 Different Types Of Hydropower Schemes 231

7.7 Ghazi-Barotha Hydropower Project 235

Abbreviations 282


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CHAPTER-I SOIL AND DAMS

1.1 INTRODUCTION

The industrial development for the progress of a nation depends mainly on theavailability of power. There are multiple means of power generation. The hydropower, for mostof the cases is the cheapest (no operating fuel cost) and the safest (no environmental pollution). Adam is the most important component of a hydropower scheme. The design and construction ofdams requires a sound knowledge of geotechnical engineering. The reservoir operation and dammonitoring are also related to geotechnical engineering. For earthfill dams, where huge quantitiesof soil are involved, basic knowledge of soil mechanics regarding compaction, seepage, pipingcontrol and filter design is very important. An exhaustive soil investigation program is requiredfor the selection of the most economical site for a dam. The properties of soil and its behavior

under changing reservoir level greatly affect the design of various components of an earthfill dam.

All the components of any hydropower project (e.g., dam, penstock, power house,tailrace and the reservoir etc.) involve soil as a construction material or foundation support. Anadequate knowledge of the properties of soil is essential for the proper design and construction aswell as operation and monitoring of these components. Geotechnical Engineering is thereforevery important for the construction and operation of the hydro power projects. Soil Mechanics is asection of Geotechnical Engineering and simply means the application of laws of mechanics andhydraulics to the soil.

1.2 SOIL

The term soil according to engineering point of view is defined as the material, by meansof which and upon which engineers build structures. The term soil includes entire thickness of

earths crust (from ground surface to bed-rock) which is accessible and feasible for practicalutilization as a foundation support or construction material. It is composed of loosely boundmineral particles of various sizes and shapes formed due to weathering of rocks. It also containorganic matter, water and air. The behavior of soil as a foundation support or as a constructionmaterial is greatly influenced by the following;

1. The moisture content present in the soil pores2. The fluctuation of groundwater table3. Seepage force of the percolating water4. Freezing and thawing phenomena5. The presence of organic matter6. History of formation of soil7. Seismicity of the area


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Soil mass generally means a collection of particles of varying sizes and shapes that arebonded together by mechanical or attractive forces. The binding power of soil particles however,is very low as compared to the binding power of rocks. The type of soil may vary from clay togravel and even to cobbles and boulders. The top soil, which usually extends to a depth of abouttwo feet contains appreciable amount organic matter and is generally considered unsuitable forCivil Engineering use.

1.3 FORMATION OF SOIL

Soil is generally formed by the disintegration and decomposition of rocks at or near theearths surface through the action of many natural, physical, mechanical and chemical agents,which break them into smaller and smaller particles (Fig: 1.1).

Top Soil (Highly Organic)

Residual Soil(Oldest soil)

Completelyweathered rock

Highly weathered rock(Mostly soil)

Moderatelyweathered rock

Slightlyweathered rock

(Some Fissures in theupper zone)

Sound massive rock

Fig: 1.1 Different stages of weathering of rocks and formation of soil.


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The disintegration of rocks is the result of wedging action of water turned to ice withinthe rock pores, because water expands during freezing. The mechanical movement of waterdestroys the bonds between mineral particles, and if the water is charged with dissolved carbondioxide from the air, a weak carbonic acid is formed. Though weak as an acid, such waterbecomes a powerful solvent, which by chemical action may dissolve or change the nature of theparent rocks constituent minerals. Previously existing igneous, sedimentary and metamorphicrocks provide the materials, in the form of minerals, from which the different types of soil areproduced The majority of soil particles are silicates of one form or another, so the silicateminerals occurring in igneous rocks may eventually turn into the soil particles.

Water, wind and gravity are the transporting agents which further work on the weatheredrocks to produce soils. Geological time is still another factor in soil formation. Over a period ofthousands of years, the beating action of rains, the grinding action of the waves and tides of thesea, combined with the transporting action of wind, flowing streams and rivers, has progressively

reduced the rock fragments and sorted them into particles varying in size between that ofboulders, at one end of the scale, and dust at the other.

1.4 TYPES OF SOIL

A geologist has an entirely different view point about the types of soil as compared to anengineer. According to a geologist the soil types are named on the particular geological agent, dueto which the soil has been formed. The name assigned to a soil on the basis of geological agentgives some idea about the engineering behaviour. According to an engineer, the soil types aresolely based on the range of particle sizes within a soil mass. Since the soil properties very muchdepend on the particle size, the name so assigned gives a general idea about its properties andbehaviour. However commonly occurring soil deposits consist of a very wide range of particlesizes i.e., a mixture of soil types. Therefore soil classification systems (as discussed in chapter-3)have been developed to classify the soil for different engineering uses. The soil types based on

geological and engineering view points separately are discussed below.

1.4.1 GEOLOGICAL CONSIDERATION:

The history of formation of a soil deposit, greatly influence its properties and behaviour.The properties of soil highly depend on the geological and climatic agents or the processesthrough which the soil deposits have been developed. The geological and the climatic forces havenever ceased to act; they still are at work and degrading the earths surface. The factor timeplays an important role in the consolidation process of cohesive soils.

Following are the types of soil based on the geological and the climatic agents or theprocesses of formation of soil.

1. Glacial soil:This type of soil is developed, transported and deposited by the

action of glaciers. The glacial deposits may be sorted, assorted or stratified. These deposits consistof rock fragments, boulders, gravels, sand, silt and clay in various proportions (i.e., aheterogeneous mixture of all sizes of particles).


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2. Residual soil:This type of soil is found on nearly flat rock surfaces where theweathering action has produced a soil with a little or no tendency to move. Residual soil alsooccurs when the rate of weathering is higher than the rate of removal. The surface soil formed dueto weathering of upper rock layers; conceal the parent intact rock below the ground surface.

3. Alluvial soil:The soil transported and deposited by water is called alluvial soil.As flowing water (stream or river) loses velocity, it tends to deposit some of the particles that itwas carrying in suspension or by rolling, sliding or skipping along the river bed. Coarser orheavier particles are dropped first. Hence on the higher reaches of a river, gravel and sand arefound. However on the lower parts, silt and clay dominate, where the flow velocity is almost zeroor very small, i.e., when the river enters the sea or a lake. Thus river deposits are segregatedaccording to size.

4. Wind blown soil or Aeolian soil:The soil transported by the geological agent

wind and subsequently deposited is known as wind blown soil or Aeolian soil. Wind can movesmall particles of soil by rolling or by carrying them and may pile up in the form of dunes. Thewind may bring dust storms in arid regions, removing the soil, which is necessary to the plant life,and causing deserts. Wind blown soil has two main types namely Dune sand and Loess.

Fig: 1.2 Sand dunes


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4-a. Dune or Dune Sand: In arid parts of the world, wind is continually formingsand deposits in the form of dunes characterized by low hill and ridge formation. They generallyoccur in deserts and comprise of sand particles which are fairly rounded and uniform in size. Theparticles of the dune sand are coarser than the particles of loess. Dune material is generally, agood source of sand for construction purposes.

4-b. Loess:Accumulations of wind blown dust (mainly siliceous silt or silty-clay)laid down in a loose condition is known as loess. The dust is originally derived from desert areasor from vegetation free areas around the ice sheets. Silt soil in arid regions have no moisture tobond the particles together and are very susceptible to the effects of wind and therefore can becarried great distances by wind storms. An important engineering property of loess is its lowdensity and high permeability. In saturated condition its strength falls significantly, such that itsstructure collapses and it consolidates under its own weight. Saturated loess is very weak andalways causes foundation problems e.g., liquefaction.

5. Colluvial soil:The accumulation formed by rock fragments and soil material

resulting from the mechanical weathering of rocks is known as colluvial soil. This type of soil isformed more or less in situ or as a result of transport by gravity over a short distance. Colluvialsoil usually exists as heaps of coarse debris at the foot of cliffs and steep slopes, the free face

Fig: 1.3 Loess (The dust blown by wind is seen at the top.)


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(slope) adopting the angle of repose for the material, usually 25-350

. The finest particles areusually removed from the heaps by percolating water.

