© 2015, TMMOB - Jeoloji Muhendisleri Odasi. All rights reserved.Gypsum precipitation related heaving generates important problems in engineering studies conducted in sulphate rich clay rocks. Formation of gypsum is generally related to the relaxation of previously existing tectonic structures due to stress relief or are related to the microcracks formed by the effect of circumferential stresses. Depending on the brittle failure principals, deformation zones are generated in underground rock structures which are excavated in sulphate rich clay rocks due to the stresses exceeding the crack initiation stress level. It is suggested that these microcracks in deformation zones are preferential pathways for gypsum precipitation. In this study, to introduce the failure mechanisms of sulphate rich clay rocks, unconfined and confined compressive strength tests, acoustic emission tests and high resolution strain measurements were conducted including microstructural and mineralogical analysis. The samples used in the study were obtained from Gipskeuper formation of Triassic age from Belchen tunnels located in the northern parts of Switzerland. The unit is typically composed of distinct clay layers and stiff anhydrite veins and/or nodules. During the studies, it is concluded that the failure processes in low deviatoric stresses are controlled by the clay matrix where the microcracks are initiated. Besides, with increasing deviatoric stresses or strain the propagating microcracks are hindered by stiff heterogeneous structures. Furthermore microcracks propagates along the boundary between the clay matrix and the stiff heterogeneous structure or penetrates the stiff heterogeneous structures (anhydrite veins). By evaluating the data obtained for larger scale, it is suggested that the stiff heterogeneous structures like anhydrite veins can limit crack propagation and prevent disintegration of rock mass structure. In this respect, even the rock mass is terminated after excessing the crack initiation stress level, the heterogeneous structure hinders sudden failure of the rock mass.