Numerical Investigation of Radial Strain-Controlled Uniaxial Compression Test of Äspö Diorite in Grain-Based Model

Abstract

A complete stress–strain curve is difficult to obtain for Class II rocks because of the abrupt failure after peak stress. Although the servo-controlled loading system has been used successfully in laboratory tests to analyze the failure of Class II rocks, improved simulation techniques are still needed to control the failure of Class II rocks. In this study, the Class II behavior of Aspo diorite was reproduced explicitly using the GBM-UDEC model. The relative differentiations in all mechanical properties between the simulation and laboratory tests were within ± 5%, such as Young’s modulus, and slope of the post-peak stress–strain curve. Based on the stress and crack generation in UCS test simulation, the failure behavior of the Aspo diorite in the post-peak stress region could be classified into three stages: (V) unstable crack growth with decreasing stress, (VI) stable crack growth with increasing stress, (VII) unstable crack growth, and failure of the rock specimen with decreasing stress. Throughout the simulation, the microcrack investigation was conducted in the aspect of number, angle, relative strength of contacts, and location. In the pre-peak stress region, the cracks between 0° and 20° with respect to axial stress was dominant with over 62% of population among total cracks. Cracks were generated preferentially in relatively weak contacts. The ratio of brakeage in the weakest contact was 22%, while the average breakage value of the other stronger contacts was 16%. In the post-peak stress region, the number of cracks generated between 20° and 40° was approximately 1.4 times higher than those between 0° and 20° at Stages V and VII. There was a significant increase in the number of cracks in the strongest contacts when the model was at the beginning of Stage V and Stage VII. In Stage VII, microcracks were generated intensively in central column of the rock model. Moreover, crack coalescence was monitored in the inner column and rock spalling, complete detachment of block in rock specimen model, was observed at the lateral side of the rock model in Stage VII. In the energy calculation, 23% of the strain energy stored in the model was extracted immediately after peak stress to prevent the violent failure of the rock and most of the energy extracted resulted from grain blocks compared to contacts.