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

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

27.10.2009 Lecture No. 22 Lecture Name: Swelling, Shrinkage and Cracking Characteristics of Soils

Sub-topics

• Swelling Characteristics
• Intrinsic expansive behaviour
• Potential expansive behaviour
• Swelling: A concern
• Swelling: A boon
• Swelling and Soil Suction
• Mechanism of swelling
• Methodologies for Identification and Determination of Swelling

Soils

• Classical methodologies
• Neo-methodologies
• Determination of Swelling Potential of Soils
• A Novel Technique to determine Swelling Characteristics of

Soils

• Shrinkage Characteristics
• Cracking Characteristics

IIT Bombay Slide 1 Environmental Geomechanics Lecture No. 22 D N Singh

The volume change due to “in-take” or “taking up” of water Exhibited by clays and fine silts Such soils are termed as “expansive soils” They shrink, as moisture content decreases and swell with increase in it Swelling caused due to Presence of swelling clay minerals (montmorillonite) in soils Exposure to water Results in Damage to structures founded on/in/over/with it

Swelling characteristics of Soils

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 2

The property by virtue of which soil holds considerable amount of adsorbed and double layer water. Determined by: The mineralogy of the soil Pore-fluid chemistry The degree of aggregation of the particle The particle size and shape however are the fundamental variables. Indicators of intrinsic expansiveness: Liquid limit Clay Fraction

Cs = ∆ e / log σ′

Intrinsic Expansive Behavior

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 3

Defined as the pressure exerted against an unyielding support due to swelling. Determined by both intrinsic expansiveness and void ratio. For a soil of low void ratio, if external agent is removed, soil asserts potential expansiveness Na- Montmorillonite exhibits high intrinsic expansiveness and very high potential expansiveness Kaolinite exhibits low intrinsic expansiveness and potential expansiveness.

Potential expansive Behavior

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 4

Swelling: A Concern

Swelling of expansive soils poses a major problem with respect to serviceability performance of light weight structures on shallow footings This occurs due to Moisture near ground fluctuates as a result of rainfall, watering of gardens Seasonal change in rainfall

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 5

Swelling: A Boon

Expansive soils are attracting greater attention as buffer and backfill materials for repository sites of high-level nuclear wastes. The function of the buffer materials is to create an impermeable zone around containers (self sealing) Buffer materials like bentonite and sand bentonite mixtures are used together A good buffer must exhibit swelling properties (and hence development

• f impermeability in the backfill)

Choice of Bentonite: Swelling characteristics of compacted bentonite and sand-bentonite mixture need to be established Estimation of the swell pressure resulting from swelling on the structures of the disposal sites. Need to establish Hydro-thermo-mechanical properties

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 6

As soil dries, the suction increases and soil shrinks As soil wets, the soil suction decreases and soil swells Soil suction = f (moisture conditions) depending on environmental factors Changes in soil suction are quite significant on the surface, where maximum soil moisture fluctuations occur. Development of “A Novel Technique” for establishing Swelling Characteristics of Expansive Soils

Swelling and Soil Suction

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 7

Mechanisms of swelling

Swelling pressure Reduction in the potential energy of the interlayer water Swelling Buildup of pressure in the trapped air of an advancing wetting front Clay swelling Balancing of forces of interaction among clay surface, ions, and water Swelling pressure osmotic pressure created by the cations Relates to the double layer theory of clay expansion

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 8

• Differential swell test
• Free swell test

Neo-Methodologies (Methods resorting to Advanced Instrumentation

• Differential thermal analysis
• X-ray diffraction method
• Electron microscopy

Methodologies (Classical) for Identification of Swelling Soils

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 9

Determination of Swelling Potential of Soils

Most of these tests are Too simple Time consuming and cumbersome Empirical & soil specific

Need for rapid and easy determination of Swelling properties/Swelling Pressure

Direct Methods

• Free swell test
• Expansion Index
• CBR Test
• Potential Volume Change Test
• Coefficient of Linear Extensibility (COLE) test

Indirect Methods Estimated from

• Plasticity Index
• Activity Ratio
• Cation Exchange Capacity

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 10

A New Method - Using Suction Measurement

Unsaturated soil Creates negative pressure or Suction (Soil Water Air) A Three Phase system- Suction State of unsaturation Higher tendency to suck water and swell Suction Swelling Pressure

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 11

Physical properties of the Soils used Property Bentonite (B) Montmorillonite (M) Color Yellowish brown Dark brown Specific Gravity 2.76 2.78 Liquid limit (%) 227 411 Plastic limit (%) 65 71 Plasticity Index (%) 162 340 Free swell Index (%) 264 410 Specific surface area (m2/g) (@150°C for 60 min.) 48 310 An oedometer is used for determining swelling pressure Ps as per the guidelines presented by ASTM D 4546

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 12

Mineral Sample w (%) Bentonite B1 13.6 B2 30.1 B3 41.3 B4 60.5 B5 69.9 Montmorillonite M1 11.4 M2 28.8 M3 55.3 M4 72.3 M5 99.8

Dry unit weight =12 kN/m3

The Methodology

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 13

Porous plate Soil sample Filter paper Acrylic cylinder Base plate Guide rods Water bath LVDT Load frame Loading arrangement Drainage Oedometer top and bottom plates Oedometer ring

1000 2000 3000 4000 5000 5 10 15 20 25 30

B1 B2 B3 B4 B5

(%) Swelling

1000 2000 3000 4000 5000

M1 M2 M3 M4 M5

Time (min.) Free swelling response of various samples

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 14

50 100 150 200 250 300

• 10
• 5

5 10 15 20 25 30 100 200 300 400 500 600 B1 B2 B3 B4 B5

(%) Compression Pa (kPa)

M1 M2 M3 M4 M5

Compression characteristics of various samples

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 15

0.0 0.2 0.4 0.6 0.8 1.0 1.2 10 10

1

10

2

10

3

10

4

10

5

10

6

Bentonite Montmorillonite

ψ (kPa) w

Variation of suction with molding water content

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 16

Mineral Sample w (%) PS (kPa) Ψ (kPa) Bentonite B1 13.6 194.26 46000 B2 30.1 114.80 9100 B3 41.3 99.22 3400 B4 60.5 44.69 410 B5 69.9 27.69 150 Montmorillonite M1 11.4 454.17 83000 M2 28.8 369.26 26000 M3 55.3 201.77 4070 M4 72.3 169.83 1500 M5 99.8 113.73 330

Results from the study

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 17

10

1

10

2

10

3

10

4

10

5

100 200 300 400 500 600 700 800 900 1000

10 12.5 15 22.5 25 A=30

Ps= A(ψ)

0.25

Bentonite (Present study) Montmorillonite (Present study) Garbulewski and Zakowicz (1995)

Ps (kPa) ψ (kPa)

Relationship between swelling pressure and suction

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 18

Inference from the Study

Knowing Suction- Swelling potential can be determined easily and rapidly Higher suction Lesser water content Higher swelling pressure

Challenges Ahead

Physico-chemico-mineralogical characterization

• f

various clay minerals should be taken up Generalize the influence of each mineral

• n the overall

swelling characteristics of the soil

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 19

Effect of valence of cations on the swelling of Na-Bentonite

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 20

Effect of concentration on the swelling of Na- Bentonite

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 21

Effect of hydrated ion size on the swelling of Na-Bentonite

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 22

Effect of pH on Swelling of Na-Bentonite

Environmental Geomechanics Lecture No. 22 D N Singh IIT Bombay Slide 23