The characteristics of expansive and collapsible soils, and considerations for foundation design and construction.

Expansive Soils

Definition

Expansive soils refer to soils or unconsolidated mudstones containning a significant amount of montmorillonite caly materials, These soils experience expansion when water is absorbed, or develop swelling pressure when constrained. 

Identification Method

Expansive Potential (EP) = 0.003 x Z x Sw > 0.5 (Expansive soils)

  • Z : Depth of the expansive layer
  • Sw = (Height of expansion due to water absorption (∆H)) / (Height of the sample(H)) x 100%

Characteristics

  • The Soil exhibits significant expansion upon moisture absorption. 
  • When constrained, the soil develops swelling pressure. 
  • The soil is highly compressible when dry. 
  • High plasticity soil 
  • Liquid Limit (LL) ≥ 40
  • Plasticity Index (PI) ≥ 15
  • These soils are geotechnically very unstable

Consideration for Foundation Design and Construction

  • Magnitude of swelling pressure:

Place the sample in a consolidation apparatus and supply water. The maximum load at which no sample expansion occurs is the swelling pressure. 

  • Thickness of Expansion Soil and Swelling Heigh:

Ensure the stability of connecting structures. 


Collapsible Soils

Definition

Collapsible soils are soils, such as loess and volcanic ash deposits, that undergo a significant reduction in volume upon saturation, even without an increase in external load. 

Identification Method

Collapse Potential (Cp) = (e1-e2)/(1+e0) x100% > 5 (Collapsible Soils)

  • e1 : Initial void ratio
  • e2 : Final void ratio after saturation
  • e0 : Initial void ratio

Characteristics

  • The unit weight (r) is low, leading to low effective stress (σ'). 
  • The high void ratio (e) results in high compressibility. 
  • Vertical cracks can occur, leading to high vertical permeability. 
  • Under Unsaturated conditions, the soil exhibits cohesion (C) and drained strength. 
  • Upon Saturation, shear strength decreases rapidly, resulting in excessive settlement. 
    •     Unsaturated State : S = c + σ'tan∅
    •     Saturated State : S = (σ - 𝒰)tan∅

Considerations for Foundation Design and Construction

  • Settlement upon Saturation

Place the sample in a consolidation apparatus and apply the unit load of the structure. 

Saturate the sample to calculate the change in void ratio(∆e). 

Calculate settlement (S) considering the thickness of the collapsible soile (Z)

    •     Settlement (S) = ∆e/1(1+e0) x Z

  • Thickness of collapsible soil : 

    • If distributed within 2 meters of the surface : Use optimum moisture content (OMC) compaction or compensatory foundation. 
    • If distributed within 10 meters of the surface: Implement ground improvement techniques. 
    • If distributed deeper than 10 meters: Install deep foundations. 
    • Deep foundations may induce negative skin friction

Conclusion

For shallow foundation design, expansive and collapsible soils, which are problematic grounds, must be thoroughly investigated using geological maps and site surveys. Understanding the soil characteristics is crucial to designing a safe structure foundation, and appropriated measures must be taken to ensure stability. 




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