Determination of Effective Stresses and the Compressibility of Soil Using Different Codes of Practice and Soil Models in Finite Element Codes Available to Purchase

Creep or secondary compression plays an important role in the deformation behavior of soft soils like peat and organic clay. For the determination of long term settlements several methods have been proposed and implemented in codes for the engineering practice. However, some of the methods used have been developed empirically. In this paper the effective stresses and the compressibility of soils are addressed for some practical cases using the methods mentioned and soil models in finite element codes. The conceptual differences in methods and calculation results are compared. A stress-strain-creep strain rate model is proposed (Plaxis Soft Soil Creep model) in which strain rates are coupled with stresses and strains. The proposed material creep model is based on the modified Cam-clay model, which has been extended with a viscoplastic formulation using a single creep parameter μ^*. The compressibility parameters κ^*, λ^* and μ^* used within the model can be determined by standard one-dimensional consolidation tests. Applying the different methods, the calculated settlements show differences in the ratio between the primary and secondary compression as well as the distribution in time of the excess pore pressures. For large settlements the introduction of the natural strain, defined as the strain related to the actual height instead of the initial height of the soil layer, gives a further improvement of the model proposed (a-b-c model).