ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering - - PowerPoint PPT Presentation

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ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering - - PowerPoint PPT Presentation

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IIT Bombay ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering DR. D. N. SINGH dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns IIT Bombay Slide 1 3.11.2009 Lecture No. 23

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SLIDE 1

ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering

  • DR. D. N. SINGH

dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns

IIT Bombay

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SLIDE 2

3.11.2009 Lecture No. 23 Lecture Name: Swelling, Shrinkage and Cracking Characteristics of Soils

Sub-topics

  • Shrinkage Characteristics
  • Importance
  • Mechanism of Swelling
  • Swelling and Shrinkage
  • Factors Influencing Shrinkage
  • Mechanism of Shrinkage
  • Some Definitions
  • Determination of Shrinkage
  • Classification of Soils (Based on swelling, shrinkage and Suction)
  • Some Important Relationships
  • Cracking Characteristics

IIT Bombay Slide 1 Environmental Geomechanics Lecture No. 23 D N Singh

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SLIDE 3

Swelling soils drying Wetting Soils Non-Swelling soils Swelling Shrinkage Montmorillonite Bentonite IIlite Vermiculite Quartz Kaolinite Dolomite Chlorite Minerals

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 2

Importance

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SLIDE 4

Mechanisms of Swelling

Swelling Inner crystalline swelling Osmotic swelling Due to the hydration of exchangeable cations of the dry clay Due to the large difference in ion concentrations close to the clay Surfaces and in pore water

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 3

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SLIDE 5

Swelling and Shrinkage are due to change in distance between the clay platelets. Increase in distance during wetting is due to development of Wanderwall forces and exchangeable cations (Montmorillonite), Hydrogen bonds (Kaolinite), Potassium bonds(Illite) and reduction in distance during drying process. Wanderwall forces are weaker than both hydrogen, potassium bonds and Double layer formation due to exchangeable cations, which causes more swelling in Montmorillonite.

Swelling and Shrinkage

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 4

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SLIDE 6

IMPORTANCE of SHRINKAGE

Cracking of soils is a function of shrinkage, thermal processes, tensile strength and fracture. Most concrete cracks are due to shrinkage. Unfavorable environment to plant growth due to root zone shrinkage. Shrinkage cracks are responsible for land slides. Overall health of pavements. Shrinkage cracks appearing on turfs/pitches Paint coatings Pottery

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 5

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SLIDE 7

FACTORS INFLUENCING SHRINKAGE

Initial bulk density Clay content Organic (carbon) content Cation exchange capacity Mica-smectite content Liquid limit Presence of salts Initial water content pH REMIDIES

Soil-cement Reinforcement Vegetation

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 6

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SLIDE 8

As moisture content decreases, capillary stresses in voids increase due to the increased surface tension. This increased surface tension tends to pull adjacent soil particles closer together resulting in overall volume decrease. The reduction in moisture content is due to (a) evaporation of water from surface of the soil in dry climates (b) lowering of the ground water table (c) desiccation of soil by trees in humid climates

Mechanism of Shrinkage

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 7

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SLIDE 9

0 hrs 24 hrs 48 hrs 72 hrs 84 hrs

Response of Bentonite

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 8

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SLIDE 10

SOME DEFINITIONS

Shrinkage: the reduction in volume of the soil, due to change in moisture content. Shrinkage limit: the boundary between the solid and semi-solid states of consistency. Volumetric shrinkage: the reduction in volume of soil mass expressed as a percentage of its dry volume when the soil mass is dried from a water content above the shrinkage limit to shrinkage limit. Shrinkage ratio: the ratio of reduction in volume of soil mass expressed as percentage of its dry volume to the corresponding reduction in water content.

IIT Bombay Slide 9

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SLIDE 11

V1, W1 water Solids Ws Plastic state water Solids Shrinkage limit state (soil is fully saturated) Ww Ws V2 V2 Air Solids Dry state Ws Remolded soil sample is prepared with moisture content > LL Then allow air drying (so that cracks don’t appear) Volumes to be measured by mercury displacement

Shrinkage Limit Determination

SL = [(w1-ws)-γw(V1-V2)]/Ws = γw{V2-(Ws/G. γw)}/Ws

Shrinkage Ratio (R) = (Ws/V2). (γw)-1

G=(R-1-SL/100)-1

= Apparent specific gravity

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 10

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SLIDE 12

1 10 100 1000 10000 5 10 15 20 25 30

Montmorillonite Bentonite

V (cc) w (%)

The change in volume of the soil with respect to the moisture content

Sample % fraction % Ac Sand Silt Clay LL PL PI SL G BT 10 8 82 305 140 165 30 2.50 2.01 MT 7 21 72 434 166 268 30 2.72 3.72

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 11

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SLIDE 13

SHRINKAGE CURVE FOR SOILS

In a shrinkage curve the inverse of bulk density (soil specific volume) is plotted to volumetric water content θ of the soil. Volumetric water content

Inverse

  • f

specific volume e

Volumetric water content

Specific volume= (1+eo)

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 12

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SLIDE 14

5 10 15 20 25 30 35 40 10 20 30 40 50 60 70 80

VH1 H1 N1 M

1

L1 1

1.5 FSI=4 2

Vw(cc)

VK(cc)

(a)

Classification of Soils

20 40 60 80 100 120 140 160

20 40 60 80 100 120 140

(b)

A=0.73(wl-20)

EH1 VH1 H1 M

1

L1

PI (%) LL (%) Degree of Swelling L1: Low M1: Moderate N1: Non swelling H1: High VH1: Very high EH1: Extremely high Vw volume of the soil in water Vk volume of the soil in kerosene FSI = (Vw-Vk)/Vk×100

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 13

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SLIDE 15

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 5 10 15 20 25 30

VH1 H1 M1 L1

FSI (%) PI(%)

(c) (d) VH1 H1 M1 L1

SL (%) PI (%)

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 14

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SLIDE 16

10 20 30 40 50 60 5 10 15 20 25 30

  • 5
  • 10
  • 15
  • 20
  • 25
  • 30

(f) (e) H1 M1 L1

SL (%) PI (%)

VH1 H1 N1 M1 L1

∆log PF/∆w

PF: soil suction

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 15

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SLIDE 17

20 40 60 80 100 10

  • 2

10

  • 1

10 10

1

10

2

10

3

10

4

10

5

20 40 60 80 100 10

  • 2

10

  • 1

10 10

1

10

2

10

3

10

4

10

5

C

B

R

2=0.25

AEV (kPa) CL (%) R

2=0.49

AEV=a⋅CL

b

(a)

A

D

(b)

R

2=0.64

AEV=c+a⋅F

b

R

2=0.71

F (%)

=30 %

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 16

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SLIDE 18

10

  • 2

10

  • 1

10 10

1

10

2

10

3

10

  • 2

10 10

2

10

4

10

  • 2

10

  • 1

10 10

1

10

2

10

3

log(AEV)=0.8+1.3⋅logS

R

2=0.62

AEV (kPa) S (%)

=0.02

log(AEV)=1.06+0.76⋅ logSp R

2=0.71

Sp(kPa)

=0.25

S: Percentage swelling Sp: Swelling pressure

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 17

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SLIDE 19

10

  • 2

10

  • 1

10 10

1

10

2

10

3

10

  • 2

10 10

2

10

4

10 20 30 40 50

log(AEV)=1.13+0.6⋅ log FSI

R

2=0.79

FSI (%)

=0.9

AEV (kPa)

log(AEV)=2.82-0.075⋅SL

R

2=0.67

SL (%)

Environmental Geomechanics Lecture No. 23 D N Singh IIT Bombay Slide 18