Multiscale physical and mechanical characterization of fault rocks with emphasis on non-destructive 3D image analysis

The presence of faults along all engineering constructions especially in underground excavation can lead to significant problems. Rocks usually considered as a strong material but when they are locating under tectonic stress, behave like a weak material because of their fractured and weathered structure. So, they are usually under influence of physical, petrographical and alteration changes. Although the general physical phenomenon for weak rocks has been predicted, so far there is a little direct experimental knowledge about the association of fault rock structure with its physicomechanical characteristics. In this study, the geomechanical characterization of fault rocks and the relationship between these characteristics are evaluated. Physical and mechanical properties and their variation considering the distance from the fault are investigated. To have a precise view, a series of laboratory experiments have been performed to identify and quantify the changes of these properties. The experiments were arranged in different categories; the physical, petrographical, and chemical change were evaluated using thin section study, X-Ray powder diffraction, and X-Ray fluorescent spectrometry. To evaluate he porosity and pore network conductivity, different methods such as fluorescent thin section microscopy, X-Ray microtomography, pycnometer analysis, mercury intrusion porosimetry and vacuum saturation techniques were used. The dynamic properties of all the samples have defined with using ultrasonic pulse velocity measurements. And, to investigate the mechanical characteristics of the samples, uniaxial compressive tests and multistage triaxial were performed. The studied samples, apart from the lithology and petrography issue, are represented three different zones in almost all the results. The first and most weak zone is devoted to 0 to 18 meters from the fault. The second zone which showed a lower degree of alteration and fractures is started from 18 to 85 meters far from the fault; and. And the last zone is between 85 to 89 meters from the fault. Petrographical and chemical characteristics, in particular, chemical index of alteration, is changed considering the factor of distance from the fault. These changes showed a good compliance with physical properties, in particular, porosity and shear wave velocity values. The improvement or decay of the properties with the average values of the compressive strength and the distance from the fault are represented by regression analysis. A significant correlation parameter is observed when average values of the compressive strength and Young’s modulus are compared with the average porosity value and chemical index of alteration. Finally, a relation between the porosity at the micro scale and petrographic index of Hoek and Brown failure criterion is evaluated. Since the cost of micro-scale physical characterization is much lower than the mechanical tests, a precise estimation of the fault rock strength based on the pore network investigation has an economical advantage. According to the result of this study, we can avoid the overestimation from the routine exist fault rock classifications. Moreover, in the thesis with utilizing the multivariate data analysis, a statistical method is presented for having a precise prediction of mechanical properties of weak rocks with having only the results of non-destructive rock analysis.

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