Dynamic compressive mechanical properties of bedding sandstone with pre-existing hole

The exposed layered rock masses with different attitudes can be found in many underground openings such as tunnel,mine roadway and chamber. With various combinations of orientation of layers and loading direction,the dynamic anisotropic mechanical and deformation properties of rock masses affect crucially the engineering stability and safety. The paper aimed to research the surrounding rock stability of roadway in inclined stratified rock mass. To inves- tigate the variation of the dynamic mechanical parameters,fracture propagation and failure modes of flawed bedding sandstone under different layer dip angles,the plate-shape bedding sandstone specimens were manufactured with a sin- gle circular hole in the specimen center. A series of dynamic impact tests were carried out using a ϕ75 mm split Hop- kinson pressure bar ( SHPB) system on seven groups of bedding sand-stone specimens ( size of 60 mm × 60 mm× 15 mm) which contain different layer dip angles φ (including 0°,15°,30°,45°,60°,75° and 90°),where the dip an- gle φ is the angle between the orientation of layers and the loading direction. The dynamic fracture evolution processes of specimens were recorded with a high-speed camera. The experimental results indicate that the strain corresponding to stress peak value ranges from 0. 008 1 to 0. 012 37. With the increase of dip angle, the dynamic compressive strength,the strain corresponding to the stress peak value and elastic modulus of specimens increase firstly and then decrease in general. Shear cracks or tensile-shear mixed cracks always initiate from the compressive stress concentra- tion areas around the holes,and then develop to macroscopic fractures. The low-dip specimens ( φ = 0°) exhibit com- bined tensile-shear failure across and along the bedding plane. The medium-dip specimens (φ = 15°,30° and 45°) ex- hibit shear failure across and along the bedding plane. The highly dipping specimens (φ = 60°,75° and 90°) exhibit X-type shear failure. Orthogonal anisotropic plate theory was applied to calculate the stress distribution around the holes. The peak stress concentration factors around the holes increase with the increase of layer dip angles φ. The max- imal compressive stresses of low and medium-dip specimens around the holes appear at the position of θ = 90°,86°, 81° and 74° (central symmetric position about the origin of θ = 270°,266°,261° and 254°),respectively,where θ is the polar angle of any point around the holes. The observed crack initiation locations of low and medium-dip specimens around the holes appear at the position of θ = 88°,85°,79° and 70° (central symmetric position about the origin of θ = 271°,264°,262° and 252°),respectively,which are roughly consistent to the position of maximal compressive stresses in theoretical analysis. Under impact loading,the load-bearing capacity of roadway surrounding rock in laminated rock is at its weakest,while the orientation of layers parallel to the loading direction. For the excavation of underground chamber with the combination of drilling-and blasting-method and bench cut method,or excavation blasting construc- tion with near-existing roadway,the load-bearing capacity of roadway surrounding rock in laminated rock will be im- proved if the direction of explosive loading can be arranged reasonably.