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Geotechnics-class notes 2012-

Reader PhD. Eng. Nicoleta ILIES

[email protected] February, 2012

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References

1. SR EN 1997-1/2006. Geotechnical design. General rules +Geotechnical investigation.

2. POPA A. FRCAS V. - Geotehnic U.T.Pres 20043. F. M. THOMLINSON - Foundations4. V.POP Geotehnic si fundatii, Lito IPCN, 19835. A. POPA, F. ROMAN Calculul structurilor de rezisten pe mediu

elastic, 20006. V. POP, col. Proiectarea fundatiilor, Lito IPCN, 1985.7. A. STANCIU, I. LUNGU Fundatii Fizica si mecanica pmnturilor,

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. ,8. T. SILION Geotechnics, Iasi, 19959. * * * STAS and romanian norms10. A. Verruijt Soil mechanics, Delft University of Technology, 2010

11. C. Venkantramaiah Geotechnical Engineering, 2006


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Geotechnics exam 6 ECTSFinal mark

30% Laboratory tests

20% Numerical aplication

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1. Geotechnics soil mechanics. General remarks The term Soil has different meanings in different scientific

To an agricultural scientist, it means the loose material on theearths crust consisting of disintegrated rock with an admixture oforganic matter, which supports plant life.

To a geologist, it means the disintegrated rock material which hasnot been transported from the place of origin.

To a civil engineer, the term soil means, the loose

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unconso ate norgan c mater a on t e eart s crust pro uceby the disintegration of rocks, overlying hard rock with or withoutorganic matter. Foundations of all structures have to be placed onor in such soil, which is the primary reason for our interest as Civil

Engineers in its engineering behaviour.


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1. Geotechnics soil mechanics. General remarks

Soil mechanics is the study of the engineering behaviour of soilwhen it is used either as a construction material or as a foundationmaterial. This is a relatively young discipline of civil engineering, systematised in its modern form by

Karl Von Terzaghi (1925), who is regarded as the Father of Modern Soil Mechanics.

An understanding of the principles of mechanics is essential to the study of soilmechanics.

A knowledge and application of the principles of other basic sciences such as physics andchemistry would also be helpful in the understanding of soil behaviour. Further, laboratoryand field research have contributed in no small measure to the development of soilmechanics as a discipline.

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construction of foundations for various structures is known asFoundation Engineering.

Geotechnical Engineering may be considered to include both soil

mechanics and foundation engineering. In fact, according to Terzaghi, it is difficult to draw a distinct line of demarcation between

soil mechanics and foundation engineering; the latter starts where the former ends.


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The Leaning Tower of Pisa, Italy, (1174 and 1350 A.D.) is an exampleof a lack of sufficient knowledge of the behaviour of compressiblesoil in those da s.

Coulomb (French Engineer) published his wedge theory of earthpressure (1776), which is the first major contribution to the scientificstudy of soil behaviour. He was the first to introduce the concept of

shearing resistance of the soil as composed of the two components -cohesion and internal friction.

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the work of Coulomb. D Arcy and Stokes were notable for their laws for the flow of water

through soil and settlement of a solid particle in liquid medium,

respectively. These laws are still valid and play an important role insoil mechanics.


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Rankine gave his theory of earth pressure (1857); he did not considercohesion, although he knew of its existence.

ouss nesq gave s eory o s ress s r u on n an e as cmedium under a point load on the surface.

Mohr( 1871) gave a graphical representation of the state of stress at apoint, called Mohrs Circle of Stress. This has an extensiveapplication in the strength theories applicable to soil.

Atterberg, a Swedish soil scientist, gave in 1911 the concept ofconsistenc limits for a soil. This made ossible the understandin of

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the physical properties of soil. The Swedish method of slices for slope stability analysis was

developed by Fellenius in 1926. He was the chairman of the SwedishGeotechnical Commission.


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Prandtl gave his theory of plastic equilibrium in 1920 which became.

Terzaghi gave his theory of consolidation in 1923 which became animportant development in soil mechanics. He also published, in 1925,the first treatise on Soil Mechanics, a term coined by him. (Erd baumechanik, in German). Thus, he is regarded as the Father of modernsoil mechanics.

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, . . .

responsible for the development of the subject as a full-fledgeddiscipline.


