Foundations on Collapsible Soils

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Foundations on Collapsible Soils

• Prof. Jie Han, Ph.D., PE
• The University of Kansas

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Outline of Presentation

• Introduction
• Basics and Evaluation
• Preventive Measures

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Collapsible Soils

• In arid and semiarid areas
• Strong if dry but collapse (quickly consolidate)
  if wet
• Sometimes called metastable soils
• Process of collapse hydroconsolidation,
• hydrocompression or hydrocollapse
• Predominantly sand and silt size particles in a
  loose
• honeycomb structure
• Water-softening cementing agents hold together
  the
• loose structure

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Driveway Drop 8 due to Collapse of Collapsible
soil
J White
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Microstructures
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Collapse of Microstructure
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Map of Collapsible Soils
KS
Dudley (1970)
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Type of Collapsible Soils

• Collapsible alluvial soils (transported by
  water) and
• colluvial soils (transported by gravity)
• Collapsible aeolian soils (transported by wind)
  sand
• dunes, (most common) loess, volcanic dust
  deposits
• Collapsible residual soils (formed in-place by
  the
• weathering of rock

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Loess in Kansas City
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Double Oedometer Tests
Insitu moisture content
?w
Strain ?
Saturated
log?
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Single Oedometer Tests
Loading (dry)
Strain ?
Wetting at constant load
?w
Loading (wet)
log?
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Example of Soil Collapse Tests
Saturated _at_ 200kPa
Dry
Wet
Crushed sand/ kaolinite 85/15
Miller et al.
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Classification of Soil Collapsibility
Potential hydrocollapse strain, ?w
Severity of Problem
0 0.01
No problem
Moderate trouble
0.01 0.05
Trouble
0.05 0.10
Severe trouble
0.10 0.20
Very severe trouble
gt 0.20
Jennings and Knight (1975)
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Hydrocollapse Strain and Normal Stress
Sudden collapse
Dry
Dry
Strain, ?
Strain, ?
Wet
Wet
log?
log?
Most collapsible soil
Loess
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Potential Hydrocollapse Strain
?w potential hydrocollapse strain
Cw hydroconsolidation coefficient
?t threshold collapse stress
?t normal stress at which hydroconsolidation
occurs
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Wetting Coefficient
Wetting Coefficient, ?
Degree of Saturation, S
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Settlement Computation
?w settlement due to hydroconsolidation
? wetting coefficient
Cw hydrocollapse coefficient
?z vertical total stress
?t threshold stress
z depth
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Preventive Measures

• Removal of the collapsible soil
• Avoidance or minimization of wetting
• Transfer of load through the collapsible soils
  to the
• stable soils below
• Injection of chemical stabilizers or grout
• Prewetting
• Compaction with rollers or vehiles
• Compaction with displacement piles
• Compaction by heavy tamping
• Vibroflotation
• Deep blasting combined with prewetting
• Controlled wetting
• Design structure to be tolerant of differential
  settlements

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Heavy Tamping/Dynamic Compaction
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Vibroflotation
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Grouting
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Transferring Structural Loads through Collapsible
Soils
Coduto (2001)