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FACTORS CONTROLLING STRESS STRAIN BEHAVIOR OF SOIL
Prof. Samirsinh P Parmar (CE-14103277- Research Scholar)Department of Civil EngineeringIndian Institute of Technology, Kanpur, IndiaMail: [email protected]
Presentation on
ADVANCED GEOTECHNICAL ENGINEERING
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OVERVIEW OF PRESENTATION
Understanding soil Stress –strain models Introduction stress-strain in soil Suitability of models. Factors affecting stress strain in soils. Future scope References
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UNDERSTANDING SOIL
METAL SOIL WATER
Crystal Structure
Viscous FreeStructure
Particulate System,Particles Sometimes free to move, sometimes not
1 cm
1 cm
1 cm 5 Million Contacts between sand within 1cm3, Hence it is impossible to build a stress –strain law for each particle and the stress strain behavior of soil is resultant of this single contact behavior. - Lambe & Whiteman (pg.19)
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STRESS STRAIN MODELSSt
ress
,
Linear and Elastic
Stre
ss,
Strain,
Non-Linear and Elastic
Strain,
Ref: Bina Nusantara
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STRESS-STRAIN MODELS
Strain,
Stre
ss,
Elasto-Plastic
Stre
ss,
Strain,
Elastic Perfectly Plastic
Ref: Bina Nusantara
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Ref: Soil Physics 2010
Stress – strain curves IHooke’s Law:
For elastic materials:
L0
DL
F
Strain , DL/L0
Stre
ss
, F/A
Slope is E: Young’s modulus
E ≡ / , so
= / E
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7L0
DL
F
Strain , DL/L0
Stre
ss
, F/A
Ref: Soil Physics ,2010
Stress – strain curves IIFor many materials (including soil, sometimes):
Elastic
Plastic Failure: Material has sheared or fractured
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STRESSES
4 Stresses commonly encountered:• Compressive
• Tensile (Considered
Negligible)
• Shear t
• Torsion (not typical in soil)
o Kinds of behavior under stress- Elastic- Plastic- Viscous- Brittle
•Soils have aspects of all of these Ref: Soil Physics 2010
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Ref: Soil Physics 2010
Strain (elastic & plastic)
•Strain is deformation (e.g. in response to stress)
•Often given as ≡ DL / L0 : a relative change in length (dimensionless)
•Young’s modulus is the ratio: E ≡ / = L0 / DL
L0
DL
Original height
L0 – DL:Height after compression
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IDEALIZED STRESS-STRAIN RESPONSE It is impossible to take into account all aspects of
soil behaviour in a conceptual stress-strain response and some idealization is necessary as shown below:
Linear ElasticOY ’
Rigid Plastic
Elastic-Perfectly Plastic
Elastoplastic
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SELECTION OF AN APPROPRIATE MODEL
Stress
OA – Linear ElasticBC – Perfectly Plastic
Strain
Stability Calculations
Settlement Calculations
OA : a linear elastic model is adequate for settlement calculations.
BC : For stability calculations that involve taking the soil to failure, a perfectly plastic model is required.
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STRESS STRAIN RELATIONSHIP IN SOILS
(a) General Stress Strain Relationship (b) Typical Stress –strain Relationship for soil
Ref: Geotechnolgy- Robetrs, pg 29
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EFFECTIVE AND NEUTRAL STRESSES IN SATURATED SOILS
Total Stress
• It is having Physical Meaning.
• Measurable.• Measured by
Earth Pressure Cell
Effective Stress σ = Total Stress - Pore water Pressure (u).
Earth Pressure cell
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FACTORS CONTROLLING STRESS-STRAIN BEHAVIOUR OF SOILS (PAULOS1989)
1. Soil composition ( soil mineralogy)2. Structure3. Shape of particles4. State (Initial)5. Loading conditions6. Degree of saturation/ Pore Pressure7. Confinement
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1 . SOIL COMPOSITION It includes mineralogy, grain size and grain size
distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
Ref: Soil Science Society, USANot only clay mineralogy but mineralogy of sands is also important
More Cation Exchange Capacity, More it can take stresses
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1 . SOIL COMPOSITION Inter granular bonds are strong based on
cation exchange capacity. Also the adsorbed water in cohesive soil
under strong bonding. More inter granular force – More stress it can
take.
Water adsorption by hydrogen bond in soil minerals
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2. STRUCTURE
Refers to the arrangement of particles within the soil mass.
The manner the particles are packed or distributed. Features such as layers, joints, fissures, slickensides
, voids, pockets, cementation, etc., are part of the structure.
Structure of soils is described by terms such as: undisturbed, disturbed, remoulded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic.
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2.STRUCTURES OF SOILClay structures: dispersed (a)flocculated (b)Dispersed (edge-face)(c) Dispersed (aggregated)(d) and the natural structure of clay (after Craig, 1990)
Loose packing Dense packing Honeycombed fabric
Ref: Holtz and Kovacs, 1981
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2.STRUCTURES OF SOIL
Structure of soil decided the Inter particle Forces and hence the shear strength.
Large volume change occurs – strain increases under constant stress.
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3. SHAPE OF PARTICLES
Ref: AGCSA Australia
Shape of Sand Particles
Angular particles offer more frictional resistance
Highly Spherical particles rolls / slides from its place offers less frictional resistance
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4. STATE (INITIAL) It includes Normally Consolidated soil
Behavior and Over consolidated soil behavior.
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4. STRESS STRAIN BEHAVIOR : STATE
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4.STRESS STRAIN BEHAVIOR : STATE
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4.STRESS STRAIN BEHAVIOR : STATE
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5. LOADING CONDITIONS
Effective stress path, i.e., drained, and undrained; and type of loading, i.e., magnitude, rate (static, dynamic), and time history (monotonic, cyclic).
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6. DEGREE OF SATURATION/ PORE WATER PRESSURE
It Includes Permeability Pore water Pressure Total vs Effective
Stress Drained / Undrained
ConditionVolume change due to uniaxial stress application with zero excess pore water pressure.
(Note: V is the volume of the soilelement at any given value of .)
(Ref. B.M Das pg.153)
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CD TEST STRESS PATH
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CU TEST
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UU TEST
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DECREASED PORE PRESSURE
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INCREASED PORE PRESSURE
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7. CONFINEMENT / DENSITY
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8. CRITICAL VOID RATIO- CONFINING STRESS(AFTER CASSAGRANDE)
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8.CRITICAL VOID RATIO- CONFINING STRESS(AFTER CASSAGRANDE)
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9. PLAIN STRAIN CONDITION
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10.THIXOTROPY
o An isothermal, reversible, time-dependent increase in strength at a constant water content.
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COMMON TYPE OF STRESS STRAIN TESTS
Ref. Lambe, pg. 117
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ADDITIONAL FACTORS NOT CONSIDERED IN PRESENTATION Material isotropy Soil-water-air combined effect on stress-
strain. Stress strain behavior under dynamic loads.
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FUTURE SCOPE
Effect of cementation due to Chemicals. Nano-technology application to study stress-strain
effects. (Soil + water + micro organism) Constitutive
relationship ( Geo environmental issue) Geotechnical Structure Health Monitoring.
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REFERENCES
Soil Mechanics , SI version- Lambe & Whitman. Pg. 29- 241.
Geotechnology, A. Roberts. Pg. 17-65. Geotechnical Engineering , A Rao, C
Venkatramiah, pg. 60-62.
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41Question's ?
Thank You