On Dynamic Characteristic Model Of Jointed Rock Mass Under Impact Loading

Due to much existence of defects such as bedding, crack and joint in natural rock mass, the underground rock mass often suffers disturbance of dynamic vibration from outside in practical engineering. Meanwhile, these non-linear structural planesalso affect fairly their dynamic mechanic properties in the rock mass. Therefore, it makes sense that the research of jointed rock mass is good for the study of dynamic stress field, damage field and underground hardened structure for protection against war weapon.This method was adopted mainly combing theoretical analysis, model testing, modern signal processing technology and so on in the article. So, the dynamic properties as well as laws of damage and crack extension for different geometrical characteristics in the jointed rock mass were researched. Based on the similar theory,jointed specimens of different form and unbroken specimenswere prefabricated by using cement mortar. Then, as to different cut-through degree, different dip angle and various groups of jointed rock mass,the dynamic compressive test under different loading speed and impact frequency was conducted respectively through drop hammer impact experimental machine, where dynamic mechanical characteristics could be illustratedfor different jointed rock mass of geometrical form. Based on rock damage theory, laws of accumulative damage for unbroken or jointed rock mass could be studied under the cyclical impact loading as well as dynamic crack propagation mechanism could be also analyzed at the same time. Compared the results of jointed specimens with the unbroken, it is discovered that the jointed characteristics have an effect on impact laws.Main contents and results for this paper are shown as follow:(1) On the basis of method of comparative analysis, the normal dynamic deformation and laws of energy conversion for unbroken specimens as well as different joint continuity during the process of impact. The peak force of unbroken specimens was large than the joint's under the same impact energy. Meanwhile, the peak force and stiffness of joint presented a negative correlation to degree of joint continuity. The dynamic compressive deformation was different and the normal amount of joint was gradually larger as the increasing of impact energy. During the process of loading, when the input energy was low at this time,the size of impact force work presented a positive correlation to joint continuity. However, the work of impact loading would increase at first and then decrease with the increasing of impact energy.(2)According to time-history curves of force and displacement as well as curves of itfor the analysis of different groups and dip angle of specimens, the jointed rock massofdynamic characteristicsfor its geometrical form were researched under various conditions of impact. The vertical displacement of dynamic compression presented a positive correlation to groups of joint, angle of dip and falling height. In single and gradient jointed specimen, the workwas the greatest under effect of dynamic loadingand the impact-resistance strength is the lowest when the angle of dip is 45°. Based on elastic damage theory, the elastic modulus of specimen decreased gradually as the increase of damage. Moreover, the dynamic deformation would transfer from elastic deformation to plastic deformation as the increase of dynamic loading gradually.(3) Based on the damage theory of rock mass, damage variation of specimen was defined by ultrasonic wave velocity. In the same energy of impact, damage degree for different groups of jointed rock mass had a common tendency to both groups of joint and impact frequency. The failure modes of specimens would transfer from tensional and shear failure to shear failure along with increase of jointed groups. When different angle of dip for jointed rock mass suffered dynamic failure, the angle between the jointed tip of crack and extension cord was around 90°. In addition, specimens were easily failure when the angle of dip is 45°. The whole process of crack extension underwent such as conception, development, lap and cut-through. The impact stress wave experienced a complex transmission reflection and part of energy was dissipated due to the existence of crack.