CHARACTERIZATION OF THE STRESS-STRAIN RELATIONSHIP OF THE HARD ROCKS OF MADDHAPARA GRANITE MINE, DINAJPUR, BANGLADESH

Bangladesh’s second underground mining and first Hard rock mining project is sited at Maddhapara, where Precambrian basement rocks, consisting igneous and metamorphic rocks of mainly Granodioritic composition, occur at a depth of 128m (borehole GDH24) to 326m (borehole GDH26). A major aquifer bearing formation, Dupi Tila, overlies the fractured crystalline basement formation. Even a slight subsidence or any fault displacement that affect both the aquifer and highly fractured and fissured basement formation may inundate the entire mine area. Hence, stress analysis in and around the excavation area is an essential task for mine design. We investigated the stress-strain relationship of the rocks of three different categories. The samples of all the three categories (category I, category –II, category III) were prepared according to ASTM (American Standard for Testing Material) concrete block. The Uniaxial Compressive Strength (UCS) values of tested rock samples of category I, II and III show that the rocks are of brittle nature and they abruptly fail at the stresses of 126.39 MPa, 59.1 MPa and 39.58 MPa respectively. The stress-strain relationship of the tested samples indicates that some of them fail within the elastic range while others fail beyond the elastic range. The average lower tangent modules values of category I, II and III are 17.91 MPa, 13.73 MPa and 8.73 MPa respectively. From the strength factor and failure trajectories analyzed using the Examine2D software, it is evident that category I rock type is the strongest and is unlikely to fail. Category II is weaker than category I and is vulnerable. Rock type Category III is the weakest and is highly vulnerable. The obtained results will be used to gain a better understanding of the stability of present structural support design.