Soil Mechanics

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Transcript of Soil Mechanics

MECHANICS OF SOILSCOURSE INTRODUCTIONAccording to Ralph Pech: Soil Engineering is an Art Soil Mechanics is an Engineering Science Three Attributes of a Successful Soil Engineer: Knowledge of Precedents (Experience) Familiarity ith S il M h i F ili it with Soil Mechanics Working Knowledge of Geology Purpose of this Course: To Familiarize the Student with the Fundamental Principles of Soil Mechanics

The Solution of Soil Engineering Problem g g

Complicating Characteristics of Soil Deposits1) Soil does not possess a linear or unique stress-strain 2) 3) 4)

5)

relationship. Soil behavior depends on pressures, ti S il b h i d d time, and environment. d i t The soil at essentially every location is different. In I nearly all cases th mass of soil i l ll the f il involved i underground l d is d d and cannot be seen in its entirety but must be evaluated on the basis of small samples obtained from isolated locations locations. Most soils are very sensitive to disturbance from sampling, and thus the behavior measured by a laboratory test may be unlike that of the in situ soil. Nearly all soil problems are statically indeterminate to high degree.

Applications in Soil Engineering 1) Analysis and Design of Earth Structures such as Dams and Embankments 2) Stability of Artificial and Natural Slopes 3) Foundations Supports for Various Structures 4) Lateral Pressures against Various Structures 5) Prediction of Water Movement through the Soil 6) Improvement of Soil Properties by Chemical and Mechanical Methods

Geotechnical Materials 1) Soils are discrete particles derived from rock minerals and have extreme variability 2) Soils are cheap and readily available construction materials 3) Soils support all structures located above and below ground

GENERAL PROCEDURE FOR MOST GEOTECHNICAL PROJECTSDefine P j t C D fi Project Concept t

purpose, schedule, location, purpose schedule location plans review of information, site inspection Subsurface investigations, soil conditions, design p g parameters physical, analytical, numerical models evaluate various solutions cost, benefit, time, reliability, environmental impact, etc. plans and specifications for recommended solutions l ti inspection of construction operations revisions of plans due to new information fp f observe long range performance

Site Reconnaissance Working Hypothesis Model for Analysis Alternative Schemes Specific Recommendations S ifi R d ti Plans and Specifications p Supervision and Consultation Performance Feedback

Figure 1.1 Examples of Geotechnical Engineering Construction

Figure 1.2 Principles of Mechanics

Fig. 1.3 Branches of Mechanics used in Geotechnical Engineering

Fig. 1.4 Compression and Distortion

The Particulate Nature of SoilThe discrete particles that make up soil are not strongly bonded together in the way that the crystal of a metal are, and hence the soil particles are relatively free to move with respect to one another. p The soil particles are solid and cannot move relative to each other as easily as the elements in a fluid. y (Lambe and Whitman, 1979) It is this basic fact that distinguishes soil mechanics from solid mechanics and fluid mechanics.

Consequences of the particulate nature of soil t f il1st consequence: Nature of soil deformation q The deformation of a mass of soil is controlled by interactions between individual particles, especially by sliding (and also adhesion) between individual particles. B b t i di id l ti l Because sliding i a nonlinear and lidi is li d irreversible deformation, we must expect that the stress-strain behavior of soil will be strongly nonlinear and irreversible. {various constitutive irreversible. soil models} 2nd consequence: Role of pore phase -- Chemical interaction Soil is inherently multiphase, and the constituents of the pore phase will i fl h ill influence th nature of th mineral surfaces and h the t f the i l f d hence affect ff t the processes of force transmission at the particle contacts. This p { interaction between the phases is called chemical interaction. {double layer water; plasticity of soils; swelling potential, compression, strength, fluid conductivity}

Consequences of the particulate nature of soil ( t f il (contd) t3rd consequence: Role of pore p q p phase -- Physical interaction y Water can flow through soil and thus interact with the mineral skeleton, altering the magnitude of the forces at the contacts between particles and influencing the compression and shear resistance of the soil. {effective stress concept and consolidation theory} 4th consequence: Role of pore phase Sharing the load When changed, When the load applied to a soil is suddenly changed this change is carried jointly by the pore fluid and by the mineral skeleton. The change in pore pressure will cause water to move through the soil, hence the properties of the soil will change with soil time (hydrodynamic time lag). {basis of consolidation theory of Terzaghi, the father of soil mechanics. This marked the beginning f b i i of modern soil engineering} d il i i }

