Microcracks in ultrabasic rocks under uniaxial compressive stress |
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| Authors: | Ioannis Rigopoulos Basilios Tsikouras Panagiotis Pomonis Konstantin Hatzipanagiotou |
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| Institution: | 1. University of Patras, Department of Geology, Section of Earth Materials, GR-265 00 Patras, Greece;2. University of Athens, Department of Geology and Geoenvironment, GR-157 84 Athens, Greece;1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;2. Key Laboratory of Deep Coal Mine Excavation Response and Disaster Prevention and Control, Anhui University of Science and Technology, Huainan 232001, China;1. South China Research Institute on Geotechnical Engineering, School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510641, China;2. University of Lille, CNRS, Centrale Lille, LaMcube UMR9013, 59000 Lille, France;3. Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China;4. State Key Laboratory Frozen Soil Engineering, Cold and Arid Region Environmental and Engineering Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China |
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| Abstract: | The initiation and propagation of microcracks under stress are highly dependent upon the mineralogical and textural characteristics of the various lithotypes. Detailed observation and quantification of microcracks before and after uniaxial compression test were conducted. A fresh olivine-rich harzburgite and a serpentinized dunite were analyzed, collected from the Pindos and Vourinos ophiolites (northern Greece) respectively, in order to compare their microcrack patterns. Quantitative analysis indicated that during uniaxial compression the intragranular microcracks, which are the dominating crack-type, are gradually transformed or organized into transgranular cracks. Some of the newly formed transgranular cracks may also be a result of the growth of existing grain-boundary cracks. The new intragranular microcracks in the olivine-rich harzburgite are oriented predominantly parallel to the compressive stress direction, while those in the serpentinized dunite show a scattered orientation presumably due to the mesh texture of this rock-type. The new transgranular cracks of both peridotites tend to be subparallel to the compressive stress direction, however, many of them show a random orientation due to the fact that they have been formed as a result of the propagation of grain-boundary cracks. The occurrence of the soft serpentine along fracture surfaces of olivine, when it is in assemblage with orthopyroxene, tends to absorb the applied stress hampering the development of microcracks in olivine. On the other hand in serpentinized peridotites, the microcracks are enhanced in olivine because it is surrounded by large amounts of the much softer and flexible serpentine. Microcracks are usually formed along the cleavage planes of orthopyroxene porphyroclasts, indicating that such crystallographic preferred orientations act as planes of weakness, controlling the direction of the crack paths. Cr-spinel grains likely comprise locations of initiation of microcracks due to their very hard nature and different mechanical behaviour relative to the surrounding silicate phases. Knowledge of the mineralogical and textural characteristics may assist in the prediction of potential development of failure surfaces of an ultrabasic rock in-service. |
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