New Constants of Fracture Angle on Quartz Sandstone

Calculation of fracture angle based on the Mohr-Coulomb concept has been generalized to all lithologies, whereas each lithology has a heterogeneous characteristic that is not the same as one another. This research aims to determine constants that calculating the fracture angle based on the Mohr-Coulomb concept for quartz sandstone. The fracture angle based on the results of calculations using mathematical formulas and measurement results in the laboratory is compared. The empirical result for analysis was obtained from direct measurement under the uniaxial test with a single fracture and compared them based on mathematical calculation. Laboratory testing was applied to 50 mm-sized core samples of sandstone. The value of the fracture angle was obtained after taking uniaxial testing with fracture angle measurement against the plane directly. The determination of the friction angle was performed using an undrained-unconsolidated triaxial method. The determination of the correlation constants was conducted by plotting the scatter graph between the friction angle and fracture angle. The correlation shows that the fracture angle has a very strong relationship with rock characteristics. The constants based on the laboratory test are higher than that of the Mohr-Coulomb concept. The increasing constants depend on several factors: mineralogy, cohesion, internal friction angle, density, void ratio, and Poisson’s ratio. The constants of the fracture angle for the quartz sandstone should be changed from 45 to 53.705. This result may be proof that generalizing the formula of fracture angle to all lithologies may not be applied due to the differences of their characteristics in defining the fracture angle.