the effect of the crack initiation and propagation on the p-wave velocity of limestone and plaster subjected to compressive loading

Since many largescale projects such as tunnels, boreholes constructed for extracting gas and oil and underground excavations for the disposal of chemical wastes are being produced in high stress rock masses; it is necessary to consider the crack initiation and propagation process in rock. various methods have been developed for monitoring the material damage and fracture. among these methods, in recent years, the use of the variation trend of the velocity of ultrasonic waves due to the unique characteristics has attracted the attention of researchers in the field of damage mechanics. in this research, the effectiveness of this method was evaluated in the monitoring of the crack initiation and propagation process in the rock. for this purpose, the effect of crack initiation and propagation on pressure wave velocity was investigated for loading at four stress levels of 30, 50, 70 and 90% of peak strength in limestone (travertine) and six stress levels of 30, 50, 60, 70, 90 and 100% of peak strength in plaster. a microscopic thinsection of limestone sample was prepared and studied in order to clarify the cracking process after compressive loading parallel with the loading direction. the results of the research indicate that, the pressure wave velocity has increased up to a certain stress level as a result of compressive loading and then, with a higher loading, the pressure wave velocity suddenly and severely decreases. accordingly, in the limestone to the stress level of 50% of the peak strength, the pressure wave velocity increased by about 5% and then decreases by about 50 percent with increasing load. similarly, in plaster artificial stone up to the loading level of 70%, the wave velocity increased by about 1.6% and then suddenly decreased. in addition, the study of microscopic thin sections showed that when the limestone is subjected to stress, the cracks initiate and propagate approximately parallel with the applied load (010֯). it was also observed that under stress around the pores in the texture of the rock, concentration of stress has occurred and, cracks around these pores have grown and penetrated into the grains by breaking the bond between the grains.