1.4.2 ENGINEERING CONSIDERATION:

The types of soil based on engineering consideration solely depend on the particle size. Sincethe engineering properties of soil are greatly influenced by the change of particle size, differentnames are assigned to particular ranges of particle sizes. The range of particle sizes specified foreach soil type, however, vary among the agencies. The soil types based on the MIT classificationare as follows.

1.4.2.1 Clay:It is composed of very fine particles, less than .002 mm in size. They are flaky inshape and therefore have considerable surface area. These surfaces carry electrical charge, whichhelps in understanding the engineering properties of clay soils. In a moist condition, clay becomes

very sticky and can be rolled into threads. Due to electrical charge, clay shows high inter-particleattraction and thus exhibits sufficient cohesion. It has very high dry strength, low erosion andgood workability under moist condition, and can be readily compacted. It has no inter-particlefriction and is, therefore, subjected to slides at high moisture contents. It is also susceptible toshrinkage and swelling. It has very low permeability. Clay soils commonly have brown colour.

Fig: 1.4 Electrical charge on clay

Particle surface having a net negative charge

Particle edges having positive & negative charges.


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1.4.2.2 Silt: It is composed of particle ranging in size between .002 and .06 mm. It has highcapillarity, no plasticity and has very little dry strength. It has particle size intermediate betweenclay and the sand. Therefore it possesses properties of both the clay and sand, i.e. it shows slightcohesion and also the friction. The colour of silty soil is mostly brown.

1.4.2.3 Sand:Durable mineral grains, usually broken crystals of quartz are known as sand. Itconsists of particles ranging in size from .06 mm to 2 mm. It commonly has a grey colour. Theseparticles may be rounded to angular in shape. It shows no plasticity, high strength in a confinedstate and has considerable frictional resistance. The frictional resistance depends upon the particleshape. Angular particles have higher frictional resistance as compared to the rounded ones. Theparticles of sand have no cohesion and therefore it is known as non-cohesive soil. Sand due to itsabove mentioned properties may be termed as, non plastic soil, frictional soil or granular soil.When sand particles are strongly bonded together by some natural cementing agent it is assandstone. It has high permeability and low capillarity. Sand is commonly used in filters and

drainage blankets for earthfill dams. In geotechnical engineering sand is frequently used in sanddrains to speed the process of consolidation. Sand is also used as main constituent in cementconcrete construction.

1.4.2.4 Gravels:They consist of particles varying in size from 2 mm to 60 mm. They form agood foundation material. They show high frictional resistance. The frictional resistance dependsupon the particle size and shape. Angular particles have higher frictional resistance as comparedto the rounded particles. The gravels produced by crushing of rocks are angular in shape, whilethose taken from river beds are sub-rounded to rounded in shape. Therefore, gravels from crushersare used in the upper layers of pavement, where wheel load stresses are higher. They show veryhigh permeability. Gravels are used in filters, relief wells and drainage blankets for earthfill dams.They also used as main constituent in cement concrete construction. When sand and silt are mixedwith gravels their bearing capacity in further increased but permeability may be decreased.

1.4.2.5 Cobbles or Boulders:Particles larger than gravel are commonly known as cobbles orboulders. Cobbles generally range in size from 60mm to 200mm. The material larger than 200mmis designated as boulders. They are used in stone masonry walls, gravity retaining walls andgabion retaining walls. They are commonly used as rip rap for earthfill dams.

1.4.2.6 Organic Matter:The main source of organic matter is the plants or animal remains thatare added to the soil when these organism die. Plants decompose at a slower rate than the animalremains. Commonly about 12of the soil from top surface has a major concentration of organicmatter. Organic matter has open spongy structure and is mechanically weak. It undergoes largevolume changes under loads and contains high natural moisture content. The strength of soil isvery much reduced when the concentration of organic matter is more than 2% and the soil isconsidered unsuitable for foundation support.

The above particle size limits as already mentioned are based on the MIT (Massachusetts

Institute of Technology) classification. The soil types based on the grain size limits according to

ASTM and AASHTO are given in the following table.


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Table: 1.1 Nomenclature of material (soil type) and range of sizes

The nomenclature for the materials assigned to the grain-size limits adopted by the

ASTM (American Society for Testing and Materials) as given in the Table: 1.1, has been used in

the unified soil classification system. The AASHTO soil classification system however, follows

the nomenclature established by the AASHTO (American Association of State Highway and

Transportation Officials) for the classification of soils.

1.5

SOIL MECHANICS

Soil Mechanics is defined as the branch of engineering science which enables anengineer to know theoretically or experimentally the behavior of soil under the action of loads

(static or dynamic), gravitational forces, water and temperature. Simply speaking it is theknowledge of engineering science, which deals with properties, behavior and performance of soilas a construction material or foundation support. Terzaghi, a famous soil scientist, defines soilmechanics as follows:

Soil Mechanics is the application of laws of hydraulics and mechanics to engineeringproblems dealing with sediments and other unconsolidated accumulations of solid particlesproduced by the mechanical and chemical disintegration of rocks.

With recent advances in Engineering science, the design and execution of large projects,which in the past were considered beyond imagination and control, have now become quitecommon. Dams for hydro power, sky scrapers, subways, maritime & off-shore structures, tunnelsthrough the sea-bed for transportation linkage of the islands, and bridges spanning the sea aresome examples of large projects.

For the design and construction of almost all such projects the engineers have to dealwith both soil and rock, either as construction material or as foundation support. Further, it is

Nomenclature(Soil Type)

Range of Sizes

ASTM AASHTOGravel 75 mm to 4.75 mm

(3in Sieve to No. 4 sieve) Larger than 2 mm

Coarse Sand 4.75 mm to 2 mm(No. 4 to No. 10 sieve) 2mm to 0.425 mm

Medium Sand 2 mm to 0.425 mm(No. 10 to No. 40 sieve) -------------------------

Find Sand 0.425 mm to 0.075 mm(No. 40 to No. 200 sieve) 0.425 mm to 0.075 mm

Silt 0.075 mm to 0.005 mm(No. 200 to ---) 0.075 mm to .002 mm

Clay Smaller than 0.005 mm Smaller than 0.002 mmColloids Smaller than 0.001 mm Smaller than 0.001 mm


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known that physical and engineering properties of soil and rock are very much dependent on thegeological processes of formation. Few decades back, it was therefore considered more logical touse a more descriptive term, i.e., Geotechnical Engineering instead of Soil Mechanics. Thus SoilMechanics, now-a-days is considered as a section of Geotechnical Engineering. Some of thecases where a soil mass is subjected to different types of forces are discussed as follows

1.6 SOIL MASS SUBJECTED TO DIFFERENT TYPES OF FORCES

To familiarize the reader, how a soil mass is subjected to different type of forces, somepractical examples are discussed below.

1.6.1 SOIL SUBJECTED TO STATIC LOAD:

Figure 1.5 shows a soil mass subjected to a static load i.e., from a gravity dam. When a dam

is constructed, the dead load is transferred to the soil. The dead load, once applied to the soil,remain constant and known as static load. Due to the static load, the supporting soil is compressedand the dam settles down. For the safety of dams, the following basic stability criteria should besatisfied.

1. There should be no shear failure of the supporting soil.2. The settlement should be within permissible limits.3. No sliding away of the dam due to water pressure, ice pressure, wave pressure, etc.4. No overturning of the dam.

Therefore, for the design of dam foundation, the information about the shear strength andcompressibility characteristics of the soil are required.

Fig: 1.5 Concrete gravity dam


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1.6.2

SOIL SUBJECTED TO DYNAMIC LOAD:

The dynamic loads on dams include wave pressure, wind pressure and the earthquakeforces and are considered in the design.

1.6.3 SOIL SUBJECTED TO WATER FORCES:

1.6.3.1 Uplift Pressure:Figure 1.6 shows uplift forces acting on a gravity dam. Water seepingthrough the soil underneath the dam with high pore pressure develops significant uplift pressure.Uplift pressure is an active force and is very important for the stability analysis of the dam. Highuplift pressure greatly reduces the stress and hence friction at the dam-foundation interface, whicheventually decreases the factor of safety against sliding away of the dam. These pressures varywith time and are related to boundary conditions and the permeability of the material. Therefore,for the design and monitoring of dams, the soil characteristics related to the permeability and

seepage are required.

