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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: samirddu@gmail.com

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

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.

41Question's ?

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