Capturing non-linear stress-strain response of brittle rocks due to closure of coring-induced micro-cracks using 3D bonded block model

The stress-strain curves of brittle rocks can be divided into five regions: 1. crack closure, 2. elastic region, 3. crack initiation, 4. crack damage, and 5. peak and post-peak region. The initial non-linear section of the stress-strain curve is known to be due to the closure of pre-existing micro-cracks. This non-linear section may or may not be present depending on the density and geometry of pre-existing micro-cracks. It is known that some of these micro-cracks may form due to the stress redistribution and tensile stresses generated inside the cores during drilling from deep and high stress grounds. The presence of such micro-cracks may affect the properties of rock specimens determined from laboratory tests. Therefore, the knowledge of the level of core damage (micro-crack density) and associated changes in the laboratory properties of brittle rocks is of paramount importance for reliable designs of deep underground excavations. In this paper, the discontinuum numerical program 3DEC and its Bonded Block Model (BBM) is used to explicitly simulate drilling-induced core damage. The laboratory test data from the well documented case of the AECL's Underground Research Laboratory (URL) is used for numerical simulation and model calibration. The numerical simulations involve: 1) calibrating a 3D BBM to the properties of undamaged Lac du Bonnet (LdB) granite under an unconfined condition, 2) simulating core drilling and associated micro-cracks in the cored specimen (i.e., BBM), and 3) uniaxially loading the damaged BBM and comparing its mechanical properties with those of damaged LdB granite. It is found that the initial region of the stress-strain curve of the damaged BBM is non-linear. This is interpreted to be due to the closure of micro-cracks generated during core drilling simulation. The results of numerical study presented in this paper demonstrate the capability of the proposed modeling approach for a realistic simulation of drilling-induced core damage and associated non-linear stress-strain response of brittle rocks

, 2020