Linking shear fractures observed in laboratory to neoformed faulting structures: example in a Carboniferous limestone

The « Petit Granit », an encrinitic Carboniferous limestone, exploited in different quarries in Southern Belgium, is a very common ornamental rock used in building and civil engineering. The tectonic context is quite well known as well as the mechanical properties. In this paper, possible correlations between tectonic framework (particularly micro-tectonics) and the rock mechanical behaviour are proposed in order to establish different heritage or permanent links between both.
In the Parautochton cover of the Brabant Massif, Carboniferous limestones occur in monoclinal structures low dipping to the south. During the Variscan orogeny, they are affected by shear tectonics inducing strike-slips faulting with associated tension gashes and stylolithes. Later, MesoCenozoic extensional dynamics has induced extensional jointing with karstic features and normal faulting.
Triaxial tests on cubic samples have been performed to assess the influence of confining stresses on the rock behaviour. The rock exhibits brittle failure on the whole range of confining stresses that were used (0 to 100 MPa). Nonetheless, with increasing confining stresses, the amount of plastic strain increases and softening appears on stress-strain curves. The intrinsic curve is not linear but can be interpreted in terms of Mohr-Coulomb criterion by using a set of successive linear portions.
Failed samples show shear fractures organized in conjugated systems, forming an angle of 30 to 60° independently of the confining stresses. Those values are similar to the one measured in-situ on neoformed faulting structures. Therefore, we can consider that a kind of heritage is transmitted to the rock and is expressed again when the material is compressed in laboratory.
Tension gashes are other important structural features observed in Soignies limestone. They have also been investigated at a local scale to determine their influence on the rock strength. In this case, punching tests have been performed at several places from the fissure centre to the intact rock. A 20% decrease of strength is found between the fissure centre and the intact rock, indicating that those features are weakness zones of the rock.