Fig: 1.6 Uplift Pressure on concrete gravity dam

H

H

h

h

Drain

Gallery

[h + k(H-h)]


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1.6.3.2 Seepage Forces:Figure 1.7 shows seepage taking place through a dam foundation. Theconstruction of hydraulic structures, e.g., weirs and dams etc., create difference between upstreamand downstream water levels. Due to this head difference water seeps down into the bed material(soil) from the upstream side of hydraulic structure at a much higher rate compared with normalriver flows. On the downstream side, the tail water level is much lower, therefore the seepagewater moves up towards the ground surface. Depending upon the hydraulic gradient causing theflow, the seepage pressure at the exit point, commonly known as exit gradient may be higher. Ifthe exit gradient exceeds the critical hydraulic gradient, boils occur and the soil particles areeroded away by upward flowing water, leading to the formation of a pipe shaped channeleventually causing piping failure. To prevent erosion of soil particles and the piping failure, reliefwells are installed to allow safe exit of water.

Fig: 1.7 Seepage & backward erosion piping through dam foundation.

Fig: 1.8 Boils on the D/S of main embankment (Mangla dam Pakistan).


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Relief wells are installed at the down stream side of a dam for safe collection anddisposal of seepage water away from the dam. Relief wells are vertically installed wells consistingof screens surrounded by a filter material designed to prevent in-wash of foundation materials intothe well. The wells have inside diameters generally between 6 and 18 inches to accommodate themaximum design flow. Relief wells intercept under-seepage and provide a controlled outlet forthe water. They are used to relieve excess hydrostatic pressures in the pervious substratumoverlain by more impervious top strata, conditions which often exist landward of dams.

Fig: 1.9 Relief well at the down stream (landside) of dam

Fig: 1.8 shows boils on the downstream of the main embankment (Mangla dam). TheMangla dam has relief wells system to collect seepage water, but the boils appeared at certainlocations and some boils disappeared after sometime. The reason may be inadequate depth andspacing of the relief wells or loss of efficiency due to clogging of the well screens. Fig: 1.9 shows

a relief well installed at the downstream of an earthfill dam. Therefore for the design of reliefwells knowledge of soil mechanics related to filter design is required.

Figure 1.10 shows a landslide in Mangla dam reservoir which basically occurred due togravitational forces acting on the soil mass. Gravitational forces are continuously acting on soilmass to pull it down to lower levels. The shear strength of soil provides a resisting force againstthe downwards movement. If the downwards gravitational force is greater than the resisting shearforce, a slide takes place. Water is the most aggressive factor responsible for the occurrence ofmost of the landslides. Water has a two ways negative effect. It reduces the resisting forces due todecrease in shear strength and at the same time it increases the gravitational forces due to increasein unit weight. The soil mass of reservoir slope which was stable before imponding, becamesaturated after imponding, the shear strength reduced, density increased and eventually the slideoccurred. The landslides, for most of the slopes in critical condition, are triggered by earthquake.

Therefore, soil properties related to permeability, seepage, drainage, shear strength andstability analysis of slopes are required to be known. A slope angle should be designed using

Flow

SeepageRelief wellPrevious Substratum

Top Stratum


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knowledge of soil mechanics to provide a reasonable factor of safety under all conditionsexpected during lifetime of a reservoir.

Fig: 1.10 Landslide in the Mangla reservoir, mainly due to increase in moisture content.

Figure 1.11 below, shows a soil mass subjected to the action of water. The storm watermoving along a soil surface without vegetation cover carries the soil particles with flow. As the

particles are gradually eroded away with the flow, small channels are developed. The channelsgradually grow in size as they attract more and more water. The rate of erosion or the sediment

Fig: 1.11 Soil subjected to action of water (Erosion).


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transport gradually increases due to gully formation. Sediment eventually fills reservoirs (quicklyin some cases) and gradually reduces storage capacity. The highest rates (in percentage terms) ofloss of storage are found in the smallest reservoirs and the lowest rates in the largest. Theworldwide average loss of storage due to sedimentation is between 0.5% and 1.0% per annum.

In many areas of the world the life span of reservoirs is determined by the rate ofsedimentation which gradually reduces the storage capacity and eventually destroys the ability toprovide water and hydel power generation.

The rate of loss of storage for a reservoir depends on the sediment yield from thecatchment which is dependent on the rate of erosion and transport of sediment by water within thecatchment. In regions where the catchments have remained stable, the rate of loss of storage isessentially constant. In regions where deforestation has occurred, the loss of storage increaseswith the rate of catchment erosion.

The basic principles of sediment transport from the upstream river system into areservoir are set out in standard texts and are not described in detail here. The location ofdeposition of sediments within the reservoir depends on the local water velocity. Coarser materialis deposited first at the upstream of reservoir often in a form that is recognized as a delta. Finermaterial may reach the dam and affect the design and operation of the outlet works. The problemis to estimate the rate and location of deposition and the period of time before it will start tointerfere with the operation on the reservoir.

Figure 1.12-15 shows a soil mass subjected to the action of river water. Whenever abridge is constructed, the piers obstruct the flow and the flow path is disturbed. The flow area atthe bridge section is reduced and the flow velocity is increased. The disturbed flow path and theincreased flow velocity result in erosion of soil around the pier known as scouring.

Fig: 1.12 Soil under the action of water (Scouring)


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The scour depth depends mainly on the flow velocity and the particle size of the riverbed material. For the design of foundation of the bridge pier, the maximum depth of scour formaximum anticipated flood during the lifetime of bridge should be known. The foundation for

the pier should be placed below this depth. If the foundation is placed above the potential scourdepth, expected during the maximum anticipated flood, the soil below the pier foundation will


Scour depth



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be eroded away, resulting in washing away of the entire bridge structure. Therefore, knowledgeof soil mechanics is required to determine the scour depth for the design of bridge foundations.

Fig: 1.15 Failure of Machnai Bridge at river Kabul, near Peshawar due to tilting of pier as a result

of excessive scouring (July 2005 floods)

1.6.4 SOIL SUBJECTED TO COMPACTION, SWELLING AND SHRINKAGEFORCES:

Figure 1.16 shows an under construction earthfill dam. Huge volumes of soil arerequired for the construction of earthfill dams. The soil is excavated from the borrow areas anddumped at the dam site and then compacted by rollers. The compaction of a soil very muchdepend on its type, the moisture content, the weight and type of roller and the number of passes ofthe rollers. The type of roller is selected based on the type of soil. The weight of roller is based onthe degree of densification required. For economic reasons the borrow areas are selected close tothe dam site. However, if the soil from the borrow areas close to the proposed dam is not suitablefor construction, then either the soil will be stabilized or may be brought from other areas. Thedam has to store water and during operation the reservoir level fluctuates due to weatherconditions. The moisture content within the dam body remains changing during its life time.Some soils show significant swelling and shrinkage due to increase and decrease of moisturecontent respectively. Therefore, for the design and construction of dams, compaction related


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characteristics of the soil and its behavior with respect to changes of moisture content arerequired.

Fig: 1.16 Different components of an earthfill dam

1.7 SALIENT FEATURES OF DAMS

The salient features of dams include type height and length of dam, reservoir and catchment area,

gross and live storage capacity of reservoir, spillway type and capacity and power generation

capacity etc. As an example salient features of Mangla Dam are given below. Brief history of the

original Mangla Dam and its present raising are given in the next sections for information of the

readers.

1.7.1 SALIENT FEATURES OF MANGLA DAM

Dam Type: Earthfill

Height: 454 ft. (above riverbed)

Length: 10,300 feet (main + intake)

Lake Area: 97.7 sq. miles

Impervious Core Transition Section Filter Section


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Catchment Area: 12,870 Sq miles

Gross Storage Capacity: 5.88 MAF

Live Storage Capacity: 5.34 MAF

Main Spillway Capacity: 1.10 million cusecs

Year of Completion: 1967

Hydropower Generation: 1,000 MW from 10 units of 100 MW each

No. of people to be displaced by raising of dam: 40,000

1.7.2 CHARACTERISTICS

Main EmbankmentType : EarthfillMaximum height : 138 mCrest length : 2560 mStorage capacity : 6587 million m3.

