Behaviour of joints in sandstones during the shear test

Abstract

In the case of soft rocks, with many beds, or subjected to tectonic tensions, fractures with the separation occur, which cause many problems to the quantitative assessment of their properties. Considering the fact that deformation properties are proportionate to the strength properties also in the case of rocks, measurement of shear stresses can become the basis for quantitative assessment of mechanical properties of rocks joints, in particular as regards their shear stiffness. In the article, the author has presented the results of laboratory studies of shear behaviour of rock joints in Carboniferous sandstones: fine- and medium grained during a shear test. He has presented the values of internal friction angle, cohesion, shear stiffness at the rock joints, as well as joint roughness coefficient. All the tests were carried out at low stresses with the help of direct shear test. The results show that the internal friction angle of fractured sandstones and of intact ones tested at low stress (up to 10 MPa) is similar. Shear stiffness Ks of joints grows up with the normal stress and correspond with the shifting on the discontinuity. For the low normal stresses Ks was obtained as follows: 320 MPa/m, 353 MPa/m and 418 MPa/m. The joint roughness coefficient (JRC) for the fine-grained sandstone amounts to 5 to 7, while for medium-grained sandstone - 6 to 8. This conclusion can be very useful for geological or engineering designing (eg. mining workings support design, underground roadways stability assessment, rock mass classification) when evaluate the shear resistance between sandstone rock blocks.