Initiation and Development of Desiccation Cracks in BentoniteSand Mixtures as a Function of PhysicoC

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Initiation and Development of Desiccation Cracks
in Bentonite-Sand Mixtures as a Function of
Physico-Chemical Factors
Tenzin Lhundup Stony Brook University, NY
Markus Tuller Soil and Land Resources Division,
PSES Department
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Outline

• Introduction
• Objectives
• Materials and Methods
• Results
• Conclusions

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Clay Soils
Clays are secondary soil minerals formed
by decomposition (weathering) of primary minerals
Swelling Clays
Non-swelling Clays
e.g., Montmorillonite
e.g., Kaolinite
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Different Types of Landfill Liners
Benson, 2000
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Composite Clay Liner
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Compacted Clay Liner
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Shrinkage Cracks Preferential Flow and Transport
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Structural Damage to Roads and Buildings

• Besides hydraulic behavior, shrink-swell
  phenomena affect many mechanical, rheological and
  engineering properties of soils and clay liners.

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Montmorillonite (Na-Bentonite)

• 21 clay mineral - one octahedral sheet
  sandwiched between two tetrahedral sheets
• High surface area (600-800 m2/g)
• Large cation exchange capacity
• Many isomorphic substitutions Mg2, Fe2, Fe3
  for Al3 in octahedral site
• Very active shrink/swell behavior

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Swelling and Changes in Lamellar Spacing
H2O osmosis from bulk soil solution due to DDL
cations
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Characterization of Clay Pore Structures
Clay Soils Exhibit a Hierarchy of Pore Structures
MESOSCALE
MACROSCALE
MICROSCALE
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Objectives

• To investigate effects of solution chemistry,
  clay content, temperature on development and
  propagation of desiccation cracks.
• To determine effects of Physico-chemical factors
  on saturated permeability of Na-bentonite sand
  mixtures.

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Materials
Used Mixtures and Permeant Liquids

• Wyoming Na-bentonite Ottawa silica sand (F35)
  mixtures in mass ratios of 0/100, 10/90, 20/80,
  30/70, 40/60, and 50/50 were used for dehydration
  and saturated permeability
• 0.05M and 0.5M NaCl solutions were used as
  permeant soil solution.

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Permeability Testing
An automated Flexible Wall Permeameter is used to
measure saturated hydraulic conductivity of
clay-sand mixtures.
FLEXIBLE WALL PERMEAMETER
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Dehydration Experiment

• Observation of crack formation (macro-scale)

Sample at its maximum water ratio compacted in
acrylic container
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X-Ray Computed Tomography
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X-Ray Image Analysis
2D and 3D images
Filtering to reduce artifacts
Binarization
Qualitative and quantitative analysis
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Binarization

• Binarization method using MATLAB

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Permeability Results Ghebrehawariat, 2005
Clay Content and Permeability
Revil and Cathles, 1999
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Recent Results
Clay Content and Permeability for 0.05M NaCl
solution
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2-D Digital Images
Digital images for samples initially saturated
with 0.05M NaCl solution
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2-D Digital Images
Digital images for samples initially saturated
with 0.5M NaCl solution
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3-D Quantitative Analysis Crack Porosity
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3-D Quantitative Analysis Aperture Distributions
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Conclusions

• Samples with high bentonite content and low salt
  content showed high crack formations.
• Permeability is strongly affected by solution
  chemistry and samples permeated with deionized
  water had the highest swelling potential and
  lowest permeability followed by 0.05 M and 0.5 M
  solutions.

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Acknowledgments

• Markus Tuller
• Thomas Gebrenegus
• Fritz Fiedler
• Alecia Hoene
• Water Resources Research Experiences for
  Undergraduates Program at the University of Idaho
  

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Reference Benson, C. H. 2000. Liners and covers
for waste containment. Japanese Geotechnical
Society. Proc. Fourth Kansai Intl. Geotechnical
Forum. Kyoto. Japan.1-40. Revil, A., and Cathles,
L. M. 1999. Permeability of shaly sands. Water
Resources Research. 35651-662
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The End