Soil Forming ProcessDefinition of Soil Soil - All materials, organic or inorganic, overlying bedrock

Soil are natural aggregates of mineral grains that can be separated by such gentle mechanical means as agitation i water, while rocks are natural aggregates of h i l it ti in t hil k t l t f minerals connected by strong and permanent cohesive forces. Based on Origin Inorganic Soil Organic Soil Inorganic Soils Residual Soil Transported Soil - located at a place where it was formed - the soil has been moved to another location by gravity, water or wind - derived from chemical and mechanical weathering - significant parts are derived from growth and decay of plant a d animal life and a a e

Alluvium river and stream deposits ( y heterogeneous mixture of g p (very g gravels, sands, and , , silts/clays) Lacustrine lake deposits; Marine salt water deposits (beach, swamps); Deltas deposits at mouth of streams and rivers Wind blown loess uniform mixture of silts, fine sands and clays

Nature of Soil

VOIDS

Soil is composed of particlesSOLIDS

Coarse-Grained Soils (large particles)

Volume >> Surface AreaFRICTION

Gravitational Force governs Behavior

(Sand/Gravel) Surface Area >> VolumeDOUBLE LAYER

Fine-Grained Soils (small particles)COHESION

clay

Surface Force (electrical) governs the behavior

clay (Clay/Silt)

Double layer expands repulsion Double layer contracts - attraction

Dispersive SoilReplace Na+ - Ca++, Mg++ + + + + Clay + + Na+ Farther repulsion + clay + + + + Clay Cl + + + + clay + + + + + +

Nearer attraction

Types of Soil

Types of Soil

Origin of Clay Minerals1) Inheritance. The clay mineral was formed by reactions

that occurred in another area, was transported to its present site, and is stable enough to remain inert in its present environment environment.2) Neoformation The clay has precipitated from solution Neoformation.

or has formed from reaction of amorphous material.3) Transformation. An inherited clay has undergone

chemical reaction. Two reactions are possible, namely, i exchange and l l ion h d layer t transformation. I f ti In layer transformation, the arrangements of octahedral, tetrahedral, tetrahedral or fixed interlayer cations are modified modified.

Importance of Soil Mineralogy in Geotechnical EngineeringIt is a controlling factor determining the sizes, shapes, and g g , p , surface characteristics of particles in the soil. It determines interactions with fluid phases. Together, these factors determine: Plasticity Swelling Compression Strength g Fluid conductivity behavior It is essential when dealing problems involving environmental g g problems, such as: safe disposal and containment of hazardous and nuclear wastes; clean up of contaminated sites; and protection of ground water Compositional characteristics of water. soils and their relation to the long-term physical and chemical properties are of most concerned.

STRUCTURE OF CLAY MINERALS

Two fundamental building laws can be noticed with clay minerals1) Silica tetrahedron unit 2) Octahedral Unit (with Al+3 or Mg+2)

(one tetrahedron shares 3 oxygens with other tetrahedrons) (each Si has one O-aton and shares 3 other oxygens SiO4 unit has neg. charge of 1

Al (OH)6-3 (each OH is shared by 2 Al-ions

Basic Structural Unit of Clay MineralBasic Silicate Unit: (1) Silicon Tetrahedron

Block Symbol =

Basic Silicate Unit: (2) Aluminum or Magnesium OctahedronBlock Symbol =

Gibbsite sheet: if cations are mainly Aluminum Brucite sheet: if cations are mainly Magnesium

KAOLINITE consists basically of repeating layers of one tetrahedral (silica) sheet and one octahedral (alumina or gibsite) sheet.

Successive layers are held together by hydrogen bonds between the hydroxyls of the octahedral sheet and the oxygen of the tetrahedral Since the hydrogen bond is very strong it prevents tetrahedral. strong, hydration and allows the layers to stack up to make up 70 to 100 layers thick. Halloysite is related to kaoline. It somehow became hydrated between layers causing distortions and random stacking of the crystal lattice so that it is tubular in shape The water can be easily shape. driven out from between layers by heating or air drying and the process is irreversible.

MONTMORILLONITE sometimes called smectite composed of 2 silica and one alumnica (gibsite) sheet

Because the bonding by van deer Waals forces ( g y (common attraction between matter) between the ) tops of the silica sheets is weak and there is a net negative charge deficiency in the octahedral sheet, water and exchangeable ions can e