SOIL MECHANICS-CH2-Soil Formation

SOIL MECHANICS LECTURESFOR 3rd CLASS – CIVIL ENG. DEPT.

Asst. Prof. Dr. Mohammed Shaker Al Shakerchy

Ph.D. in Geotechnical EngineeringFaculty of Engineering / University of Kufa

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SOIL MECHANICS LECTURES – 3RD CLASS – CIVIL ENG. DEPT.

LECTURE TWOSOIL FORMATION

2SOIL MECHANICS LECTURES by Dr. Mohammed Sh. M. Al Shakerchy

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SOIL MECHANICS- KCE3241SOIL FORMATION

The main items in this lecture are:

ORIGIN OF THE SOIL

TYPES OF PARENT ROCK

WEATHERING OF ROCK

SOIL MECHANICS LECTURES by Dr. Mohammed Sh. M. Al Shakerchy

Soils are the results of geological events (except for the very small amount produced by man). The nature and structure of a given soil depends on the geological processes that formed it:

- breakdown of parent rock: weathering, decomposition, erosion. - transportation to site of final deposition: gravity, flowing water, ice, wind. - environment of final deposition: flood plain, river terrace, glacial, marine. - subsequent conditions of loading and drainage - little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.

ORIGIN OF THE SOIL



All soils originate, directly or indirectly, from solid rocks in the Earth's crust:

1- igneous rocks crystalline bodies of cooled magma e.g. granite, basalt, dolerite, gabbro, syenite, porphyry

2- sedimentary rocks layers of consolidated and cemented sediments, mostly formed in bodies of water (seas, lakes, etc.) e.g. limestone, sandstones, mudstone, shale, conglomerate

3- metamorphic rocks formed by the alteration of existing rocks due to heat from igneous intrusions (e.g. marble, quartzite, hornfels) or pressure due to crustal movement (e.g. slate, schist, gneiss).

TYPES OF PARENT ROCKS



ROCK CYCLE



1- Physical weathering Physical or mechanical processes taking place on the Earth's surface, including the actions of water, frost, temperature changes, wind and ice; cause disintegration and wearing. The products are mainly coarse soils (silts, sands and gravels). Physical weathering produces Very Coarse soils and Gravels consisting of broken rock particles, but Sands and Silts will be mainly consists of mineral grains.

2- Chemical weathering

Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or alteration. The results of chemical weathering are generally fine soils with separate mineral grains, such as Clays and Clay-Silts. The type of clay mineral depends on the parent rock and on local drainage. Some minerals, such as quartz, are resistant to the chemical weathering and remain unchanged.

WEATHERING OF ROCKS



Sedimentary Soil: The individual particles were created at one location, transported, and finally deposited at another location.

◦ Sediment Formation: the formation of sediment is by the physical

and chemical weathering of rocks on the surface of the earth. Generally silt, sand, and gravel sized particles are formed by the physical weathering of rocks and clay-sized particles are formed by the chemical weathering of rocks.

◦ Sediment Transportation: sediments can be transported by any of five agents (they are water, air, ice, gravity, and organisms). The effects of transportation on sediments are Alters particle shape, size, and texture by abrasion, grinding, impact, solution.

Sorts the particles.◦ Sediment Deposition: the particles are deposited to form sedimentary

soil. The causes of deposition in water are: - velocity reduction, solubility decrease, and electrolyte increase.

SOIL FORMATIONS



Residual Soil:◦ Residual soil results when the products of rocks weathering are not transported as sediments but accumulate in place. If the rate of rock decomposition exceeds the rate of removal of the products of decomposition, an accumulation of residual soil results.

◦ The factors influencing the rate of weathering are: climatic (like temperature and rainfall), time, type of source rock, vegetation, drainage, and bacterial activity.

SOIL FORMATIONS



Fill:◦ Fill is man-made soil deposit. A fill is actually a “sedimentary” deposit for which man carried out all of the formation processes.

◦ The fill soil can be left as dumped (such as rock toe in the earth dam), or can be processed and densified – compacted – as for the core in the earth dam.