Intake EmbankmentType : EarthfillMaximum height : 79.8 mCrest length : 579 mCrest width : 44.8 m

Sukian DamType : EarthfillMaximum height : 43.9 m

Crest length : 5151 mCrest width : 9.1 m to 12.2 m

Jari DamType : EarthfillMaximum height : 83.5 mCrest length : 2072 mCrest width : 12.1 m

Main SpillwayType : Submerged orificeDischarge capacity : 31, 150 cumecs

Emergency Spillway

Type : WeirControl : Erodible bundCapacity : 6,513 Cumecs


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TunnelsNumber : 5, steel and concrete lined in bedrock.Diameter : 9.14, 9.44, 7.92 mLength : 475 m

Power HouseThe power house has 10 vertical shaft francis type turbines, each having a generating 100 MW.

1.7.3 GENERAL DESCRIPTION

Mangla Dam Project is a multi-purpose project designed and built to store and controlwaters of river Jhelum. The primary purpose of the project is to provide replacement storagecapacity for irrigation under the terms of Indus Basin Water Treaty.

At present Mangla power plant is generating 800 MW but has an ultimate capacity of

1000 MW. In order to ensure the safety of the project, periodic inspections of the embankment arecarried out by NESPAK. The experts analyze exhaustive monitoring data and carry out physicalinspection of all structures and the reservoir rim. In a project of such large size several minorproblems if left unattended, can have far-reaching consequences. In one of such inspection carriedout few years back, NESPAK suggested several measures such as careful investigation of boilingat the toe of main dam. In order to check the excessive seepage through Sukian dyke, upstreamblanket for the area was suggested. An inspection report discussing the inspection along withfindings and recommendations was submitted to the client.

1.7.4 HISTORY OF DEVELOPMENT

In April 1948, India diverted the flow of three rivers (Ravi, Sutlej and Beas), whichthreatened irrigated cultivation in Pakistan. In an effort to mitigate the consequences of possibleinterference by India resulting in non supplies of the canals feed by those rivers, a program of link

canal construction to enable the transfer of water between rivers was undertaken.

Until 1967, the entire irrigation system of Pakistan was fully dependent on unregulatedflows of the Indus and its major tributaries. The agricultural yield was very low mainly due tolack of water during critical growing seasons. The core of water shortage problems being, theseasonal variation in river flows and the absence of storage reservoirs to conserve the vastamounts of surplus water during periods of high river discharge.

Mangla Dam was the first development project undertaken to reduce the shortcomingand strengthen the irrigation system.

1.7.5 THE DAM PROJECT

Mangla Dam was the 12th largest dam in the world when constructed. It was constructed

in 1967 across the River Jhelum, about 60 miles southeast of the federal capital, Islamabad. Themain structures of the dam include 4 embankment dams, 2 spillways, 5 power-cum-irrigationtunnels and a power station.


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Since its first impounding in 1967, sedimentation to the extent of 1.13 MAF hasoccurred, and the present gross storage capacity has reduced to 4.75 MAF from the actual designof 5.88 MAF.

The live capacity has been reduced to 4.58 MAF from 5.34 MAF. This implies areduction of 19.22 % in the capacity of the reservoir. The project was designed primarily toregulate the river flows in terms of increasing the amount of water for irrigation from RiverJhelum and its tributaries. Its secondary function was to generate electrical power from theirrigation releases. The project was not designed as flood control structure, although some benefitin this respect also arises from its use for irrigation and water supply.

1.7.6 FINANCIAL BENEFITS

A brief summery of financial benefits for the previous years (1996 1999) is given as an

example.The Indus River System Authority (IRSA) indented 4.21 MAF of water releases for

irrigation purpose during 1999-2000, against 5.1 MAF during the previous year, worth Rs 3,789million at a rate of Rs. 900 per acre-feet. In addition, the Mangla Power Station generated3,184.77 million kilowatt hours (MKWH) of electricity, worth Rs. 955.43 million at a rate of Rs.0.30 per kwh unit. The financial benefits for the years 1996 and onwards are given below:

Year Water Power Total Benefits

July toJune

StorageReleas

Rs. 900 perAc-Ft

GenerationRs.0.3 per

Ac-Ft Benefit, Rs.Million

MAFBenefit, Rs.

MillionMKWH

Benefit, Rs.Million

1996-97 4.98 4,482 5,665.63 1,699.69 6,181.69

1997-98 4.36 3,924 6,103.72 1,831.11 5,755.11

1998-99 5.1 4,590 4,778.53 1,433.56 6,023.56

1999-00 4.21 3,789 3,184.77 955.43 4,744.43

1.7.7 MANGLA WATERSHED MANAGEMENT PROJECT

The main objective of the project is to prolong the reservoir life through improvedmethods of land-use and implementation of watershed management practices in the catchment.The project, in addition to reducing the entry of sediments into the reservoir, has also improvedthe following:

Socio-economic conditions of the public living in the area by improvement ofland with consequent increase in agriculture

Forest and range-land products

Increase in sub-soil water resources and perennial stream flows

Minimizing runoff with consequent reduction in flood hazards and Environmental protection of the area


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The total catchment area of the River Jhelum is 12,870 square miles. Out of the totalarea, 56% lies in the Indian-held Kashmir territories and the remaining 44% in Pakistan and AzadJammu and Kashmir. Within Pakistan, only 3,433 square miles is covered by this project, whichincludes the critical sediment source areas.

The watershed management practices include reforestation of bare and denuded lands,development of range-lands, improvement of cultivated fields by land leveling/improvement ofterraces and structural works such as silt trap storages, spillways, check dams, retaining diversionwalls and gully control structures.

1.7.8 RAISING OF MANGLA DAM

A joint venture of consultants comprising NESPAK, Barqaab, Binnie and Partners andHarza has been awarded the contract to undertake the feasibility for raising Mangla Dam by 40

feet. The proposal for raising of Mangla Dam was part of the Final Completion Report submittedby Binnie and Partners in 1971. This will raise the elevation of the dam from 1,234 feet to 1,274feet and subsequently increase the conservation level from 1,202 feet to 1,252 feet and theminimum operating capacity of reservoir from 5.88 MAF to 9.6 MAF. But finally the dam wasraised from 1,234 feet to 1,264 feet with increase of reservoir level from 1,202 feet to 1,242 feetand the minimum operating capacity of reservoir from 5.88 MAF to 9.6 MAF.

According to the investigations made during 1999, the capacity of Mangla has reducedby 19.22% due to silting i.e from 5.88 MAF to 4.75 MAF. Concerned by this, the governmentinitiated the raising of Mangla Dam as a fast-track project on 14-8-2000. The raising of Mangladam will make the main dam 494 feet high, providing an additional 1,000 GWH or an 18%enhancement and 3.1 MAF of additional storage under normal conditions. The Government ofPakistan has allocated 53 billion rupees for this project between 2001-06.

0

5000

10000

15000

20000

MillionRupees

2001-02 2002-03 2003-04 2004-05 2005-06

Fiscal Year

Budget Allocation for Mangla Raising Project


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1.7.9 RESETTLEMENT ISSUES7.0 THE RESSETLEMENT ISSUE

During the construction of the Mangla dam, 65,100 acres of land was submerged. Thisled to the resettlement of the residents of old Mirpur town and the affected people were providedaccommodation in the newly developed town of Mirpur. Most of the people were accommodatedhowever, some grudges remained after resettlement.

An important concern on the raising of Mangla dam was the resettlement of an estimated40,000 people living in 7,000 houses. Rs. 20 billion has been allocated for population resettlementin the Rs 53 billion project. WAPDA has developed a policy and compensation package forresettlement of the affected.

