Experimental Study On Unloading Mechanical Properties Of Irregular Columnar Jointed Rock Mass

The columnar joint is a kind of primary tensile fracture structure commonly seen in basalt.Due to the existence of columnar joints,the complete rock block is cut into regular or irregular prismatic blocks,forming columnar jointed rock mass,which is rich in multiple groups of joint cutting surfaces and micro-cracks,and has good mechanical properties by close occlusions before excavation.However,after excavation,it is easy to be disturbed,shearing,gnawing and damaged,and the combination with the excavation face will even form a large loose small block group,and the mechanical properties will deteriorate rapidly,with significant unloading and relaxation characteristics.In this paper,the method of indoor physical model test and theoretical analysis were combined to study the unloading mechanical characteristics of irregular columnar jointed rock mass.The main research results are as follows:(1)In this paper,the deformation characteristics of the method of unloading the load are studied by means of the method of the single column,and the results show that: the normal dilatancy phenomenon is obvious in the specimen of rough rock joints under the condition of unloading.Under the action of two levels of normal stress,the dilatancy angle increases with the increase of joint roughness,and decreases with the increase of normal stress.The dilatancy displacement decreases with the increase of normal stress.The dilatancy angle of the unloading failure shows a similar "N" shape with the overall change of joint roughness,and the dilatancy angle and dilatancy displacement of the unloading failure are larger than those before the unloading.The peak strength of the normal unloading test of rough rock joints shows an inverted "V" shape with the change of joint roughness JRC.At the same time,two typical failure modes of rough rock joint specimens in normal unloading tests are obtained,namely incomplete shear failure and complete shear failure of the convex body on the joint surface.(2)The uniaxial compression and conventional triaxial compression tests were carried out on the specimens of irregular columnar jointed rock mass with different column dip angles.The results show that the compressive strength and deformation modulus of irregular columnar jointed rock mass under the loading condition show a "J" shaped curve with the change of dip angles.At the same time,three typical failure modes of irregular columnar jointed rock mass under uniaxial and conventional triaxial compression conditions are obtained,namely,shear failure of vertical columnar joint(β=0°),sliding failure along columnar joint plane(β=15°～75°),and fracturing failure along column(β=90°).(3)With different dip angle β irregular columnar joint rock specimens,in the uniaxial compression and conventional triaxial compression test on the basis of study on unloading test,get the unloading stress-strain curve,on the basis of the irregular columnar joints and unloading rock mass mechanics properties and its failure mode is analyzed,the results show that: the peak unloading strength of irregular columnar jointed rock mass changes in a "J" shape with the dip angle of column;When β=0°～30°,75°,volume expansion is easy to occur,β=60°,90°,volume compression is easy to occur.At the same time,three types of typical failure modes of irregular columnar jointed rock specimen under unloading condition are the same as those under loading condition.(4)The failure modes,deformation characteristics and mechanical properties of irregular columnar jointed rock specimens under two loading conditions were compared: the failure mode of irregular columnar jointed rock specimen under unloading condition is the same as that under loading condition,but the difference is slight.At the same time,the volumetric strain under unloading condition is smaller than that under conventional triaxial compression condition,and the volumetric strain increases only when the dip angleβ=75°.The cohesion c and internal friction angle φ of irregular columnar jointed rock mass in the unloading test are different from those in the conventional triaxial compression test under different column inclination angles.