Recovery of Porosity and Permeability for High Plasticity Clays

Stress history is normally evaluated based on an evaluation of geological history and if possible combined with oedometer tests where preconsolidation stress σp and compaction properties are measured. Often the constrained modulus M is used to define the stiffness, and the value will typically depend on the stress state of the sample. The stress state of a sample is typically defined by overconsolidation ratio OCR defined as the ratio between vertical preconsolidation stress and actual vertical stress (OCR = σp/σ0). Janbu (1963), Maine & Kulhawy (1982) and others have published typical relations that describe stress state and constrained modulus as functions of OCR, plasticity index and other quantities. They are all based on the assumption that the soil sample “remembers” previous load levels, and that stiffness of the sample is consequently increased due to the preloading. Definition and applicability of the reconsolidation stress is now questioned when high plasticity calys are considered. Even if it is known, that a clay layer has been exposed to a high stress level, for instance due to the weight of an ice shield, the actual stress level according to that preloading might not be the relevant parameter when estimating stiffness properties for use in basin modeling or other models.