◦Borrow Soil: is the soil obtained from a source or made by blasting, transported by land vehicle (such as truck, scraper, pan or bulldozer) or water vehicle (barge) or pipe, and then deposited by dumping.

SOIL FORMATIONS




SOIL FORMATIONSFill – Soil Transported


Soil Structure refers to the orientation and distribution of particles in a soil mass (also called “fabric” and “architecture”) and the forces between adjacent soil particles.

This is limited to small, plate shaped particles, and to the orientation of individual particles. This includes bedding orientation, stratification, layer thickness, the occurrence of joints and fissures, the occurrence of voids, artifacts, tree roots and nodules, the presence of cementing or bonding agents between grains.

Structural features can have a major influence on in situ properties.◦ Vertical and horizontal permeability will be different in alternating layers of fine and coarse soils.

◦ The presence of fissures affects some aspects of strength. ◦ The presence of layers or lenses of different stiffness can affect stability.

◦ The presence of cementing or bonding influences strength and stiffness.

SOIL STRUCTURE



a- grained structures ,b. sheetlike structures, andc. composite structures

a. Single grained structure

b. Honeycomb structure

e. coarse grained skelton structure

f. cohesive matrix structure

SOIL STRUCTURE

c. flocculated structured. dispersed structure



The civil engineer working with soil must design his structure not only for the properties of the soil as it exists at the start of the project but also for the entire design life of the structure. He needs to know both the properties of the soil at the start of the project and how these properties will vary during the design life.

The factors influencing the changes in soil behavior are: stress, time, water, and environment.

ALTERATIONS OF SOIL AFTER FORMATION




DESCRIPTION OF THE INDIVIDUAL SOIL PARTICLE

COMPOSITION OF SOIL PARTICLES

APPEARANCE OF A SOIL

PARTICLEPARTICLE SIZE

SHAPE

OF GRAINS

DEGREE OF ROUNDNESS

, TEXTURE, AND

COLOR

FORCES ON SOIL PARTICLE



The nature and arrangement of the atoms in a soil particle is called “soil composition”. The composition have a significant influence on permeability, compressibility, strength, and stress transmission in soils, especially in fine-grained soils. There are three main groups of clay minerals:





kaolinites (include kaolinite, dickite and nacrite) formed by the decomposition of orthoclase feldspar (e.g. in granite); kaolin is the principal constituent in china clay and ball clay.

illites (include illite and glauconite) are the commonest clay minerals; formed by the decomposition of some micas and feldspars; predominant in marine clays and shales (e.g. London clay, Oxford clay).

montmorillonites (also called smectites or fullers' earth minerals) (include calcium and sodium montmorillonites, bentonite and vermiculite) formed by the alteration of basic igneous rocks containing silicates rich in Ca and Mg; weak linkage by cations (e.g. Na+, Ca++) results in high swelling/shrinking potential

The most abundant and important minerals are silicates. Over (90 %) of the weight would be silicate minerals .






Basic Structural UnitsIncluding: -1.Silicon Tetrahedron (1-Si and 4O)

2.Aluminum Octahedron (1-Al and 6OH)

3.Magnesium Octahedron (1-Mg and 6OH)


One Layer Sheetsuch as: Silica (4-Si + 10-O), Gibbsite (4-Al + 12-OH), and Brucite (6-Mg + 12-OH)

4-O

4-Si

6-O

Silica




Two Layer SheetSuch as: Serpentine (6-Mg + 8-OH + 4-Si +10-O) and Kaolinite (4-Al + 8-OH + 4-Si +10-O)

A number of sheets are stacked one on top of another to form an actual crystal, for example, the Kaolinite particle contains about (115) of the two-layers sheet units.

Three Layers SheetThe three layer sheet minerals are formed by placing one Silicon the top and on the bottom of either a Gibbsite or Brucite sheet. Such as Pyrophyllite and Muscovite.




