Strengthening and stress relaxation of Opalinus Clay

► Stress relaxation testing to identify non-dilatant long-term deformation behaviour. ► To suppress artefacts several loading cycles were performed for pre-compaction. ► After pre-compaction, stress relaxation does not depend on texture or draining. ► Stress-strain-behaviour is discussed on basis of common constitutive equations. ► Non-dilatant long-term stress relaxation converges to a non-zero stress threshold.