1.7.10 SOCIAL ISSUES

Kashmiri Group Vows to Stop Mangla Dam Extension (The News International 11-3-05). Arif Chaudhry, spokesman for the Anti-Mangla Dam Extension Committee, has said thatthey will use all possible means, including diplomacy, dialogue, protest, hunger strike and roadclosure, to stop the extension of Mangla Dam. He was responding to questions at a seminar on'People's perspective on Mangla Dam Extension Project' organized by the SustainableDevelopment Policy Institute here on Friday. Naeem Iqbal of the Pakistan Network of Rivers,Dams and People conducted the proceedings. Arif Chaudhry said that the present unrest amongpeople would further aggravate if no attention was given to their demands. He said that if the damis raised, it would displace the people of Mirpur. He said that 80,000 to 85,000 people weredisplaced during the construction of Mangla Dam in the 1960s and some of the affected personswere still waiting payment of compensation. He said that seven promises were made, includingfree electricity to Mirpur at the time of the construction of the dam, but except for sending 300people to England, no other promise was fulfilled. He said that even this was done by the Britishconstruction company to win support for the project. He said that raising the dam would lead to

many socio-economic problems as it would adversely affect most of the immigrants living inEngland who constructed beautiful houses in Mirpur. He said Kashmiris living in England do notbury their family members in the UK but bring their dead bodies to Mirpur. He regretted that allthis would be submerged in the lake. He said his organization had presented 12 alternatives to thegovernment but no attention was paid to them as these reduced chances of corruption. He said thatthe price of one kanal land in Mirpur is Rs. 2.5 million while the government allocated Rs. 50,000per kanal in the budget. Afsar Shahid, president of the Kashmir Freedom Movement, said that it isbetter to take people into confidence. He said that the issue could only be solved throughconsensus.

The issues mostly have settled down and the construction of raising project hascommenced since 20-06-04 and the scheduled completion date was 19-09-07. It should be kept inmind that these issues are very important for smooth execution and timely completion of any suchproject involving resettlement and compensation to public.

The Maps, drawings and pictures of the Dam, reservoir and different dam structures aregiven on the next pages:


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Fig: 1.17 Map showing location of the Mangla Dam on Jhelum River, Pakistan

Reservoir

Power intakes

Main Dam

Erodible bund

Main spillway

Power station

Power tunnels

Sukian DykeTo Mirpur

River Jhelum

Bong Canal


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Fig: 1.18 Layout Plan showing Mangla dam and other structures

Fig: 1.19 A View of main spillway of Mangla dam

Tailrace


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Fig: 1.20 Dam cross-section showing various components of Mangla dam

Fig: 1.21 Main spillway of Mangla dam, view of upper chute

5Free drainage gravel

2-6Rip rap class III

3-6Rip rap class I

Washed GravelRolled

clay

Rolledsandstone

type A

5Foundation Excavation

Rolledsandstonetype A

2-0Cobbles a boulders

4-6Gravel fill

3-0Foundation gravel drain

2-0Foundation coarse filter

D/S


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Fig: 1.22 Emergency Spillway of Mangla dam

Fig: 1.23 Reservoir of Mangla dam


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Fig: 1.24 Arial view of Spillway of Mangla dam

Fig: 1.25 Arial view of Powerhouse and tailrace of Mangla dam

Tailrace


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Fig: 1.26 Mangla dam raising project, additional reservoir areas shown by dark colour


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Figure-1.2

7Housing

colonyfortheaffecteesofMangladamraisingproject


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Fig: 1.28 Designed Cross-section of raised Mangla dam

1.8 TYPES OF DAMS

Based on material used, following are the main types of dams.1. Masonry Dam2. Concrete Dam3. Earth-fill Dam4. Rock-fill Dam

Masonry dam were used for low height small reservoirs in the past and presently are quiteuncommon. The above dams are further classified based on shape and arrangement of theircomponents.

1.8.1 TYPES OF CONCRETE DAMS

Based on shape, following are the types of concrete dams.1. Concrete Arch Dam2. Concrete Gravity Dam3. Concrete Buttress Dam

Arch dam Gravity dam Buttress dam

Fig: 1.29 Types of Concrete Dam


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1.8.2 TYPES OF EARTH-FILL DAMS

Based on the arrangement of components and mode of construction, following are the types ofearth-fill dams.

1. Homogeneous Earth-fill Dam2. Modified Homogeneous Earth-fill Dam3. Zoned Earth-fill Dam4. Hydraulic-fill Dam

The first three dams are constructed using conventional earth moving and compaction machineryand based on mode of construction they are known as rolled-fill dams. While hydraulic-fill is aspecial technique for raising and constructing dam embankment. In this method soil is transportedto the dam site by pumping soil water mixture through pipes.

1.8.3 TYPES OF ROCK-FILL DAMS

Based on the arrangement of components, following are the types of rock-fill dams.

1. Rock-fill dam with central core2. Rock-fill dam with sloping core

3. Rock-fill dam with diaphragm

Homogeneous Earth-fill Dam Modified Homogeneous Earth-fill Dam

Zoned Earth-fill Dam

Fig: 1.30 Types of Earth-fill Dam


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1.9 ARCH DAMS

The dam is built in an arch, and most of the water pressure is resisted by the abutment rock. Anarch dam is a curved dam which depends upon arch action for its strength. They are quite thinnerand hence require less material than any other type of dam. They are suitable for sites that arenarrow and have strong abutments.

Fig: 1.32 Arch dam

Rock-fill dam with Central Core Rock-fill dam with upstream Core

Rock-fill dam with Diaphragm

Fig: 1.31 Types of Rock-fill Dam


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Fig: 1.33 Arch dam

Arch dams are usually made of reinforced cement concrete and are very suitable for narrow

gorges with strong abutments. The gorge often has a V-shape. Arch dams require much lessconcrete than gravity dams. The best design is a double-curved arch.Arch dams are generally classified as thin, medium, and thick, depending on the ratio of the widthof the base (b) to the height (h). The following general criterion is usually followed.

Thin arch when the ratio b/h < 0.2 Medium thick arch when the ratio 0.2 < b/h < 0.3

Thick arch when the ratio b/h > 0.3

There is no generalization for the ratio between the width at the crest (c) and the width at the base(b). Historically, c/b = 1 (the same thickness at the base and crest) has even been used. Butusually c/b ratio is equal to 0.5.If the height of an arch dam is known, the following simple equations can be used to find theother dimensions for a simulation:

Crest width = height * 0.2

Base width = height * 0.3


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1.9.1 COMPONENTS OF ARCH DAM

1. Abutment2. Axis3. Central Angle4. Crest5. Cross Section6. Downstream Face7. Foundation8. Heel

9. Height10. Plan View11. Radius12. Reservoir13. Span14. Toe15. Upstream Face16. Width

Fig: 1.33 Cross Section & Plan View

1.9.2 FORCES ON ARCH DAM

The main forces on a dam include the forces of the reservoir water, uplift force and the weight of

concrete. There are many other forces that may act on an arch dam and are listed below:

There may be water on the downstream side of the dam as well; this water will have the

same sort of vertical and horizontal forces on the dam as the water on the upstream side

Internal hydrostatic pressure: in pores, cracks, joints, and seams

Temperature Variations

Chemical Reactions

Silt pressure; silt will build up over time on the upstream side; silt provides about 1.5

times the horizontal pressure of water and twice the vertical pressure of water Ice load on the upstream side

Wave load on the upstream side

Central angle

Toe

Abutment

Radius

Downstream face

Width

Span

Upstream face

Foundation Heel

Downstream face

AxisHeight

Width

Reservoir

Crest

Toe


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Eart

hquake Loads

Settlement of the foundation or abutments

Other structures on top of the dam -- gates, a bridge, cars

Creep of concrete: deformation of the concrete when under a constant load for a long

period of time

1.10 EMBANKMENT DAM

Embankment dams are massive dams made of earth or rock. They rely on their weight to resist the

flow of water, just like concrete gravity dams. Embankment dams may be made of earth or rock,

both of which are pervious to water that is, water can seep through it. These dams usually have

some sort of water proof insides (called the core). The core material is usually more watertight

than the rock or earth that is on the outside of the dam, but the core material is still not totally

impervious to water. There are two types of embankment dams namely rock-fill dam (when the

dam body mainly comprise of rocks) and earth-fill dam (when the dam body mainly comprise of

earth).

1.10.1 ROCK-FILL DAM

The main body of these dams consists of rocks and gravels. A relatively impervious element is

included in the dam body to control seepage. It is best suited in the area where rocks are available.


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1.10.2 EARTH-FILL DAM

These dams are built using soil (clay/silt, sand and gravels) as a main constituent material. Theycan be built in locations where the ground is relatively soft.

