geotechnics - c1

Reader PhD. Eng. Nicoleta ILIE

[email protected]

5 March, 2013 1

Geotechnics -class notes 2013-

References

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

2. POPA A., FRCAS V., - Geotehnic, U.T.Press, 2004 3. F. M. THOMLINSON - Foundations 4. V.POP Geotehnic si fundatii, Lito IPCN, 1983 5. A. POPA, F. ROMAN Calculul structurilor de rezisten pe mediu

elastic, 2000 6. V. POP, col. Proiectarea fundatiilor, Lito IPCN, 1985. 7. A. STANCIU, I. LUNGU Fundatii Fizica si mecanica pmnturilor,

Ed. Tehnic, 2006 8. T. SILION Geotechnics, Iasi, 1995 9. * * * STAS and romanian norms 10. A. Verruijt Soil mechanics, Delft University of Technology, 2010 11. C. Venkantramaiah Geotechnical Engineering, 2006

Geotechnics exam 5 ECTS

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Final mark

30% Laboratory tests

20% Numerical aplication

50% Theory

1. Geotechnics soil mechanics. General remarks

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The term Soil has different meanings in different scientific fields. It has originated from the Latin word Solum. To an agricultural scientist, it means the loose material on the

earths crust consisting of disintegrated rock with an admixture of organic matter, which supports plant life.

To a geologist, it means the disintegrated rock material which has not been transported from the place of origin.

To a civil engineer, the term soil means, the loose unconsolidated inorganic material on the earths crust produced by the disintegration of rocks, overlying hard rock, with or without organic matter. Foundations of all structures have to be placed on or in such soil, which is the primary reason for our interest as Civil Engineers in its engineering behaviour.


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Soil mechanics is the study of the engineering behaviour of soil when it is used either as a construction material or as a foundation material. 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 soil

mechanics.

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

The application of the principles of soil mechanics to the design and construction of foundations for various structures is known as Foundation 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 use of soil for engineering purposes dates back to prehistoric times. Soil was used not only for foundations but also as construction material for embankments. The knowledge was empirical in nature and was based on trial and error, and experience.

The hanging gardens of Babylon were supported by huge retaining walls, the construction of which should have required some knowledge, though empirical, of earth pressures.

The large public buildings, harbours, aqueducts, bridges, roads and sanitary works of Romans certainly indicate some knowledge of the engineering behaviour of soil. This has been evident from the writings of Vitruvius, the Roman Engineer in the first century, B.C.

Mansar and Viswakarma, in India, wrote books on construction science during the medieval period.


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The Leaning Tower of Pisa, Italy, (1174 and 1350 A.D.) is an example of a lack of sufficient knowledge of the behaviour of compressible soil, in those days.

Coulomb (French Engineer) published his wedge theory of earth pressure (1776), which is the first major contribution to the scientific study 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.

Poncelet, Culmann and Rebhann were the other men who extended 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 in soil mechanics.


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

Boussinesq (1885) gave his theory of stress distribution in an elastic medium under a point load on the surface.

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

Atterberg, a Swedish soil scientist, gave in 1911 the concept of consistency limits for a soil. This made possible the understanding of 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 Swedish Geotechnical Commission.


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Prandtl gave his theory of plastic equilibrium in 1920 which became the basis for the development of various theories of bearing capacity.

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

Later on, R.R. Proctor and A. Casagrande and a host of others were responsible for the development of the subject as a full-fledged discipline.


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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 the granary on the border of Danube River

1967 Bucharest first National Conference of Geotechnics and Foundations (every 4 years since 1967)

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


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SOIL MECHANICS - APPLICATIONS

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 a soil through the foundation for the structure. Thus, the foundation is an important part of a structure, the type and details will be decided only with the knowledge and application of the principles of soil mechanics.

2. Underground and Earth-retaining Structures

Underground structures (drainage structures, pipe lines, tunnels and earth-retaining structures: retaining walls) can be designed and constructed only by using the principles of soil mechanics and the concept of soil-structure interaction.


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SOIL MECHANICS - APPLICATIONS

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 the traffic loads.

Problems peculiar to the design of roads are the effect of repetitive loading, swelling and shrinkage of sub-soil and frost action. Consideration of these and other factors in the efficient design of a road is a must and one cannot do without the knowledge of soil mechanics.


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SOIL MECHANICS - APPLICATIONS 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 itself

is used as the construction material, requires a thorough knowledge of the engineering behaviour of soil especially in the presence of water.

Knowledge of slope stability, effects of seepage, consolidation and consequent settlement as well as compaction characteristics for achieving maximum unit weight of the soil in-situ, is absolutely essential for efficient design and construction of embankments and earth dams.


