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Soil Properties - I

Amit Prashant

Indian Institute of Technology Gandhinagar

Short Course on

Geotechnical Aspects of Earthquake Engineering 04 – 08 March, 2013

Regional Soil Deposits of India

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Alluvial deposits

Marine deposits

Desert Soils

Laterites and lateritic soils

Black cotton soils

Boulder deposits

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Soil: A Complex Material

Particulate Media

Three Phase Relationship

Volume Change Characteristics

Principal of Effective Stress

Pressure Dependency

Transient Response

Highly Non-linear Response

Spatial Variability

Others

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Particle Shapes and Sizes

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Rounded Subrounded

Subangular Angular

Sands and Gravels Particle Size > 75 mm 17 mm

2 mm

Clays Size > 2 mm

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Soil Groups Based on its Particle Size

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Fine grain soils

Coarse grain soils

0.002 300 80 4.75 0.075

Grain size (mm)

Boulder Clay Silt Sand Gravel Cobble

Granular soils or Cohesion less soils

Cohesive soils

Non-Clay minerals Clay minerals

0.425 2.0

Fine Medium Course Fine Course

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Grain Size Distribution

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Poorly Graded

Well Graded

Gap Graded

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Three Phases in Soils

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S : Solid Soil particle

W: Liquid Water (electrolytes)

A: Air Air

v

s

Ve

VVoid ratio,

Air

Water

Solid

ra

rw

rs

Particle Assemblage: Void Ratio

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5

Relative Density

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max

max min

r

e eD

e e

Void ratio, e

1.0

0.8

0.6

0.4

0.2

0

emax

Dr = 0%

e

0%<Dr <100%

emin

Dr = 100%

Atterberg Limits

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The presence of water in fine-grained soils can significantly affect associated engineering behavior, so we need a reference index to clarify the effects.

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Typical Values of Atterberg Limits

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(From Mitchell, 1993)

Plasticity Chart

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H L M

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Sensitivity

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strengthshearUnconfined

disturbedStrength

dundisturbeStrengthSt

)(

)(

Clay particle

Water

w > LL

Thixotropy

Loses strength when remolded; Gains strength while at rest

Structure re-adjusts when left undisturbed

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R

A

S

SRatioStrength cThixotropi

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F

F

Displacement

Ax

ial

Fo

rce

, F

k

1

Time

De

form

ati

on

, F

Time

Ax

ial

Fo

rce

, F

Compressibility and Time Effects

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Displacement

Sh

ea

r F

orc

e, T

f

W

F

N

Tf

W

F

N Tf

.fT Nm

. tann

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Model of Contact Friction: Rigid Plastic

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F sh

w

s

wh

F

. .s s w w sN h h h .s sN h

sh w

s

wh

.F Nm .F Nm

Area = 1

Total Stress

Effective Stress

Water Pressure

.w wu h

N

Area

N

Area

.F Nm

. tan

.tanu

. .s s w sN h h

.w wN N h

. .s s w wN h h

Effective stress

Effective Stress Profile in Soil Deposit

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10

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Example

Determine the effective stress distribution with depth if the water table is 2 m below ground surface

evaluate distribution

of total head with depth

subtract elevation head from total head to yield pressure head

calculate distribution with depth of vertical “total stress”

subtract pore pressure (=pressure head x γw) from total stress

Steps in solving effective stress problems: 38

190

233

80

100

110

133

. tann

n

Failure Criteria for soils

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11

N

N

F

F Glue

Particle Friction

N

N

F

F Glue

Particle Interlocking

sParticle-to-Particle Friction angle

Increase in Friction angle due to Interlocking Effect Non-linearity due to

reduced interlocking with increase in normal stress

Interlocking Effect

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N

N

F

F Glue

c

. tann

.tannc

Cohesion and Cementation Effect

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Relative Density, max

max min

r

e eD

e e

Increase

in Dr

Effect of density or void ratio

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Overconsolidation Ratio, vo

v

OCR

Maximum overburden stress ever induced

Increase in OCR

For Peak Shear Strength as Failure

OCR constant for each line

For varying OCR but constant

vo

Effect of Pre-compression history

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13

Initially loose configuration Contractive

Dilative Initially loose configuration

Increase in Pore Water Pressure

Drained Shearing

Slow Loading

Undrained Shearing

Fast Loading

Decrease in Pore Water Pressure

Volume Change or Evolution of Pore Water Pressure During Shearing

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Stress strain behavior of soils

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Drained / Slow Loading Undrained / Fast Loading

q

q

qV

V

ve

ve

'p

q

Total and effective Stress Path

q

q

qu

ve

ve

'p

q

Contraction

Dilation

Effective Stress Path

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Stress Strain Curve for soils

q

q

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2Gmax

2G

q

q

First yield point

Elastic zone

Peak Shear Strength

Strain hardening zone

Softening zone Zone of instability

Steady State Shear Strength

Gmax = Maximum Shear Modulus

G = Secant Shear Modulus

Strain Dependent Shear Modulus

Linear Elastic Model

Nonlinear Elastic Model

%

Include Plasticity

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15

Oil Tank

Flood Flood

Transient Effects on structures

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16 m 18 m

10 m

1

2

10 m

Under Filter

1

2

b = 18 kN/m3; sat = 20 kN/m3 ' = 18º (CU test) Top line of water seepage

Transient Effects on structures

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1

2

3

4

5

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Bore Holes

Spatial Variability of soil strata

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Thank You