含孔洞层状砂岩动态压缩力学特性试验研究

隧道、矿山巷道和硐室等地下岩石工程中揭露的层状岩体往往具有不同的产状,层理弱面的方向与主要动荷载作用方向存在多种组合,相应的动态各向异性力学特性和变形破坏特征对地下岩石工程安全稳定具有至关重要的影响。针对冲击载荷下倾斜层状岩体中巷道围岩稳定性问题,选取一种层理构造显著的黄砂岩,其中层理倾角φ为层理面与加载方向之间的夹角,加工制备倾角分别为0°,15°,30°,45°,60°,75°和90°的7组预制中央圆形孔洞板状试样(尺寸为宽度60 mm×高度60 mm×厚度15 mm),在75 mm杆径分离式霍普金森压杆(SHPB)试验平台上进行冲击压缩试验,并使用高速摄影仪实时记录试样动态裂纹扩展演化过程,研究不同层理倾角条件下预制中心孔洞层状岩石的动态力学参数、裂纹扩展演化过程及最终破坏模式等动态压缩力学特性变化规律。结果表明,峰值应力处试样破坏的峰值应变在0. 008 1～0. 012 37变化,随着层理倾角的增加,试样动态抗压强度、弹性模量及峰值应变整体均呈先增大后减小的变化规律;初始起裂裂纹总是从孔洞周边压应力集中处萌生,随后逐渐形成宏观裂纹,宏观裂纹为剪切裂纹或拉剪复合裂纹;倾角0°试样发生局部沿层理和局部穿越层理的复合张剪破坏,倾角15°～45°试样发生局部沿层理和局部穿越层理的剪切破坏,倾角60°～90°试样最终发生穿越层理的类X型剪切破坏;利用正交各向异性板理论计算孔洞周边应力分布,发现随着层理倾角的增加,孔洞周边应力集中系数的峰值也逐渐增大,且层理倾角为0°,15°,30°,45°的试样孔洞周边最大压应力出现在θ(θ为孔洞周边任意一点的极角)为74°,81°,86°,90°及关于原点中心对称的254°,261°,266°,270°处,同时试验中观测到相应的层理倾角试样分别在88°,85°,79°,70°及关于原点对称的271°,264°,262°,252°处萌生剪切裂纹,与理论分析结果吻合较好。层理方向与冲击载荷平行时,层状岩体中巷道围岩对冲击载荷的承载能力最弱。针对钻爆法分台阶开挖硐室或爆破施工中存在近距既有巷道,应合理布置爆破载荷的方向,避免层理方向与爆破载荷之间的夹角过小而导致巷道失稳。

Experimental study on 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 investigate 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 single circular hole in the specimen center.A series of dynamic impact tests were carried out using a 75 mm split Hopkinson 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 angle 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 concentration areas around the holes,and then develop to macroscopic fractures.The low-dip specimens (φ=0°) exhibit combined tensile-shear failure across and along the bedding plane.The medium-dip specimens (φ=15°,30° and 45°) exhibit 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 maximal 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 construction with near-existing roadway,the load-bearing capacity of roadway surrounding rock in laminated rock will be improved if the direction of explosive loading can be arranged reasonably.