Relationship between point load index and mode II fracture toughness of granite

Abstract

Experimental and numerical methods were used to investigate the relationship between point load index and mode II fracture toughness of granite. A punch-through shear test was used to measure the mode II fracure toughness of granite. Point load test was performed to measured the point load index of jointed granite. Three granite samples with dimension of 20 mmx150 mmx40 mm consisting parallel non-persisent joint were prepared in the laboratory for punch test. Also six recangular specimen with echelon joint was prepared for point load test. Cuncurrent with experimental tests, numerical simulations have been done for punch test by PFC2D and poin load test by PFC3D. Numerical model for punch test has dimension of 100 mmx120 mm. similar to those for joints configuration systems in the experimental test, three models with different rock bridge lengths were prepared. Also, numerical model for point load test has dimension of 100 mmx100 mmx40 mm. six models consisting non-persistent joint were prepared. The punch testing results showed that the failure process was mostly governed by the rock bridge lengh. The shear strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge length. The strength of samples decreases by increasing the joint length. The point load testing results showed that the tensile cracks initiate beneath the loading cone and propagates through the intact rock till coalescence with notch tips. The value of point load index has close relationship with mode II fracture toughness obtained by punch test. The failure pattern and failure load are similar in both methods i.e., the experimental testing and the numerical simulation methods.