ABSTRACT
Effective strength parameters φ’ and c’ cannot be measured in tests. They are derived from the linear equation defining the tangent to a point on the failure envelope and cannot be addressed individually. For years Danish Practice has been to assess the effective stress failure envelope from the linear stress path in undrained triaxial compression tests (CAUc), or alternatively assuming that the effective friction angle is a function of Plasticity Index (using φ’=A-B·logIP) in combination with effective cohesion as function of the undrained shear strength (c’=C·cu). The paper discusses the shortcomings of these practices for Danish clay tills and presents a model that ensures that paired values of φ’ and c’ can be consistently addressed from curved failure envelopes that are uniquely defined by void ratio and Plasticity Index.
