ABTRACT: Gypsum precipitation related heaving generates important problems in engineering studies conductedin sulphate rich clay rocks. Formation of gypsum is generally related to the relaxation of previouslyexisting tectonic structures due to stress relief or are related to the microcracks formed by the effect ofcircumferential stresses. Depending on the brittle failure principals, deformation zones are generated inunderground rock structures which are excavated in sulphate rich clay rocks due to the stresses exceedingthe crack initiation stress level. It is suggested that these microcracks in deformation zones are preferentialpathways for gypsum precipitation. In this study, to introduce the failure mechanisms of sulphate rich clayrocks, unconfined and confined compressive strength tests, acoustic emission tests and high resolutionstrain measurements were conducted including microstructural and mineralogical analysis. The samplesused in the study were obtained from Gipskeuper formation of Triassic age from Belchen tunnelslocated in the northern parts of Switzerland. The unit is typically composed of distinct clay layers andstiff anhydrite veins and/or nodules. During the studies, it is concluded that the failure processes in lowdeviatoric stresses are controlled by the clay matrix where the microcracks are initiated. Besides, withincreasing deviatoric stresses or strain the propagating microcracks are hindered by stiff heterogeneousstructures. Furthermore microcracks propagates along the boundary between the clay matrix and the stiffheterogeneous structure or penetrates the stiff heterogeneous structures (anhydrite veins). By evaluatingthe data obtained for larger scale, it is suggested that the stiff heterogeneous structures like anhydriteveins can limit crack propagation and prevent disintegration of rock mass structure. In this respect, eventhe rock mass is terminated after excessing the crack initiation stress level, the heterogeneous structurehinders sudden failure of the rock mass