Fig: 1.34 Rock-fill dam with concrete upstream face


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Fig: 1.35 Earth-fill dam

1.10.3 COMPONENTS OF EMBANKMENT DAM

1. Crest2. cross section3. downstream face4. foundation5. Heel6. height

7. plan view8. reservoir9. Span10. toe11. upstream face12. width

Crest

Width

FoundationToeHeel

Reservoir

Upstreamface

Height

Cross Section of Dam


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1.10.4 FORCES ON AN EMBANKMENT DAM

The main force on an embankment dam is the force of the water. The weight of the dam is also a

force, but each material has a different weight, so it is not shown here as one force the way it is on

the concrete dams. The uplift force is also acting on the embankment dam, but some of the water

seeps into the dam so the force is not the same as on a concrete dam.

Fig: 1.37 Forces on an embankment dam

Free water surfaceof seepage line

Unstable zone

Water surface

Homogeneous

Impermeable foundation

Fig: 1.36

Upstream face

Plan View of Dam

Downstream face

Length

Width

Crest


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Other forces that may act on an embankment dam are:

There may be water on the downstream side of the dam as well; this water will have thesame sort of vertical and horizontal forces on the dam as the water on the upstream side


Silt pressure; silt will build up over time on the upstream side; silt provides about 1.5times the horizontal pressure of water and twice the vertical pressure of water

Ice load on the upstream side Wave load on the upstream side

Earthquake Loads


Other structures on top of the dam -- gates, a bridge, cars

1.11 BUTTRESS DAMS

In Buttress dams the dam face is held up by a series of supports. The face may be flat or curved.The buttress head may be flat, as shown above, or rounded. Usually, buttress dams are made ofreinforced concrete. The buttresses may be hollow or solid.

Fig: 1.38 Buttress Dams

Free water surface

Drainage blanket

with a drainage blanket


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1.11.1 COMPONENTS OF BUTTRESS DAM

1. Abutment2. Axis3. Buttress4. Buttress head5. Crest6. Cross section7. Downstream face8. Foundation

9. Heel10. Height11. Plan view12. Reservoir13. Span14. Toe15. Upstream face16. Width


FoundationHeel

Upstream face

Axis

Width

Reservoir Crest

Toe

Downstream face

Dam

Upstream face

Span

Abutment

Downstream face

Width

Buttress head

Buttress


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1.11.2 FORCES ACTING ON A BUTTRESS DAM

The main forces on a dam include the forces of the reservoir water, uplift force and the weight ofconcrete. There are many other forces that may act on a buttress dam and are listed below:



Temperature Variations

Chemical Reactions Silt pressure; silt will build up over time on the upstream side; silt provides about 1.5

times the horizontal pressure of water and twice the vertical pressure of water

Ice load on the upstream side


Earthquake loads Settlement of the foundation or abutments

Other structures on top of the dam -- gates, a bridge, vehicles, creep of concrete:deformation of the concrete when under a constant load for a long period of time.

1.12 GRAVITY DAMS

Gravity dams resist the horizontal thrust of the water entirely by their own weight. These damsare typically used to block streams through narrow gorges. Because it is their weight holding thewater back, large amount of concrete is used for gravity dams. These dams can be very expensivedepending upon the large amount of material they use. Generally, the base of a concrete gravitydam is equal to approximately 0.7 times the height of the dam:

Fig: 1.40 Gravity Dam


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Fig: 1.40 Gravity Dam

1.12.1 COMPONENTS OF GRAVITY DAM

1. Abutment2. Axis3. Crest4. Cross section5. Downstream face6. Foundation7. Heel

8. Height9. Plan view10. Reservoir11. Span12. Toe13. Upstream face14. Width


Bedrock

Reservoir

Apron

Training wall

Overflow section

Gate

Foundation

Heel

Axis

Crest

Toe

Upstream face

Span

Abutment

Downstream face

Width

Width Reservoir

Height

Downstream face


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1.12.2 FORCES ON GRAVITY DAM

The main forces on a dam include the forces of the reservoir water, uplift force and the weight ofconcrete. Beside these main forces, there are many other forces that may act on a gravity damwhich are listed below:



Temperature Variations Chemical Reactions

Silt pressure; silt will build up over time on the upstream side; silt provides about 1.5times the horizontal pressure of water and twice the vertical pressure of water

Ice load on the upstream side


Earthquake Loads


Other structures on top of the dam -- gates, a bridge, vehicles

Creep of concrete: deformation of the concrete when under a constant load for a longperiod of time.


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CHAPTER-II PHYSICAL PROPERTIES OF SOIL

2.1 INTRODUCTION

This chapter is designed to introduce the reader to the general terms and definitionsroutinely used by geotechnical engineers. These terms, and definitions, primarily describe theimportant physical and index properties of soils. Most of the physical properties are interrelatedand simple equations have been developed so that if some of the properties are known, the othercan be determined by the use of equations. Several of the equations will be used so frequentlythroughout the text that it will be most appropriate to memorize them. However it is worthwhileto mention that these equations are quite large in number and perhaps may be difficult toremember them all. Therefore the students must practice to derive the required relationships(equations) from the basic definitions.

The fundamental physical properties of soil are colour, structure, particle size and shape,

specific gravity, unit weight, porosity, void ratio, soil phases, moisture content, and consistency,

which are mainly important for the selection of material for construction of embankments. These

properties are briefly discussed in the following paragraphs.

2.2 COLOUR

It is the most common property of soil. Soil exist in nature in a wide variety of coloursdepending upon the particular type of soil mineral, organic contents, the amount of colouringoxides and the degree of oxidation. Black colour of soil is due to the presence of manganesecompounds, green or blue colour due to ferrous compound, red, brown or yellow due to iron, andgrey due to organic matter. For identification purposes the colour of moist soil in the natural state

is generally noted.

2.3 SOIL STRUCTURE

Arrangement or grouping of soil particles depending upon their size and shape in variouspatterns of structural framework is called soil structure. This arrangement is usually developedduring the process of sedimentation or rock weathering.

Soil deposits at the face of earth have been developed by many natural processes of

accumulations of soil particles over historical period of time. During the process of accumulation,

soil particles arrange (group) themselves in different patterns, depending upon their size and

shape (mass to surface area ratio). For coarse grained or non-cohesive soil, mass to surface area

ratio is relatively higher therefore, the effect of gravity has major influence on the arrangement of

particles, and the effect of electrical charge on the particle surfaces is negligible.


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The fine-grained soil (mainly clay) because of their low mass to surface area ratio is

more affected by the electrical forces acting on their surfaces compared with gravity forces, and

therefore the particles arrange (group) themselves in different patterns.

Terzaghi grouped the most common patterns of soil structure into the following threeprincipal groups.

i. Granular or Single-grained structureii. Flocculent Structureiii. Dispersed structure

i Granular or single-grained structure:

Cohesion-less soil (coarse-grained soils and silts > .01mm) tend to form a single-grained

structure which may be loose or dense (Fig: 2.1). In single grained structure, each grain is in

contact with several of its neighbors in such a way that the aggregate is stable even if there are no

forces of adhesion at the point of contact between the grains. Single-grained soil structures are

formed when soil grains independently settle slowly in quiet water. However experience indicates

that it is possible for sands or silts to develop an unusual loose or honeycomb structure (Fig: 2.2).

Honeycomb structure may develop due to settlement of grains in soil-water suspension, or from a

loosely dumped moist soil, where grains develop a particle-to-particle contact that bridge over

relatively large void spaces and can resist the overburden pressure. Such deposits in coarse-

grained soil may be unstable when subjected to shock or vibrations, resulting in quick volume

reduction and loss of strength.

The risk of instability or loss of strength is reasonably reduced if some cementing at points of

particle contacts exists. The cementation over geologic time period may occur as percolating

water precipitates various carbonates and other materials.

Fig: 2.1 Granular soil Structure.


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ii. Flocculent Structure:

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

ratio. The clay particles carry a negative electrical charge on their surfaces. The affect of

electrical forces is more than the gravity forces. Clay deposits developed from particles settling

out of soil-water suspension (either in fresh-water or salt-water) tend to form a flocculent

structure. A flocculent structure is developed when the edge of one clay particle is attracted to

the flat face of another (i.e., edge-to-face contact) (Fig: 2.2). The structure of clay soil settling

out in marine environment (salt-water, which acts as an electrolyte) is more flocculent than clay

in fresh water.

