An elasto-plastic description of two distinct volume change mechanisms of soils

In critical state soil mechanics, the volume change behavior of soils has typically been analyzed based upon conventional e- log p' relationships. However, compaction/densification of loose sand, for example, can sometimes occur even without any significant increase of mean effective stresses. This study presents a model which considers the fact that volume change can occur due to decay/collapse of the structure of soils. Taking into consideration the differences between clay and sand, this study models super-subloading surfaces together with rotational hardening using the modified Cam-clay model. The effects of decay of the soil structure, loss of overconsolidation and evolution of anisotropy are mutually discussed concerning their relationship with ongoing plastic deformation. Fundamental constitutive model responses are illustrated in the present study particularly for compaction of sand. Repeated application of low-level shear stress upon loose sand yields a huge amount of volume compression, which is due to the rapid collapse of the initial soil structure. Repetition of the loading also results in a rapid increase of the overconsolidation ratio. Drained and undrained shear behavior of the sand naturally changes remarkably along this densification/compaction procedure, which is also consistently predicted using a single set of soil parameters. Elasto-plastic behavior of the same sand at various densities is thus totally described in the present study in a single theoretical framework based on soil parameters independent of density.