FrameworksQuartz, a framework Silicate structure, has a very low ratio of oxygen to silicon. It is thus one of the most weather resistance minerals. The Feldspars have higher oxygen to silicon and can change through weathering into clay minerals, while these minerals sometimes occur in clay particles, they are most common in silt size and larger.




Unit of measurement of particle thickness is angstrom (Ao)

10 Ao = 1 mmm = 1000 mmmm = 1000 m




Particle size depends on the dimension that was recorded and how it was obtained. Two ways of determining particle size are: - sieve analysis for particle longer than (0.06) mm.

hydrometer analysis for smaller particles.

•APPEARANCE OF A SOIL PARTICLEDESCRIPTION OF THE INDIVIDUAL SOIL PARTICLE

PARTICLE SIZE



Very coarsesoils

BOULDERS >200 mm

COBBLES 60 - 200 mm

Coarsesoils

GRAVELCoarse 20 - 60 mmmedium 6 - 20 mmfine 2 - 6 mm

SAND

coarse 0.6 - 2.0 mmmedium 0.2 - 0.6 mm

fine 0.06 - 0.2 mm

Finesoils

SILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

CLAY <0.002 mm

•APPEARANCE OF A SOIL PARTICLETABLE (2): SHOWS DIFFERENT SIZES OF SOIL TYPES.



The majority of soils may be regarded as: -◦equidimensional: the particle size could be given by single number only as a cube or sphere. This situation for soil particles in the silt range and coarser.

◦Sheetlike: as in clay size particles.

•APPEARANCE OF A SOIL PARTICLESHAPE OF GRAINS



Rounded: Water- or air-worn; transported sediments Irregular: Irregular shape with round edges; glacial sediments (sometimes sub-divided into 'sub-rounded' and 'sub-angular') Angular: Flat faces and sharp edges; residual soils, grits Flaky: Thickness small compared to length/breadth; clays Elongated: Length larger than breadth/thickness; broken flagstone Flaky & Elongated: Length<Breadth<Thickness; broken schists and slates




The degree of roundness refers to the sharpness of the edges and corners of a particle.

Surface texture is minor feature of surface of a particle, independent of size, shape, or degree of roundness. The terms used to describe surface texture are dull or polished, smooth or rough, striated, frosted, etched, or pitted.

•APPEARANCE OF A SOIL PARTICLE

DEGREE OF ROUNDNESS, TEXTURE, AND COLOR:



Color is a useful particle characteristics to the geologist working in mining, but it is of little value to the soil engineer. The colors and their indications in the followings: -◦Red color: due to the presence of non-hydrated Fe2O3.◦Clear yellow and brown colors: indicates good drainage.◦Bluish grey colors: reduced or bivalent iron compounds.

◦Black and dark brown colors: are characteristic of organic matter.

◦White colors: due to preponderance of silica, lime, gypsum, and relatively pure clay deposits.

•APPEARANCE OF A SOIL PARTICLECOLOR



Specific surface is the ratio of surface area per unit wight. Surface forces are proportional to surface area (i.e. to d²). Self-weight forces are proportional to volume (i.e. to d³).

Mineral/Soil

Grain width,

d (mm)

Thickness

Specific Surface ,m²/N

Quartz grain 100 d 0.0023

Quartz sand 2.0 - 0.06 »d 0.0001 - 0.004Kaolinite 2.0 - 0.3 »0.2d 2Illite 2.0 - 0.2 »0.1d 8

Montmorillonite 1.0 - 0.01 »0.01d 80

•APPEARANCE OF A SOIL PARTICLESPECIFIC SURFACE



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FORCES ON SOIL PARTICLE

sadd

Fm R’ A’ Fw Fa Free Body Diagram

Air channel

d

water

mineral

Fm: the force when the contact is mineral – mineral.

Fa: the force where the contact is air – mineral or air- air.

Fw: the force where the contact is water – mineral or water – water.

R’: the electrical repulsion between the particles.

A’: the electrical attraction between the particles. SOIL MECHANICS LECTURES by Dr. Mohammed Sh. M. Al Shakerchy


SOIL MECHANICS-CH2-Soil Formation

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