6.6.2 Drained analysis，undrained analysis，and part...
Drained analysis refers to methods where the excess pore water pressure is assumed to be completely dissipated in analysis（i.e.ue＝0）and the soil goes through a volume change accordingly.Drained analyses are mainly applied to granular soils and the long⁃term behavior of clayey soils.Thus，effective stress analysis and effective stress parameters should be adopted.Undrained analysis refers to methods where the excess pore water is not dissipated at all in analysis（ue≠0）.If the soil is in the saturated state，the volume change would be none.
In the hyperbolic model，the undrained analysis for short⁃term behavior of saturated clay has to adopt the total stress analysis.That is，the analysis should adopt su and assumeφ＝0.The undrained analysis of unsaturated clay directly adopts the results of the triaxial UU test，cT andφT.For granular soils，it is only the effective stress analysis that has to be adopted.That is，c′andφ′are the required parameters.
In the elastoplastic model，the undrained analysis for short⁃term behavior of saturated clay can adopt either the effective stress analysis or the total stress analysis.If adopting the effective stress analysis，the yielding function has to be capable of representing the fact that the soil strength increases with the increase of the average effective stress（p′）.The yielding function can be the Mohr⁃Coulomb model（input parameters c′andφ′），the Drucker⁃Prager model，the Cam⁃clay model，etc.The input parameters are all the effective stress parameters and the stiffness of pore water also has to be fed.If adopting the total stress analysis，the yielding function should be capable of simulating the fact that the soil strength does not increase with the increase of the average total stress（p）.The yielding function can be the Mohr⁃Coulomb model（su，φ＝0），the von Mises model，the Tresca model，etc.The input parameters are all the total stress parameters.
In some cases，the behaviors of clay are neither completely drained nor perfectly undrained.Rather，they are in between.They are partially drained behaviors.The analysis of the partially drained behavior of clay can be carried out through undrained analysis，which takes advantage of the known excess pore water at each stage.To obtain the pore water pressure at a certain stage，one of the methods is to install a piezometer to measure the pore water pressure，which may partially dissipate at a construction stage.Then use the equation，σ′＝σ－u，to derive the effective stress of the soils.Given the effective stress of soils，according to the normalized behavior of soils（su／σ′v＝constant），the undrained shear strength of the soils can then be obtained.Then carry out an undrained analysis of the excavation for each stage to obtain the partially drained behavior of soils.Another method is to obtain the undrained shear strengths for each stage by means of laboratory or in situ shear strength tests first，which are then used for undrained analysis to analyze the partially drained behavior.