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Romania:

1936 first detailed geotechnical studies about soil behaviour for

CFR Palace in Bucharest Test performed by K. Terzaghi in Viena laboratory

1939 Bucharest first geotechnical laboratory in Romania

Due to the important geotechnical works necessary for thegranary on the border of Danube River

1967 Bucharest first National Conference of Geotechnics and

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1990 Romanian Society of Geotechnics and Foundations (SRGF)affiliated to International Society of Soil Mechanics andGeotechnical Engineering (ISSMGE)


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FIELDS OF APPLICATION OF SOIL MECHANICS

The knowledge of soil mechanics has application in many fields of Civil Engineering

1. Foundations

The loads from any structure have to be ultimately transmitted to asoil through the foundation for the structure. Thus, the foundation isan important part of a structure, the type and details will be decided

only with the knowledge and application of the principles of soilmechanics.

-

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.

Underground structures (drainage structures, pipe lines, tunnels andearth-retaining structures: retaining walls) can be designed andconstructed only by using the principles of soil mechanics and the

concept of soil-structure interaction.


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The knowledge of soil mechanics has application in many fields of Civil Engineering.

3. Roads Design

Roads Design may consist of the design of flexible or rigid elements.

Flexible - depend more on the subgrade soil for transmitting thetraffic loads.

Problems peculiar to the design of roads are the effect of repetitive

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oa ng, swe ng an s r n age o su -so an ros ac on.Consideration of these and other factors in the efficient design of aroad is a must and one cannot do without the knowledge of soilmechanics.


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The knowledge of soil mechanics has application in many fields of Civil Engineering.

4. Excavations Embankments and Dams

Excavations require the knowledge of slope stability analysis; Deep excavations may need temporary supports, the design of

which requires knowledge of soil mechanics.

The construction of embankments and earth dams, where soil itselfis used as the construction material, requires a thorough knowledgeof the engineering behaviour of soil especially in the presence of

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wa er. Knowledge of slope stability, effects of seepage, consolidation and

consequent settlement as well as compaction characteristics forachieving maximum unit weight of the soil in-situ, is absolutely

essential for efficient design and construction of embankments andearth dams.


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Soil behaviour

The knowledge of soil mechanics, assuming the soil to be an ideal, , ,

experimental determination of soil properties, is helpful in predictingthe behaviour of soil in the field.

Soil being a particulate and heterogeneous material, does not lenditself to simple analysis. Further, the difficulty is enhanced by the factthat soil strata vary in extent as well as in depth even in a small area.

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successful foundation engineer.

It is difficult to draw a distinguishing line between Soil Mechanics andFoundation Engineering; the later starts where the former ends.


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2. Soil composition

1. Structure of soils

The structure of a soil may be defined as the manner of.

In a broader sense, consideration of mineralogical composition, electricalproperties, orientation and shape of soil grains, nature and properties of soilwater and the interaction of soil water and soil grains, also may be includedin the study of soil structure, which is typical for transported or sediments

soils. Structural composition of sediment soils influences, many of their

important engineering properties such as permeability,

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compressibility and shear strength. The following types of structure are commonly studied:

(a) Single-grained structure

(b) Honey-comb structure

(c) Flocculent structure


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2. Soil composition


a. Single-grained Structure

-characteristic ofcoarse grainedsoils, with a particle sizegreater than 0.02mm.

Gravitational forcespredominate - the surfaceforces, hence grain to grain

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contact results.

The deposition may occur in aloose state, with large voids orin a dense state, with less of

voids.


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2. Soil composition

1. Structure of soilsb. Honey-comb Structure

This structure can occur onl in fine-

grained soils, especially in silt. Due to therelatively smaller size of grains, besidesgravitational forces, inter-particle surfaceforces also play an important role in the

process of settling down. In the formation of a honey-comb

structure, each cell of a honey-comb is

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grains. The structure has a large void space and

may carry high loads without asignificant volume change. The structurecan be broken down by externaldisturbances.


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2. Soil composition

1. Structure of soilsc. Flocculent Structure This structure is characteristic of fine-

gra ne so s suc as c ays. n er-par c eforces play a predominant role in thedeposition. Mutual repulsion of theparticles may be eliminated by anappropriate chemical; this will result in

grains coming closer together to form afloc. Formation of flocs is flocculation. But

the flocs tend to settle in a hone comb

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structure, in which in place of each grain,a floc occurs. Thus, grains grouping around void spaces

larger than the grain-size are flocs andflocs grouping around void spaces larger

than even the flocs result in theformation of a flocculent structure.