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Soil behaviour

The knowledge of soil mechanics, assuming the soil to be an ideal material, elastic, isotropic and homogeneous, coupled with the experimental determination of soil properties, is helpful in predicting the behaviour of soil in the field.

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

A through knowledge of soil mechanics is a prerequisite to be a successful foundation engineer.

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

2. Soil composition

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1. Structure of soils

The structure of a soil may be defined as the manner of arrangement and state of aggregation of soil grains. In a broader sense, consideration of mineralogical composition, electrical

properties, orientation and shape of soil grains, nature and properties of soil water and the interaction of soil water and soil grains, also may be included in 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, compressibility and shear strength.

The following types of structure are commonly studied: (a) Single-grained structure

(b) Honey-comb structure

(c) Flocculent structure

2. Soil composition

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1. Structure of soils

a. Single-grained Structure

Single-grained structure is characteristic of coarse grained soils, with a particle size greater than 0.02mm.

Gravitational forces predominate - the surface forces, hence grain to grain contact results.

The deposition may occur in a loose state, with large voids or in a dense state, with less of voids.

2. Soil composition

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1. Structure of soils b. Honey-comb Structure This structure can occur only in fine-

grained soils, especially in silt. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces 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 made up of numerous individual soil grains.

The structure has a large void space and may carry high loads without a significant volume change. The structure can be broken down by external disturbances.

2. Soil composition

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1. Structure of soils c. Flocculent Structure This structure is characteristic of fine-

grained soils such as clays. Inter-particle forces play a predominant role in the deposition. Mutual repulsion of the particles may be eliminated by an appropriate chemical; this will result in grains coming closer together to form a floc.

Formation of flocs is flocculation. But the flocs tend to settle in a honeycomb structure, in which in place of each grain, a floc occurs.

Thus, grains grouping around void spaces larger than the grain-size are flocs and flocs grouping around void spaces larger than even the flocs result in the formation of a flocculent structure.

2. Soil composition 1. Structure of soils

Very fine particles or particles of colloidal size (< 0.001 mm) may be in a flocculated or dispersed state.

The flaky particles are oriented edge-to-edge or edge-to-face with respect to one another in the case of a flocculated structure. Flaky particles of clay minerals tend to from a card house structure (Lambe, 1953), when flocculated.

When inter-particle repulsive forces are brought back into play either by remoulding or by the transportation process, a more parallel arrangement or reorientation of the particles occurs. This means more face-to-face contacts occur for the flaky particles when these are in a dispersed state (honey comb structure).

In practice, mixed structures occur, especially in typical marine soils.

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2. Soil composition

2. Soil profile / soil horizon A deposit of soil material, resulting from one or more of the

geological processes, is subjected to further physical and chemical changes which are brought about by the climate and other factors prevalent subsequently. Vegetation starts to develop and rainfall begins the processes of leaching and eluviation of the surface of the soil material.

Gradually, with the passage of geological time profound changes take place in the character of the soil.

These changes bring about the development of soil profile.

Thus, the soil profile is a natural succession of zones or strata below the ground surface and represents the alterations in the original soil material which have been brought about by weathering processes. It may extend to different depths at different places and each stratum may have varying thickness. 20

2. Soil composition

2. Soil profile / soil horizon

O - Rich in humus and organic plant residue. This is usually eluviated and leached; the ultrafine colloidal material and the soluble mineral salts are washed out of this horizon. It is dark in colour and its thickness may range from a few centimetres to half a meter. This horizon often exhibits many undesirable engineering characteristics and is of value only to agricultural soil scientists.

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2. Soil composition


A - Surface soil: The A-horizon is sometimes referred to as the zone of accumulation. The material which has migrated from the O-horizon by leaching and eluviations gets deposited in this zone. There is a distinct difference of colour between this zone and the dark top soil of the O-horizon. This soil is very much chemically active at the surface and contains unstable fine-grained material. The thickness of A-horizon may range from 0.50 to 0.75m.

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2. Soil composition


B Subsoil: The material in the B-horizon is in the same physical and chemical state as it was first deposited by water, wind or ice in the geological cycle. The thickness of this horizon may range from a few centimetres to more than 30m. The upper region of this horizon is often oxidised to a considerable extent.

C Bed rock: Layer of large unbroken rocks. This layer may accumulate the more soluble compounds.

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2. Soil composition

3. Texture of soils The term Texture refers to the appearance of

the surface of a material, such as a fabric. It is used in a similar sense with regard to soils.

Soil texture refers to particles lay down, in a certain area of the investigated soil.