Clay deposits with flocculent structure have high void ratio, low density, high water

content and high permeability. The structure however is quite stable and resistant to external

forces that can be maintained as long as the electrical charges on the edges of the particles remain

opposite in sign to those on the faces. However, due to change of environment surrounding the

particles, such as the salt being leached from the deposit, the inter-particle attraction and hence

the strength is drastically reduced.

iii Dispersed Structure:

The dispersed structure is developed when the edges and faces of the clay particles have

similar electrical charge. The particles repel each other and the orientation is nearly parallel (Fig:2.3). The dispersed structure also develops as a result of remolding by the transportation process

Fig: 2.2 Honeycomb and flocculent structure

(a) Hone comb structure in a (b) Flocculated-t e structure (ed e to face


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(man-made earth fills). The particle arrangement that develops from remolding has a more

parallel orientation of particles. A flocculated structure with the addition of moisture content and

application of compaction energy is changed to a dispersed structure.

2.4 PARTICLE SHAPE AND SIZE

Particle size and shape very much influence the engineering properties of soil. Particles of

coarse-grained soil (sand, gravel, boulder etc.) are generally bulky in shape, i.e., their length

width and thickness are approximately equal. Different shapes are commonly termed as angular,sub-angular, sub-rounded and rounded (Fig: 2.4). The shapes of the particles however depend on

the rock type, their age, weathering and transportation processes. The newer particles are

generally angular and rough surfaced. With the passage of time and as a result of weathering and

transportation processes, the edges are broken and the particles change finally to rounded shape.

Sub-angular and sub-rounded are the transition stages. The angular and rough surfaced particles

possess better engineering properties compared with those of rounded and smooth particles. Some

of the rocks, upon weathering produce flaky particles. The presence of flaky particles in a

granular soil mass has significant effect on the engineering properties e.g., void ratio, density and

compressibility. The flaky & elongated particles bridge over open spaces, which can resist

overburden pressure. Therefore they produce relatively large void ratios and loose soil mass (Fig:

2.5). The flakes, however, are incapable of resisting the applied loading. They bend or break and

allow rearrangement of particles under applied loading, which some times produce undesirabledeformations.

Fig: 2.3 Dispersed structure.

(a) Dispersed-type structure (face to face (b) Remolded or dispersed

Clay Particle Silt particle


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The silt particles although classified as fines are still bulky in shape and have the same

mineralogical composition as the coarse grained soil.

The clay particles are flaky in shape (flat plate-like shape). Their length and width are

many-many times greater than the thickness. The clay particles originate from crystalline

minerals. Due to their distinct mineralogical composition they exhibit inter-particle attraction

and bonding with water molecules. As a result the behavior of clay soil drastically changes with

change of moisture content. At different moisture contents, but at the same void ratio, a clay soil

may behave as a liquid, plastic or a solid mass.

Individual clay particle seldom exists. Due to cohesive forces, they group together, to

form a cluster. The clay particles are very small in size (less than .002mm or 2). However it

Rounded ElongatedFlaky

Fig: 2.4 Particle shapes.

Sub-RoundedAngular Sub-Angular

Fig: 2.4 Particle shapes.


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must be kept in mind that the properties of clay (cohesion and plasticity) are due to the type of the

mineral (i.e., clay mineral) and not due to its small size. The particles of non-clay minerals

although smaller than .002mm, do not exhibit the clay properties (i.e., cohesion and plasticity).

Actual soil deposits consist of soil particles having variation in particle sizes. The

variation of particle sizes may be small to large. An ideal particle size distribution (well-graded)

produces an optimum particle arrangement and upon compaction produce a dense and strong soil

mass. While a mass of soil having particles of nearly the same sizes (uniformly or poorly-graded),

produces a loose packing due to absence of small particles to fill the voids between bigger

particles.

2.5 SPECIFIC GRAVITY

The specific gravity of any substance is defined as the ratio of the unit weight of that

substance, to the unit weight of water at 4oC.

The above definition simply means that how many times a substance (or material) is

heavier than water. For example the specific gravity of mercury of 13.6 means that if equal

volumes of mercury and water are taken than mercury will be 13.6 times heavier than water.

Fig: 2.5 Elongated particles bridging the gap


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ws

s

w

s

sV

WG

==

Similarly specific gravity of gold is 19.3 or one can say that gold is 19.3 times heavier

than water. A geotechnical engineer is commonly interested in the specific gravity of the soil

grains (or solids), which is defined as the ratio of unit weight of soil grains, to the unit weight of

water. It is denoted by Gs and expressed as:

(2.1)

Where, s= unit weight of the soil solids (no pores)

s= Ws/Vs (2.2)

Where Ws is the weight of soil solids, which is equal to the dry weight. And Vs is the

volume of soil solids (no pores).

The term bulk specific gravity or mass specific gravity is also used and it is expressed as

G = b/w (2.3)

Where b= bulk density of soil.

Average values of Gs for soil solids range from 2.50 to 2.70, and it depends on the

mineral making the soil particles. If the mineral composing the soil is heavier the specific gravity

will be greater. A soil mass may be composed of a single mineral or have been developed by amixture of various minerals. Any mineral soil has a unique value of specific gravity, which is

independent of state of soil deposit (i.e., moisture content, compaction etc.).

The bulk specific gravity however depends on the state of soil deposit. It is variable i.e.,

a lower value for loose soil and a higher value for dense soil but can never be more than the

specific gravity of soil solids.

The specific gravity is a very important soil property and is extensively used for the

determination and calculation of many other soil properties; some of them are listed below.

1- Particle size analysis by hydrometer test

2- Porosity and void ratio3- Unit weight

4- Critical hydraulic gradient in studying the quick condition especially to check


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piping failure of hydraulic structures, or heaving of soil while excavating below

water table.

5- Degree of saturation or zero-air-voids in the studies of compaction of soil.

It is therefore very important to pay serious attention and care to the determination of

specific gravity of soil.

Specific gravity of some common soil minerals and various soil types are given in the

following table.

Table: 2.1 Specific Gravity of some Minerals and Soil types

Minerals Specific Gravity Soil-type Specific Gravity

Dolomite 2.8-2.9

Feldspar 2.5-2.6

Gypsum 2.2-2.4

Illite 2.60

Quartz 2.60-2.65

Talc 2.7-2.8

Kaolinite 2.6-2.63Magnetite 5.17-5.18

Calcite 2.8-2.9

Bentonite clay 2.13-2.18

Chalk 2.63-2.73

Clay 2.45-2.90

Humus 1.37

Loess 2.65-2.75

Peat 1.26-1.8

Silt 2.68-2.72Quartz sand 2.60-2.65

Lime 2.7

2.6 SOIL PHASES

A soil mass is a collection of solid particles of different sizes and shapes, which form aporous medium. Depending upon seasonal variations these pores may be filled with air or wateror both. The phase of a soil means any homogeneous part of a soil mass different from other partsin the mass and clearly separated from them.

Since soil is a porous medium consisting of three different homogenous parts (e.g.,solid particles, water and air), a given volume of soil mass may be regarded as a mass consisting

of three fundamental phases, namely: Solid phase, Liquid phase and Gaseous or vapour phase.


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In cold regions, the pore water in the upper soil layers freezes due to accumulation ofsnow on the ground surface.

In the studies of these soils, four phases can be defined as under.

1. Solid phase2. Liquid phase3. Gaseous phase

4. Ice phase.The volumetric proportion of different phases can be studied by phase diagram. It must

be kept in mind that there is no real means of separating the soil phases as shown in the Fig: 2.6.

Fig: 2.6 Schematic diagram indicating different soil phasesSolid particleWater invoids

Air in voids

Water

Water

SolidsSolids

Air

Volume of soilmass = V

Partially Saturated3-phase soilmass

Fully Saturated2-phase soil mass

Va

Vw

Vs Vs

Vw

Water

Air

Air

Solids Solids

Ice

Vs Vs

Vw

Va

Va

Vi V = Vs+Vv

Vv = Vw+ Va + Vi

Vv= Vw(Saturated)

Vv= Va(Dry)

Fully dried2-phase soilmass

Frozen soil4-phase soilmass

Fig: 2.6 Schematic diagram indicating different soil phases


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VG

Wn

w

s

= 1

V

Vn s=1

w

sG

=

s

s

s

ws

s

V

Wwhile

V

WG ==

soilofdry weightWhile s == wG

WV

w

ss

2.7 POROSITY

A soil mass is a porous medium consisting of solid particle, and the pores or voids. Thetotal volume of soil mass is the summation of volume of solid particles and the volume of pores orvoids. The volume of the pores or voids depends on the soil density or degree of packing and isreduced considerably by compaction.