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2. Soil composition


Very fine particles or particles of colloidal size (10% - difficultground conditions (c)

Lens shaped layers - difficult ground conditions (d)24


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2. Soil composition

4. Soils as three-phase system

Soil is a complex physical system.

A mass of soil includes: solid particles or soil grains and

the void spaces that exist between the particles.

The void spaces may be partially or completely filled with water orsome other li uid. Void s aces not occu ied b water or an other

liquid are filled with air or some other gas. Because the volume occupied by a soil mass may generally be

expected to include material in all the three states of matter (solid,

liquid and gas), soil is, in general, referred to as a three-phasesystem25


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2. Soil composition


A soil mass as it exists in nature is a

more or less random accumulation ofsoil particles, water and air-filledspaces.

For purposes of analysis it isconvenient to represent this soil massby a block diagram, called Phase-diagram

It may be noted that the separation ofsolids from voids can only beimagined.

The phase-diagram provides aconvenient means of developing theweight-volume relationship for a soil.

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2. Soil composition


n so vo s r comp y wwater, the gaseous phase being absent, thesoil is fully saturated or saturated.

When there is no water at all in the voids, the

voids will be full of air or other gas, the liquidphase being absent ; the soil it is dry. (It maybe noted that the dry condition is rare innature and may be achieved in the laboratorythrough drying).

In both these cases, the soil system reducesto a two-phase system

These are special cases of the three-phasesystem. 27

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2. Soil composition

5. Soil solid particles

Soil mechanics approach particulate materials (soils) found in theground, that are not cemented and not greatly compressed.

Soils usually have a sedimentary origin, however, they can alsooccur as the result of rock weathering without any transport of

the particles.

The soil particles can have varying sizes, shapes and mineralogy,

although these properties are usually interrelated.

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2. Soil composition


The larger sized particles are generally composed of quartz andfeldspars, minerals that have high strengths and the particles are

fairly round.

The smaller sized particles are generally composed of the clayminerals (montmorillonite), minerals that have low strengths.

One of the most important aspects of particulate materials is thatthere are voids between the particles. The amount of voids is also

influenced by the size, shape and mineralogy of the particles.

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2. Soil composition


The extent and properties of the soil on the site have to bedetermined for any construction project.

Cheap and simple tests are required to give an indication of theengineering properties such as stiffness and strength for

preliminary design.

To achieve this task, continuous samples are recovered fromboreholes, drilled to a depth that will depend on the scale of the

project.

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2. Soil composition


Observation of the samples deteremine each soil and then

classification tests are erformed for these different strata.

The extent of the different soil layers can be determined by

correlating the results from different boreholes.

An indication of the engineering properties is given on the basis of

particle size. This approach is used because the engineering

e v u s s w ve y sm p es, usu y n n ng y

minerals, is significantly different from the behaviour of soils withlarger particles. Clays can cause problems because they are

relatively compressible, drain poorly, have low strengths and can

swell in the presence of water.31


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2. Soil composition


soil types/ particle sizes are somewhat arbitrary, but the following

scale is in use worldwide

Most soils contain mixtures of sand, silt and clay particles, so the

range of particle sizes can be very large.

Not all particles less than 2m are comprised of clay minerals, and

some clay mineral particles can be greater than 2m. (A micron,

m, is 10-6m).

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2. Soil composition


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2. Soil composition

5. Soil solid particles Coarse-grained soils: sands, gravels and larger particles.

For these soils the grains are well defined and may be seen by the eye.

their geological origins.

Fine-grained soils: silts and clays with particles smaller than 63 m.

Silts - These can be visually differentiated from clays because they have the propertyof dilatancy. If a moist sample is shaken in the hand water will appear on the surface.

If the sample is then squeezed in the fingers the water will disappear. Their gritty feel

can also identif silts.

