7.5 The Soft Soil Model
As for soft soils，we consider near⁃normally consolidated clays，clayey silts and peat.A special feature of such materials is their high degree of compressibility.This is best demonstrated by oedometer test data as reported for instance by Janbu（1985）in his Rankine lecture.Considering tangent stiffness moduli at a reference oedometer pressure of 100 kPa，he reported for normally consolidated clays，Eoed＝1 to 4 MPa，depending on the particular type of clay considered.The differences between these values and stiffnesses for Nc⁃sands are considerable as here we have values in the range of 10～50 MPa，at least for non⁃cemented laboratory samples.Hence，in oedometer test，normally consolidated clays behave ten times softer than normally consolidated sands.This illustrates the extreme compressibility of soft soils.
A feature of soft soils is the linear stress⁃dependency of soil stiffness.According to the Hardening Soil model we have
at least for c＝0 and－σ′3＝Knc0σ′1，a linear relationship is obtained for m＝1.Indeed，on using an exponent equal to unity，the above stiffness law reduces to
For this special case of m＝1，the Hardening Soil model yields
＝λ∗
，which can be integrated to obtain the well⁃known logarithmic compression lawε＝－λ∗ln－σ′1( )for primary oedometer loading.
For many practical soft⁃soil studies，the modified compression index will be known and user can compute the oedometer modulus from the relationship：
From the above considerations，it would seem that the Hardening Soil model is quite suitable for soft soils.Indeed，most soft soil problems can be analyzed by using this model，but the Hardening Soil model is not suitable for very soft soils with a high compressibility，i.e.E
／E
＜0.5.
For such soils，the Soft Soil model may be used.
Some features of the Soft Soil model are listed below：
•Stress dependent stiffness（logarithmic compression behavior）.
•Distinction between primary loading and unloading⁃reloading.
•Memory for pre⁃consolidation stress.
•Failure behavior according to the Mohr⁃Coulomb criterion.