Six non conventional in situ shear tests were carried out in order to investigate the strength properties of the Shale Limestone Chaotic Complex bimrock at the Santa Barbara disused open-pit mine. The testing procedure ensures that the failure surface is free to develop in a tortuous way along block/matrix contacts, thus allowing for the evaluation of the bimrock strength parameters by taking into account the influence of blocks, and ultimately to overcome the size limitation of laboratory specimens. The evaluated operative strength parameters show an overall larger friction angle and a lower cohesion compared to that of the clayey matrix, in agreement with the common bimrock mechanical behaviour.
The Shale-Limestone Chaotic Complex bimrock (hereafter referred as SLCC) outcrops over a wide slope in the Santa Barbara disused lignite open-pit mine (Italy). During the past years, in fact, the SLCC has been the object of several studies that assumed it as a homogeneous body governed by the mechanical properties of the clayey matrix only (D'Elia et al., 1988; Tommasi, 1996; D'Elia et al., 2006). In order to overcome the inadequate size of laboratory specimens and namely to take into account the influence of blocks, six non conventional in situ shear tests were carried out on specimens of 0.3m3 in volume. The testing procedure ensures that the shear plane is free to develop inside the specimen and to negotiate in a tortuous way along the block/matrix boundaries; thus differing from the ISRM Suggested Method for In Situ Shear Tests (ISRM, 2007). This aspect has a very important role in shear tests performed on bimrocks: the main consequence for the presence of blocks, in fact, is an increase in tortuosity of the shear plane that causes the increase in shear strength.