Modelling virgin compression line of compacted unsaturated soils

  • Zhong Han
  • Sai K. Vanapalli
  • Wei-lie ZouEmail author
  • Xie-qun Wang
  • Jun-feng Zhang
Research Paper


In this paper, the volumetric collapse of an unsaturated soil, upon soaking to saturation under a certain stress level, is referred to as soaking collapse. The soaking collapse for a soil under virgin condition is assumed equal to the difference between the soil’s virgin compression line (VCL) and its normal consolidation line (NCL) at saturated condition based on results of oedometric constant water content compression and soaking tests performed on a compacted Nanyang expansive clay. A one-parameter model is proposed to describe the variation of the soaking collapse with the pre-soaking degree of saturation under virgin condition once (1) a soil’s yielding point can be clearly defined and (2) the degree of saturation during virgin compression is known. This model can be used to predict the VCL from the NCL. Data of the Nanyang expansive clay, along with published data of nine soils that were derived from constant water content or constant suction compression tests (including oedometric and triaxial compression), were used to calibrate and validate the model. It is shown that the model, when combined with a degree of saturation-volume model and the information of yielding and NCL, can predict reasonable VCLs for all examined soils and suitably capture several characteristics of the VCL including the nonlinearity and pressurised saturation. A constant model parameter of 1.5 is found suitable for all examined soils. The model is also used to predict the VCLs of two compacted soils from the constant degree of saturation compression, which is a recently developed testing technique to evaluate the compression behaviour of unsaturated soils, with reasonable agreement achieved.


Compacted soil Partial saturation Virgin compression Volumetric behaviour 

List of symbols


Specific gravity


Plasticity index


Specific volume on the NCL at lnσ′ = 0


Net mean stress


Coefficient of determination




Degree of saturation


Degree of saturation at saturation


Degree of saturation at yielding


Specific volume


Soaking collapse


Specific volume at pressurized saturation


Specific volume at saturation


Water content


Liquid limit


Dry unit weight


Maximum dry unit weight


Model parameter


Recompression index


Compression index for unsaturated soil


Compression index for saturated soil


Collapse ratio


Collapse ratio at yielding


Effective stress (for saturated soil) or net stress (for unsaturated soil)


Net stress at pressurized saturation


Net vertical stress

ψ, ϕ, n, m

Model parameters of Gallipoli et al. [14] model



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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Zhong Han
    • 1
    • 2
  • Sai K. Vanapalli
    • 2
  • Wei-lie Zou
    • 1
    • 3
    Email author
  • Xie-qun Wang
    • 4
  • Jun-feng Zhang
    • 5
  1. 1.School of Civil EngineeringWuhan UniversityWuhanChina
  2. 2.Department of Civil EngineeringUniversity of OttawaOttawaCanada
  3. 3.School of Civil EngineeringXijing UniversityXi’anChina
  4. 4.School of Civil Engineering and ArchitectureWuhan University of TechnologyWuhanChina
  5. 5.Country Garden Holdings Co., LtdFoshanChina

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