V = Vs + Vv (2.4)

V = Total volume of soil massVs= Volume of solid particles of soilVv= Volume of voids, which may be filled with air or water or both

The ratio of volume of all the voids Vv to the total volume of the soil mass V is

known as the porosity. It is denoted by n and expressed in percentage.

(2.5)

In the above basic formula, it is difficult to determine the term Vv by any simplemeans. The porosity n may be expressed in terms of other physical properties of soil and then itwill be easily determined. The relationships can be developed as follows.

Putting the value of Vsin Equation- 2.6

(2.7)

All the terms on the right hand side of Eq-2.7 can be easily determined and hence n

can be calculated. Porosity varies in the range of 0


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V

VV

V

V

e

vs ==+1

1

ws

ss

s

w

G

WVwhile

W

VGe

== 1

en

+=

1

11

n

ne

=

1

e

e

e

en

+=

+

+=

11

11

Since it is practically impossible to eliminate all the voids therefore porosity can neverbe zero. The increase in the volume of voids increases the total volume by the same amounttherefore porosity can never be 100 percent. Porosity helps in the studies of seepage throughsoil.

2.8 VOID RATIO

It is defined as the ratio of volume of voids present in the soil to the volume of solidparticles in a soil mass. It is denoted by e.

(2.8)

Relationship with other soil properties

(2.9)

The values on the right hand side of the equation (2.10) can be determined easily andhence e can be calculated.

Again from Eq.-2.9

Similarly it can be derived that

1=

==ss

s

s

v

V

V

V

VV

V

Ve

sV

Ve =+1

s

v

V

V

soilinsolidsofvolume

soilinvoidsofvolumee ==

nV

V

e

v ==+

111

1

(2.10)

(2.11)

(2.12)


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100=soildryofweight

waterofweightm

wbe

GesweightUniti

+

+=

1

)(

e

Gw

d += 1

e

Gew

sat+

+=

1

)(

e

G wsub

+

=

1

)1(

e

GigradienthydraulicCriticalii c

+

=

1

1,

minmax

max,Reee

eeDdensitylativeiii

=

The void ratio is expressed as a number and the values vary within the range.0


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It is an important physical property since the behavior of soil is very much influencedby changes of moisture content. At large changes of moisture content the behavior of soil isentirely changed e.g., a soil which behaves as a solid at low moisture contents change to liquidstate at high moisture content and the shear strength is practically reduced to zero. Increase inmoisture always increases the unit weight of a dry soil. The moisture in the voids of a soil massoccurs in a variety of forms. Depending upon the form of occurrence they are given differentnames e.g.,

a- Hygroscopic Moisture

It is also known as adsorbed moisture, contact moisture or surface bound moisture. Thisform of soil moisture exists as a very thin film of moisture surrounding the surfaces of individualsoil particles and is held by the force of adhesion. Practically the moisture present in an air driedsoil sample may be termed as hygroscopic moisture. The value of hygroscopic moisture however

depends on the atmospheric temperature, relative humidity and the type of soil. In fine grainedsoil such as clays, due to large specific surface, hygroscopic moisture is high (up to 20% ormore) while in coarse grained soil (sand) it is relatively low due to limited amount of specificsurface. The approximate values of hygroscopic moisture for various soils are as under:

1- Sand 1-2 percent2- Silt 7-9 percent3- Clay 17-20 percent

The values are just approximate and vary with humidity and temperature etc.Hygroscopic moisture is not affected by gravitational forces, capillary forces and air drying atordinary temperature. Hygroscopic moisture film is bound so rigidly to the particle surfaces thatit can not be removed even by centrifugation. It does not exert any hydrostatic pressure. The

difference between the weight of an air-dried sample to its weight after oven drying at + 105Cgives the amount of hygroscopic moisture present in the soil.

b. Film Moisture

The thickness of moisture film around soil particles varies depending upon theconditions such as weather etc. The moisture film attached to the soil particles, above the layerof hygroscopic moisture film, is known as film moisture. It is held by the molecular forces and isnot affected by gravity. It can move from points of higher potentials (heat or electric) to lowerones or from points of thicker to thinner films. The amount of film moisture depends on thespecific surface i.e., higher the specific surface higher will be the film moisture and vice-versa.

c. Capillary Moisture

It is defined as the moisture which is held within the voids of capillary size. Thecapillary moisture is continuously connected to the groundwater table. It rises above the water

table and is held by the surface tension force of the menisci at the top of water columns incapillary tubes formed due to interconnected pores in soil. The voids are completely filled withwater and the soil is fully saturated. The height or thickness of capillary saturated zone above the


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satv

w

v

w

m

m

W

W

V

VS ===

groundwater table depends on the size of soil particles. Finer the particles, greater is thethickness of capillary zone.Capillary water can be removed from the soil by drainage when thequantity of water present in the voids is in excess of that retained by the surface tension forces.

d. Chemically Bound Moisture

It is the moisture contained chemically within the mineral particles and can be removedonly by chemical process which breaks the crystalline structure of the mineral. The chemicallybound water does not influence the physical properties and behavior of soil and therefore is notcommonly determined.

The moisture content determined through oven drying method (or any other method) byEq.-2.22 includes adsorbed moisture, film moisture and only that portion of capillary moisture,which is held within the voids by surface tension forces. All other forms of water (not discussed

here) will be drained out by gravity as the soil sample is extracted from the ground (from surfaceor sub-surface layers). Chemically bound moisture is not important for common soil engineeringproblems and therefore is not determined.

The range of moisture content is

0 m


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sv

wva

vVV

VV

V

VAorA

+

==

)1(

)1(

eV

SVA

s

v

+

=

e

enwhileSnA

+==

1)1(

e

se

A +

= 1

)1(

1000


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shear strength) at high moisture content. The consistency of a soil means its physical state withrespect to the moisture content present at that time.

Four consistency states are commonly defined for clays (cohesive soils)

1. Solid state2. Semi-solid state3. Plastic state4. Liquid state.

Atterberg, a Swedish soil scientist defined the boundaries of the above four states interms of limits as follows.

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

semi-solid state.2- Plastic Limit: It is the moisture content at which a soil changes from semi-solid state to

plastic state.

3- Liquid Limit: It is the moisture content at which a soil changes from plastic state toliquid state.

The transition from one state to the next however is gradual, and according to abovedefinitions it is quite difficult to know the value of moisture content at which the change of stateoccurs. The definition that clearly states the moisture content at which the change of state occurswill be given later.

The most important of these limits are the liquid and the plastic limits, which indicatethe range of plastic state. The range of plastic state means, the upper and lower bounds of

moisture content within which the soil behaves similar to a plastic material. It is the numericaldifference between the liquid and plastic limits and is known as plasticity index.

Due to the plastic behavior of fine-grained soils, these limits are related to the plasticity,which is a major characteristic of fine-grained (clay) soils. It is defined as the property thatenables a material to undergo large irrecoverable deformations without cracking or crumbling.Since the plasticity, greatly influence the engineering properties, such as shear strength andcompressibility, it is therefore used as a basis for the classification of fined grained soils.

As discussed earlier the plasticity does not depend on the size of the particles. Rockflour, for example, practically exhibits no plasticity, where as clay having the same size willexhibit a marked plasticity. Bentonite and kaolinite clays having almost similar particle sizes havedifferent plasticity values. Actually many factors, such as, the size, shape, nature of the claymineral and the nature of the adsorbed layer, control the plasticity. Where the average specific

surface is high (more fine and flaky e.g., Montmorillonite clay having approximate size,length=0.1-0.5, & thickness=0.001-0.01m), the plasticity may be extremely high and the soilextremely compressible.


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2.12-a SHRINKAGE LIMIT

It is defined as that moistur

Final Draft Introduction to Soil and Dam Engg. 12-3-2009

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