Analysis of the failure mechanisms of a weak rock through photogrammetrical measurements A. M. Ferrero 1 , M. Migliazza 1 , R. Roncella 1 and G. Tebaldi 1 1 Dept of Civil Engineering - University of Parma, annamaria.ferrero@unipr.it ABSTRACT. This research has been dedicated to the experimental study of a French Pleistocene marl through uniaxial compressive tests on prismatic specimens. Induced deformations were measured during testing by classical extensometers applied on the rock surface and by means of photogrammetrical measurements. The choice of this material was forced by its relative good characteristics in terms of material homogeneity when compared with other kinds of marl. The specimen shape was dictated by the necessity to induce plane deformation during testing to be measured by using optical methods. Specific software, implementing a correlation algorithm able to track high deformation fields and model crack propagation was developed and applied to determine displacement and deformation maps at each photogram on the whole specimen surface. Photogrammetrical method has been validated by the comparison with classical extensometer results. This measurement tool has been specifically dedicated to the identification of the microcrack formation during the performed tests for a better understanding of weak rock triggering failure phase before crack propagation start. The interpretation of experimental tests on the light of material physical and mechanical features is illustrated in the paper together with all difficulties encountered in the specimen’s preparation phases due to the material peculiar nature. INTRODUCTION Weak rocks and hard soils are traditionally defined as material showing a mechanical behaviour in between those associated with classical soils and rocks [1, 2]. They often show a complex mechanical behaviour mostly not uniform and anisotropic due to the presence of defects or pre-existing discontinuities, they are strongly influenced by the load law and by the deformation rate, by water content etc...[3, 8]. Moreover classical experimental devices applied for soil and rocks are often unable to test these kinds of materials for the unusual range of strength and deformability involved [9, 10]. Consequently, specific laboratory instrumentations need to be used and unconventional experimental tools to be utilised to measure stress – strain behaviour of these materials [11, 12]. In particular, the local evaluation of the strain induced by the acting stress can not be easily done by conventionally system like extensimeter since they do not guarantee a perfect cohesion with the rock surface at increasing loads. On the other hand, the failure mechanism of this material is characterised by strain localization [13, 14] even for loads