Soil texture can be: Homogeneous with the same type of soil on the

entire layer thickness (a)

Layered on the active area there are few different layers. Horizontal layers (b)

Inclined layers, with the slope >10% - difficult ground conditions (c)

Lens shaped layers - difficult ground conditions (d) 24

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 or some other liquid. Void spaces not occupied by water or any 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-phase system

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2. Soil composition

4. Soils as three-phase system A soil mass as it exists in nature is a

more or less random accumulation of soil particles, water and air-filled spaces.

For purposes of analysis it is convenient to represent this soil mass by a block diagram, called Phase-diagram

It may be noted that the separation of solids from voids can only be imagined.

The phase-diagram provides a convenient means of developing the weight-volume relationship for a soil.

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2. Soil composition

4. Soils as three-phase system

When the soil voids are completely filled with

water, the gaseous phase being absent, the soil 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 liquid phase being absent ; the soil it is dry. (It may be noted that the dry condition is rare in nature and may be achieved in the laboratory through drying).

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

These are special cases of the three-phase system. 27

2. Soil composition

5. Soil solid particles

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

Soils usually have a sedimentary origin, however, they can also occur 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 and feldspars, minerals that have high strengths and the particles are fairly round.

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

One of the most important aspects of particulate materials is that there 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 be determined for any construction project.

Cheap and simple tests are required to give an indication of the engineering properties such as stiffness and strength for preliminary design.

To achieve this task, continuous samples are recovered from boreholes, 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 performed 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 behaviour of soils with very small particles, usually containing clay minerals, is significantly different from the behaviour of soils with larger particles. Clays can cause problems because they are relatively compressible, drain poorly, have low strengths and can swell in the presence of water.

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2. Soil composition


Particle size definitions: The precise boundaries between different 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


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

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

The individual particles may vary from perfectly round to highly angular reflecting 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 property of 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 identify silts.

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

Organic -These may be of either clay or silt sized particles. They contain significant amounts 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 soil surface, but layers of peat may be found at a certain depth. These are very poor soils for most engineering purposes.

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2. Soil composition


Procedure for grain size determination Different procedures are required for fine and coarse-grained material.

Detailed procedures are described in the Romanian or European norms. They will be demonstrated in a laboratory session.

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Coarse particles: Sieve analysis is used to determine the distribution of the larger grain sizes. The soil is passed through a series of sieves with the mesh size reducing progressively, and the proportions by weight of the soil retained on each sieve are measured. There are a range of sieve sizes that can be used, and the finest is usually a 63m sieve. Sieving can be performed either wet or dry. Because of the tendency for fine particles to clump together, wet sieving is often required with fine-grained soils.

2. Soil composition


Fine particles To determine the grain size distribution of material

passing the 63m sieve the hydrometer method is commonly used (sedimentation analysis).

The soil is mixed with water and a dispersing agent, stirred vigorously, and allowed to settle to the bottom of a measuring cylinder.

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

An hydrometer is used to record the variation of specific gravity with time, applying Stokes Law, which relates the velocity of a free falling sphere to its diameter

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

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2. Soil composition


Grading Curves

The results from the particle size determination tests are plotted as grading curves.

They show the particle size plotted against the percentage of the sample by weight that is finer than 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.

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0 .0 0 0 1 0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 0

0

2 0

4 0

6 0

8 0

1 0 0

P a r tic le s ize (mm)

% F

in

er

W - Well graded material

U - Uniform material

P - Poorly graded material

C - Well graded with some clay

F - Well graded with an excess of fines

2. Soil composition


Grading Curves

The use of names to describe typical grading curve shapes and positions has developed as the suitability of different gradings for different

purposes 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. For this reason soils of this type are preferred for road bases.

An important property of a granular or coarse-grained soil is its degree of uniformity. The grain-size distribution curve of the soil itself indicates, by its shape, the degree of soil uniformity. A steeper curve indicates more uniform 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.

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2. Soil composition

5. Soil solid particles Grading Curves

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

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

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

The soil is said to be

very uniform, if cU < 6;

of medium uniformity, cU = 6 to 15; and

very non-uniform or well-graded, if cU > 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


Ternary diagram / ternary plot

The proportions of gravel, sand, silt, and clay in any soil.

SR EN ISO 14 688

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1 gravel percentage

2 sand percentage

3 fine particles percentage

4 clay percentage

5 fine soils (clay+silt)

6 mixt soils (clayey or silty gravel and sand)

7 granular soils (gravel and sand)

2. Soil composition

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2. Soil composition

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When defining a soil, all the fractions have to be named, considering their importance:

Examples: Sandy gravel

sa Gr Fine gravel with coarse sand

c sa F Gr Silt with medium sand

m sa Si Coarse sand with fine gravel

f gr C Sa 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

See you next time!!!

geotechnics - c1

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