Tensile strength and fracture toughness of sedimentary rocks at different buried depths in Shendong coal field

To investigate the tensile strength and fracture properties of sedimentary rock buried at different depths in Shendong coal field,the acoustic emission ( AE) parameters of sedimentary rock specimens at dif-ferent depths in Shendong coal field were monitored by AE monitoring system during the Brazilian splitting experiment. The variation of tensile strength,model I fracture toughness and the ratio k(tensile strength with respect to fracture toughness) of rock specimens with respect to the buried depth was discussed. The damage evolution,failure features and precursory fea- tures of specimens were also analyzed. The results indicate that the tensile strength and the fracture toughness increase with the increase of the buried depth and have power function relationship to the buried depth. In general,the tensile strength and the fracture toughness of rocks buried depth above -300 m are lower than 0. 5 MPa and 0. 15 MPa· m1 / 2 ,while those buried depth deeper than -300 m are higher than 1. 0 MPa and 0. 15 MPa·m1 / 2 ,respectively. The ratio k also increases with the rise of the buried depth,and has a logarithmic function relationship with the depth. The k values of rocks with the buried depth over -300 m are 2-3. The k values are 3-6 for the rocks with the buried depth close to -400 m,while the k values are relatively discrete for the rocks with the buried depth deeper than -500 m. In addition,the failure modes were observed similarly existing in the tested specimens,tension-shear mixed failure is the dominant pattern. However,the shear failure occurs at different time for the specimens with the various buried depths. For example,the shear failure begins to appear in the post-peak stage for the rocks with the buried depth less than -300 m,while the shear failure appears in the pre-peak stage for the rocks with the buried depth larger than -300 m. The failure precursors of the tested rocks appear normally before the peak strength,except the difference of the trig- gered stress. The precursors of the specimens (except medium-coarse sandstone) with the depth less than -300 m ap- pear at the stress reaching 70% -90% peak stress. For the specimens with the buried depth larger than -300 m,the precursors appear at the stress larger than 92% of the peak stress. This indicates that the warning time of the specimen failure decreases with the increase of the buried depths.