Clays - Clays exhibit plasticity, they may be readily remoulded when moist, and if itis let to dry can attain high strengths

Organic -These may be of either clay or silt sized particles. They contain significantamounts of vegetable matter. The soils as a result are usually dark grey or black and

have a noticeable odour from decaying matter. Generally they appear only at the soilsurface, but layers of peat may be found at a certain depth. These are very poor soils

for most engineering purposes.34


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2. Soil composition

5. Soil solid particles Procedure for grain size determination

Different procedures are required for fine and coarse-grained material.

Detailed procedures are described in the Romanian norms or Europeannorms.

These will be demonstrated in a laboratory session.

Coarse particles:Coarse particles:SieveSieve analysisanalysis isis usedused toto determinedetermine thethe distributiondistribution ofof thethelargerlarger graingrain sizessizes..

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meshmesh sizesize reducingreducing progressively,progressively, andand thethe proportionsproportions bybyweightweight ofof thethe soilsoil retainedretained onon eacheach sievesieve areare measuredmeasured..ThereThere areare aa rangerange ofof sievesieve sizessizes thatthat cancan bebe used,used, andand thethe finestfinestisis usuallyusually aa 6363mm sievesieve.. SievingSieving cancan bebe performedperformed eithereither wetwet orordrydry..

BecauseBecause ofof thethe tendencytendency forfor finefine particlesparticles toto clumpclump together,together,wetwet sievingsieving isis oftenoften requiredrequired withwith finefine--grainedgrained soilssoils..

2 Soil composition


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2. Soil composition

5. Soil solid particles Fine particles

To determine the grain size distribution of material

commonly used (sedimentation analysis). The soil is mixed with water and a dispersing agent,

stirred vigorously, and allowed to settle to thebottom of a measuring cylinder.

As the soil particles settle out of suspension thespecific gravity of the mixture reduces.

An hydrometer is used to record the variation of, ,

which relates the velocity of a free falling sphere toits diameter

The test data provide particle diameters and the %by weight of the sample finer than a particular

particle size.36

2 Soil composition


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2. Soil composition

5. Soil solid particles Grading Curves

The results from the 100

particle size determinationtests are plotted as grading

curves.

They show the particle size

plotted against thepercentage of the sample

by weight that is finer than 0

20

40

60

80

%F

iner

that size. The results are

presented on a semi-logarithmic plot.

The shape and position of

the grading curve are used

to identify some

characteristics of the soil.37

0.0001 0.001 0.01 0.1 1 10 100

Particle size (mm)

W - Well graded materialU - Uniform materialP - Poorly graded material

C - Well graded with some clayF - Well graded with an excess of fines

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2. Soil composition


has developed as the suitability of different gradings for differentpurposes has become apparent.

For example, well graded sands and gravels can be easily compacted to

relatively high densities which result in higher strengths and stiffnesses. Forthis reason soils of this type are preferred for road bases.

An important property of a granular or coarse-grained soil is its degree ofun orm y . e gra n-s ze s r u on curve o e so se n ca es, y

its shape, the degree of soil uniformity. A steeper curve indicates moreuniform soil.

From the typical grading curves it can be seen that soils are rarely all sand

or all clay, and in general will contain particles with a wide range of sizes.38

2 Soil composition


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2. Soil composition


Certain properties of granular or coarse-grained soils have been related to.

Quantitatively, the uniformity of a soil is defined by its Coefficient ofUniformity

where d60 = 60% finer size and d10 = 10% finer size, or effective size.

The soil is said to be

, U

of medium uniformity, cU= 6 to 15; and very non-uniform or well-graded, ifcU> 15.

On the average,

for sands cU= 10 to 20,

for silts cU= 2 to 4, and for clays cU= 10 to 100 (Jumikis, 1962) 39

2 Soil composition


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2. Soil composition

5. Soil solid particles Ternary diagram / ternary plot

The ro ortions of ravel sand silt

and clay in any soil. SR EN ISO 14 688

1 gravel percentage

2 sand percentage

3 fine particles percentage

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5 fine soils (clay+silt)6 mixt soils (clayey or silty gravel andsand)

7 granular soils (gravel and sand)

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2. Soil composition

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2. Soil composition When defining a soil, all the fractions have to be named, considering

their importance:

Examples: an y grave

sa Gr Fine gravel with coarse sand

c sa F Gr

Silt with medium sand m sa Si

Coarse sand with fine gravel

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Silty fine sand si F Sa

Silt with fine gravel and coarse sand

f gr c sa Si Clay with medium sand

m sa Cl


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