<p>Clay-rich rocks such as marls and claystones are of great interest for many applications which include the storage of CO₂ or the storage of spent nuclear fuel. These rocks are either regarded as potential host rocks or as caprocks. As both CO₂ storage and a nuclear waste repository require a long-term stability of the rocks under changing stress conditions, Young’s modulus is a critical parameter as it describes the stiffness of the material. Up until now, Young’s modulus is regarded as a constant with large error bars in numerical models even though it depends on several parameters. In this study, a constitutive equation for Young’s modulus is suggested whose incorporation into numerical models can considerably reduce the uncertainty with which these models are fraught. In our triaxial experiments on Opalinus Clay and Passwang Marl, Young’s modulus depends not only linearly on the effective mean stress but also on the deformation the sample has experienced during the experiment. As a proxy for this deformation acts the true axial strain which is recorded during the experiments. Our results show that using the constitutive equation for Young’s modulus, which takes these dependencies into account, the uncertainties in numerical models can be reduced significantly.</p>