There exists disunity

even wrong phenomenon in geotechnical community when determining and choosing strength index. The detailed several aspects are shown as follows: (1) the classification of strength test; (2) the measurement of undrained strength; (3) the strength index determination and choice for loading project; (4) the strength index determination and choice for unloading project. In above mentioned several aspects

it’s pointed out that unconsolidated-undrained test is fit to be named “natural consolidated-undrained test”. All kinds of methods measuring natural foundation soil strength are discussed and compared. Theoretical analysis of modified effective stress method and effective consolidation stress method created by Chinese scientists is made; and the results indicate that both methods conservatively estimate in-situ strength. The correspondent confining pressure formula of isotopic consolidation to model K0 consolidated state and the strength increase formula by effective consolidation stress method adopting CIU strength index equivalent to CK0U strength index are derived. The “undrained” effective stress analysis

QRS method

undrained strength analysis method are introduced. It’s pointed out that SHANSEP method adopted by undrained strength analysis can well model strength increase of foundation soil during construction stage and is fit for China to adopt. The application occasions of loading -unloading - loading way to get the unloading strength index commonly used in China are analyzed.